Make a Vessel That Will Echo Back to You
3. PRINCIPLES OF INSTRUMENTS FOR FISHERIES ACOUSTICS
3.1 The Echo-Sounder
3.2 The Analog Echo-integrator
3.3 Digital Echo-Integrators
3.4 Test Instruments
Echo-sounders transmit a pulse of acoustic energy down towards the seabed and measure the total time taken for it to travel through the water, i.e. the outwards and return journey. If the measured time is one second and it is known that the speed of acoustic waves is 1500 m/s, the depth is obviously (1500 x 1)/2 metres = 750 m.
By using a recorder with slowly moving paper to display the time of transmission and then the echoes as they return, a past history of the depth and of the sea bed topography is built up. If the system is sufficiently sensitive it will also display the echoes from fish, but this gives merely an indication of their relative abundance. Instruments capable of making quantitative acoustic measurements are needed, together with methods of turning these into figures of the absolute fish abundance. To do this, echo-sounders with precise characteristics have evolved. Their signals are coupled to a specially developed instrument, the echo-integrator, which selects and processes them in various ways. In this section we first consider the echo-sounder.
3.1 The Echo-Sounder
3.1.1 Time base
3.1.2 Transmitter
3.1.3 Transducers and Acoustic Beams
3.1.4 Receiver Amplifier
3.1.5 Displaying and Recording Signals
3.1.6 Recording Paper
There are many units, each with distinct functions, which combine to form a complete system for the measurement of acoustic signals related to aquatic biomass. The echo-sounder comprises a transmitter, transducer, receiver amplifier and timebase/display. Figure 17 is a block diagram showing the interconnection of these units. Blocks 1,2,4 and 5 are usually contained within the same cabinet and it often requires only the connection of the transducer (block 3) to enable soundings of depth to be taken. The operation is as follows.
The timebase (block 1) initiates an electrical pulse to switch on (modulate) the transmitter, which in turn produces a pulse of centre frequency (f) and duration (p), to energise the transducer (block 2). Electrical energy is converted by the transducer into acoustic energy in a pulse of length cp which is beamed into the water, insonifying objects in its path. Echoes from these objects return, to be converted back to electrical pulse signals by the reverse process in the transducer. These signals are normally very small so they are amplified, but in a selective way, relative to the time they occurred after transmission (time-varied-gain, TVG). This compensates for the power losses when travelling out and then back to the transducer. After the TVG process, signals are demodulated (detected) i.e. the information they contain, amplitude and duration is extracted. In this form signals can mark a paper, or be processed by an echo-integrator. Now we consider the units in detail.
3.1.1 Time base
One function of a time base (block 1) is to provide the 'clock' which sets the accuracy of depth measurement, the other is to control the rate (P) at which transmissions are made.
In section 2.7 we saw that, except for extreme conditions, the effects of salinity and temperature on the speed of an acoustic wave are not very significant for fisheries surveys. This means that the speed of the timebase 'clock' can be set in relation to a nominal speed of acoustic waves and 1500 m/s has been adopted for most marine purposes. This speed is exact for a temperature of 13°C and a salinity of 35‰ (see Figure 9). At the extreme temperatures shown on this figure (but with the same salinity of 35‰) depth errors of about 3% would occur, i.e. at 30°C the recorded depth would be 3% shallower than the true depth and the opposite at 0°C. The timebase may consist of a 'constant' speed motor driving a pen across recording paper, or an electronic circuit controlling the spot of light moving over the face of a cathode-ray tube. In either case it is also used to initiate the 'trigger' pulse which marks the point of transmission i.e. zero on the depth scale.
The trigger pulse is so-called because it 'fires' or 'triggers off' the transmission from the echo-sounder. This is important because it must always occur at a precisely defined interval of time, chosen so that the rate of transmission (P) pulses per second, sometimes called pulse repetition frequency (PRF) is suitable for the depth of water to be surveyed. That is, a long enough interval between pulses for all the echoes resulting from one transmission to have returned, before the next transmission. This factor is controlled by the depth selector of the echo-sounder, i.e. the manufacturer arranges a suitable PRF for each depth scale.
3.1.2 Transmitter
The transmitter (block 2 of Figure 17) is triggered from the timebase at a rate of P, pulses per second. Each 'trigger' starts the pulse duration circuit (symbol t), it runs for a selected time and during this time the actual echo-sounder frequency is coupled to the power amplifier which is in turn connected to the transducer. A specific number of cycles at the correct frequency are released by the pulse duration circuit. If the frequency is 38 kHz we know from section 2.7 that the periodic time, t (time taken to complete one cycle) is t = f-1 i.e.
t = 1/38000 = 26 x 10-6 seconds or 26m s.
Figure 17.
If 20 cycles are transmitted, the pulse duration
t = 20 x 26 m s = 520 m s or 0.52 ms.
We know that acoustic waves travel at a speed (c) of 1500 m/s so the distance covered in this time is
ct (12)
which in the present example is
1500 x 520 x 10-6 = 0.78 m pulse length
i.e. the actual physical length of the pulse in the water.
This is an important parameter of a fisheries echo-sounder because
(a) it determines the vertical (depth) resolution between targets, i.e. between one fish and another, or between a fish and the sea bed. The minimum distance between any objects X and Y, sufficient for their echoes to be separated isct /2 (13)this is shown in Figure 18 and discussed further in Section 9.4.2. The shorter is t, the better the resolution.
(b) it affects the transmitted energy. The longer the pulse in the water the greater is the chance of detecting targets at long distances because the average power is increased.
Figure 18.
There are physical limitations to the minimum pulse duration which can be used and to the amount of power it is possible to transmit, which are not related to the transmitter.
A power amplifier within the transmitter raises the power output to some hundreds of watts, or even a few kW and this power level must remain exceptionally constant. It is measured with the transducer connected, either by taking the peak-to-peak voltage, converting it to rms, then squaring it and dividing by the transducer resistance RR (see section 3.1.3 about RR).
(14)
or, it may be more convenient to read peak to peak voltage directly, then
Power = (V2 peak-to-peak)/8RR (15)
3.1.3 Transducers and Acoustic Beams
Although there are separate transmitter and receiver circuits within all echo-sounders it is normal to use only one transducer for both transmission and reception. A transducer can be described as an energy converter; during transmission its input is electrical and its output is acoustic; for reception the input is acoustic and the output electrical. It is similar in function to a combined loudspeaker and microphone, but the different acoustic properties of water mean that it is not possible to use the same designs. Also, a much higher efficiency of energy conversion is possible in water than in air. When used for transmission the transducer is known as a projector, and when receiving, it is called a hydrophone. Underwater transducers use an effect whereby the actual dimensions of a piece of material change under the influence of either a magnetic (magnetostrictive), or electric (electrostrictive) field. If the field follows the electrically applied oscillations the resulting change in dimensions will generate acoustic pressure variations at the same frequency. The opposite effect occurs when an acoustic echo acts on the face of a transducer, the dimensions change, producing a voltage across the terminals which varies in sympathy with the echo.
In the region close to the transducer face the axial acoustic intensity varies in a complex way between maximum and minimum levels. As the transducer expands, it exerts pressure on the water immediately in contact with it, thus causing compression. When the transducer contracts, the pressure is reduced, causing rarefaction. These effects of compression and rarefaction are projected forward, still contained within dimensions equal to those of the transducer face until a distance, as illustrated in Figure 19 is reached. The volume contained within this distance and the dimensions of the transducer face is known as the near-field.
Figure 19.
Within the near-field (sometimes called the Fresnel diffraction zone) and far-field for that matter, the distance from any edge of the transducer face to a point on the axis is greater than the distance from the face along the axis to the same point. If we consider the variation in distance to a given point for all the vibrations leaving the transducer face it is possible to visualise the interference effects which arise and cause the maxima and minima of acoustic intensity to occur. For practical purposes the near-field ends and the far-field begins at a distance R of
R = 2L2 l -1 (16)
where
L is the length of the longest side of a transducer, or its diameter
l is the wavelength
both L and l in metres.
The minimum distance for measurements is shown in Chapter 7, Figure 44.
Acoustic intensity from a projector is greatest on the axis of the beam (Figure 20), it decreases as the angle from the axis increases, until the first zero of the response pattern is reached. Beyond the angle of this zero is the first sidelobe which itself goes to zero at a still greater angle and the pattern continues, each sidelobe having a progressively smaller response the greater its angle from the axis.
Figure 20.
The beam angle is not usually measured to the first zero for reference purposes, it is always measured to the angle where the response is half that on the axis.
10 log 1/2 = -3 dB
and the reference angle is quoted as the half angle q /2 to the half power level, i.e. from the axis to the angle where the response is -3 dB. In Figure 20 a polar diagram of an actual transducer response is shown which illustrates the relationship of the main lobe and sidelobes, when L >> l the full beam angle q can be calculated to a good approximation from
q = 57.3 l L-1 (17)
where
L and l are in m
q is in degrees
57.3 is the number of degrees in a radian
l is the wavelength
L is the diameter of a circular face, or the length of a rectangular face.
By re-arranging this we can find the length of the active face of the transducer whose pattern appears in Figure 20.
L = 57.3 l q -1 (18)
Of course, if the transducer is rectangular it will have a different beam angle in the front to back direction to that in the side-to-side direction. However, assuming the above transducer is circular (diameter L) and is resonant at 38 kHz,
l = cf-1 = 1500 ÷ 38 x 103 = 3.95 x 10-2 m
L = 57.3 x 3.95 x 10-2 ÷ 12.5 = 0.18 m
A general rule with transducers is that, the narrower the beam the larger is the transducer.
A property of transducers, related to the beam angle, is the directivity index DI. For the present purpose it can be defined as the ratio of acoustic intensity transmitted or received by a transducer of full beam angle q, to that of an omni-directional transducer. In other words it is a measure of the extent to which transducers can concentrate transmitted or received acoustic power. Figure 21 illustrates this.
Figure 21. (a)
Figure 21. (b)
Figure 21. (c)
For a circular transducer the approximate expression for DI is
DI = 10 log(2p al -1)2 (19)
where
a = radius in m
l = wavelength in m
Applying this to the transducer above
DI = 10 log((6.28 x 0.18/2) ÷ 3.95 x 10-2)2 = 23 dB
When the transducer is square or rectangular and the length of the shortest side,
L >> l, then
DI = 10 log 4p A l -2 (20)
where A = area of transducer face
if the beam angle is known, but the area is not
DI = 10 log 4p /(q 1/57.3)(q 2/57.3) (21)
where
q 1, (degrees) is the full beam angle in one direction
q 2, (degrees) is the full beam angle in the other direction.
An important property of transducers is their frequency response. Transducers used for fisheries survey purposes are resonant at a particular frequency, often called the echo-sounder frequency e.g. 38 kHz. But if they only responded to this one frequency it would be necessary to use an infinitely long transmission which would make echo-sounding impossible. At the other extreme, if we tried to use an infinitely short pulse, the transducer would need to respond to an infinite number of frequencies. This is because a square pulse is made up from an infinite number of different frequency sine waves. Fortunately, a reasonable shape of pulse can be achieved with a relatively small, finite number of frequencies so a compromise can be made.
The design and construction of a transducer determines its frequency response, or bandwidth (BW) as it is known. Bandwidth is defined as the number of Hz between the frequency, at each side of the resonant frequency, where the transducer response is -3 dB of the maximum. It is not possible to change the transducer bandwidth which means that
(a) there is a minimum pulse duration
(b) there is a maximum receiver amplifier bandwidth. (See next section.)
The shape of the bandwidth curve is controlled by a factor called Q.
Q = Resonant frequency/f2 - f1 (22)f2 is the highest frequency where the response = -3 dB
f1 is the lowest frequency where the response = -3 dB.
Typically Q might be 10 to 15 for a 38 kHz transducer.
In order to pass a pulse without reducing its amplitude and excessively distorting its shape the minimum bandwidth must be
BW = 2t -1 (23)
Assuming Q = 10 and f = 38 kHz (the resonant frequency)
BW = 3.8 kHz
the value of pulse duration to match this is
t = 2/(BW)-1 = 2/3.8 x 103 = 526 x 10-6 ie 526 m s or 0.526 ms
Note that, whilst it is necessary to have a wide bandwidth to preserve pulse shape, the greater the bandwidth the more noise is let into the receiving system. This point is discussed in chapter 4.
Two other properties of transducers are important to the full understanding of their use and application to fisheries surveys; the electrical impedance and the efficiency of energy conversion. In section 2.1 the resistance R of an electrical circuit was the filament of a lamp (the energy converter). Power in the circuit was related to the square of the voltage or current in proportion to the resistance. The function of a transducer is extremely complex but in principle the method of calculating the power input is similar to that applying to the lamp. A transducer does not present a simple resistance at its terminals, instead it has an impedance. This term is used when there is a combination of resistance and reactance (resistance to AC) in a circuit. The effect of the reactance is frequency dependent but it does not dissipate power, it impedes the flow of current according to the frequency. Its effect is cancelled by use of an equal reactance with an opposite sign. What we need is the value of the effective resistance, usually called the radiation resistance (RR) of the transducer. It is not a simple operation to measure RR but manufacturers normally provide this figure to enable power calculations to be made.
Transducer efficiency (h) is defined as the percentage of power output to power input whether this is electrical to acoustic (transmission), or the reverse (reception). Typically the efficiency of magnetostrictive transducers is 20 to 40% and electrostrictive types, 50 to 70%.
The sensitivity of a transducer (SRT) as a receiver of acoustic waves is expressed in terms of the number of dB with reference to one volt for each micropascal of pressure, i.e. dB/1 Volt/1m Pa. It is normal for SRT to have a value somewhere in the range of -170 to -240 dB/1 Volt/1 m Pa (-170 is the most sensitive of these). An approximate figure is given by
SRT = 20 log (2.6 x 10-19 h A RR) 1/2 dB/1 Volt/1 m Pa (24)
where
h is % (e.g. 50% = 0.5)
A is the transducer face area in m2
RR is the radiation resistance in ohms.
This is an appropriate point to consider the receiving system beyond the transducer.
3.1.4 Receiver Amplifier
This is block 4 of Figure 17, usually the most complex electronic unit in the echo-sounder. A diagram illustrating the receiver amplifier principal functions appears as Figure 22. The purpose of the complete unit is to amplify the signals VRT received from the transducer in a precisely controlled manner and to present them to the following instruments (the echo-integrator or echo-counter) at a suitable level of amplitude for further processing.
Figure 22.
Starting at the input of block 1 of Figure 22, the transducer output is electrically matched to the input of the receiver, i.e. in terms of impedance and frequency bandwidth. Sometimes the receiver bandwidth is controlled by means of a switch to closely match the transmitted pulse duration t, BW » 2t -1. Although quoted at the -3 dB response points on either side of resonance in the same way as a transducer, the receiver bandwidth is often controlled until the response is at least 40 dB down on the maximum. Usually a 'bandpass' form of response is provided because it only allows those frequencies which lie within the wanted band to pass from the input, thus minimising the effects of high level wideband interference.
Overall amplification, or gain factor G is defined as
G = 20 log VR/VRT dB (25)
where
VR is the output voltage
VRT is the minimum detectable voltage from the transducer.
The overall receiver response is defined as the voltage VR (dB/1 Volt) relative to an acoustic intensity of 1 m Pa at the transducer face. Gain must be precisely controlled in relation to depth and blocks 1 and 2 of Figure 22 automatically vary the tuned amplifier gain relative to the time after transmission. This is known as time varied gain TVG and the circuits comprising it are the TVG generator and controller, see sections 4.2; 7.2.2. At the beginning of each sounding period the transmitter trigger pulse also starts the TVG generator control circuit (block 2) after a fixed delay, often at 3 m depth but it can be less.
Modern TVG circuits operate digitally; for each small time increment there is a corresponding change of gain in the amplifier, the rate of change depending on which TVG law is in use, see section 4.2 for details. With a correctly functioning TVG the calibrated output voltage VR from the receiver amplifier is independent of the depth to the target, preferably to an accuracy of ±0.5 dB or better at any depth over which the TVG is designed to operate. This is of course provided that the TS of a target does not itself vary with depth.
In addition to the trigger pulse which initiates timing at the beginning of each sounding period, there is another input to the TVG. This is the absorption coefficient a for which the, TVG circuits must compensate. A value for a is determined at the start of a survey and switched, or keyed, into the TVG circuit where it remains the same until conditions change sufficiently that it must be updated, see section 2.6.1.
All amplifiers produce some noise, i.e. with no input signal from the transducer, or with merely a matched resistor replacing it, there will be some noise at the output; the receiver self noise. This electrical noise must always be below the lowest level of acoustic noise likely to occur from a very low sea state when the ship is stationary, or, when working at higher frequencies, the thermal noise level, see section 4.7. Receiver self-noise can be quoted as less than -n dB/1 Volt referred to the input terminals but with a TVG amplifier is not constant. Modern receiver amplifiers generally have input sensitivities of 1 m V or less, i.e. -120 dB/1 Volt or less.
The maximum depth at which a given size of target can be detected is the point where it is just distinguished above the noise level, but for acoustic survey purposes the SNR must be greater than 10 dB. At the other extreme there is a maximum size or density of target with which the receiver can cope at short range due to the saturation level of the circuits. Receiver saturation is defined as the condition when the output voltage no longer follows the input voltage linearly, i.e. the gain factor is not constant. It is vital that the receiver voltage response (gain) is linear between the extremes of signal level (³ 120 dB) likely to be encountered under practical survey conditions. The difference between the minimum useable signal at the receiver input and the maximum input signal which does not cause saturation is the dynamic range. A typical output signal dynamic range might be between 50-80 dB. For measurement purposes the output voltage VR is always taken from the calibrated output but there is usually another amplifier which processes the signals for display purposes, either a paper recorder or a rectified 'A' scan cathode ray tube display.
3.1.5 Displaying and Recording Signals
Once amplified, the echo signals are still in the form of a pulse comprising a certain number of cycles at the echo-sounder frequency, Figure 23(a). For display purposes only this pulse at the echo-sounder frequency is further amplified then demodulated, otherwise known as 'detected', or 'rectified', Figure 23(b). This process removes all traces of the echo-sounder frequency, and, either the positive half of the negative half of the pulse. The result is a uni-directional DC waveform which can be used to mark a paper record, or to deflect the beam of a cathode-ray tube (rectified 'A' scan). An unrectified 'A' scan CRT would take its signals from the calibrated output.
Figure 23.
Signals cannot be displayed intelligibly without a timebase. The function of a time base was described earlier although it is usually an integral part of a display. There are multi-stylus 'comb' recorders which use an electronic time-base, but some recorders of scientific echo-sounders still have a mechanical timebase. In these systems a motor and gearbox drive a marking stylus across electrosensitive wet or dry paper which is slowly drawn over a metal plate, at 90° to the path of the stylus.
As the stylus rotates, or moves past the zero mark on the recorder scale the transmitter 'trigger' contacts operate, causing an acoustic pulse from the transducer. Whilst the stylus continues to move across the paper, echo signals start to return and mark the paper at the instant they arrive. When the stylus reaches the zero mark again, the paper has been drawn along so that successive soundings are just separated from one another giving the familiar record. A recorder timebase normally generates time marks and for acoustic survey purposes it is important to have an input from the ship's log to mark the paper at the end of each nautical mile or some other unit of time or distance.
3.1.6 Recording Paper
Moist paper is sensitive to weak signals and has a good dynamic range relative to dry paper (the ability to show a range of different colouring according to the signal strength). It is still widely used despite a number of disadvantages. These are
1. Moisture content must be carefully controlled during manufacture
2. Careful packaging and storage before use
3. Must be 'sealed' in the recorder to retain moisture
4. Shrinks when it dries
5. Fades quickly and discolours if exposed to light.
Stylus pens for moist paper have 'thick' polished tips and are applied to the paper at a constant pressure. Compensation is made for the change of marking density with change of speed of rotation. Dry paper is prepared with electrically conductive surfaces and a filling of fine carbon powder between them. A fine wire stylus conducts a high voltage to break down the front surface paper and make a dense black mark. Although this marking process is difficult to control and consumes the stylus, less storage problems occur before and after use. Dynamic range is about 10 dB whereas nearer 20 dB is claimed for the moist paper. Multistylus recorders can use either wet or dry paper.
3.2 The Analog Echo-integrator
3.2.1 Demodulator
3.2.2 Amplifier
3.2.3 Threshold
3.2.4 Depth and Interval Selection
3.2.5 Voltage Squarer
3.2.6 Voltage Squared Integrator
3.2.7 Display of Integrated Signals
Echo-integrators were first used in the late 1960's when only analog techniques were practicable. Despite the introduction of a number of digital integrators many analog units are still in use. Because of this the essential functions of signal processing and echo-integration are first described by reference to the Simrad QM system. A brief description of the main features of the digital units is then given in section 3.3.
An echo-integrator receives all signals from the calibrated output of the echo-sounder, see diagram 1 of Figure 24. These signals require further processing and the facility for the operator to select sections, or intervals of the water column at depths which can be adjusted to make the echo-integrator into a practical tool. Because of this there are many circuit functions, of which only one is strictly an integrator, but it is convenient to place them together and call the resulting system of units an echo-integrator. The term integrator is used in its mathematical sense of measuring the area under a curve of voltage versus time. Time is usually proportional to the distance moved by the survey vessel and the voltage output is proportional to fish density. A block diagram showing the main functions of an echo-integrator appears in Figure 24(a) and the associated waveforms in 24(b).
Figure 24. (a) Block schematic diagram of an analog echo-integrator (b) the wave forms associated with each block
3.2.1 Demodulator
When the TVG controlled signals from the calibrated output of the echo-sounder reach the echo-integrator, they still consist of sinewaves at the echo-sounder frequency. It has been shown that a sinewave has equal positive and negative values and the information it carries (the modulation) is in the form of equal positive and negative changes of amplitude. The integral of a sinewave is zero, so before integration the information must be changed to a different form. This process is known as demodulation, sometimes called detection, or rectification. Figure 23(a)(b) and block 2 of Figure 24.
This completely removes either the positive or the negative portions of the signal so that only variations between zero and one polarity occur, but these are still at high frequency. A further process filters out the high frequency half-cycles and we are left with the average voltage (i.e. an 'outline' of the signals) of varying amplitude according to the signal strength. In section 3 of Figure 24 there is an illustration of the waveform in section 1 when it has been demodulated. After this process there may be a need to amplify the signals.
3.2.2 Amplifier
Survey conditions in regard to density of fish and depth at which they occur can vary widely so it is sometimes useful to have an amplifier (block 3) to increase the amplitude of the signals by a precisely known amount. If a thin layer of widely spaced targets is to be integrated, the signals may be very small so that the subsequent processing cannot be carried out efficiently. Any change of signal amplitude is important so a switched type of control is necessary allowing say, 0-10-20-30 dB of amplification to be used. These steps of gain correspond to amplitude changes of 1, 3.16, 10 and 31.6 times respectively.
3.2.3 Threshold
This function, block 4 of Figure 24, is linked to the gain control of the amplifier to ensure similar operation at each setting of the latter. The effect of the threshold control is to vary the zero reference of the DC waveform by a small amount so as to suppress noise, which although at a low level, may exist throughout the full depth interval, thus giving rise to a significant integrated output. Of course the threshold setting must be taken into account when the final results are being calculated. However, in order to make the processing after the threshold as accurate as possible, the amount subtracted from each signal above the threshold level is added again but exact compensation cannot be achieved. The threshold control should never be used unless it is absolutely essential. When used with analogue integrators it seriously biases the obtained results in a manner which cannot be reproduced. The effect of any threshold is difficult to calculate so use of a threshold is inadvisable for quantitative measurements.
3.2.4 Depth and Interval Selection
Although the echo-integrator accepts signals from the whole water column it is necessary to have a means of excluding the transmission and the bottom echo from being integrated and this is the function of block 5, Figure 24. It is desirable to be able to select specific depth layers within the water column and to vary the extent of the layer and the depth at which it starts.
In the early equipments thumbwheel switches controlled the settings, usually in increments of 1 m. Thus a depth interval 2 m wide could be placed at a depth of 100 m for integration. The action of the depth and interval selector is initiated by the same trigger pulse which operates the transmitter and starts the TVG. It causes a circuit to operate for a duration of time proportional to the depth at which integration is required to start. When this time is reached, the first circuit causes another to operate for a time proportional to the depth interval required, this is sometimes known as an electronic signal gate. Even though the depth interval has been selected the signals are still not ready for integrating.
3.2.5 Voltage Squarer
Seen as block 6 of Figure 24 this performs one of the most critical functions in an echo integrator. It is necessary because the signal voltages V are still proportional to acoustic pressure p. Density of fish is proportional to acoustic intensity which is proportional to p2.
Using the relationships and the analogies discussed in Chapter 2, i.e.
V is analogous to p and V2 µ W
W is analogous to I so p2 µ I
we can say that by squaring the voltages they become proportional to intensity. The effective gain steps of 3.2.2 are then 1, 10, 100, 1000 times, corresponding to 0, 10, 20, 30 dB respectively.
3.2.6 Voltage Squared Integrator
When the echo signal voltages have been squared, they go to block 7 of Figure 24. It is here that the energy, represented by the area under the squared voltage curve, is put into its final form of a DC voltage whose amplitude at any given time is proportional to the acoustic intensity of the signal. In Figure 24 there are two signals selected by the INTERVAL gate, the deeper of the two is partly lost because it is not completely inside the gate. The DC waveform in block 7 shows how the integrator voltage increases as the first echo rises to its maximum then falls again. When this echo finishes, the DC is maintained at the level it has reached until the next signal occurs. As shown in the waveform of block 7 the level then rises again when the second signal occurs, in this instance the rate of increase is greater than that due to the previous signal. This is because of the larger amplitude.
At this point integration is complete for the one sounding period illustrated. Although echo-integrators usually have a facility for display of single sounding integrals it is of limited value and the normal arrangement is to allow the integrals to accumulate over a given time period, or a nautical mile, after which the integrator is reset and the DC voltage starts again from zero.
3.2.7 Display of Integrated Signals
The simplest form of display possible is a DC voltmeter of either the analogue or digital type (see Chapter 7 for details) but this is not very convenient, eg when reset occurs the reading is lost. Usually a recording voltmeter is provided which displays and records the integrator output on heat-sensitive paper. In this way the variations in echo intensity can be related to positions along the ships track.
3.3 Digital Echo-Integrators
3.3.1 Simrad QD Integrator
3.3.2 Biosonics DE1 120 Integrator
3.3.3 AGENOR Integrator
3.3.4 Furuno FQ Integrator
The most recent instruments developed for fish stock assessment purposes are based on digital techniques. These have similar functions to the analog system described in section 3.2 but digital instruments have greater versatility and are inherently more accurate.
Computer technology which forms the basis of digital systems is becoming commonplace in everyday life, but because of its relatively recent application to fisheries acoustics, it may pose problems to those installing, operating and maintaining such equipment until they become fully familiarised with it. Digital techniques and computer technology give high speed, accurate operation, avoiding the drift and stability problems inherent to sensitive analog systems. A digital circuit has two states only, OFF or ON, corresponding to 1 or 0 respectively. These are known as binary digits (or bits).
Signals from the echo-sounder are analog, they are changed by means of an analog-to-digital converter, (ADC) into a 'word' comprising a numbers of bits, e.g. the Simrad and Biosonics digital integrators use 12 bit words. A description of the functions carried out in an echo-integrator was made easy using the Simrad QM as an example because the waveforms throughout the system illustrate what is happening.
In a digital unit after the ADC there is nothing of this sort to visualise, there are merely the digital words being acted upon according to the inbuilt programs or operator inserted instructions.
Many of the operational features of analog integrators are found in digital systems, but they also have additional ones. The difference immediately obvious between the systems is the manner in which they are controlled. Instead of a large number of front panel controls with which to set the various equipment functions, the operator of the digital unit is provided with a computer style keyboard to type in the instructions. Inside is a computer plus a microcomputer, or microprocessor, memories for the program, the interface, a separate data memory and a data-logger sets out results on a typed record sheet.
3.3.1 Simrad QD Integrator
The QD equipment comprises two small rack-mounting units and a keyboard. Part of the system is called the QX Integrator Pre-processor which although specifically designed for use in conjunction with the QD in one version, can form the interface between the scientific echo-sounders and any general purpose computer in other versions.
The QX accepts inputs by push-button selection, or by software instruction, from one of four echo-sounders in the frequency range of 10-200 kHz. If the QX510/QD or QX525/NORD 10 are used, the echo-sounder can be selected by the data terminal. These combinations accept signals with a dynamic range not exceeding 70 dB, -50 to +20 dB relative to 1 Volt, ie 3 mV to 10 V. From the echo-sounder comes the bottom pulse, a transmitter trigger pulse, a digital 'hold' for the echo signal level, and an inhibit signal for echoes below the threshold level. If the input signal level exceeds +17 dB/1 V, ie 7 volts, a light-emitting diode (LED) flashes on the front panel and a warning is sent to the QD. The echo-sounder signals are converted from analog to digital form before being squared, but the threshold can be applied to either the analog or the digital part of the circuit, or both. A high-performance demodulator, a 12 bit ADC, a fast-operating signal squaring unit and an accumulator for signals prior to integration are contained in the QX.
Figure 25, shows the connection to external items of equipment needed for a complete system. The labels of blocks representing major operational functions are self-explanatory, but it is not possible to judge the practical versatility, or flexibility of the system from this figure. A description of the functions starts with the way that signals are 'sorted by depth' in the QD.
Figure 25.
1. Depth Intervals or 'layers' as they are described (to avoid confusion with other types of interval in this system) can be programmed to operate down to 1000 m. Eight such layers are available in transmission-locked mode, they have a depth accuracy of 0.1 m and are sampled at each 2.5 cm of depth, ie every 33 m s in time. To set up the depth-sampling layers, the operator enters instructions through the keyboard, for the depth of start and finish of each layer and lines at the required positions appear on the echo-sounder record. The pattern of depth layers cannot be changed whilst the system is integrating, for modification the 'initial' setting up procedure must again be used. Each layer may have a different threshold ascribed to it if necessary. Any two depth layers can be selected to display their integrated output in mm deflection on the echo-sounder paper record.
2. In addition to the eight depth layers referred to above, there are two bottom-locked layers which require a bottom signal of good quality, ie having a clean, fast-rising leading edge and must exceed a given amplitude. If no suitable bottom signal is received, or if strong fish echoes are likely to be mistaken for the bottom the system prevents integration. The method ensuring that the bottom contour is followed properly whilst acoustic conditions permit, depends on the generation of a so-called 'window'. Its operation may be visualised by considering a square pulse which starts just before the bottom signal and ends just after it. When the water depth is greater than 10 m, the window circuit seeks a bottom signal between +25% or -12.5% of the depth recorded by the previous bottom signal. If there are three consecutive transmissions without a bottom signal appearing in the window, it then opens from 1 to 1000 m to search for this signal, and, once found, holds it in the window again.
When positively identified, the bottom signal can safely be used as the time-reference to bottom-lock a layer to within 0.1 m of the bottom. In the QD the first bottom-locked layer can extend from 0.1 m to 100 m above the bottom. The second bottom-locked layer can be set to any height above the first within the overall limit of 127 m. If the operator does not wish to 'lock' the system to the minimum height of 0.1 m there is an off-set instruction of 0 to 1 m which can be used. In exceptionally shallow conditions of 10 m, or less, the window looks for bottom signals within ± 50% of the last recorded depth. A data logger prints results on a record sheet, but, in addition the integrated signals from two selected 'layers' appear in analog form (mm deflection) on the echo-sounder paper record, adjacent to those echoes from which they are processed.
3.3.2 Biosonics DE1 120 Integrator
This is contained in one unit having a front panel mounted keyboard plus some analog controls. It can work in conjunction with echo-sounders operating over a wide range of frequencies but its input signals must be demodulated. In Figure 26(a) the integrator is shown as part of a complete acoustic survey system and Figure 26(b) is a block diagram of the echo-integrator hardware. Input signals of a maximum level of 7.5V pass through an ADC and are processed according to the internal program and the operators instructions.
Figure 26. (a)
Figure 26. (b)
The unit can be put into operation by pressing the RESET button which causes 'SELECT SYS MODE' to appear on the screen above the keyboard. One of the three system modes can then be selected by rotary switch.
1. Integrator Manual Bottom Tracking
2. Integrator Automatic Bottom Tracking
3. Data Logger
after which the MODE change key is pressed and the system is ready to accept parameters to be entered via the keyboard after prompts which appear on the screen. Most of the prompts appear with what is called a default value already entered for the parameter, if this value is correct, pressing the ENTER key will retain it and bring up the next prompt. Finally, when all parameters have been entered, 'SELECT MODE' appears, and the rotary switch turned to RUN followed by ENTER so that integration can begin.
Thirty depth intervals can be specified. The DE1 120 samples its input voltage every 134.2 m s, equal to 0.1 m depth increments for c = 1490 m/s. Sampled voltages above the threshold are converted to a 12 bit word by the ADC. Echo voltages appearing in each depth interval are squared and summed over the 0.1 m depth increments. After the specified number of transmissions a final sum-of-squares value is calculated for each depth interval and the values obtained are used to calculate fish density from the expression
l xf = Sxf.A.Bx(P.Nx)-1
where
l xf = fish density for the (x) interval in kg.m-3 or fish.m-3 depending on units of the constant AP = number of transmissions per sequence
Nx = number of 0.1 m increments per (x) Interval
Bx = constant for TVG correction in (x) interval
where
t = pulse duration in seconds
c = speed of acoustic waves
s bs = average back-scattering cross-section of a single fish in m2.kg-1 or m2.fish-1
po = rms pressure of transmitted pulse in m Pa.1m-1
gx = transducer, cable, echo-sounder system gain in V.m Pa-1. 1m-1
means squared beam pattern weighting factor.
If a relative abundance survey only is being undertaken it is sufficient to let A = 1.
A paper printer forms part of the instrument from which the recorded data issues at the end of each sequence. These data are also available in ASCII (American Standard Code for Information Interchange) format at an RS232 output port for computer processing.
3.3.3 AGENOR Integrator
Also a self-contained unit, this integrator can operate from echo-sounders working at frequencies between 10 and 50 kHz. Demodulated analog signals from the echo-sounder are sampled every 133.3 m s, equal to 0.1 m depth increments when c = 1550 m/s. An ADC changes the sampled voltages to 12 bit words.
System parameters relative to the survey are entered via the front panel keyboard prior to the start of a survey but they can be modified at any time although the effects do not occur until the next sequence. Modified parameters are printed out each time by the in-built printer and appear at the RS232 port. A block diagram of the system is shown in Figure 27.
Figure 27.
When AGENOR is switched on the prompt "AGENOR VERS-O" appears and the operator selects the "CHGT PARAM" mode to enable the relevant parameters to be entered. The first line of parameters are displayed on the screen and also a cursor which can be incremented or decremented by keys for entry of new values. Key ¯ stores the completed current line after which the next line of parameters is shown.
There are 14 programmable parameters some of which are given below.
2, 3 and 4, Number of transmissions: Number of minutes per sequence: Number of 0.1 nautical miles per sequence
5. Threshold, referred to the ADC; is chosen by the operator looking at the demodulated signal.
6. Time interval for which automatic bottom tracking operates.
10. Acquisition mode
1: sequence stopped and new one started when number of transmission set in (2) is reached.
2: Sequences are repeated when number of minutes (3) is reached.
3: Sequence is stopped when log number (4) is reached.
11. Number of depth intervals (1 to 10) referred to the surface for which signals will be integrated.
12, 14 Constants A and B:
A is an overall scale constant from a combination of factors including c and s. It relates the sum of squared voltages to fish densities and has units of kg.m-3V2 or fish.m-3V2.B is a non-dimensional scale factor to correct for variations in the echo-sounder TVG.
There are also two depth intervals which are bottom-locked, they are called 11 and 12.
To start the system running, PAUSE is selected, the sequence number, the last automatic bottom value and the manual bottom value are then displayed. The bottom window is set over the bottom echo by the operator to obtain the initial value for automatic bottom tracking. When "ACQUISITION" is selected, data processing starts and at the end of each sequence data are printed out. The major part of the software calculates the average acoustic target density by unit surface (Rsj) or volume (Rvj) for each depth interval during a sequence of transmissions.
3.3.4 Furuno FQ Integrator
The Furuno FQ comprises a dual frequency echo-sounder and echo-integrator shown in the block diagram of Figure 27A. Echoes at each frequency are corrected by TVG before processing by an ADC and storage in the memory. A total of 3 bottom-locked and 9 transmission locked layers can be simultaneously integrated. One of each of these layers has the volume back-scattering strength printed out on the echo-sounder recording paper whilst the other ten values are listed on a printer output.
Figure 27A.
The rate of echo sampling is constant at 1024 times which on the 100 m range means every 98 mm and on the 500 m range every 490 mm. A vertical distribution of mean volume back-scattering strength (MVBS) in decibels with a dynamic range of 50 dB is registered in graphical form at every log marker position.
For the measurement of school aggregation density there are two possible methods. These are
i. from the vertical distribution graph read the MVBS at the centre of the school and add 10 log l/lG where l is the log interval and lG is the horizontal length of the school shown on the recorder.ii. select the aggregation average mode. Then the cross-sectional area of the school (SA) is calculated automatically within the integration layer wherein the school has occurred. 10 log l (integration layer)/SA is then added to the MVBS for log interval l.
3.4 Test Instruments
3.4.1 Multimeters
3.4.2 Oscilloscopes
3.4.3 Signal Generators
3.4.4 Electronic Counters
3.4.5 Hydrophones
3.4.6 Projectors
3.4.7 Calibration of Test Instruments
As methods for the assessment of fish stocks by acoustic means have improved, so the need for greater precision in making the measurements has arisen and this is reflected in the accuracy with which the various parts of the equipment must perform their functions. Test equipment used to check these functions must be of known reliability and precision before being used on the calibration and measurement processes.
For any type of electronic equipment it is important to ensure that the correct supply and signal voltages are being applied. In this context, supply voltages relate both to the power supply of the ship and to the non-signal voltage levels which occur throughout the circuits making up the complete instrument. Development of test instruments has kept pace with the general trends in electronics so there is no difficulty in making accurate electrical measurements. It is mainly in the area of acoustic calibration that problems occur. These are due to the practical difficulties encountered in the alignment of standard targets, projectors and hydrophones in the acoustic beam and to the lack of stable characteristics of the latter devices.
Whatever type of measurement is made, it is vital that the readings are taken correctly. When making acoustic or electrical measurements, whether it be the small signal output of a hydrophone, or the output of a high-power transmitter, it is necessary to ensure that the values used for calculation are Root-Mean-Square (rms). However, it is much easier to read peak, or peak-to-peak values from the calibrated amplitude scale of an oscilloscope, so for convenience these values are taken and converted to rms (section 2.3).
3.4.1 Multimeters
i) Analogue
Instruments are called multi-meters if they are capable of measuring a number of functions by the connection of their input leads to different sets of terminals on the meter, or, more usually by turning a rotary switch. Modern multi-meters can measure AC or DC voltages and currents, often from microvolt (m V) or microamp (m A) levels, ie 10-6, up to kilovolts (kV) i.e 103 times, and to tens of amps. They also incorporate an ohmmeter to measure resistance of components or circuits, from 1 ohm (W) to perhaps 10 MW. Analogue types are so-called because they show the quantity being measured in relation to a scale.
The majority of analogue meters use moving coil construction with a thin pointer positioned over the scale. This has a disadvantage when reading the scale due to 'parallax error', caused by the observer being unable to judge when his line of sight is perpendicular (exactly 90°) to the scale and pointer. A slight angle to the perpendicular position results in a reading being too high, or too low. To assist in overcoming this difficulty all good quality meters are fitted with a strip of mirror embedded with the scale. If the observer looks at the reflection of the pointer in the mirror, then moves his head until the reflection is hidden by the pointer, he has reached the best position to read the scale accurately.
In order to get adequate resolution, the scale is made as long as possible, > 10 cm, and the ranges split into divisions which can be selected by a switch, eg 0-3V, 0-12V, 0-60V, etc, similarly for current, 0-12 m A, 0-6 mA etc. and resistance 0-2 kW, 0-200 kW etc. The electrical tolerance on these scales would typically be 2%, ie a reading should be to ±2% of the full-scale value.
An important factor with all analogue meters is the amount of loading they impose on the circuit being tested. Across the terminals of a meter there is a resistance due to the moving coil and the scaling components, this must be sufficiently high to avoid changing the actual value being measured. Typically, a good modern meter has a figure of between 20,000 W per Volt to 100,000 W per Volt, which means that each full scale value is multiplied by the resistance quoted in kW, ie 10V scale x 20 kW = 200 kW. For most purposes, other than some tuned and Field Effect Transistor (FET) circuits, 20 to 100 kW per volt is adequate.
When a fault has occurred in a circuit, as indicated by a low, or high voltage reading, the power is switched OFF and the ohm-meter section of the multi-meter is often used to investigate the circuit conditions. For this operation the meter provides a voltage at its terminals, which, when applied between particular points, will drive a current through the circuit proportional to the resistance encountered. This resistance, measured in ohms, is displayed in analogue or digital form by the meter.
Experience and knowledge of the circuit function is essential for correct interpretation of resistance readings. This is because many circuit elements, such as transistors and diodes, present a different resistance to the meter depending on the polarity of the applied voltage, ie the test leads, also, the windings of transformers have a different resistance to DC than to AC of a given frequency.
ii) Digital Multimeters (DMM)
As the name implies, these meters display the measured quantity in decimal form by digits, either by Nixie tube, Light Emitting Diode (LED) or Liquid Crystal Display (LCD). They are designed with a very high input impedance of 10 MW to avoid the circuit loading problem inherent with most analogue meters. The accuracy on DC voltage is normally ± 0.1% of the reading, ± 1 digit, and on AC voltages and DC currents is 0.75% of the reading ± 1 digit.
3.4.2 Oscilloscopes
Very little work can be pursued on modern electronic equipment without the use of an oscilloscope. An oscilloscope is an instrument based on the ability of a cathode-ray tube (CRT) to display oscillatory voltages. It does this by deflecting an electron beam, directed at a fluorescent screen, simultaneously in two mutually perpendicular planes. When DC coupled, oscilloscopes can also measure steady voltages. The detailed functioning of a CRT is beyond the scope of this manual.
Despite a multiplicity of controls (see. Figure 28), the oscilloscope has a basically simple function which is to display, for the purpose of measurement, the form of voltage variation in electronic circuits against time (their waveform). Figure 3 shows a sine wave in terms of its peak-to-peak voltages related to angle. The rate of change of angle is of course proportional to frequency. It is the purpose of an oscilloscope to measure the variation of waveform over a very wide range of frequencies and voltages.
Figure 28.
The primary controls of an oscilloscope are, TIMEBASE, usually calibrated in microseconds per cm (m s/cm), milliseconds per cm (ms/cm), seconds per cm (s/cm) and VOLTAGE. Voltage calibration ranges from microvolts per cm (m V/cm), millivolts per cm (mV/cm), to Volts per cm (V/cm). In some cases the calibration graticule may be less than 1 cm, then the marking will be m s/division etc. Other controls are concerned with aspects of the waveform presentation rather than the fundamentals of the waveform itself. However, unless the user is able to control the presentation of a waveform, it will appear in a form unrecognisable to the human eye. One of the most important controls, and the one which is most effective in 'stopping' or 'holding' the waveform is the TRIGGER.
It is not unusual for the TRIGGER function to be divided amongst a number of knobs or push-buttons. Many oscilloscopes are constructed in modular form with separate plug-in modules for amplifiers, timebases, and triggering facilities, which might contain up to 20 front panel controls. This apparent over-complication is due to the need for the 'holding', or 'synchronising' of waveforms having different polarity, amplitude, frequency and repetition rate, and the requirement to examine certain parts of the waveform, e.g. to compare it with another waveform simultaneously, or sequentially and so on.
DELAY: This feature normally employs two timebases, one of which is called the delaying 'sweep'. A typical operation might involve the operator selecting, by means of the delaying sweep, a particular delay time. When this is reached, the second (delayed) timebase starts, and runs at perhaps ten times the speed of the first, thereby giving greater resolution of the selected portion of the waveform. More than one trace, or beam, is useful with this function so that the expanded portion can be compared with the whole waveform.
POSITION: There are two oscilloscope controls for precise positioning of the trace, horizontally (the time axis, X) and vertically (the voltage axis, Y) i.e. the waveform can be aligned in both X and Y planes with the scaled graticule. Vertical position controls are usually attached to an amplifier module, whilst the horizontal position control is often associated with the timebase module.
C.R.T. CONTROL: Quality of trace is determined by the setting of controls for brilliance, focus and astigmatism. Brilliance, or intensity is a control to be carefully used, because excessive brightness can result in burning the phospher on the screen. Focus sharpens the trace, enabling detail to be seen and making measurements easier, providing that the (often pre-set) astigmatism controls are adjusted to their optimum positions (these are used to obtain the 'roundest' spot from the electron beam). Most oscilloscopes have a control which gives variable illumination to the graticule allowing the scale to be easily read, or photographed.
DUAL-BEAM/DUAL-TRACE: A dual-beam oscilloscope contains two independent deflection systems within the same CRT, it can therefore display two input signals simultaneously even if they are non-recurrent and of short duration. These oscilloscopes are not now generally available.
The dual-trace incorporates electronic switching to alternately connect two input signals to a single deflection system. This allows a better comparison to be made because only one timebase and one set of deflection plates are used. Recent developments allow as many as eight traces to be displayed.
STORAGE: Two forms of storage are now used, CRT and digital. Both allow accurate evaluation of slowly changing phenomena, but the CRT type is preferable for viewing quickly changing waveforms as in underwater acoustics. As the name indicates, CRT storage is within the tube, either on a mesh or a special phospher and the PERSISTENCE control allows the selection of gradation between the bright trace and the dark background and also controls the time for which a stored image can be retained.
Digital storage relies on waveform sampling, ie taking signal values at discrete time intervals, and quantising, which is transforming the value into a binary number before transferring it into a digital memory. This storage method gives crisp, clear displays for unlimited periods, it can suffer from aliasing i.e. the sample data pulse train does not accurately represent the input signal. Most digital storage oscilloscopes sample often enough to display a 'clean' waveform from echo-sounders if operators take care to set the sampling rate correctly to avoid aliasing.
PROBES: The probes, although plug-in devices, must be regarded as an essential part of an oscilloscope system. They are designed to prevent significant loading of the circuit under test and are usually selected on the basis of adequate frequency and voltage response. For voltage amplitude measurements the capacitance and resistance of a probe form a voltage divider with the circuit being tested. At echo-sounder frequencies the resistive component is of major importance and needs to be at least two orders of magnitude greater than the impedance at the circuit point being examined.
It is also possible to measure transmission current with oscilloscope probes, something which is likely to become of increasing importance as the need to ensure even greater precision in the measurement of acoustic parameters. Current probes have a different form of construction and method of connection, to voltage probes, for, whereas the latter are connected directly to the terminals of a circuit, the current probe is clipped over the wire through which the current is flowing, (ie there is no 'metallic' contact).
3.4.3 Signal Generators
Although this instrument is a transmitter of electrical frequencies it differs from the echo-sounder transmitter in most respects excepting for the generation of frequencies. The signal generator provides signals (transmissions) accurately controlled in frequency and amplitude which can be varied over a wide band of frequencies and levels of voltage whilst remaining pure in waveform.
It is the purpose of a signal generator to provide the means of electrically calibrating receiving amplifiers in terms of their sensitivity, dynamic range and bandwidth. A wide range of precisely controlled output voltage level is necessary, preferably from < 1 m V to > 10 V. The signal generator should be capable of producing at the echo-sounder frequency CW and pulses (bursts) of controlled variable duration, which, by means of a time delay (depth control) can be set anywhere in the full depth scale of the echo-sounder under test. Accuracy and stability are of prime importance.
Figure 29 illustrates the essential features of a signal generator. Block 1 is the oscillator which generates CW at the frequency selected by the switch (coarse range) and the tuning dial. This oscillator must have the properties of low harmonic distortion and high frequency stability. Its output is fed to an electronic gate, Block 2, controlled by the square waves from Block 3 for pulse mode, or, bypassed completely for CW mode. Block 3 has a control by which it is possible to vary pulse duration to simulate the transmitted pulse.
Figure 29.
There are two modes of operation for block 3, 'free-run' and 'triggered'. In free run, the rate at which pulses are produced can be varied within limits. When in triggered mode, only one pulse occurs for each revolution of the recorder stylus, in response to the echo-sounder trigger pulse. However, the time (depth) at which it occurs can be set by the time delay control (block 4), initiated by the recorder trigger pulse.
The output of the gate is amplified (block 5), then fed to an attenuator (block 6), calibrated in voltage or dB. An essential feature of the attenuator is low output impedance so that signals can be injected into the transducer/receiver input circuits without adversely affecting them. When injecting signals, especially of m V level it is necessary to avoid introducing electrical interference to the circuit and a good method is to use an inductive form of coupling into one of the leads between the transducer and the receiver. Such an arrangement reduces the impedance introduced into the circuit, typically by 100 times, from say 0.1 W to 0.001 W.
Care must be taken to prevent direct coupling between the signal generator circuits and those of the receiver amplifier under test, or false measurements may result. It is normally sufficient to ensure that the earthing arrangements for the two are correct and that the correct cable from the signal generator is used for connection to the receiver.
The signal generator should include fine frequency control of tuning, because of the relatively narrow bandwidth of receivers. However, the precise frequency to which the generator is tuned can best be obtained by use of a frequency counter. This instrument is discussed in section 3.4.4, it gives a direct digital reading of frequency when connected to a CW output. The importance of a frequency counter is best illustrated by considering a practical example.
An echo-sounder is tuned to the resonant frequency of its transducer, 38.75 kHz, and has a bandwidth of 2.2 kHz to the -3 dB points. By using a frequency counter it is easy to set the signal generator, first to 37.65 kHz (-1.1 kHz), then to the centre frequency, 38.75 kHz and lastly to 39.85 kHz (+1.1 kHz). It would be extremely difficult to achieve acceptable accuracy if an analogue dial or scale were used.
3.4.4 Electronic Counters
The type of electronic counter used in fisheries acoustics is one which can make a precise count, or measurement, of frequency. It gets its name because the measurement is made by counting the number of sinewaves which occur in a given period of time. This number is displayed digitally, usually in kHz. Frequency counters of this type have become sophisticated devices but are quite simple to use. Controls are limited to selection of the number of digits to be displayed, selection of the mode of operation (if timing and other measurements are possible) and the input level. The latter is particularly important in some of the older instruments because if the input level was set too low or too high readings were erratic.
It is difficult to use this form of counter to measure the frequency of a pulse transmission or echo. Manufacturers normally provide a CW output of the transmitter oscillator where this can be done and signal generators can be switched into CW for the same purpose.
3.4.5 Hydrophones
These are the sensor devices, defined as transducers, which provide electrical signals in response to waterborne acoustic waves. When a hydrophone is placed in the acoustic field (beam) of an echo-sounder transducer, it responds to the pressure fluctuations and produces a proportional voltage across its terminals. A conversion factor is supplied by the hydrophone manufacturers which enables the voltage to be related to acoustic pressure at the frequency being used. This is usually in the form of the number of decibels relative to one volt which can be measured for each micropascal of pressure, dB/1 V/1 m Pa. In the past it was given as dB/1 V/1 m b) but the microbar (m b) has been superseded and 100 dB must be added to m b figures to bring them to m Pa. For example, a typical figure of -75 dB/1 V/1 m b when converted to the SI unit is -175 dB/1 V/1 m Pa.
Modern calibration hydrophones are designed to have an omni-directional response in one plane, but often have some undesirable directionality in the other. They are made from physically small electrostrictive elements encased in acoustically transparent, but watertight material. Usually they have a wide frequency band response but some change in the characteristics can occur with change of temperature. Calibration normally includes the length of connecting cable supplied. This cable must neither be shortened, nor lengthened, unless proper allowance can be made for such changes.
3.4.6 Projectors
A projector is a transducer which, when supplied with electrical power produces pressure waves corresponding to the frequency at which it is driven. Projectors for calibration purposes normally have an omni-directional response over a wide band of frequencies. The same transducer can also be used as a hydrophone, if it has reversible characteristics. Care must be taken to avoid overloading when operating in the projector mode because this can strain the material and therefore change the hydrophone calibration. The projector calibration factor is related to a given electrical driving power for which the acoustic pressure can be calculated, usually in the form dB/1 m Pa/1 V. A typical figure might be 228 dB/1 m Pa/1 V. If the calibration is given in terms of the now discontinued unit it would be 128 dB/1 m b/1 V.
3.4.7 Calibration of Test Instruments
The most important factors in maintaining calibration, and a good performance from any item of test equipment are, care in using it, in handling it, and particularly in transporting it. Before using any test instruments, it is necessary to make some simple checks to be sure that they are functioning properly. Failure to do this may result in much time being wasted, through both the recording of incorrect data and attempts to find non-existent faults in survey equipment.
Tests on multimeters are quite simple. The ohm-meter ranges can be checked to see if the pointer (or digits in a digital meter) can be zeroed. If not, the most likely reasons are that the battery is low, or, the leads are broken or making bad contact at the terminals, matters which can be rectified easily. The accuracy can then be roughly checked by measuring a few close tolerance resistors whose values are chosen to check the instrument at various points throughout the scales.
Checking the function and calibration of the voltmeter sections may be more difficult. Direct current (DC) scales can be roughly checked on known dry battery voltages or, more accurately on laboratory or bench type power units. However, if the instrument is of good quality and it has been well treated, (ie not overloaded, dropped, or subjected to severe vibration in the case of moving coil meters), it is unlikely that its accuracy will have deteriorated. Current measuring scales can be checked by switching to the highest current scale, then inserting the meter in series with a circuit of known potential difference and resistance so that the current which should be indicated can be calculated. Starting any measurement using the highest range of voltage and current is a sensible precaution.
For an alternating current meter it is necessary to be sure what the scale is indicating. Normally calibration is in terms of the rms value of a true sine wave (see 2.3).
Deviations from a pure sinewave, (distortion) will cause some error in the reading by an amount depending on the 'form factor'. This is only obtainable by waveform analysis. Observation of the waveform with an oscilloscope will indicate any obvious distortion which might affect the result.
Once the electronic equipment has been thoroughly checked and calibrated, the acoustic calibration can be considered. The various methods of achieving this are discussed in chapter 7.
Make a Vessel That Will Echo Back to You
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