The present invention relates to a method and an apparatus for the measurement of fluid flow velocities, for instance in a slurry trans- port (mixture liquid/solid material) or in a pneumatic transport (dry gas/solid material) , but also for the measurement of the velocity of a liquid or gas and mixtures thereof.

In a known device, disclosed in the British Patent Specification 1,359,159 at least two transmitter transducers, situated at a predeter- mined mutual distance seen in the flow direction of the fluid, transmit signals through said fluid to at least two receiver transducers, also situated at a predetermined mutual distance seen in the flow direction of the fluid, in which each receiver transducers receives mainly the signal from the belonging transmitting transducer and in which the signals supplied by the receiver transducers are correlated, in order to determine the fluid flow velocity.

In a slurry-transport in dredger conduits irregularities occur continuously in the flowing fluid, such as vortexes in the water carry¬ ing the sand and sludge or other material. Moreover this carried material will not be divided evenly, but will appear as clouds, which continuously change in composition and shape.

The signals transmitted by the transmitter transducer through this mixture are modulated continuously by said irregularities between the transmitter transducers and receiver transducers and these irregulari- ties can be used after demodulating the received signal by means of cross correlation methods for the measurement of the travel velocity of the mixture. In case the distance in the flowing direction of the mix¬ ture between the transducers is sufficiently small, said irregularities will have changed to a very small extent only, so that by comparing the demodulated signals from the two receiver transducers by the said cross correlation method, it is possible to determine the time which the mix¬ ture needs to travel between the pairs of transducers, from which the fluid flow velocity can be calculated then.

In the method known from above said British Patent Specification use is made of a continuous signal transmitted by the transmitter transducer, from which signal it is separated the amplitude modulation by detection, which modulation Is used for correlation then.

After thorough research it appeared, that such continuous signal not seldom results in an interferention pattern of standing waves in the dredger conduits. In particular in case of small conduit diameters and low specific mass such interferention patterns can be very detri- mental for the correlation measurement. Because this interferention pattern is determined also by the mixture and need not be the same for the pairs of transducers (transmitter/receiver) this phenomenon has not been recognized as an unwanted interference.

Therefore one of the main objects of the present invention is to remove the results of the received signals from the interferention patterns of standing waves.

According to the present invention the above object is reached by transmitting pulse-shaped signals by the transmitter transducers having such mutual time separation, that within this time separation standing waves caused by reflections will be nearly completely die down.

In order also to exclude from the velocity measurement interferen¬ tion patterns generated by pulse-shaped signals of too long duration, preferably samples are measured only of the signal received by the receiver transducer as soon as possible after reception, by the receiver transducer of the front edge of the transmitter signal.

In connection with the wanted equivalence of the two signal pro¬ cessing channels, each comprising a transmitter transducer and a receiver transducer, preferably use is made of the same frequency for these two channels. In order to avoid cross talk between the signal processing channels the pulse-shaped signals are transmitted according to the present invention by the one transmitter transducer within a time separation between the pulse-shaped signals of the other transmit¬ ter transducer, in which the time separation between pulses of dif¬ ferent transmitter transducers have been chosen such, that within this time separation standing waves caused by reflections will be died down nearly completely.

In order to avoid loss of information upon using pulse-shaped sig¬ nals, the - ulse repetition rate will be at least two or three times as high as the highest frequency which after demodulation of the received signal supplies Information as to the velocity of the fluid.

Other signals which may be detrimental for the measurement are caused by collisions of for Instance sand grains against the trans-

ducers and against the walls of the fluid conduits. These signals form a nearly continuous noise spectrum, extending from a very low frequency to very high frequencies, containing the operation range of frequencies used in the present invention, which means the range at both sides of the frequency signal transmitted by the transmitter transducer, which can be regarded as a carrier wave and ^" hence also the side bands of the modulation signals at both sides of this carrier frequency signal.

This broad band noise will change continuously from location to location and will not appear in the form of a pattern carried by the current, such as above said vortexes, so that this noise cannot be used for signals to be correlated.

As far as there is anything to be correlated in this broad band noise, this signal mainly will carry information concerning the water layers passing directly in front of the receiver transducers and not across the total cross section of the conduit, because of the increas¬ ing attentuation, depending on the distance before the head of the receiver transducer. The amplitude of this noise yet may be strong enough, so that in a broad range at both sides of said carrier signals side bands will occur, which may conceal the useful signals. This broad band noise is present all the time and independent of the transmitted send signal.

It appeared now that the interference caused by this broad band noise can be reduced considerably by sampling the received signal imme¬ diately before and immediately after the front edge of the transmitted pulseshaped signal will reach the receiver transducer and by using a sufficiently small distance between these samples.

As remarked above in the method known from said British Patent use is made of the amplitude modulation of the signal transmitted by the transmitter transducer brought about by the physical phenomena in the moving fluid, which signal, as far as the receiver transducer is con¬ cerned can be seen as a carrier wave.

It appeared that this amplitude modulation is forming a part only of the total influence exerted on the carrier wave by the fluid.

When an acoustic wave is transmitted across a slurry stream, the acoustic results hereof will be the following:

1) water vortexes will exert upon the acoustic velocity of the crossing acoustic wave an accelerating or delaying influence. By Doppler-

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phenomena increase and decrease of frequencies will result, which means side bands above and below the transmitted carrier wave frequency. 2) Sand clouds and/or slurry clouds possibly carried along will a.o. cause a more or less strong attenuation of the crossing ultrasonic audio wave, which means also that additional side bands will be added.

The vortexes and sand clouds or slurry clouds are carried on by the water stream and will change little only over a sufficiently small distance between the pairs of transducers and therefore will be suffi¬ ciently useful as sources for signals to be correlated for velocity measurement.

The frequencies of the side bands caused by the vortexes mainly are dependent on the velocity of the pumped mixture, the pipe cross section and the dimensions (cross section) of the receiving trans¬ ducers. Theoretically all frequencies will occur from zero to infinite. The reasonably workable signals are provided, however, by the smallest vortexes which can be ascertained. These vortexes will have a diameter which is as large as the cross section of the receiver transducers, which means a few centimetres. This means that alternatively an accelerating and a delaying crossing path for the acoustic wave will occur having a thickness equal to the cross section of the receiver transducer. When this pattern passes with the highest velocity to be measured, which means practically 10 metres per second, this will result in a maximum processing frequency, which also can be mentioned the deviation frequency such as in FM techniques. This deviation frequency can be computed as follows: ^v m f _h = 2 d _t •

Herein is f^ = maximum frequency in Hz

V _m = maximum flow velocity (10 m per second) d _t = diameter of the receiver transducer (3 cm) From above formula it appears, that upon using the said values for the velocity and diameter the maximum frequency to be measured will be about 160 Hz. Using this frequency the carrier wave can be shifted to

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an upper side band and to a lower side band with respect to the fre quency position of the carrier wave transmitted by the transmitte transducer. This mechanism can be compared also with a tape recorder in which the highest frequency depends on the head slit and the tap velocity.

The frequency distance of these side bands up to the position o the carrier wave frequency depends on the velocity component of th vortexes in the direction of propagation of the audio signal, whic means perpendicular to the flow direction. The most extreme situation will occur in case the vortex compo nents in the direction of the audio ray are equal to the velocity i the conduit and in the same direction or in counter direction. Thi will result in that the audio velocity V of the crossing wave in cas of a flow velocity of 10 m per second will vary from 1500 + 10 t 1500 - 10 per second (the audio velocity in water being estimate 1500 m per second) .

• In case the length of the crossing path is 1 and the transmitte frequency f within this path, there will be 1/v x f waves varying I number 20/1500. This variation may occur in 1/400 second. Approximately this is the time duration of half a period of f^ This means that in case of a conduit diameter of 150 mm and a carrie wave having a frequency of 0.25 MHz a deviation of 400 x 20/1500 1/v x f =» about 130 Hz.

This is an extreme situation which not occurs continuously, bu this gives an idea of what might occur. Despite the fact that the vor texes are influencing the received frequency this should not be seen a frequency modulation in the usual sense of this term. In the mechanis which is working here, in principle there is no relation between th upper side band and the lower side band. Considerable differences ma occur between these side bands, because the vortexes nearly never wil be purely symmetrical. The side bands are generated mutually indepen dent and preferably should be reproduced and processed separately i the receiver.

The variable attentuatiou of the crossing ultrasonic audio wave b the sand clouds and slurry clouds will result in a sort of amplitud modulation. Because moreover by the presence of sand and slurry th physical characteristic of the fluid will change, thus also will

change the propagation velocity of the audio and this also will result in a particular pattern of side bands having no symmetrical character. This can be regarded as a sort of frequency modulation.

Because the pattern of side bands for said reason will not be sym- metrical preferably a single side band technique will be used.

The presence of all above information carrying signals is posi¬ tively tight to the presence of a transmitted carrier wave, because the influence upon this carrier wave will result in the generation of said side bands. In contradistinction the broad band noise mentioned before will be present always and will be independent of a transmitted trans¬ mitter signal.

Therefore, in a further preferential method for the measurement of the fluid velocity, upper side band and/or lower side bands will be separated from the signals received by the receiver transducer with respect to the carrier wave transmitted by the transmitter transducer and for the correlation use will be made of pairs of two thus formed upper side bands and/or lower side bands.

In principle a device for the measurement of the flow velocity of a fluid will consist of four parts shown in Figure 1: 1) the take up part, consisting of a transport conduit 70 having dia¬ metrically placed transmitting and receiving transducers 10, 11 and 12, 13 respectively, transforming electrical signals into ultrasonic audio vibrations and vice versally into electrical signals again.

2) the pre-processor 71, supplying the electrical signals to the trans- mitting transducer 10 and 11 and processing the electrical signals received by the receiver transducers 12 and 13 in such a way that signals to be correlated are generated.

3) the cross correlator 72, performing the time measurement between the correlatable signals supplied by the pre-processor 71. 4) a display device 73, displaying in an analogue or digital way the ascertained velocity.

The pre-processor 71 should fulfil a number of conditions in order to perform the method of the present invention.

In order to exclude the standing wave patterns pulse-shaped sig- nals should be used in the conduits. After each signal pulse reflec¬ tions should be died down before the following signal pulse is trans¬ mitted. Because of the necessary equality of the signal processing

channels these preferably work at the same frequency. In order to avoid cross talk then the pulses should be transmitted alternatively by the transmitter transducer. The measurements then will be a sampling pro¬ cess of the results obtained per pulse-shaped signal. In order not to avoid loss of information the repetition rate should at least be twice or three times as high as the highest frequency which may occur as use¬ ful information.

Moreover, the information containing signals should be separated as much as possible from the broad band noise signals generated in the transport conduit itself. Because as above said, there is no relation between the upper side bands and the lower side bands the receiver system should be designed as a true single side band receiver. Using the above values for the conduit diameter, transducer diameter and flow velocity, the low frequency band width should extend between 2 and 150 Hz.

Because the pre-processor is followed by the cross correlator 72, which is capable to ascertain all repeating phenomena, all time depen¬ dent phenomena should be defined completely, so that no jamming time functions will reach the correlator. Reckoning with the above wishes the pre-processor according to the invention will have to apply the following demands: the signal transmitted by the transmitter transducer in first in¬ stance is generated by a generator having a frequency which is four times as high. This alternating voltage of about 1 MHz is divided by four by means of two D-flipflops. The following voltages will be ob¬ tained by a suitable combination of the signals generated herewith:

1) a staircase voltage of the wished frequency, capable to control the transmitter output stage through a simple low pass filter;

2) two block voltages having a mutual phase shift of 90°, which can be used as reference control signal for the multiplying stages in the receiver.

The device known from above said British Patent 1,359,151, com¬ prises generally at least two transmitter transducers, located behind one another in the flow direction of the fluid, two receiver trans- ducers, also located behind one another in the flow direction of the fluid, each mainly intended for receiving signals transmitted by a be¬ longing transmitter transducer and influenced by the medium, and cor-

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relation means connected with the receiver transducer, in order to derive the fluid flow velocity from the time difference between the signals supplied by the receiver transducer.

The device of the present invention is characterized by means for separating an upper side band and/or a lower side band from signals supplied by the each transmitter transducer with respect to the signals transmitted as carrier wave by the transmitter transducer in the fluid and means for supplying to the correlation means the two thus separated upper side bands and/or lower side bands, in order to determine the fluid velocity from their time difference.

Preferably the above device comprises means for supplying to the transmitter transducer pulse-shaped signals having such mutual time separation, that between the pulses all standing waves caused by reflections will die down. Moreover, the means for transmitting the pulse-shaped signals are designed such, that the transmitter transducer will transmit a pulse¬ shaped signal alternatively.

Preferably the device of the present invention comprises: pulse generator means for supplying to two separate gate pulse conductors, one for each signal processing channel, gating pulses, the duration of which corresponds with the pulse-shaped signals transmitted by the transmitter transducers, which, pulses are supplied alternatively to gate pulse conductors of each signal processing channel; oscillator means for generating two carrier wave signals having equal frequency but a phase difference of 90° , in which a separate one of these two carrier wave signals is supplied to each signal processing channel; first gate means in each signal processing channel, comprising two inputs, one of which being connected with the gate pulse conductor for receiving the gate pulses, the other input being connected with the oscillator means for receiving the carrier wave signals, the output signal consisting of a carrier wave signal pulse train, supplied to the transmitter transducer in the signal processing channel; two product detectors in each signal processing channel, each first input of which receiving signals supplied by the receiver trans¬ ducer, each second input of which receiving the carrier wave signal, however, with a phase shift of 90° between the signals for each second

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input of these two product detectors;

90° phase shift means connected with the output of each of the product detectors; signal adding means, supplying the sum signal of the output signal from a product detector and the 90° shifted output signal of the other product detector, which sum signal corresponds with one side band of the signal demodulated by the product detectors; signal subtracting means, supplying the difference signal of the output signal from a product detector and the 90° shifted output signal from the other product detector, which difference signal corresponds with the other side band of the signal demodulated by the product detectors; first delay means in each signal processing channel, the input of which receiving gate pulses, the output supplying a first sample con- trol signal having such a time delay, that the first sample control signal will appear immediately before the reception by the receiving transducer of the front edge of the pulse-shaped carrier wave signal; second delay means in each channel, the input of which receiving the gate signals, the output supplying a second sample control signal having such a second time delay, that the second sample control signal will appear immediately after the reception by the receiver transducer of the front edge of the pulse-shaped carrier wave signal; four sample hold means in each signal processing channel, one pair of which receiving the output signal of the first product detector and the other pair of which receiving the output signal of the second pro¬ duct detector, one of the sample hold means of each pair also receiving the first sample control signal and the other of the sample hold means of each pair also receiving the second sample control signal; subtracting means for each pair of sample hold means, supplying the difference between the output signals of the pairs of sample hold means, which difference contains nearly exclusively information con¬ cerning the modulation of the carrier wave signal in the fluid; third delay means in each signal processing channel, the Input of which receiving gate signals, the output supplying a third sample con- trol signal, having such a third time delay, that this third sample control signal appears after the front edge of the difference output signal supplied by the subtraction means;

two further sample hold means in each signal processing channel, one input of which receiving the third sample control signal, the other Input of each of the sample hold means being connected with one of the subtracting means, such that the sample hold means exclusively supply a signal corresponding with the pure difference signal of each of both subtraction means.

The output of each product detector is connected to a low pass filter having such a rise time, that this filter can readily follow the information contents of the product detector output signal and in which each third sample hold circuit is connected with a band filter, passing exclusively signals containing information concerning the modu¬ lation imparted to the carrier wave signal by the medium.

The invention will now be further elucidated by means of the draw¬ ings, showing embodiments of a device according to the present inven- tion.

Figure 2 shows a schematic block diagram of a device of the pre¬ sent invention, with the exception of the correlation device and the delay device;

Figure 3 shows in a time diagram signals appearing in different points of the circuit of Figure 2. The horizontal axis forms a time scale, In which 1 cm corresponds with about 100 μ sec;

Figure 4 shows a schematic block diagram of the circuit for sepa¬ rating the side bands.

In the receiver, receiving the signals from the receiver trans- ducers, preferably use is made of multiplying synchronous detectors or product detectors, supplying as output signal the product of the input signals, being the signals of the differential frequency or beat fre¬ quency and with instantaneous amplitude, depending on the instantaneous phase difference between these both input signals. These product detec- tors are followed by low pass filters, having a maximum pass frequency of about 3500 Hz, said filters also having a rise time of about 100 micro seconds.

The dimensions of the receiver transducers limit, as explained above, the signals relating to the mixture velocity to the said proces- sible frequencies of about 160 Hz. This means, that the information contents of a particular kind within the rise time of this low pass filter and having a maximum frequency of 3500 Hz cannot or hardly

change. It is especially remarked, that the above said values, such as dimensions, numbers, frequencies, and times relate to an example being tested in practice, the satisfying operation of which has been proved in a dredger conduit having a cross section of 150 mm. The block generator 1 in Figure 2 generates symmetrical, stair¬ cased signals in counterphase, having a frequency of 500 Hz, in Figure 3 shown on lines a and b. At each turnover or step of these signals, a trigger signal is sent to the monostable circuit 2, vide Figure 3, line c, which monostable oscillator 2 as a result supplies a pulse having a length of about 300 micro seconds, vide line d in Figure 3. This pulse is combined in the "AND"-gates 3 and 4 with signals from oscillator 1. Gates 3 and 4 supply as a result alternatively a signal of 300 micro seconds via the gate pulse lines 5 and 6 to both signal processing channels, vide Figure 3, lines e and f. The high frequency oscillator 7 generates an alternating voltage of about 1 MHz, which after division by four results in a frequency of 250 Hz, supplied in both outputs, however, having a mutual phase dif÷- ference of 90°. These two signals are fed respectively to the inputs of the transmitter output amplifiers, 8 and 9 respectively. The pulses on lines 5 and 6 (e, f) will open the transmitter out¬ put amplifiers 8 and 9 during a time of about 300 illi seconds, within which the amplified signals from oscillator 7 are supplied to the transmitter transducers 10 and 11.

Having a pipe diameter of 150 mm and an acoustic velocity in water of 1500 m per second, the crossing time for an acoustic wave will be 100 micro seconds. It should be added hereto the travel time of the signal through the transducers, which for each transducer Is about 5 micro seconds. The total time between the electrical transmission and the electrical reception is therefore 110 micro seconds. The wave trains shown on lines g and h in Figure 3, transmitted by the transmitter transducers 10 and 11, reach the receiving pre-ampli- fiers 14 and 15 respectively, vide Figure 3, lines j and k, after a delay of 110 micro seconds through the receiver transducers 12 and 13. The amplified received signals from the pre-a plifiers 14 and 15 are supplied to the product detectors 16, 17 and 18, 19 respectively, each followed by its own low pass filter 20, 21 and 22, 23 respective¬ ly, having a cut off frequency of about 3500 Hz, as said above, corres-

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ponding with a rise time of about 100 micro seconds. Explained in a more simple way this means, that this filter at each moment will deliver a signal voltage, which is the average of what has been receiv¬ ed within the preceding 100 micro seconds. As remarked above after each receiver pre-amplifier 14 and 15 two parallel product detectors 16, 17 and 18, 19 are used, receiving the signals mutually phase-shifted 90°, having a frequency of 250 kHz. Then follow two parallel branches in which these signals having a mutual 90° phase shift remain in tact. This is done to obtain separate upper side bands and lower side bands, as will be explained lateron.

The signals in the gate pulse lines 5 and 6, shown on lines e and f of Figure 3, are supplied to delay circuits 24, 25. These consist of monostable multivibrators, triggered by the front edge of signals e, f and returning after a delay time of 95 micro seconds, supplying in turn a trigger signal to the monostable multivibrators 26 and 27 respective¬ ly, each generating a pulse of 6 micro seconds. These pulses are supplied to sample hold circuits 28, 30 and 32, 34 respectively. During this pulse time of 6 micro seconds the opened sample hold circuits sample the signals supplied by filters 20, 21 and 22, 23 respectively. The value thus obtained remains present and constant in the output of each sample hold circuit until replacement by the next sample.

The delay time of 95 micro seconds has been chosen such, that the trigger pulse on line appears just before the reception of the front edge of the received signals, vide lines j, k in Figure 3, in the pro- duct detector 16, 17 and 18, 19 respectively. The signal thus obtained in the sample hold circuits therefore contains information only relat¬ ing to the broad band noise, hence without the side bands resulting from modulation of the carrier wave In the fluid.

The signals e and f in lines 5 and 6 are supplied next to delay circuits 36 and 37, each followed by a multivibrator 38, 39. The delay circuits 36 and 37 will be triggered by signals e and f and generate a pulse having a length of 250 micro seconds. At the moment of appearance of the trailing edge of this pulse, vide line p, the monostable cir¬ cuits 38 and 39 are triggered, each supplying again a pulse having a duration of 6 micro seconds. As shown in Figure 3 this last pulses appear on line q after the front edge of the signal received by the product detectors, vide line j.

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110 Micro seconds after the front edge of the transmitted pulse, line g, the wished side band information will arrive in the filters 20, 21 and 22, 23 respectively mixed with broad band background noise. In order to obtain the wished information at the output of said last men- tioned filters it has to be waited at least the rise time of these fil¬ ters, which means at least 100 micro seconds. This has been taken into account upon selecting the length of the delay time of the delay cir¬ cuits 36 and 37 of 250 micro seconds, vide line p.

The output signals of filters 20, 21 and 22, 23 respectively are supplied to sample hold circuits 29, 31 and 33, 35 respectively, which then from that moment will supply a signal consisting of broad band noise plus the wished side band information. These signals are supplied to the differential amplifiers 40, 41 and 42, 43 respectively, supply¬ ing the difference between the output signals of the sample hold cir- cults 28, 30, 32, 34 on the one hand and 29, 31, 33, 35 on the other hand. The differential signals of these differential amplifiers contain after a transient time nearly information only concerning the wished side bands. The broad band noise signals are cancelled out here.

The output signals of these differential amplifiers are supplied again to further sample hold circuits 44, 45 and 46, 47 respectively. These sample hold circuits are controlled again by trigger signals from the multivibrators 50 and 51 having a length of 6 micro seconds, which each are controlled again by delay circuits 48, 49, each having a delay of 400 micro seconds. This time is of sufficient length, so that the differential amplifiers have reached a stable condition after the last transient signals. Hence the sample hold circuits 44, 45 and 46, 47 respectively supply pure signals containing information of the side bands, each from an upper side band as well as from a lower side band. The output signals from the sample hold circuit 44, 45 and 46, 47 respectively are supplied to band pass filters 52, 53 and 54, 55 res¬ pectively, having a pass band of 2 - 150 Hz.

These filters are followed by circuit parts 56-67, separating and amplifying the upper side bands and lower side bands, such that the amplifier outputs 66 and 67 supply four signals having different in- formation contents, which can be supplied to the cross correlation cir¬ cuits. Hereafter the operation of these separating circuits will be elucidated further, vide Figure 4, in which the same reference numbers

are used for corresponding parts as in Figure 2. In order not to over¬ load this Figure 4 it shows the necessary parts only.

In Figure 4 reference number 7 refers to oscillator 7 In Figure 2, generating two signals having a frequency of 250 kHz, mutually phase- shifted over 90°. These two, 90° phase-shifted signals are supplied again to mixer circuit 16, 17. The output signals from these mixer cir¬ cuits reach, after passing different blocks, the outputs of the filters 52 and 53, not shown here. In the intermediate blocks these signals are sampled only, but this is not of importance for the operation of the side band separation.

Apart from the useful signal mixer circuit 16 also receives the signal from oscillator 7 without 90° phase-shift, whereas the mixer stage 17 receives the signal with a 90° phase-shift.

The mixer stage 17 is followed in the circuit of Figure 4 by a phase-shift circuit imparting an additional phase-shift of 90° to the output signal of the second mixer stage 17. The signals thus obtained are supplied next to the adding circuit 60 and to the subtracting cir¬ cuit 61 respectively. The adding circuit supplies a signal s, the sub¬ tracting circuit the signal v. This circuit operates as follows.

First an upper side band will be considered, which means a receiv¬ ed signal of frequencies which are a little higher than the carrier wave or the reference, followed by a lower side band having frequencies which are a little lower than this carrier wave or reference. A) Reception of the upper side band

The oscillator 7 supplies the reference signal

01 = cos α

02 = cos (α + 90)

Reception of signal E, a little higher in frequency than the oscil- lator, hence:

E = cos (α +o)

The first mixer stage 16 supplies:

Ml = E x 01 = cos (α -ho) x cos α = 0.5 cos (2α +&) + 0.5 cos and because of the low pass filter 20:

Ml -= 0.5 cos

The second mixer stage 17 supplies:

M2 = E x 02 - cos (o + t) x cos (α + 90)

= 0.5 cos (2α + 90 + y. ) + 0.5 cos (6 - 90) and because of filter 21: M2 = 0.5 cos (c - 90) M2 will have a 90° additional phase shift, hence: M2' = 0.5 cos &

S - Ml + M2' = cos C , hence the single upper side band V = Ml - M2' = 0, hence no output, from the upper side band B) Reception of lower side band The oscillator 7 supplies the reference signal:

01 = cos α

02 = cos (α + 90)

Now it is received signal E, which is a little lower in frequency than the oscillator, hence: E = cos (α -/

The first mixer stage 16 supplies: Ml = E x 01 = cos (α - / x cos α

= 0.5 cos (2α -y + 0.5 cos and because of the low pass filter 20: Ml = 0.5 cos f

The second mixer stage 17 supplies:

M2 = E x 02 = cos (α - ) x cos (α + 90)

= 0.5 cos (2o + 90 - ) + 0.5 cos (^+ .90) and because of filter 21: M2 = 0.5 cos (γ + 90)

M2 will have a 90° additional phase shift, hence: M2' = 0.5 cos ( + 180) - +0.5 cosy Hence:

S = Ml + M2' = 0 and V = Ml - M2' = cos^

The above shows that the lower side band will result in a signal only in the output V and not in S. Thus this circuit offers the possi¬ bility of separate reception of upper side band and lower side band in the outputs S and V respectively. A phase shift of 90° over the wished band of 2 - 150 Hz cannot be realized physically by means of a filter. Instead two filters can be used, giving a mutual difference in phase shift of 90° over a large

frequency area.

The filters 52 and 54 are followed therefore by the broad band phase shift filters 56, 58 respectively, whereas the filters 53 and 55 are followed by filters 57 and 59 respectively, giving an additional 90° phase shift. The signals from the filters 56 and 57 supply, adding in the adding circuit 60 the upper side band and subtracting In the subtracting circuit 61 the lower side band. The same applies to the other reception channel. The adding circuit 62 supplies the upper side band and the subtracting circuit 63 the lower side band. The upper side bands and/or lower side bands can be selected by means of switches 64 and 65 in order to be applied by means of ampli¬ fier 66 and 67 to a cross correlator 72 In Figure 1 for further proces¬ sing and displaying the fluid velocity by means of the display circuit 73. By using the upper side bands and lower side bands separately and two signal processing channels, each comprising a transmitter trans¬ ducer and a receiver transducer, the same effect can be obtained as having four signal processing channels without using side band sepa¬ ration techniques. By using the above discussed circuit a high reliability and accuracy of the velocity measurement can be obtained, which experiment¬ ally appears to be within 2%. The background noise is suppressed very strongly and errors resulting from standing waves are nearly completely absen .

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