Thus acoustic flowmeters must be capable of operating with very low signal to noise ratios and as a result practical systems must rely upon complex signal processing techniques. These techniques can only be implemented at considerable cost and as a result acoustic flowmeters have not been widely used. It would be possible to improve the signal to noise ratio by introducing a band pass filter into the signal processing system. The sort of frequencies that have been proposed for use in flowmeters are typically from a few hundred Hertz to a few thousand Hertz. The reason for this is that provided the acoustic wave length is larger than the diameter of the pipe through which the monitored flow is passing the performance of the flowmeter is independent of the flow profile. Given such frequencies a significant improvement in the signal to noise ratio can only be achieved by using a band pass filter having a very narrow pass band, typically a pass band only a few Hertz wide. Unfortunately it is not a practical proposition to manufacture such a filter using discrete components. The tolerances required would be two rigid and such a filter would drift off frequency when exposed to temperature variations.

It has been known for many years in radio frequency systems to adopt the "Weaver" method to generate single side band signals. The Weaver method relies upon the use of low pass filters to reject unwanted sidebands. It is an object of the present invention to apply low pass filters to the problems outlined above with regard to the

signal to noise ratio encountered in acoustic flowmeters.

According to the present invention, there is provided an acoustic flowmeter comprising an acoustic signal source, means for launching a signal output from the source into a flow path, means for detecting the signal after transmission through the flow path, means for measuring the time taken for the signal to be transmitted through the flow path, and means for calculating the rate of flow within the flow from the measured transmission time, wherein the detecting means comprises a transducer which is responsive to acoustic signals transmitted through the flow path to generate a first signal, means for mixing the signal with a second signal to generate a third signal, the first and second signals being of different frequency, means for mixing the first signal with a fourth signal to generate a fifth signal, the second and fourth signals being at the same frequency and in quadrature, a pair of low pass filters to each of which a respective one of the third and fifth signals is applied, each low pass filter having a cut-off frequency selected such that signals at a frequency substantially equal to or less than the difference in frequency between the first and second signals are passed, means for mixing the filtered third signal with the second to generate a sixth signal, means for mixing the filtered fifth signal with the fourth signal to generate a seventh signal, and means for combining the sixth and seventh to generate an eighth signal which is representative of the signal transmitted through the flow path.

The signal to noise ratio of the eighth signal is very much larger than the signal to noise ratio of the first signal as a result of rejection of unwanted signal components in the low pass filters.

It is a relatively easy matter to produce low pass filters having the required filter characteristics. In particular, commercial low pass elliptic switched capacitor filters are readily available. The system in accordance with the invention may be regarded as a translation of the low pass filter to the frequency of the second signal together with a mirroring of the low pass filter characteristic around the second frequency. The result is equivalent in performance to the provision of a band pass filter with its centre frequency at the frequency of the second signal and a band width equal to twice the cut off frequency of the low pass filters.

Preferably the eighth signal is applied to a phase locked oscillator to provide in effect a memory the content of which is available in the event of a temporary loss of signal.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is the acoustic spectrum of noise in a pipe through which a fluid is flowing and into which a signal at 316.25 Hz has been launched;

Figure 2 illustrates the signal to noise ratio achieved using a simple acoustic pick up to detect the signal represented in Figure 1; Figure 3 illustrates the improvement in the signal to noise ratio which can be achieved using a narrow band-pass filter;

Figure 4 illustrates an embodiment of the present invention which enables characteristics such as those illustrated in Figure 3 to be achieved;

Figure 5 illustrates the characteristic performance of low pass filters illustrated in Figure 4 and the equivalent band-pass filter characteristics achieved by the use of low pass filters in Figure 4; Figure 6 illustrates a circuit for generating the fixed frequency signals utilised in the embodiment of Figure 4; and Figure 7 illustrates results obtained with an embodiment of the present invention.

Referring to Figure 1, this illustrates the acoustic spectrum of noise in a pipe together with a signal of 316.25 Hz launched into that pipe. The launched signal is detected by a conventional transducer and if the output of that transducer is not further processed the signal to noise ratio is very poor due to the high level of the noise floor as illustrated in Figure 2. Figure 3 illustrates how the signal to noise ratio could be greatly enhanced if it was possible to provide a narrow band pass filter centred on the wanted signal frequency of 316.25 Hz. The improvement in the signal to noise ratio is a corollary of the narrow band noise floor.

The signals illustrated in Figures 1 and 2 are those which are obtained using conventional acoustic flowmeter techniques in a noisy environment. The conditions illustrated in Figure 3 are unfortunately not achievable at sensible cost as it is not possible to produce a band

pass filter with a suitably sharp narrow pass band. At the frequencies shown typically a pass band of only 6 Hertz would be required.

Referring now to Figure 4, this illustrates an embodiment of the invention in which the wanted signal that is launched into the monitored flow is at a frequency of 1758 Hz. The signal is received by a loudspeaker configured as a microphone and the output of that transducer is applied as a first signal to an input 1. The first signal is the combination of the wanted signal at 1758 Hz and the accompanying acoustic noise. The detector circuitry illustrated in Figure 4 comprises a local oscillator the output of which is at a frequency 1762 Hz and is applied to input 2. The local oscillator signal is applied to a phase shifter 3 having outputs 4 and 5 to which signals at 1762 Hz are applied in quadrature. That is to say there is a 90° phase shift between the signals appearing on outputs 4 and 5.

The signal on output 4 represents a second signal which is multiplied in a mixer 6 with the first signal to generate a third signal that is applied to a low pass filter 7. The signal on output 5 represents a fourth signal which is applied to a mixer 8 with the first signal so as to produce a fifth signal that is applied to a low pass filter 9. The mixers 6 and 8 may be for . example LM1596/LM1496 balanced modulator - demodulators available from National Semiconductor Corporation and the filters 7 and 9 may be LTC1064-1 low noise, eighth order, clock sweepable elliptic low pass filters available from Linear Technology. The low pass filters receive a clock signal applied to input 10 at 439.5 Hz which gives the filters a cut¬ off frequency of 4.395 Hz. The outputs of the mixers 6 and 8 carry signals at the frequency of the sum and difference of the signals applied to inputs 1 and 2. The difference (beat) frequencies are passed by the filters 7 and 9 and the other signals are rejected. Thus the outputs of filters 7 and 9 are at 4 Hz.

The output of the filter 7 is applied to a mixer 11 with the second signal appearing on the output 4 of the phase shifter 3. The output of the filter 9 is applied to a mixer 12 with the fourth signal appearing at the output 5 of the phase shifter 3. The outputs of the mixers 11 and 12 can be considered as sixth and seventh signals which are combined to form an eighth signal. This occurs in an amplifier 13

to produce a reconstituted version of the original signal without the noise. A phase locked oscillator 14 is then locked onto this signal. The output of the phase locked oscillator is at 1758 Hz, that is the frequency of the wanted signal.

In effect, the circuit operates to translate the low pass filters in frequency such that the low pass filter characteristics are mirrored about the frequency of the local oscillator. The result is effectively a band pass filter with its centre frequency at the frequency of the local oscillator. As the filters are of the switched capacity type and cannot respond to DC signals the local oscillator frequency is also suppressed.

It will be appreciated that the two sides of the circuit need to be balanced and that in order to maintain good frequency stability all signals should be derived from a common source. Such an arrangement is illustrated in Figure 6 in which a stable 10.245 MHz crystal source 15 supplies a first divider 16 which outputs a frequency at 1762 Hz and a second divider 17 which outputs a frequency of 1758 Hz. A third divider 18 generates an output at 439.5 Hz.

By way of further explanation of the circuit of Figure 4, the above mentioned first to eight signals can be represented as follows:

First Signal cos W _c t

Second Signal cos W _m t

Third Signal cos (W _c t - W _m t) - cos (W _c t + _m t)

Fourth Signal cos (W _m t + 90)

Fifth Signal cos (W _c t - [W _m t + 90] - cos (W _c t + [W _m t + 90])

Sixth Signal cos W _c t - cos (2W _m t - W _c t)

Seventh Signal cos W _c t + cos (2W _m t - W _c t)

Eighth Signal 2 cos W _c t

To summarise the advantages of the invention, the good signal to noise ratio characteristics enable acoustic systems to operate on relatively low power levels. An embodiment of the invention has been tested measuring a gas flow of 36 metres per second in a four inch pipe with only 62 mW of power launching the signal into the gas flow. Results obtained are illustrated in Figure 7. It will be seen that the acoustic velocity (the output of the meter embodying the invention) accurately tracks the reference velocity (the velocity of the flow

determined by other means) Earlier acoustic gas flow measurement systems have used much higher power to overcome the signal to noise ratio problem, for example 70 watts.

In the illustrated embodiment of the invention the switched capacitor filters that are used avoid phase drifts since the clock signal is derived from a crystal. Furthermore, switched capacitor filters have added immunity from noise due to the use of sampling. Finally, the use of a phase locked oscillator that is locked to the transmitting frequency means that the stability of the whole circuit is only dependent on the stability of the 10.245 MHz crystal.