GB2362058A - Equalising transfer functions of linear electro-acoustic systems - Google Patents

Abstract

An electro-acoustic system utilizes a technique in which the source terminating impedance (Rt, fig 1) connecting to the output terminals of a power amplifier of the system is minimised or eliminated in order to improve efficiency and then compensate for the increased phase and amplitude distortions introduced by this removal or minimisation by introducing a phase and amplitude equaliser 41 on the input to the amplifier. A signal receiver of the system comprises a transducer and a signal shaping circuit for producing an output signal having linear phase response over a selected frequency band. The method is particularly useful for hydrophones.

Description

2362058 1 METHOD AND APPARATUS FOR EQUALISING TRANSFER FUNCTIONS OF LINEAR

ELECTRO ACOUSTIC SYSTEMS, The present invention relates to equalisation techniques which help to improve the overall phase linearity, efficiency and amplitude response of transfer functions particularly in electro-acoustic systems. Transducers in a very broad sense, are devices for converting energy from one form to another.

An underwater electro-acoustic transducer converts electrical energy into mechanical energy or vice versa using the Piezoelectric effect. For a transmitter or (projector), electrical energy is converted into mechanical energy and transferred into a transmission medium such as water as a sound (pressure) wave. A receiver (hydrophone) on the other hand converts received acoustic pressure waves into an electrical signal. In general, piezoelectric transducers is are narrowband devices. That is, they only work efficiently and with linear phase over a very narrow band of their overall frequency response. This causes phase and amplitude distortion of any linear signal being transduced whether on transmit or receive. The result of this, is that the output signal of the system is no longer a good replica of the input signal.

The analysis of a transducer's performance in an electrical system is best understood by knowing its electrical equivalent circuit. The equivalent circuit of a piezoelectric transducer is made up of passive components such as inductors, capacitors and resistors as is shown in Fig 1. If the device is to be used as a projector a tuning coil (Lo) is then placed across the input terminals to tune out the ceramic plate capacitance and an impedance matching transformer Tl to match the load to the impedance required by the power 2 amplifier for optimum power transfer. The resistor Rt in the circuit represents the termination impedance which together with the load impedance RI forms the termination impedances described in all classical analogue filter design tables.

The problem of piezoelectric transducers introducing phase and amplitude distortion is well known and there have been and still are atiempts to overcome these problems. For example the use of composite materials and the introduction of extra mechanical components attached to the piezoelectric ceramic improve the situation somewhat but still do not offer significant improvement.

An alternative approach is to use the matching networks externally associated with the transducer and modify these. As an example, and using classical methods it is possible to take a transducer electrical equivalent circuit and add components to form tuned circuit sections. This combination forms is an N'th order bandpass filter where N is the total number of tuned circuits in the complete system. This has the effect of creating an overall wider bandpass system response with improved phase linearity when used with the correct terminating resistance Rt. As a consequence, these methods are very inefficient when one is trying to transmit power due to the power loss in the resistor Rt.

The present invention relating to the transmit part of the system proposes a technique in which the source terminating impedance Rt connecting to the output terminals of a power amplifier of the system is minimised or eliminated in order to improve efficiency and then compensate for the increased phase and amplitude distortions introduced by this removal or minimisation by introducing a phase and amplitude equaliser on the input to 3 the amplifier. It is assumed that the source impedance of the amplifier Rs is negligible and does not influence the behaviour of the system response. The advantage of this technique is that the system has the same overall effect as the correctly terminated filter but without the power loss.

In order that the present invention be more readily understood, an embodiment thereof will now be described with reference to the accompanying drawings in which; Figure 1 shows an electrical equivalent circuit for a conventionally matched electrical transducer circuit with terminating resistor Rt.

Figure 2 shows an electrical equivalent circuit for an overall N=3 transducer + matching circuit with terminating resistor Rt.

Figures 3a and 3b show the phase and amplitude transfer function of C Fig 2 without Rt and without equalisation.

Figure 4a shows a block diagram of an equalised signal generation is system according to the present invention.

Figure 4b shows a block diagram of a modification to the system shown in Fig 4a.

Figures 5a and 5b show the phase and amplitude compensating transfer function necessary to equalise the responses in Figs 3a and 3b.

Figures 6a and 6b show the overall system phase and amplitude transfer functions of the system by applying the technique of the present invention shown in Fig 4.

Figure 7 shows the output signal response of the unequalised system without Rt for an input signal consisting of 10 cycles of a sine wave.

Figure 8 shows the output response of the equalised system without Rt for an input signal consisting of 10 cycles of a sine wave incorporating the 4 invention referred to in Fig 4.

Figure 9 shows the reduction in VA taken from a power amplifier for different matching/transducer configurations.

Figure 10 shows a block diagram of an equalised receiver (Hydrophone) system.

Figure 11 shows the Amplitude responses of Fie, 10.

Figure 12 shows the Phase responses of Fig 10.

Figure 13 shows the electrical signal out of the hydrophone for an input signal consisting of 5 cycles of a sine wave.

Figure 14 shows the electrical signal out of the equaliser where the input signal comes from the hydrophone via the undistorted pre-amp.

The preferred embodiment of the present invention will be described in relation to underwater sonar systems. Broadband sonar improves acoustic replication of an input electrical signal especially pulses of a short time is duration which would be necessary in a shallow water conflict to rocess P against reverberation. As the bandwidth of the transmitted signals is broadened it is important to ensure that the receiving hydrophone has sufficient phase and amplitude correction so as not to distort the in-water signal. The reduction in power taken by the transducer is also important when the size of the power amplifier needs to be minimised. In order to reduce this power requirement, we propose to remove or substantially reduce the dissipative termination impedance of the matching network. It should also be noted that this impedance does not then form part of the source impedance of the power amplifier which should be low enough so as not to affect the overall system response. Normally Rs is insignificant compared to Rt. An electrical equivalent circuit of a conventional narrowband transducer system with tuning coil and impedance transformer is shown in Fig 1. An electrical equivalent circuit of a correctly terminated broadband transmitting transducer and matching system is shown in Fig 2. Its corresponding phase and amplitude C transfer functions when Rt is removed are shown Figs 3a and 3b respectively. It is these transfer functions that have to be equalised. A block diagram of the preferred embodiment to achieve system equalisation is shown in Fig 4a in which a signal generator 40 generates an output signal. This signal is applied to the phase and amplitude equaliser 41 whose transfer function shown in Figs 5a and 5b causes the signal to be distorted in opposition to the transfer function provided by the transducer and matching 43. This transfer function was previously shown in Figs 3a and 3b. It is assumed that the linear amplifier 42 does not introduce amplitude or phase distortion into the system. The combined overall system transfer function is shown in Figs 6a, and 6b. The unequalised output response when 10 cycles of a sine wave form the input signal from 40 is shown in Fig 7. When the equaliser 41 is added to the system the output signal is much improved and is shown in Fig 8. The extra amplitude equaliser 44 can be used to adjust the overall amplitude response without affecting the phase characteristic. Thus it is possible to extend the flat amplitude response over a wider band of frequencies but at the expense of an increased input power requirement.

A further aspect of this embodiment is that due to the improvement in the linear phase characteristic of the system the group delay of the system is much improved. It should be noted that the group delay is the differential of the phase response and for a narrowband system as shown in Figl, the poor non-linear phase response causes the group delay to vary quite considerably. For the group delay to be constant the phase response has to be 6 linear.

Having improved the efficiency and quality of the transmitted waveforms it is appropriate to consider whether the receiver transducer fidelity can be improved. It has been discovered that by considering the phase and amplitude response of the hydrophone it is possible to equalise this as well.

The embodiment of this system is shown in Fig 10 and consists of a receiver hydrophone 100 which converts a received acoustic pressure wave into a voltage Vhyd. This low level signal normally in the order of mV is applied to the receiver amplifier 101. This is usually a low noise operational amplifier with a linear gain and phase response over the operating frequency range sufficient so not to introduce any unwanted distortions into the system. An equaliser 102 whose phase and amplitude transfer ftinction acts in opposition to that of the hydrophone produces an output signal which is a much closer replica of the in water pressure wave. The corresponding system transfer is functions and their resultant are shown in Figs II and 12. Note the much improved linear phase and amplitude response. This in turn causes the group delay to be more constant.

As a result of the improvement in the transmitter and receiver as described above, sonargrams can be achieved with a fidelity that has not been achieved before.

Although the preferred embodiment has been described in relation to its use as a sonar system operating in the range 200 to 400Hz, it will be appreciated that the above techniques can and has been used over different frequency bands, with different Nth order systems and with other types of transducers where their electrical equivalent circuits can be evaluated.

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Claims (8)

CLAIMS:

1. A signal transmission arrangement comprising a signal generation for generating a signal, the generator having little or no source tenninating impedance, a signal shaping circuit for shaping the generated signal to produce an output signal having linear phase response over a selected frequency band, and a transducer which receives the output of the shaping circuit and transmits signal into a transmission medium.

2. An arrangement according to claim 1, wherein the signal shaping circuit is arranged to compensate for phase errors resulting from the absence of a source impedance.

3. An arrangement according to claim 1 or 2 wherein the signal is shaping circuit includes means for compensating for errors in amplitude response to produce a flat amplitude response over a selected frequency band.

4. A signal transmission arrangement substantially as herembefore described with reference to Figs 4 to 9 of the accompanying drawings.

5. A signal receiver arrangement comprising a receiver transducer and a signal shaping circuit for producing an output signal having linear phase response over a selected frequency band.

6. A receiver arrangement according to claim 5, wherein the signal shaping circuit includes means for producing a relatively flat amplitude 8 response.

7. A receiver arrangement substantially as hereinbefore described with reference to Figs 10 to 14 of the accompanying drawings.

8. A sonar system comprising a signal transmission arrangement according to any one of claims 1 to 4 and a signal receiving arrangement according to any one of claims 5, 6 or 7.