INTERFEROMETRIC SIGNAL PROCESSING APPARATUS

Abstract: An interferometric signal processing apparatus (10) producing an output signal from a first input signal (34) and a second input signal (34), said input signals (34) having substantially equal carrier frequencies, comprising: a bridge (12) having a first arm (26) and a second arm (28), each arm having a first end (30) and a second end (32), the first and second input signals (34) being input to the first end (30) of the first and second arms (26, 28), respectively; a device-under-test (36) provided the first arm (26); a carrier suppression means (14) connected to the second ends (32) of the first and second arms (26, 28) to produce a carrier-suppressed signal at its output (A); an amplifier (16) arranged to amplify said carrier-suppressed signal; and a mixing means (22, 24) responsive to the amplified carrier-suppressed signal and a carrier-dominated signal to produce the output signal; wherein the differential group delay between: the first end (30) of the first arm (26) and the output (A) of the carrier suppression means (14); and the first end (30) of the second arm (28) and the output (A) of the carrier suppression means (14) is less than or equal to 1000/fo seconds, where fo is the time-averaged mean value of the carrier frequencies of the input signals (34).

Description: TITLE INTERFEROMETRIC SIGNAL PROCESSING APPARATUS TECHNICAL FIELD This invention relates to an interferometric signal processing apparatus. BACKGROUND ART Interferometric signal processing apparatuses use an interferometric technique to minimise the noise contributions of the signal processing elements acting on the input signals, so that finer details of the differences between the input signals can be analysed with less limitation. Carrier suppression is used to increase the sensitivity of interferometric signal processing apparatuses. Interferometric signal processing apparatuses have been used in two configurations. The first configuration is described by A. L. Lance, W. D. Seal and F. Labaar at page 273 in 'Infrared and Millimeter Waves', Vol 11 , 1984, published by Academic Press, Inc. The interferometer described consists of a bridge in which a delay line forms one arm of the bridge and a phase shifter and an attenuator form the other arm of the bridge. The two arms of the bridge are then combined to achieve carrier suppression. Both arms of the bridge are fed with a signal from a unit-under-test. The interferometer described by Lance et al measures FM noise in the unit- - under-test. The delay line acts to introduce a differential group delay between the signals present at the power combiner, so that if the frequency of the signal produced by the unit-under-test varies with time, the signals appearing at the power combiner will have different frequencies. The output from the power combiner will be in accordance with the phase difference between the signals. The sensitivity of the interferometer described by Lance et al is proportional to the differential group delay created by the delay line. Consequently, a significant group delay is considered desirable. The second configuration is described by E. Ivanov in a paper titled 'A new mechanism for paramagnetic back-action effect in a gravitational wave antenna with a microwave cavity transducer' at pages 1737-1742 in J. Phys. D: Appl. Phys, Vol 28, 1995, The interferometer described consists of a bridge in which a microwave transducer, operating as a displacement sensor for a Nb gravity- wave detector, is radiatively coupled into one arm of the bridge via a pair of microwave antennae and a circulator, and the other arm of the bridge has a phase shifter and an attenuator. The two arms of the bridge are combined to achieve carrier suppression. Both arms of the bridge are fed with a signal from a microwave signal source. The interferometer described by Ivanov is used to increase the sensitivity of the measurements from the transducer by utilising carrier suppression. The phase shifter in the other arm of the bridge is used to ensure that the signals appearing at the power combiner are 180° out of phase to achieve carrier suppression. Sensitivity is of primary importance in measuring displacement of the Nb gravity- wave bar. Transducers with a high Q are used to increase the sensitivity of the measurements, however such high Q transducers introduce a significant group delay into the one arm of the bridge. In turn, this results in a significant differential group delay in the bridge of the interferometer described by Ivanov, ~ with differential group delays of 10μs or more at 10GHz. The significant differential group delay in the bridge places stringent requirements on the stability of the microwave signal source used. If the frequency of the microwave signal source varies with time, the output from the power combiner will include signals which are due to noise of the source, including its PM, FM and AM noise. This can significantly limit the sensitivity of the displacement measurements. In both of the above configurations, the device in the one arm of the bridge acts as a frequency discriminating element and alters the relative phase shift of the signal travelling through it depending on the frequency of the signal. The sensitivity of the interferometer described by Lance et al is directly proportional to the group delay of the delay line. Similarly, in the interferometer described by Ivanov the sensitivity of the transducer is proportional to its Q factor. Consequently the sensitivity of the transducer is proportional to the group delay of the transducer, since the group delay of the transducer is substantially equal to the Q factor of the transducer divided by the carrier frequency. SUMMARY OF THE INVENTION The interferometric signal processing apparatus of this invention produces an output selectively containing information about the amplitude, phase or both amplitude and phase noise produced by a device-under-test (DUT). The invention seeks increase the frequency bandwidth over which the apparatus is useable and to reduce the limitations set by the noise contributions of the signal source by minimising the differential group delay. According to a first aspect of this invention, there is provided an interferometric signal. processing apparatus producing an output signal from a first input signal and a second input signal, said input signals having substantially equal carrier frequencies, comprising: a bridge having a first arm and a second arm, each arm having a first end and a second end, the first and second input signals being input to the first end of the first and second arms, respectively; a device-under-test provided the first arm; a carrier suppression means connected to the second ends of the first and second arms to produce a carrier-suppressed signal at its output; an amplifier arranged to amplify said carrier-suppressed signal; and a mixing means responsive to the amplified carrier-suppressed signal and a carrier-dominated signal to produce the output signal; wherein the differential group delay between: the first end of the first arm and the output of the carrier suppression means; and the first end of the second arm and the output of the carrier suppression means is less than or equal to 1000/f o seconds, where f 0 is the time-averaged mean value of the carrier frequencies of the input signals. In the previous statement and throughout this specification, the term 'device- under-test' is intended to mean anything that is responsive to a signal at the carrier frequency, and may include a number of components or a circuit network. ~ Preferably, a delay means provided in at least one arm of the bridge to reduce the differential group delay. Preferably, the carrier suppression means comprises a power combiner, a phase shift means and an amplitude matching means, the phase shift means and the amplitude matching means being arranged such that the power combiner produces the carrier-suppressed signal from signals input thereto. In one arrangement, said carrier-dominated signal is a further signal produced by the power combiner. In an alternative arrangement, said carrier-dominated signal is one of said input signals. In a further alternative arrangement, said carrier-dominated signal is a third signal having substantially the same carrier frequency as the first and second input signals. Preferably, said output signal is used in a control system, a feedback system or a read-out system. Preferably, the apparatus further comprises a plurality of mixing means to produce a plurality of output signals. By altering the phase difference between the carrier-suppressed signal and the carrier-dominated signal, it is possible to control whether the output signal corresponds to the amplitude of, the phase difference between, or both the amplitude of and the phase difference between the input signals. For example, if the carrier-suppressed signal and the carrier-dominated signal are in phase (ie. a phase difference of 0°) when input to the mixing means, the output signal will correspond to the amplitude of the input signals. Throughout the specification, this arrangement is referred to as an 'amplitude sensitive mode'. Further, if the carrier-suppressed signal and the carrier-dominated signal are in quadrature (ie. a phase difference of 90°) when input to the mixing means, the output signal will correspond to the phase difference between the input signals. Throughout the specification, this arrangement is referred to as a 'phase sensitive mode'. Furthermore, if there is a phase difference of between 0° and 90° between the carrier-suppressed signal and the carrier-dominated signal when input to the mixer, the output signal will correspond to both the amplitude of and the phase difference between the input signals in proportions corresponding to the cosine and sine of the phase difference, respectively. Advantageously, if two mixing means are utilised in the apparatus and are arranged so that one of said means is set to a phase sensitive mode and the other said means is set to an amplitude sensitive mode, the apparatus can simultaneously measure both phase and amplitude changes induced by the device-under-test. This arrangement is particularly advantageous in that the outputs of the two mixers contain all of the amplitude and phase information needed to fully characterise the properties of the device-under-test. A further advantage of the apparatus is that it provides increased immunity to amplitude and phase noise in the signal source, nor is its sensitivity limited by the intrinsic noise of the mixers when compared with existing apparatuses. Preferably, said apparatus has two mixing means, one arranged in an amplitude sensitive mode and the other arranged in a phase sensitive mode, to produce a pair of output signals corresponding to the amplitude and phase noise induced by the device-under-test. One or both of the output signals can then be used in ~ a feed back control system in association with a voltage controlled attenuation means and/or a voltage controlled phase shift means to reduce one or both of the amplitude and phase noise in the device-under-test. In addition, one or both of the output signals can also be used in a further feed back control system to control operation of the phase shift means and the amplitude matching means of the carrier suppression means to maximise the carrier suppression, or to control certain characteristics of the device-under-test. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described with reference to six embodiments thereof and the accompanying drawings, in which: Figure 1 shows an interferometric signal processing apparatus according to a first embodiment of the invention; Figure 2 shows an interferometric signal processing apparatus according to a second embodiment of the invention; Figure 3 shows an interferometric signal processing apparatus according to a third embodiment of the invention; Figure 4 shows an interferometric signal processing apparatus according to a fourth embodiment of the invention; Figure 5 shows an interferometric signal processing apparatus according to a fifth embodiment of the invention; Figure 6 shows an interferometric signal processing apparatus according to a sixth embodiment of the invention; and Figure 7 shows an interferometric signal processing apparatus according to a seventh embodiment of the invention. DESCRIPTION OF THE BEST MODE OF THE INVENTION ln figure 1 there is shown an interferometric signal processing apparatus 10 comprising a bridge 12, a carrier suppression means 14, an amplifier 16, a pair of phase shifters 18 and 20 and a pair of double balanced mixers 22 and 24. The bridge 12 has a first arm 26 and a second arm 28, each of which has a first end 30 and a second end 32. The first end 30 of each arm 26 and 28 is connected to a signal source in the form of an oscillator 34. A DUT (device- under-test) 36 is provided in the first arm 26 of the bridge 12. The carrier suppression means 14 comprises a power combiner 38 in the form of a 3dB 90° hybrid, a phase shifter 40 and an attenuator 42. The phase shifter 40 and the attenuator 42 are provided in the second arm 28 of the bridge 12. The power combiner 38 is connected to the second ends 32 of the arms 26 and 28. The phase shifter 40 and the attenuator 42 are arranged such that the power combiner 38 produces a carrier-suppressed signal at its output 'A' and a carrier-dominated signal at its output 'B'. In other embodiments, the phase shifter 40 and the attenuator 42 may be provided in the first arm 26. The output 'A' of the power combiner 38 is connected to the input of the amplifier 16. The output of the amplifier 16 is connected to each of the mixers 22 and 24. The output 'B' of the power combiner 38 is connected to each of the phase shifters 18 and 20. The output of the phase shifter 18 is connected to the mixer 22 and the output of the phase shifter 20 is connected to the mixer 24. The phase shifter 18 and the mixer 22 are arranged in a phase-sensitive mode. That is, arranged such that the signals appearing at the mixer 22 are in _ quadrature such that the mixer 22 produces an output corresponding to the phase difference between the signals input to the power combiner 38. The phase shifter 20 and the mixer 24 are arranged in an amplitude-sensitive mode. That is, arranged such that the signals appearing at the mixer 24 are in phase, such that the mixer 24 produces an output corresponding to the amplitude of the signals input to the power combiner 38. To minimise the FM noise contribution from the oscillator 34 appearing at the output 'A' of the power combiner 38, the differential group delay between the first end 30 of the first arm 26 and the output 'A' of the power combiner 38, and the first end 30 of the second arm 28 and the output 'A' of the power combiner 38 is minimised. The differential group delay must be less than 100ns, and ideally will be close to 0ns. Minimising the differential group delay also increases the bandwidth of measurements that can be taken. To achieve a minimal differential group delay, it is necessary to match the group delay from the first end 30 of the first arm 26 to the output 'A' of the power combiner 38 with the group delay from the first end 30 of the second arm 28 to the output 'A' of the power combiner 38. In this embodiment, it is assumed that the DUT 36 has a smaller group delay than the combined group delay of the attenuator 42 and the phase shifter 40. Accordingly, a delay means 43 is provided in the first arm 26 of the bridge 12 to minimise the differential group delay. The delay means used in the embodiment is a length of cable, however it should be appreciated that other devices may be used with equal efficacy, such as transmission line, phase shifters or trombone phase shifters. In order to achieve a minimal differential group delay, the group delay of each arm 26 and 28 is measured with a vector network analyser and the difference calculated therefrom. Alternatively, the group delay of each arm 26 and 28 can be estimated from measured or specified device characteristics. The group delay in at least one of the arms 26 and 28 is then adjusted by adding or removing a length of transmission line of known delay per unit length. Fine tuning of the differential group delay can be achieved using a variable-delay transmission line, such as a phase shifter, a tombone phase shifter, a reactive phase shifter or a voltage controlled phase shifter. Where the differential group delay between the arms 26 and 28 is minimal, compensation for this group delay may be achieved using the phase shifter 40. Where the differential group delay is larger, a delay means must be inserted into one of the arms 26 or 28. A second embodiment of the invention is shown in figure 2. The second embodiment is directed towards an interferometric signal processing apparatus 50, with like reference numerals to denoting like past to those described in the first embodiment. The second embodiment differs from the first embodiment in that the first end 30 of the first arm 36 is connected to a first signal source 52 and the first end 30 of the second arm 28 is connected to a second signal source 54. Further, in this embodiment it is assumed that the phase shifter 40 can compensate for the differential group delay. It should be appreciated that a delay means can be inserted into the bridge 12 if the differential group delay so required. In addition, the output 'B' of the power combiner 38 is terminated using a matching impedance 56. A carrier-dominated signal is provided to the mixers 22 and 24 from a third signal source 58. A delay means 59 is provided between the third signal source and the mixers 22 and 24. The delay means 59 is arranged such that there is a minimal group delay between the signals input to the mixers 22 and 24. The first signal source 52, the second signal source 54 and third signal source 58 all have substantially equal carrier frequencies. The phase-sensitive output produced by the mixer 22 is input to a filter 60, the output of which is connected to the phase shifter 40 to control operation thereof. Similarly, the amplitude-sensitive output produced by the mixer 24 is connected to a filter 62, the output of which is connected to the attenuator 42. By utilising a feed back control system in the form of the filters 60 and 62, operation of the phase shifter 40 and the attenuator 42 are controlled such that carrier suppression achieved by the power combiner 38 is substantially maximised. A further advantage of the feedback control system is that variations in the phase and amplitude response of the DUT 36 with time, temperature, power or frequency are compensated for, ensuring carrier suppression. The output signals from the mixers 22 and 24 can also be used to monitor the phase and amplitude noise induced by the DUT 36 or in a signal analysis or control system. A third embodiment of the invention is shown in figure 3, with like reference numerals denoting like past to those described in relation to the previous embodiments. The third embodiment is similar to the second embodiment, except that the delay means 59 and the feed back controlled system is not provided. Further, in this embodiment it is assumed that the group delay in the first arm 26 is significantly greater than the group delay in the second arm 28. Accordingly, a delay means 43 is provided in the second arm of the bridge 12. A fourth embodiment of the invention is shown in figure 4, with like reference numerals denoting like past to those described in relation to the earlier embodiments. The fourth embodiment is directed towards a interferometric signal processing apparatus 80 of a generally similar form to the first embodiment. The fourth embodiment differs from the first embodiment in that the signal source for each arm of the bridge and the mixer is the same source, and the phase shifter 20 and the mixer 24 are omitted from the interferometric signal processing apparatus 80. Further, the output 'B' of the power combiner 38 is terminated by a matching impedance 52. The carrier-dominator signal for the mixer 22 is provided from the oscillator 34. Still further, a delay means 43 is provided in each arm 26 and 28 of the bridge 12. In this manner, the differential group delay is minimised, and the differential group delay of the signals input to the mixer 22 is also minimised. A fifth embodiment of the invention is shown in figure 5, with like reference numerals denoting like past to those described in relation to the previous embodiments. The fifth embodiment is directed towards an interferometric signal processing apparatus 100 that is similar to the interferometer of the first embodiment, except that the phase shifter 20 and the mixer 24 in the apparatus of the first embodiment are omitted from the interferometric signal processing apparatus 100. Further, the device-under-test in the fifth embodiment comprises an amplifier 102. A coupler 104 is provided in the first arm 26 after the amplifier 102 to couple a portion of the output of the amplifier 102 from the bridge 12. The fifth embodiment includes a feed back control system in the form of a filter 106 and a voltage-controlled phase shifter 108. The voltage-controlled phase shifter 108 is provided in the first arm 26 before the amplifier 102. The filter 106 is responsive to the phase-sensitive output of the mixer 22. The output of the filter 106 is connected to the voltage-controlled phase shifter 108 to _ control the operation thereof. The feed back system acts to substantially eliminate phase noise introduced by the amplifier 102. - 13 - A sixth embodiment of this invention is shown in figure 6, with like reference numerals denoting like past to those described in the previous embodiments. The sixth embodiment is directed towards an interferometric signal processing apparatus 120. The interferometric signal processing 120 includes a feed back control system comprising a filter 122 and a voltage-controlled attenuator 124. The voltage- controlled attenuator 124 is provided in the first arm 26 before an amplifier 126. The device-under-test in the sixth embodiment is the amplifier 126, which is provided in the first arm 26 of the bridge 12. A coupler 128 is provided in the first arm 26 after the amplifier 126 to couple a portion of the output of the amplifier 126 from the first arm 26. The output coupled by the coupler 128 is input to a resonant cavity 130, the output of which is input to a phase shifter 132. The output of the phase shifter 132 forms the signal source for the interferometric processing apparatus 120. In effect, the resonant cavity 130, the phase shifter 132, the amplifier 126 and the coupler 128 form a loop oscillator. A further coupler 134 is provided in the loop oscillator to couple an output therefrom. The amplitude-sensitive output from the mixer 24 is input to the filter 122, the output of which is used to control the voltage-controlled attenuator 124. In effect, the feed back controlled system in the interferometric processing apparatus 120 acts to stabilise the amplifier 126 and thereby stabilise the amplitude of the output of the loop oscillator. _ A seventh embodiment of this invention is shown in figure 7, with like reference numerals denoting like past to those described in the previous embodiments. The seventh embodiment is directed towards an interferometric signal processing apparatus 140. The apparatus 140 is similar to the apparatus 50 of the second embodiment, except that the delay means 59 is not provided. In addition, the DUT 36 in the apparatus 140 includes a second output 142 The apparatus 140 includes a first feed back control system in the form of filters 60 and 62 as described in relation to the second embodiment. The apparatus 140 further includes a second feed back control system in the form of filters 106 and 122. The filters 106 and 122 are responsive to the outputs of the mixers 22 and 24, respectively. The outputs of the filters 106 and 122 are input to the DUT 36 to control operation thereof. The filters 60 and 62 act to maximise the carrier suppression, whilst the filters 106 and 122 act to compensate for phase and amplitude noise in the DUT 36. It should be appreciated that in some circumstances, the frequency ranges filtered by the filters 60 and 62 and the filters 106 and 122 should not overlap to avoid the first and second feed back control systems from competing. It should be appreciated that the present invention is not limited to the particular embodiments described above.

Claims: CLAIMS 1. An interferometric signal processing apparatus producing an output signal from a first input signal and a second input signal, said input signals having substantially equal carrier frequencies, comprising: a bridge having a first arm and a second arm, each arm having a first end and a second end, the first and second input signals being input to the first end of the first and second arms, respectively; a device-under-test provided the first arm; a carrier suppression means connected to the second ends of the first and second arms to produce a carrier-suppressed signal at its output; an amplifier arranged to amplify said carrier-suppressed signal; and a mixing means responsive to the amplified carrier-suppressed signal and a carrier-dominated signal to produce the output signal; wherein the differential group delay between: the first end of the first arm and the output of the carrier suppression means; and the first end of the second arm and the output of the carrier suppression means is less than or equal to 1000/f o seconds, where f 0 is the time-averaged mean value of the carrier frequencies of the input signals. 2. An interferometric signal processing apparatus as claimed in claim 1 , further comprising delay means provided in at least one arm of the bridge to reduce the differential group delay. 3. An interferometric signal processing apparatus as claimed in claim 2, wherein said delay means is provided in the second arm of the bridge. 4. An interferometric signal processing apparatus as claimed in claim 2, wherein said delay means is provided in the first arm of the bridge. 5. An interferometric signal processing apparatus as claimed in any one of the preceding claims, wherein the differential group delay is less than or equal to 100/f o seconds. 6. An interferometric signal processing apparatus as claimed in any one of the preceding claims, wherein the differential group delay is less than or equal to 10/f o seconds. 7. An interferometric signal processing apparatus as claimed in any one of the preceding claims, wherein the carrier suppression means comprises a power combiner, a phase shift means and an amplitude matching means, the phase shift means and the amplitude matching means being arranged such that the power combiner produces the carrier-suppressed signal from signals input thereto. 8. An interferometric signal processing apparatus as claimed in claim 7, wherein said carrier-dominated signal is a further signal produced by the power combiner. 9. An interferometric signal processing apparatus as claimed in any one of claims 1 to 7, wherein said carrier-dominated signal is one of said input signals. 10. An interferometric signal processing apparatus as claimed in any one of claims 1 to 7, wherein said carrier-dominated signal is a third signal having substantially the same carrier frequency as the first and second input signals. 11. An interferometric signal processing apparatus as claimed in any one of the preceding claims, further comprising a plurality of mixing means arranged to produce a plurality of output signals. 12. An interferometric signal processing apparatus as claimed in claim 11 , wherein one of said mixing means is arranged in a phase sensitive mode and another of said mixing means is arranged an amplitude sensitive mode. 13. An interferometric signal processing apparatus as claimed in any one of the preceding claims, further comprising a first feed back control system responsive to at least one output signal to reduce one or both of the amplitude and phase noise in the device-under-test. 14. An interferometric signal processing apparatus as claimed in claim 13, wherein said first feed back control system includes voltage-controlled attenuation means provided in said first arm of the bridge, and a first filter circuit responsive to said output signal to control the voltage-controlled attenuation means. 15. An interferometric signal processing apparatus as claimed in claim 13 or 14, wherein said first feed back control system includes voltage-controlled phase shift means provided in said first arm of the bridge, and a second filter circuit responsive to said output signal to control the voltage-controlled phase shift means. 16. An interferometric signal processing apparatus as claimed in claim 7 or any one of claims 8 to 15 as dependent on claim 7, further comprising a second feed back control system responsive to at least one output signal to control operation of at least one of the phase shift means and the amplitude matching means of the carrier suppression means to maximise carrier suppression. 17. An interferometric signal processing apparatus as claimed in claim 16, wherein said second feed back control system controls operation of both of the phase shift means and the amplitude matching means of the carrier suppression means.

Inventors: IVANOV EUGENE NIKOLAY, TOBAR MICHAEL EDMUND, WOODE RICHARD ALEC, IVANOV, EUGENE NIKOLAY, TOBAR, MICHAEL, EDMUND, WOODE, RICHARD, ALEC
Applicants: IVANOV EUGENE NIKOLAY, POSEIDON SCIENT INSTR PTY LTD, TOBAR MICHAEL EDMUND, UNIV WESTERN AUSTRALIA, WOODE RICHARD ALEC, IVANOV, EUGENE NIKOLAY, POSEIDON SCIENTIFIC INSTRUMENTS PTY. LTD., THE UNIVERSITY OF WESTERN AUSTRALIA, TOBAR, MICHAEL, EDMUND, WOODE, RICHARD, ALEC
Application year: 1997
Main classification: G01R17
IPC codes: G01B9, G01R27, H03D9