BACKGROUND ART lnterferometric signal processing apparatus 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 difference between the input signals can be analysed with less limitation. Interferometers make use of carrier suppression to achieve their sensitivity.

Interferometers have been used since Michelson and Morely to make precision measurements using electromagnetic waves. Interferometers perform a real time vector sum and difference of the two signals incident on their input ports.

Interferometers have two outputs. At one output the output corresponds to the sum of half of the power of each input port, this is the"Summing"or Sigma port.

The other output corresponds to the difference of the power of each input port, this is the"Difference"or Delta port.

However, one of the draw backs of interferometers is that by their nature (based on the laws of conservation of energy) they suffer from losing one half of the information (power) contained in the two signals incident on the interferometer in the"summing"port.

This loss of power results in a reduced sensitivity of the interferometer, and in sophisticated systems the loss of half of the information can severely limit system performance.

One example of an interferometer is described in US patent 5, 841, 322. In Figure 3, a reflected signal and a transmitted signal are input to a 3dB power combiner to

produce a carrier-suppressed signal at one port of the power combiner. The sensitivity of this interferometer configuration is limited because half of the power is wasted by the power combiner ; one half of the power of the signals present at the input ports of the power combiner are summed and output at port 60, which is not used. Further, power is lost in coupling signals from the loop oscillator.

Arthur L. Whitwell et al described a frequency discriminator bridge for measuring FM noise in a paper entitled"A New Microwave Technique for Determining Noise Spectra at Frequencies Close to the Carrier", in the Microwave Journal, November 1959. The frequency discriminator bridge described by Whitwell et al consists of a four port bridge in which one port is designated an input port, a second port designated an output port, a third port of the bridge is connected to a cavity and a fourth port of the bridge is connected to a variable attenuator and a tuneable short circuit. Whitwell et al describes that the variable attenuator and tuneable short circuit are used to ensure an equal division of power between the third and fourth arms of the bridge and to ensure that any reflections arising in the third and fourth arms as a result of mismatches between the cavity at the carrier frequency are self-cancelling at the output port.

This configuration uses only reflected signals. Moreover, by equally dividing power between the arms of the bridge, the sensitivity of the interferometer is limited for a given power input.

DISCLOSURE OF THE INVENTION Throughout the specification, unless the context requires otherwise, the word "comprise"or variations such as"comprises"or"comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

In accordance with a first aspect of this invention, there is provided an interferometric signal processing apparatus, responsive to a first signal having a carrier frequency, comprising :

coupling means responsive to the first signal to produce a second signal and a third signal therefrom ; frequency dispersive means connected to said coupling means and responsive to the second signal and the third signal to produce at least a reflected second signal from the second signal and at least a reflected third signal from the third signal ; said reflected second signal and said reflected third signal being input to said coupling means which produces a carrier-suppressed signal therefrom ; and mixing means responsive to the carrier-suppressed signal and to a carrier-dominated signal to produce an output signal.

In one arrangement, the frequency dispersive means comprises a bi-directional frequency dispersive element having a pair of ports, said second signal being incident on one port and said third signal being incident on the other port to produce a reflected second signal and a transmitted third signal at said one port and a reflected third signal and a transmitted second signal at said other port.

Preferably, the first signal is input to a first port of the coupling means to produce the second and third signals at second and third ports of the coupling means, respectively, said one port of the frequency dispersive element being connected to the second port of the coupling means and said other port of the frequency dispersive element being connected to the third port of the coupling means.

Preferably, the frequency dispersive element comprises a resonator.

Preferably, the ports of the resonator are coupled with, = 0. 5.

Preferably, a first phase shifter and a first attenuator are provided between the second port of the coupling means and the frequency dispersive element.

Preferably, a second phase shifter and a second attenuator are provided between the third port of the coupling means and the frequency dispersive element.

In an alternative arrangement, said frequency dispersive means comprises : a frequency dispersive element responsive to the second signal to produce the reflected second signal ; and mismatched termination means responsive to the third signal to produce the reflected third signal.

Preferably, the first signal is input to a first port of the coupling means to produce the second and third signals at second and third ports of the coupling means, respectively, said frequency dispersive element being connected to the second port of the coupling means and said mismatched termination means being connected to the third port of the coupling means.

Preferably, the coupling means is arranged such that a majority of the first signal is produced as the second signal at the second port and a minority of the first signal is produced as the third signal at the third port.

Preferably, the frequency dispersive element is close to critically coupled.

Preferably, a first phase shifter and a first attenuator are provided between the second port of the coupling means and the frequency dispersive element.

Preferably, the mismatch termination means comprises a short circuit, and a second phase shifter and a second attenuator are provided between the third port of the coupling means and the short circuit.

Preferably, the coupling means comprises a four-port coupler.

Preferably, an amplifier is provided before the mixing means to provide an amplified, carrier-suppressed signal to the mixing means.

Preferably, said apparatus further comprises a control circuit responsive to the output signal to produce a control signal, and one of the first phase shifter or the second phase shifter being responsive to the control signal to control operation thereof and maintain carrier suppression.

Preferably, said apparatus further comprises a control circuit responsive to the output signal to produce a control signal, and one of the first attenuator or the second attenuator being responsive to the control signal to control operation thereof and maintain carrier suppression.

In accordance with a second aspect of this invention, there is provided an oscillator comprising a signal source that produces a first signal having a carrier frequency, and an interferometric signal processing apparatus according to the first aspect of this invention, responsive to the first signal.

Preferably, said oscillator further comprises a control circuit responsive to the output signal from the mixing means and arranged to produce a control signal therefrom, the signal source responsive to the control signal in producing the first signal to reduce amplitude and/or phase noise in the first signal.

In accordance with a third aspect of this invention, there is provided an oscillator comprising an amplifier, a filter, and a circulator arranged in a loop, and an interferometric signal processing apparatus according to the first aspect of this invention, responsive to a first signal from the oscillator having a carrier frequency, wherein the frequency dispersive element of the interferometric signal processing apparatus is provided in said loop of the oscillator such that a portion of the second or third transmitted signal from the frequency dispersive element circulates around said loop.

Preferably, the first signal is input to a first port of the coupling means to produce the second and third signals at second and third ports of the coupling means, respectively, said one port of the frequency dispersive element being connected to the second port of the coupling means and a directional coupler is provided in the loop at the other port of the frequency dispersive element, whereby a portion

of the second transmitted signal passes though the loop and the remainder passes to the third port of the coupling means.

Preferably, said oscillator further comprises a variable attenuator and/or a variable phase shifter provided in said loop of the oscillator, and a control circuit responsive to the output signal from the mixing means to produce a control signal, said control signal being input to the variable attenuator and/or phase shifter to control operation thereof to reduce amplitude and/or phase noise in the oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood with reference to the following description of seven embodiments thereof and the accompanying drawings, in which : Figure 1 shows a cavity-stabilised oscillator incorporating an interferometric signal processing apparatus according to a first embodiment of the invention ; Figure 2 shows a cavity-stabilised oscillator incorporating an interferometric signal processing apparatus according to a second embodiment of the invention ; Figure 3 shows a cavity-stabilised oscillator incorporating an interferometric signal processing apparatus according to a third embodiment of the invention ; Figure 4 shows a loop oscillator incorporating an interferometric signal processing apparatus according to a fourth embodiment of the invention ; Figure 5 shows a loop oscillator incorporating an interferometric signal processing apparatus according to a fifth embodiment of the invention ; Figure 6 shows a Pound-stabilised loop oscillator incorporating an interferometric signal processing apparatus according to a sixth embodiment of the invention ; and Figure 7 shows loop oscillator incorporating a Pound-stabilised interferometric signal processing apparatus according to a seventh embodiment of the invention.

BEST MODE (S) FOR CARRYING OUT THE INVENTION The first embodiment is directed towards a cavity-stabilised oscillator that incorporates an interferometric signal processing apparatus (interferometer) 10.

The oscillator and the interferometer 10 are shown in figure 1. The interferometer 10 shown in Figure 1 is also arranged to be used as a read-out system since the interferometer provides signals corresponding to amplitude and phase noise in the signal source.

The interferometer 10 comprises a four-port coupler 12 in the form a 3dB hybrid power combiner having first through fourth ports 12a-12d, respectively. A signal source S produces a first signal that is input to the first port 12a of the four port coupler 12, which produces a second signal at the second port 12b denoted by the arrow labelled with the numeral 2 and a third signal at third port 12c denoted by the arrow labelled with the numeral 3.

The interferometer 10 further comprises a cavity resonator 14 having two ports A and B. The ports A and B of the cavity resonator 14 are connected to the ports 12b and 12c of the four-port coupler 12, respectively.

A first phase shifter 16 and a first attenuator 18 are provided between the second port 12b of the four-port coupler 12 and the port A of the cavity resonator 14.

Similarly, a second phase shifter 20 and second attenuator 22 are provided between the third port 12c of the fourth port coupler 12 and the port B of the cavity resonator 14.

In the embodiment, the cavity resonator 14 is used as a frequency dispersive element. That is, the cavity resonator 14 alters the phase of signals passing through the cavity resonator 14 or reflected at ports A and B depending upon the frequency of the signal.

An amplifier 24 is connected to the fourth port 12d of the four port coupler 12.

The output of the amplifier 24 is split and input to the RF ports of two mixers 26a

and 26b. A carrier-dominated signal is provided to the LO port of the mixers 26a and 26b. In the embodiment, the carrier-dominated signal is the first signal obtained via the coupler 28 and phase shifter 30. The mixers 26a and 26b produce outputs at their IF ports which are base-band signals corresponding to the phase noise, amplitude noise or combination thereof according to the phase difference between the signals appearing at the LO and RF ports of the mixers 26a and 26b.

If the signals appearing at the LO and RF ports of a mixer are in phase (i. e. a phase difference of 0 or 180 degrees) the output at its IF port will correspond with the amplitude noise present in the first signal. If the signals appearing at the LO and RF ports of a mixer are in quadrature (i. e. a phase difference of 90 degrees), the output at its IF port will correspond with the phase noise in the first signal.

Phase differences between in phase and quadrature will correspond with the output of the mixer representing a mixture of the amplitude noise and phase noise.

In the embodiment, the mixer 26a is arranged such that the signals appearing at its LO and RF ports are in phase so that the output of the mixer, shown in figure 1 as 1, represents the amplitude noise in the signal source S. Further, the mixer 26b is arranged such that the signals appearing at its LO and RF ports are in quadrature so that the output of the mixer 26b, shown in figure 1 as Q, represents the phase noise in the signal source S. The phase difference between the signals appearing at the LO and RF ports of the mixers 26a and 26b can be achieved by adjusting the relative path length of the signals. In alternative embodiments, it should be readily appreciated that the phase shifter 30 can be replaced with a pair of phase shifters, one before each of the mixers 26a and 26b to allow separate adjustment of the relative phase of each of the mixers 26a and 26b.

In operation, the source signal is input to the first port 12a of the four port coupler 12, which results in the second and third signals 2, 3 appearing at the ports 12b and 12c.

The second signal 2 passes through the first phase shifter 16 and the first attenuator 18, into the port A of the cavity resonator 14. Some of the second signal 2 will be transmitted through the cavity resonator 14 to appear at the port B as a transmitted second signal presented by the arrow labelled 2T, and a portion of the second signal 2 will be reflected at the port A to produce a reflected second signal denoted by the arrow labelled 2R.

Similarly, the third signal 3 passes through the second phase shifter 20 and the second attenuator 22, into the port B of the cavity resonator 14. A portion of the third signal 3 is transmitted through the cavity resonator 14 to produce a transmitted third signal denoted by the arrow labelled 3T at port A, and a portion of the third signal 3 will reflected at port B to produce a reflected third signal denoted by the arrow labelled 3R.

The transmitted third signal 3T and the reflected second signal 2R pass through the first attenuator 18 and the first phase shifter 16 and are input to the second port 12b of the four port coupler 12. Similarly, the reflected third signal 3R and the transmitted second signal 2T pass through the second attenuator 22 and the second phase shifter 20 and are input to the third port 12c of the four port coupler 12.

The four port coupler 12 acts to produce a signal corresponding to the vector difference between the signals appearing at the second and third ports 12b and 12c at the fourth port 12d, and to produce a signal corresponding to the sum of the signals input to the second and third port 12b and 12c at the first port 12a.

Consequently, the signal appearing at the fourth port 12d is a carrier-suppressed signal and the signal appearing at the port 12a is carrier-dominated signal.

The carrier-suppressed signal at the port 12d is amplified by the amplifier 24 and input to the RF ports of the mixers 26a and 26b. A carrier dominated signal is supplied to the LO ports of the mixers 26a and 26b via the coupler 28 and phase shifter 30.

The cavity resonator 14 is arranged with the coupling of ports A and B being pA=PB=0. 5, since the coupler 12 is a 3dB coupler and divides the power in the first signal equally between the second and third signals 2 and 3, respectively. In practice, other forms of coupler 12 may be used, and the coupling on the ports of the cavity resonator 14 would then be adjusted such that the power of the reflected and transmitted signals 2R + 3T, and 2T + 3R, input to the coupler 12 produce a carrier-suppressed signal at the input to the amplifier 24.

Although in the embodiment the carrier-dominated signal is derived from the source signal, it should be appreciated that the carrier-dominated signal could also be the signal appearing at the port 12a.

Advantageously, the interferometer configuration used in this embodiment makes use of reflected and transmitted signals form the cavity resonator 14, and therefore offers increased sensitivity compared with existing interferometer configurations.

The oscillator, in addition to the interferometer 10 and the signal source S, further comprises first and second control circuits 40a and 40b, respectively. The first and second control circuits 40a and 40b are responsive to the outputs of the mixers 26a and 26b via switches 42a and 42b, respectively. The control circuits 40a and 40b apply appropriate signal conditioning to the output of the mixers 26a and 26b and apply the condition signals to the signal source S in order to control the amplitude and phase noise in the signal source S, respectively. One example of how this can be achieved is by the use of voltage control attenuators and voltage control phase shifters within the signal source S.

The switches 42a and 42b allow the independent, selective enabling of the control circuits 40a and 40b. Further, the switches 42a and 42b can be configured to pass the signals I and Q to an output for external measurement. In other embodiments, the switches 42a and 42b may be omitted or replaced with a permanent connection, as desired.

Thus, in this embodiment, the signal source S is stabilised to the limits imposed by the stability of the cavity resonator 14 and the noise of the remaining components in the interferometer 10 and the oscillator. A coupler 44 is provided between the signal source S and the coupler 28 to provide an oscillator output at 46.

The second embodiment is directed towards a cavity-stabilised oscillator 100 incorporating an interferometric signal processing apparatus 110, and is shown in figure 2. Like reference numerals used to denote like parts to those in the first embodiment, with 100 added thereto.

The interferometer 110 of the second embodiment differs from the interferometer 10 of the first embodiment in that the cavity resonator 114, the phase shifters 116 and 120 and the coupler 112 are arranged such that the carrier-suppressed signal is produced at the first port 112a and a carrier-dominated signal is produced at the fourth port 112d.

The interferometer 110 of the second embodiment further comprises a matched termination 132 connected to the fourth port 110d to dissipate the carrier- dominated signal.

The interferometer 110 further comprises a circulator 134 provided between the signal source S and the first port 112a of the coupler 112. The circulator 134 is arranged such that the first signal from the signal source S passes through the circulator 134 to the first port 112a, whilst the carrier-suppressed signal produced at the first port 112a passes from the circulator 134 to the amplifier 124 that is connected to the circulator 134.

In this embodiment, it should be appreciated that the carrier-dominated signal for the mixers 126a and 126b could be provided from the fourth port 112d if desired.

The oscillator 100 of this embodiment operates in a similar manner to the oscillator described in relation to the first embodiment.

The third embodiment is directed towards a cavity-stabilised oscillator 200 incorporating an interferometric processing apparatus 210. As shown in figure 3, the oscillator 200 and the interferometer 210 in this embodiment are of a similar form to those shown in the second embodiment, and like reference numerals are used to denote like parts to those in the second embodiment with 100 added thereto.

The third embodiment differs from the second embodiment in that the cavity resonator 214 used in the fifth embodiment has a single port A that is connected to the second port 212b of the coupler 212. Further, the third port 212c is connected to the second attenuator 222 and the second phase shifter 220, and is then terminated by a mismatched termination in a form of a short circuit 236.

It should be appreciated that term mismatched termination can refer to any non-impedance matched termination. Any non-impedance matched termination will produce a reflected signal that may also be phase shifted. In some embodiments, if the phase shift present from the mismatched termination is of a desired quantity, the second phase shifter 220 may not be necessary. However, it is preferred that the second phase shifter 220 is retained to allow fine adjustments and tuning of the interferometer.

Although the interferometer 210 shown in figure 3 appears to be quite different from the configuration of the interferometer 110 shown in figure 2, the two are quite similar. The interferometer 210 of the current embodiment can be considered a special case of the interferometer 110 shown in figure 2, where the coupling of port B of the resonator is set to pub=0 and the coupling of port A of the resonator approaches pA=1.

Further, the interferometer 210 differs from the interferometer 110 in the second embodiment in that the first attenuator 118 and the first phase shifter 116 are not present in the interferometer 210. Thus, the cavity 214 is directly connected to the port 212b of the coupler 212. Amplitude and phase matching of the reflected signals from the cavity resonator 214 and the short circuit 236 can be achieved by

adjusting the second phase shifter and second attenuator 220 and 222, respectively.

The interferometer 210 operates in the same general manner as the interferometer 110 described in figure 2 with the following exceptions. The second signal appearing at port 212b passes in to the port A of the resonator 214.

Since the coupling of port A is close to 1, most of the carrier power in the second signal passes in to the resonator 214. A reflected signal, 2R, is produced at port A of the resonator 214 due to the non-perfect coupling of port A and also due to any non-carrier components in the second signal. The reflected signal 2R passes in to the second port 212b of the coupler 212.

The third signal produced at port 212c passes through the second attenuator 222 and the second phase shifter 220 and to the short circuit 236. A reflected signal, 3R, is produced at the short circuit 236 because of the impedance mismatch, and the reflected signal 3R passes through the second phase shifter 220 and the second attenuator 222 to pass into the third port 212c of the coupler 212. The coupler 212 acts on the reflected signals from the resonator 214 and the short circuit 236 to produce a carrier suppressed signal at the first port 212a and carrier-dominated signal at the fourth port 212d.

In this embodiment, although the interferometer 210 is does not make use of transmitted signals, the sensitivity of the interferometer can be improved by arranging the coupler 212 so that the majority of the first signal appearing at port 212a passes through to port 212b as the second signal, whilst a small portion of the first signal passes to the third port 212c as the third signal. The coupling at port A of the resonator 214 can be tuned to provide an insertion loss of 20dB or more. As a result, the power of the carrier present in the signal reflected from port A is 20dB less than the carrier power present in the second signal at port 212b, while the noise close to the carrier is reflected without significant attenuation.

Since the majority of the carrier power is absorbed into the cavity resonator 214, the reflected signal appearing at port 212c does not need to contain much power in order to achieve carrier suppression. By providing more power to the resonator 214, the interferometer 210 can achieve increased sensitivity.

The fourth embodiment is directed towards a loop oscillator 300 incorporating an interferometric signal processing apparatus (interferometer) 302. As shown in figure 4, the loop oscillator 300 comprises an amplifier 304, a circulator 306, a filter 308, a voltage-controlled attenuator 310 and a voltage-controlled phase shifter 312 arranged in a loop. A directional coupler 314 is provided between the filter 308 and the voltage-controlled attenuator 310 to provide an output signal from the loop oscillator 300 at 316.

The interferometer 302 comprises a four-port coupler 318 in the form a hybrid power combiner having first through fourth ports 318a-318d, respectively. The four-port coupler 318 is provided in the loop of the oscillator 300 after the circulator 306, such that a first signal from the amplifier 304 passes through the circulator 306 to the first port 318a of the four-port coupler 318. The four-port coupler 318 produces a second signal at the second port 318b denoted by the arrow labelled with the numeral 2 and a third signal at third port 318c denoted by the arrow labelled with the numeral 3.

The interferometer 302 further comprises a cavity resonator 320 having two ports A and B. The cavity resonator 320 is also provided in the loop of the oscillator 300, with the port A of the cavity resonator 320 connected to the second port 318b of the four-port coupler 318, and the port B of the cavity resonator 320 connected to the filter 308.

The interferometer 302 further comprises a phase shifter 322, an attenuator 324 and a mismatched termination 326 in the form of a short circuit that are connected to the third port 318c of the four-port coupler 318.

In the embodiment, the cavity resonator 320 is used as a frequency dispersive element. That is, the cavity resonator 320 alters the phase of signals passing through the cavity resonator 320 or reflected at port A depending upon the frequency of the signal.

A matched termination 328 is connected to the fourth port 312d of the four-port coupler 318.

An amplifier 330 is connected to the circulator 306 such that any signals output from the first port 318a of the four-port coupler 318 are input to the amplifier 330 via the circulator 306.

The output of the amplifier 330 is split and input to the RF ports of two mixers 332a and 332b. A carrier-dominated signal is provided to the LO port of the mixers 332a and 332b. In the embodiment, the carrier-dominated signal is the signal in the loop oscillator obtained via a directional coupler 334 provided in the loop of the oscillator 300 at the output of the amplifier 304. Two phase shifters 336a and 336b are provided between the directional coupler 334 and the LO port of the mixers 332a and 332b, respectively. The mixers 332a and 332b produce outputs at their IF ports that are base-band signals corresponding to the phase noise, amplitude noise or combination thereof in the loop oscillator, according to the phase difference between the signals appearing at the LO and RF ports of the mixers 332a and 332b.

If the signals appearing at the LO and RF ports a mixer are in phase (i. e. a phase difference of 0 or 180 degrees) the output at its IF port will correspond with the amplitude noise present in the first signal. If the signals appearing at the LO and RF ports of a mixer are in quadrature (i. e. a phase difference of 90 degrees), the output at its IF port will correspond with the phase noise in the first signal. Phase differences between in phase and quadrature will correspond with the output of the mixer representing a mixture of the amplitude noise and phase noise.

In the embodiment, the mixer 332a is arranged such that the signals appearing at its LO and RF ports are in phase so that the output of the mixer, shown in figure 4 as 1, represents the amplitude noise in the loop oscillator 300. Further, the mixer 332b is arranged such that the signals appearing at its LO and RF ports are in quadrature so that the output of the mixer 332b, shown in figure 7 as Q, represents the phase noise in the loop oscillator 300.-The phase difference between the signals appearing at the LO and RF ports of the mixers 332a and 332b can be achieved by adjusting the phase shifters 336a and 336b, respectively.

In operation, a portion of the signal output from the amplifier 304 is coupled from the loop via the directional coupler 334 and input to the phase shifters 336a and 336b. The remainder of the signal from the amplifier 304 passes through the circulator 306 and is input to the first port 318a of the four-port coupler 318. This results in the second and third signals 2, 3 appearing at the ports 318b and 318c.

The second signal 2 is incident on port A of the cavity resonator 320. Some of the second signal 2 will be transmitted through the cavity resonator 320 to appear at port B as a transmitted second signal presented by the arrow labelled 2T, and a portion of the second signal 2 will be reflected at the port A to produce a reflected second signal denoted by the arrow labelled 2R.

Similarly, the third signal 3 passes through the phase shifter 322 and the attenuator 324 is incident on the short circuit 326. A portion of the third signal 3 will be reflected at the short circuit 326 because it is a mismatched termination, to produce a reflected third signal denoted by the arrow labelled 3R.

The transmitted second signal 2T passes from port B of the cavity resonator 320 to the filter 308, and from there through the variable attenuator 310 and the variable phase shifter 312 to the amplifier to complete the loop. The centre frequency of the cavity resonator 320 determines the frequency of the loop oscillator 300.

The reflected second signal 2R is input to the second port 318b of the four-port coupler 318. The reflected third signal 3R passes through the attenuator 324 and the phase shifter 322 and is input to the third port 318c of the four-port coupler 318.

The four-port coupler 318 acts to produce a signal at the first port 318a corresponding to the vector difference between the signals appearing at the second and third ports 318b and 318c, and to produce a signal corresponding to the sum of the signals input to the second and third port 318b and 318c at the fourth port 318d. Consequently, the signal appearing at the first port 318a is a

carrier-suppressed signal and the signal appearing at the fourth port 318d is carrier-dominated signal.

The carrier-suppressed signal at the first port 318a passes to the amplifier 330 via the circulator 306. The carrier-suppressed signal is amplified by the amplifier 330 and is input to the RF ports of the mixers 332a and 332b. A carrier dominated signal is supplied to the LO ports of the mixers 332a and 332b via the coupler 334 and phase shifters 336a and 336b. The mixers 332a and 332b produce output signals I and Q, respectively, corresponding to the amplitude and phase noise in the loop oscillator 300.

The oscillator 300 further comprises first and second control circuits 338a and 338b that are responsive to the outputs of the mixers 332a and 332b via switches 360a and 360b, respectively. The control circuits 338a and 338b apply appropriate signal conditioning to the output signals I and Q and apply the conditioned signals to the variable attenuator 310 and the variable phase shifter 312 in order to control the amplitude and phase noise in loop oscillator 300, respectively, using negative feedback.

The interferometer 302 in this embodiment makes use of reflected signals only from the second and third signals 2 and 3. Advantageously, by adjusting the four- port combiner 318 such that more power is produced at the second port 318b than at the third port 318c, the sensitivity of the interferometer 302 can be increased. By using this technique, the sensitivity of the interferometer 302 may be sufficient that the amplifier 330 may be omitted.

The fifth embodiment is directed towards a loop oscillator 400 incorporating an interferometric signal processing apparatus 402. As shown in figure 5, the loop oscillator 400 and the interferometric signal processing apparatus 402 are of the same general form as the oscillator 300 and interferometer 302 described in the fourth embodiment. Like reference numerals are used to denote like parts to those used in the fourth embodiment, with 100 added thereto.

This embodiment differs from the fourth embodiment in that the attenuator 410 is not a voltage-controlled attenuator. Further, the attenuator 422 that is connected to the third port 418c of the four-port coupler 418 is a voltage-controlled attenuator in this embodiment. The output from the control circuit 438a is input to the voltage-controlled attenuator 422 to maintain carrier suppression at the input of the amplifier 430.

Further, the phase shifter 424 is a voltage-controlled phase shifter. A control signal can be applied to the voltage-controlled phase shifter 424 from an input 440. The control signal from the input 440 allows for external adjustment of the oscillator frequency, allowing the oscillator to be phase locked to an external source.

The sixth embodiment is directed towards a Pound-stabilised loop oscillator 500 having an interferometric signal processing apparatus 502 incorporated therewith.

As shown in figure 6, the loop oscillator 500 and the interferometer 502 are of the same general form as the oscillator 400 and interferometer 402 described in the fifth embodiment. Like reference numerals are used to denote like parts to those used in the fifth embodiment, with 100 added thereto.

The sixth embodiment differs from the fifth embodiment in that the loop oscillator 500 in the sixth embodiment is a Pound-stabilised oscillator and accordingly includes a local oscillator 550. Outputs from the local oscillator 550 and the control circuit 538b are feed into a summing circuit 552 which provides a summed signal to the voltage-controlled phase shifter 512. By feeding the output from the local oscillator 550 to the voltage-controlled phase shifter 512, the loop frequency of these signals in the loop oscillator 500 varies with the signal from the loop oscillator 550. This technique is referred to as Pound-stabilisation.

Because the loop oscillator 500 is Pound-stabilised, information concerning the signal in the loop will appear at both the baseband frequency and at the oscillation frequency of the local oscillator 550. Accordingly, the output signal from the mixer 532b, in addition to being input to the control circuit 538b, is input to an amplifier 554. The output of the amplifier 554 is input to the RF port of a further mixer 556.

The local oscillator 550 provides the LO input for the mixer 556. The output of the mixer 556 provides information concerning any drift in the frequency of the oscillator 500 from the centre frequency of the cavity resonator 520, and is labelled Q'in figure 6. This signal Q'is input to a further control circuit 538c, the output of which is also passed to the summing circuit 552 to eliminate this drift and improve the long term stability of the oscillator 500.

The seventh embodiment is directed towards a loop oscillator 600 incorporating an interferometric signal processing apparatus (interferometer) 602. As shown in figure 7, the loop oscillator 600 and interferometer 602 are of a similar form to the loop oscillator 500 and interferometer 502 described in relation to the sixth embodiment. Accordingly like reference numerals are used to denote like parts to those in the sixth embodiment, with 100 added thereto.

The current embodiment differs from the sixth embodiment in that the loop oscillator 650 is not fed to the voltage-controlled phase shifter 612 in the loop oscillator 600. Instead, the local oscillator 650 is fed to the phase shifter 624 of the interferometer 602. This arrangement allows the frequency in the loop oscillator 600 to operate without being modulated by the local oscillator 650, but still provide the advantages of a Pound-stabilised system in order to provide signals Q and Q'to the voltage-controlled phase shifter 612.

Further, the short circuit 626 in the sixth embodiment has been replaced with a directional coupler 658 in this embodiment that is provided at port B of the cavity resonator 620. In this arrangement, the interferometer 602 acts as a bi-directional interferometer, with signals passing through the cavity 620 in both directions, and the interferometer making use of both transmitted and reflected signals from the cavity resonator 620.

It should be appreciated that this invention is not limited to the particular embodiments described above. For example, it would be readily appreciate by those skilled in the art that the I and Q signals can be used to control other aspects of the oscillator or interferometer, as desired. Further, it would be readily apparent that Pound-stabilisation of an oscillator could be achieved using other configurations.