Microwave oscillator

Abstract: A microwave oscillator (10) includes an oscillator (12) having an output and a control port (44) and,a feedback circuit (20) disposed between the output and the control port (44). The feedback circuit (20) includes a modulated laser (30) having an input responsive to a portion (16) of a signal from the output of the oscillator (12), and a photo detector (34) having an input responsive to an output signal from the modulated laser (30) delayed (32) by a predetermined amount of time. A detector (28) has a first input responsive to the output of the photo detector (34), a second input responsive to a portion of the output signal from the oscillator (12) shifted (26) in phase to be in phase quadrature with the signal at the first input of the detector (28). The output of the detector (28) is coupled to the control port (44) of the oscillator (12). The output signal from the detector (28) is only zero when there is no difference between the phase shifted signal from the phase shifter (26) and the delayed signal from the photo detector (34).

Description: This invention relates to a microwave oscillator comprising: a controlled oscillator having an output port and a fine control port; output coupling means connected to the output port of the controlled oscillator; and a feedback circuit coupling the output coupling means to the fine control port of the controlled oscillator, the feedback circuit including a phase shifter coupling the output coupling means to a first input of a phase detector, a delay means coupling the output coupling means to a second input of the phase detector, and the output of the phase detector being coupled to the fine control port of the controlled oscillator. It is well known that many advanced electronic systems for communication and navigation require circuits, including oscillator circuits, having low-noise for low noise applications. Such low noise applications including space communications, electronic warfare countermeasures and modern radar techniques. In a microwave oscillator, frequency modulation (FM) noise is a problem due to the noise characteristics of the components of the oscillator. In many applications, it is advantageous to reduce the FM noise characteristics of the microwave oscillator. For example, lower noise is a key to extending the maximum detection range of a radar missile. Various techniques for lowering the FM noise of a microwave oscillator have been described. For example, a technique for degenerating low frequency FM noise arising within a microwave oscillator is described in U.S. Patent No. 5,032,800, issued July 16, 1991, entitled Tunable Oscillator with Noise Degeneration (which patent is assigned to the same assignee as this application). In U.S. Patent No. 5032800 a microwave oscillator is described which includes an amplifier and a feedback circuit disposed about the amplifier. The feedback circuit includes a resonator having a first port and a second port and a I voltage-controlled phase shifter having a input port, an output port and a control port, and has the input port of the voltage-controlled phase shifter connected to the output port of the amplifier and the output port of the voltage-controlled phase shifter coupled to a port of the resonator. The oscillator further includes a circuit responsive to signals from the output of the voltage-controlled phase shifter and the first port of the resonator to provide a control signal to the control port of the voltage-controlled phase shifter for degenerating low frequency FM noise arising within the amplifier. Although such a technique is useful in many applications, the noise performance of a microwave oscillator is a very important consideration and lower noise is a key to extending the maximum detection range of a radar. For example, in a doppler radar, noise generated at base band frequencies is upconverted and occurs in the region from the carrier at the order of expected doppler frequency shifts and will reduce the subclutter visibility of the radar. Therefore, it is desirable to provide a very precise, yet easily changeable microwave frequency signal source with a highly pure stable frequency output signal. U.S. Patent No. 4048580 describes a microwave oscillator of the kind defined hereinbefore at the beginning, in which a voltage controlled oscillator generates a microwave signal, a portion of which is extracted by a directional coupler and supplied thereby to a power divider. The portion of the microwave signal extracted by the directional coupler is divided by the power divider between an upper feedback loop where it is changed in phase and amplitude by respective set amounts regardless of the frequency, and a lower feedback loop where it is shifted in phase by an amount proportional to the frequency. These two phase shifted signals are supplied to first and second inputs of a phase detector. The amount of phase difference detected by the phase detector is proportional to the frequency. A differential amplifier compares a bias input with the phase detector output signal and applies an output signal which holds the voltage controlled oscillator to the desired frequency. The bias input voltage can be adjusted and defines the pull-in range. The lower feedback loop is in the form of a coaxial cable delay line. A temperature controlled oven holds the temperature of the delay line constant by using a thermal sensor mounted within the delay line. In W089/08848 it is observed that there is a need for equipment which can accurately measure phase noise in doppler radar equipment, and that the measurement can be made by using a frequency discriminator in which a delay line, phase shifter, and detector are used to decorrelate the phase noise present in the R.F. signal source, and measuring the resulting base band signal on a spectrum analyser. It is there further observed that limitations are imposed by the delay line. In particular, it is noted there that although long delays can be achieved with long lengths of coaxial cable, very high losses result, making measurement of low noise levels very difficult, and that, in addition, coaxial cables are usually quite bulky, making it difficult to change cables when a different length delay is desired. W089/08848 describes a radar test set incorporating a frequency discriminator which uses a fibre optic cable to produce the needed delay. The radio frequency signal to be tested is divided by a power divider into two portions which are supplied respectively to the fibre optic cable delay line system and an adjustable phase shifter. The delayed and phase shifted signals are applied to the two inputs of a mixer which has its output coupled through a low noise amplifier to a wave analyser. The fibre optic cable delay line system consists of a laser diode which is modulated directly or indirectly by the portion of the signal to be tested supplied by the power divider, a length of fibre optic cable, which provides a desired delay, and an optical detector which supplies a voltage proportional to the intensity of the modulated and delayed light to one input of the mixer. The adjustable phase shifter is so adjusted that the input signals to the mixer are nominally in quadrature. The mixer then acts as a phase detector and outputs a steady D.C. current when no noise is present in the signal to be tested. When noise is present, it arrives at the mixer through the phase shifter before it arrives through the delay line system. The noise therefore results in a change in the relative phase difference between the signals at the mixer, which consequently changes the amplitude of its output current. The low noise amplifier amplifies the extracted phase noise signal and presents it to the wave analyser which may be a spectrum analyser which displays the frequency distribution of the phase noise and the relative amplitude of each portion in the distribution. According to the present invention, a microwave oscillator of the kind defined hereinbefore at the beginning is characterised in that the delay means comprises a modulated laser coupled to the output coupling means and adapted to be modulated by the output signal from the controlled oscillator, a photo detector, and a fibre optic delay line of predetermined length coupling the modulated output of the modulated laser to the photo detector with a delay determined by the predetermined length, and in that the output of the phase detector is coupled to the fine control port of the controlled oscillator by an inverter switchable between inverting and non-inverting states and adapted to ensure that the feedback circuit provides negative feedback. A preferred embodiment of this invention takes the form of a tunable microwave oscillator having low FM noise. In a preferred embodiment of this invention the said portion of the signal from the output of the oscillator is shifted in phase to be in phase quadrature with the signal from the photo detector at the phase detector. With such an arrangement, a microwave oscillator having better FM noise performance than known microwave oscillators is provided. A preferred embodiment of the present invention may be regarded as a microwave frequency signal source which includes an oscillator, responsive to a control signal, for providing a microwave frequency output signal, a laser, responsive to a portion of the microwave frequency output signal, for providing a modulated optical signal, the modulated optical signal further being time delayed by a predetermined amount of time, a photo detector, responsive to the time delayed modulated optical signal, for providing a microwave feedback signal having a phase, and a phase detector, responsive to the microwave feedback signal and a portion of the microwave frequency output signal having a phase in phase quadrature with the phase of the microwave feedback signal, for providing the control signal to the oscillator. With this arrangement, a microwave frequency signal source is provided having an output signal with the phase noise imparted to the signal provided therefrom being reduced and accordingly the output signal from the microwave frequency signal source will have a lower frequency modulation noise level. Since a portion of the microwave frequency output signal is phase detected with a portion of the microwave frequency output signal delayed in time by a predetermined amount of time, the output of the phase detector provides a signal which will decrease noise in the oscillator. Reduction in noise contributed by the circuit elements will provide a concomitant reduction in the frequency noise of the output signal from the microwave frequency signal source. The invention will now be described by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a block diagram of a low noise oscillator which does not embody the present invention; Fig. 2 is a block diagram of an embodiment of a low noise oscillator according to this invention; and Fig. 2A is a block diagram of an alternative embodiment of a portion of the low noise oscillator shown in Fig. 2. Referring now to FIG. 1, a microwave frequency signal source 10 is shown to include a microwave, for example a C-Band, voltage-controlled oscillator (VCO 12). The VCO 12 provides an output signal with a predetermined frequency in response to a coarse control signal fed to a coarse control port 42 of the VCO 12 and a fine control signal fed to a fine control port 44 of the VCO 12. The output of the VCO 12 is connected to an input of a power divider 14 having a first and a second output. The first output of the power divider 14 is connected to an input of a coupler 16 having a first and a second output. The first output of the coupler 16 provides the output of the microwave frequency signal source 10. The second output of the power divider 14 is connected to a first input of a frequency discriminator 18. The second output of the coupler 16 is connected to a second input of the frequency discriminator 18. The output of the frequency discriminator 18 is connected to an input of a video amplifier 22 having also an output. The output of the video amplifier 22 is connected to an input of a shaping filter 24 having also an output which is connected to the fine control port 44 of the VCO 12. It should be appreciated that the power divider 14, the coupler 16, the frequency discriminator 18, the video amplifier 22 and the shaping filter 24 provides a feedback loop 20 for the VCO 12. The frequency discriminator 18 includes a voltage-controlled phase shifter 26 having an input connected to the second output of the power divider 14, the voltage-controlled phase shifter 26 also having an output and a control port. The output of the voltage-controlled phase shifter 26 is connected to a first input of a detector 28 having also a second input and an output. The frequency discriminator 18 further includes a modulated laser 30 having an input which is connected to the second output of the coupler 16. An output of the modulated laser 30 is connected to an input end of a fiber-optic cable 32 which provides a predetermined time delay. An output end of the fiber-optic cable 32 is connected to an input of a photo detector 34 having also an output which is connected to an input of an amplifier 36. An output of the amplifier 36 is connected to the second input of the detector 28. An output of the detector 28 provides the output of the frequency discriminator 18 and is connected to the input of the video amplifier 22. With such an arrangement, the feedback loop 20 provides the fine control signal to the fine control port 44 of the VCO 12 to reduce frequency modulation (FM) noise in the output signal of the voltage-controlled oscillator 12. Thus, dither in the output frequency of the output signal of the VCO 12 will be reduced. With the above described arrangement, an output signal at the output of the VCO 12 is fed to the input of the -power divider 14 wherein a first portion of the signal is fed to the input of the coupler 16 and a second portion of the signal is fed to the input of the voltage-controlled phase shifter 26. A first portion of the input signal fed to the input of the coupler 16 is provided as the RF output signal of the microwave frequency signal source 10 and a second portion of the input signal fed to the input of the coupler 16 is fed to the modulated laser 30. The modulated laser 30 is capable of providing a modulated optical output signal at the output thereof in response to a modulating input signal. The optical output signal of the modulated laser 30 is amplitude modulated (AM) at a modulation rate in accordance with the frequency of the input signal. Thus, AM modulation in the optical output signal is indicative of the frequency of the RF output signal of the microwave frequency signal source 10. The optical output signal is fed to the input end of the fiber-optic cable 32 here providing a fiber-optic delay line with a time delay of 2.5 microseconds. The signal at the output end of the fiber-optic cable 32 is a delayed optical signal with the characteristics of the optical signal fed into the fiber-optic cable 32 but having a time delay of 2.5 microseconds. The delayed optical signal is fed to the input of the photo detector 34 wherein the photo detector 34 provides an output signal (microwave feedback signal) having a frequency corresponding with the frequency of the RF output signal of the microwave frequency signal source 10 but delayed in time by 2.5 microseconds. The output signal of the photo detector 34 is fed to the amplifier 36 wherein said signal is amplified and then fed to the second input of the detector 28. As described hereinabove, the second portion of the VCO output signal at the second output of the power divider 14 is fed to the input of the voltage-controlled phase shifter 26. The voltage-controlled phase shifter 26 provides a predetermined amount of phase shift to the signal fed to the input thereof as determined by a control signal fed to the control port thereof. The amount of phase shift imparted upon an input signal is determined such that the signal fed to the first input of the detector 28 will be shifted in phase to be in phase quadrature with the signal at the second input of the phase detector 28. The detector 28 provides an output signal having a voltage indicative of phase difference between the two signals fed thereto. It should be appreciated, if two signals having the same frequency, but with one of the signals having a phase difference of 90° with the other signal, are fed into the detector 28 (here a phase detector) then an output signal of the detector 28 will have a non-varying nominal predetermined voltage here equal to zero volts. If the frequency of one of the input signals varies a small amount, then the two input signals will have a phase difference of something other than 90° and the output signal of the detector 28 will have a voltage different than the non-varying nominal predetermined voltage, the amount of difference indicative of the amount of phase difference between the two input signals of the detector 28. The signal at the first input of the detector 28 is indicative of an instantaneous RF output signal, meaning the RF output signal without having been subjected to a desired time delay. The signal at the second input of the detector 28 is indicative of a time delayed RF output signal, meaning the RF output signal having been subjected to a time delay, here the time delay being 2.5 microseconds. Thus, the output signal of the phase detector 28 will have a voltage with a magnitude indicative of the difference between the phase of the instantaneous RF output signal and the phase of the time delayed RF signal. As such, if the frequency of the instantaneous RF output signal varies, the phase of the signal at the first input of the detector 28 varies whereas the phase of the signal at the second input of the detector 28 remains constant since the signal at the second input is indicative of the RF output signal of 2.5 microseconds earlier, resulting in a larger variation in the magnitude of the voltage change of the output signal of the detector 28. (It should be appreciated, the longer the time delay, the larger the voltage change of the time output signal.) The output signal from the detector 28 is fed to the video amplifier wherein said signal is amplified. The signal from the video amplifier 22 is fed to the shaping filter 24 wherein said signal is filtered and fed to the fine control port 44 of the VCO 12. A DC offset voltage may be summed into a second input port of the video amplifier 22 in order to satisfy the biasing requirement of the fine control port 44 of the voltage-controlled oscillator 12. As described hereinabove, with a larger variation in the magnitude of the voltage change of the output signal of the detector 28 for a predetermined amount of frequency variation, the fine control signal at the fine control port 44 can more readily vary the output frequency of the output signal of the VCO 12 to offset such a change. The microwave frequency signal source 10 also includes a frequency determining circuit 38, which in response to a frequency select signal from a controller (not shown), provides a control signal to the coarse control port 42 of the VCO 12. The control signal at the coarse control port 42 of the VCO 12 is used to determine the output frequency of the output signal of the microwave frequency signal source 10, correlating to the frequency select signal fed to the frequency determining circuit 38. The frequency determining circuit 38 further provides the control signal to the control port of the voltage-controlled phase shifter 26. The latter ensures that a 90° phase difference between the input signals at the input ports of the detector 28 is maintained as the selected output frequency of the output signal of the microwave frequency signal source 10 is changed. It should now be appreciated that the feedback loop 10 is also a frequency-locked loop (FLL) which causes the frequency of the signal-out of the voltage-controlled oscillator (VCO 12) to be nominally locked to the frequency which corresponds to the zero crossing of the output signal of the phase detector 28 and as selected by the coarse control port 41 of the VCO 12. An undesired change of frequency in the RF output signal will result in a change in the output signal of the frequency discriminator 18. The latter will result in a change in the fine control signal fed to the fine control port of the VCO 12, thus minimizing undesired changes in the frequency of the output signal of microwave frequency signal source 10. Referring now to FIG. 2, a microwave oscillator 100 is shown to include a voltage-controlled oscillator (VCO 112) having an output, a coarse control port 152 and a fine control port 158. The output of the VCO 112 is connected to an input of a power divider 114 having a first and second output. The first output of the power divider 114 is connected to an input of a coupler 116 having a first and second output. The first output of the coupler 116 provides the output of the microwave oscillator 100. The second output of the power divider 114 is connected to an input of a voltage-controlled phase shifter 126 having an output and a control port. The output of the voltage-controlled phase shifter 126 is connected to an input of a coupler 150. A first output of the coupler 150 is connected to a first input of a phase detector 128 having also a second input and an output. A second output of the coupler 150 is connected to a first input of a phase detector 118 having also a second input and an output. The second output of the coupler 116 is connected to an input of a modulated laser 130. An output of the modulated laser 130 is connected to an input end of a fiber-optic delay line 132 provided by a fiber-optic cable and here having a delay of one microsecond, the fiber-optic delay line 132 further having an output end which is connected to an input of a photo detector 134. An output of the photo detector 134 is connected to an input of an amplifier 136 having also an output which is connected to an input of a coupler 140. A first output of the coupler 140 is connected to the second input of the phase detector 128. A second output of the coupler 140 is connected to an input of a -90° phase shifter 142 having also an output which is connected to the second input of the phase detector 118. The output of the detector 128 is connected to a second input of an amplifier 120 having also a first input and an output. The output of the phase detector 128 is also connected to an input of an amplifier 144. An output of the amplifier 144 is connected to an input of an integrator 146 having also an output which is connected to the control port of the voltage-controlled phase shifter 126. The output of the amplifier 144 is also connected to an input of a selectable inverter 148 having also an output and a control port. The selectable inverter 148 will either invert or not invert a signal fed thereto as determined by a control signal B fed to the control port thereof. To provide the control signal B to the control port of the switchable inverter 148, the output of the phase detector 118 is connected to the first input of the amplifier 120 and the output of the amplifier 120 is connected to the control port of the switchable inverter 148. The output of the switchable inverter 148 is connected to a first input of a loop amplifier 122 having also a second input and an output. The output of the loop amplifier 122 is connected to an input of a shaping filter 124 having also an output which is connected to an input of a switch 158 having also an output and a control port. The output of the switch 158 is connected to the fine control port 156 of the VCO 112. A DC offset bias signal may be summed into the second input of the loop amplifier 122 to satisfy the biasing requirement of the fine control port 156 of the voltage-controlled oscillator 112. The above-described circuitry provides the elements for a frequency discriminator 101 which provides a feedback loop 160 for the VCO 112. A frequency determining circuit 138, which in response to a frequency select signal from a controller (not shown), provides a control signal to the coarse control port 152 of the VCO 112. The frequency determining circuit 138 further provides a control signal A to the control port of the switch 158. With the above-described arrangement, an output signal at the output of the VCO 112 is fed to.the input of the power divider 114 wherein a first portion of the signal is fed to the input of the coupler 116 and a second portion of the signal is fed to the input of the voltage-controlled phase shifter 126. A first portion of the signal fed to the input of a coupler 116 is provided as the RF output signal of the microwave oscillator 100 and a second portion of the signal fed to the input of a coupler 116 is fed to the modulated laser 130. The modulated laser 130 is capable of providing a modulated optical signal at the output thereof in response to an input signal. The optical signal of the modulated laser 130 is amplitude modulated (AM) at a modulation rate in accordance with the frequency of the input signal. The optical signal is fed to the input end of the fiber-optic delay line 132 here providing a time delay of one microsecond. The signal at the output end of the fiber-optic delay line 132 is a delayed optical signal with the characteristics of the optical signal fed into the fiber-optic delay line 132 but having a time delay of one microsecond. The delayed optical signal is fed to the input of the photo detector 134 wherein the photo detector 134 provides an output signal having a frequency corresponding to the frequency of the RF output signal of the microwave oscillator 100 but delayed in time by one microsecond. The modulated laser 130, the fiber-optic cable 132 and the photo detector 134 can be provided by assembly 131. Here, assembly 131 is a Fiberoptic Delay, manufactured by Ortel Corp., 2015 West Chestnut Street, Alhombra, CA 91803, and having a part number 5002B-0100. The output signal of the photo detector 134 within the assembly 131 is fed to the amplifier 136 wherein said signal is amplified and then fed to the input of the coupler 140. A first portion of the signal at the input of the coupler 140 is fed to the second input of the detector 128 and a second portion of the signal at the input of the coupler 140 is fed to the input of the -90° phase shifter 142. The -90° phase shifter 142 provides 90° of phase shift to a signal fed to the input thereof at the output thereof. The signal at the output of the -90° phase shifter is fed to the second input of the detector 118. As described herein above, the second portion of the VCO output signal at the second output of the power divider 114 is fed to the input of the voltage-controlled phase shifter 126. The voltage-controlled phase shifter 126 provides a predetermined amount of phase shift to the signal fed to the input thereof as determined by a control signal fed to the control port thereof. The signal at the output of the voltage-controlled phase shifter 126 is fed to the input of a coupler 150. A first portion of the signal fed to the input of the coupler 150 is fed to the first input of the detector 128 and a second portion of the signal at the input of the coupler 150 is fed to the first input of the detector 118. The amount of phase shift imparted upon a signal by the voltage-controlled phase shifter 126 is determined such that the signal fed to the first input of the detector 128 will be shifted in phase to be in phase quadrature with the signal at the second input of phase detector 128. The detector 128 (here a phase detector) provides an output signal having a voltage indicative of the phase difference between the two signals fed thereto. In the present embodiment, the signal at the first input of the detector 128 is indicative of an instantaneous RF output signal, meaning the RF output signal without having been subjected to a desired time delay and the signal at the second input of the detector 128 is indicative of a time delayed RF output signal, meaning the RF output signal having been subjected to a time delay, here the time delay being one microsecond. Thus, the output of the phase detector 128 will have a voltage with a magnitude indicative of the difference between the phase of the instantaneous RF output signal and the phase of the time delayed RF signal. The output signal from the detector 128 is fed to the input of the amplifier 144 and to the second input of the amplifier 120. The amplifier 144 amplifies the signal at the input thereof and provides said signal at the output thereof which in turn is fed to the input of the selectable inverter 148 and the input of the integrator 146. The integrator 146 integrates the signal at the input thereof and provides said integrated signal at the output thereof which in turn is fed as the control signal to the voltage-controlled phase shifter 126. The integrator 146 will automatically drive the phase shifter 126 until an approximately zero volts average signal is obtained at the detector 128 output. The resulting effect is to automatically maintain phase quadrature for the two inputs to detector 128. The signal at the input of the selectable inverter 148 is either inverted or not inverted by the selectable inverter 148 as determined by the control signal B fed to the control port of the selectable inverter 148. The output signal of the selectable inverter 148 is fed to the input of an amplifier 122 wherein said signal is summed with a DC bias signal as necessary and amplified. The output signal of the amplifier 122 is fed to the input of the shaping filter 124 wherein said signal is filtered and in turn fed to the input of a switch 158. The switch 158 is either in the open or closed position as determined by a control signal A at the control port thereof and when said switch is in the closed position, the signal fed to the input thereof is in turn fed to the fine control port 156 of the VCO 112. Digressing briefly now here for a moment, it will be noted by those of skill in the art that the frequency discriminator 101 will provide an appropriate DC voltage signal to the fine control port 156 of the VCO 112 as the output signal of the VCO 112 varies in frequency. The frequency discriminator 101 provides a transfer function in the form of a sinusoidal discriminator curve. The slope of such discriminator curve at the zero-crossing is controlled by the amount of time delay provided by the fiber-optic delay line 132. It should be appreciated that as the frequency of the output signal of the VCO 112 increases, the frequency discriminator 101 will first provide a DC output signal with a negative going characteristic at a first zero crossing as the frequency increases and then provide a DC output signal with a positive going characteristic at a second zero crossing as the frequency increases. Such characteristics then repeat themselves. The latter introduces an ambiguity. At a frequency with a zero crossing having a negative going slope, as the frequency of the output signal of the VCO 112 increases, the DC output signal provided by the frequency discriminator 101 decreases. At a frequency with a zero crossing having a positive going slope, as the frequency of the output signal of the VCO 112 increases, the DC output signal provided by the frequency discriminator 101 increases. Accordingly, the selectable inverter 148 provides either an inverted signal or a non-inverted signal, as necessary, to provide negative feedback (stability). To correct for the ambiguity, negative feedback is restored by selecting either the inverting or the non-inverting characteristic of the selectable inverter 148 as determined by the control signal B at the control port thereof. The control signal B for the selectable inverter 148 is provided by the output of the amplifier 120. As described hereinabove, one of the two inputs to the amplifier 120 is provided by the output of the detector 128 and the other one of the two inputs to the amplifier 120 is provided by the output of the detector 118. It should be appreciated by those of skill in the art, when the frequency discriminator 101 is operating at a negative going zero crossing, the voltage of the signal out of the detector 128 will be greater than the voltage of the signal out of the detector 118 and when the frequency discriminator 101 is operating at a positive going zero crossing, the voltage of the signal out of the detector 128 will be less than the voltage of the signal out of the detector 118. Thus, if the output signal of the detector 128 is used as a reference signal at the input of the amplifier 120, then the output signal B of the amplifier 120 will be a positive signal when the frequency discriminator 101 is operating a positive going zero crossing and the output signal B of the amplifier 120 will be negative when the frequency discriminator 101 is operating at a negative going zero crossing. Using the output signal B of the amplifier 120 as a control signal for the selectable inverter 148, the selectable inverter 148 is then selected to be either in the inverting mode or the non-inverting mode as necessary. The latter ensures the feedback loop 160 is configured to exhibit negative feedback (stability). Referring now also to FIG. 2A, an alternative technique is shown to control the selectable inverter 148. A sinusoidal sweep oscillator 172 having an output which is connected to an input of a switch 174 provides a sinusoidal output signal. An output of the switch 174 is connected, via a DC blocking capacitor 176, to a first input of a summing network 178. An output of the summing network is connected to the control port of the voltage-controlled phase shifter 126. A switch 182 is positioned between the output of the amplifier 144 and the input of the integrator 146. A switch 180 is connected between the output of the integrator 146 and a second input of the summing network 178. Completing the alternate embodiment is a video quadrature detector 184 having a first and a second input with the first input connected to the output of the amplifier 144 and the second input connected to the output of the sinusoidal sweep oscillator 172. An output of the quadrature detector 184 provides a control signal to control a flip-flop 186 having an output which is connected to the control port of the selectable inverter 148. The output signal of the sinusoidal sweep oscillator 172 is connected to the control port of the voltage-controlled phase shifter by closing switch 174 and opening switches 158, 180 and 182. The latter sweeps the voltage-controlled phase shifter 126 resulting in a sinusoidal voltage signal at the output of the phase detector 128. The sinusoidal voltage signal at the output of the phase detector 128 and the sinusoidal sweep signal from the output of the sinusoidal sweep oscillator 172 are fed as inputs to the video quadrature detector 184. The output signal controls the flip-flop 186 to provide the control signal to the selectable inverter 148 to ensure negative feedback is provided within the frequency discriminator 101. With the latter completed, switch 174 is opened and switches 180 and 182 are closed. Any DC output signal from the phase detector 128 is integrated by the integrator 146 and fed to the control port of the voltage-controlled phase shifter 126 to change the insertion phase change provided by the voltage-controlled phase shifter 126 during the DC output signal of the frequency discriminator 101 to zero volts. The switch 158 is then closed and the feedback loop 160 changes to the tracking mode to maintain low noise. To set the operating frequency of the microwave oscillator 100, the frequency determining circuit 138 provides a control signal A to the switch 158 to set the switch 158 in the open position. The frequency determining circuit 138 further provides the control signal to the coarse control port 152 of the VCO 112 to set the output frequency of the output signal of the microwave oscillator 100. Once enough time has elapsed for the VCO 112 to settle at the new frequency as determined by the coarse control signal at the control port 152, the frequency determining circuit 138 provides a control signal A to the switch 158 to close the switch 158. The latter connects the output signal of the frequency discriminator 101 to the fine control port 156 of the VCO 112 activating the feedback loop 160. Referring momentarily back to FIG. 1, the fiber-optic cable 32, there providing a time delay of 2.5 microseconds, controls the frequency spacing of the output frequency of the VCO 112. Thus, as the control signal fed to the control port 42 of the VCO 12 is adjusted, the output frequency of the VCO 12 will jump every 400 kilohertz (corresponding to the reciprocal of 2.5 microseconds). Fine tuning of the frequency between the 400 kilohertz jumps is accomplished by adjusting the control signal A of the voltage-controlled phase shifter 26. Thus, the time delay of the fiber-optic cable 32 controls the coarse frequency spacing of the output signal of the VCO 12. Referring now again to FIG. 2 and with the latter in mind, a technique was shown to change the coarse control frequency spacing from 1/T , where T is the time delay of the fiber-optic delay line 132, to 1/2T. Since the feedback loop 160 of the frequency discriminator 101 can be locked on either the negative going crossing or the positive going crossing, the frequency spacing of the microwave oscillator 100 is set by 1/2T, here 500 kHz. If the frequency discriminator 101 is not at a zero crossing, the output of the amplifier 144 will cause the output of the integrator 146 to ramp, thereby shifting the voltage-controlled phase shifter 126 and causing the output of the amplifier 144 to move towards zero volts, indicating a crossover point has been reached. In this manner, the frequency discriminator 101 is controlled to always operate at a crossover (automatically) thereby yielding the best FM noise performance for the microwave oscillator 100. Having described a preferred embodiment of the invention, it will now become apparent to one of skill in the art that other embodiments incorporating its concepts may be used. For example, in radio astronomy, very low phase noise is required very close to the radio frequency carrier. In such an application, a fiber-optic delay line with a time delay greater than 100 microseconds could be utilized. Furthermore, a frequency discriminator could have three or more time delay loops, if necessary.

Claims: A microwave oscillator comprising: a controlled oscillator (112) having an output port and a fine control port (156); output coupling means (114,116) connected to the output port of the controlled oscillator (112); and a feedback circuit (160) coupling the output coupling means (114,116) to the fine control port (156) of the controlled oscillator (112), the feedback circuit (160) including a phase shifter (126) coupling the output coupling means (114,116) to a first input of a phase detector (128), a delay means (131) coupling the output coupling means (114,116) to a second input of the phase detector (128), and the output of the phase detector (128) being coupled to the fine control port of the controlled oscillator (112), characterised in that the delay means (131) comprises a modulated laser (130) coupled to the output coupling means (114,116) and adapted to be modulated by the output signal from the controlled oscillator (112), a photo detector (134), and a fibre optic delay line (132) of predetermined length coupling the modulated output of the modulated laser (130) to the photo detector (134) with a delay determined by the predetermined length, and in that the output of the phase detector (128) is coupled to the fine control port (156) of the controlled oscillator (112) by an inverter (148) swichtable between inverting and non inverting states and adapted to ensure that the feedback circuit (160) provides negative feedback. A microwave oscillator according to claim 1, characterised in that the switchable inverter (148) is controlled by the output of a comparator (120) having a first input (I) supplied by the output of the said phase detector (128) and a second input (Q) supplied by the output of a further phase detector (118) having a first input coupled to the output of the phase shifter (126) and a second input coupled to the photodetector (134) by a 90° phase shifter (142). A microwave oscillator according to claim 1, characterised in that the switchable inverter (148) is controlled by the output of a flip-flop (186) driven by a quadrature detector (184) having a first input supplied by a sinusoidal sweep oscillator (172) and a second input supplied by the output of the phase detector (128). A microwave oscillator according to any preceding claim, characterised in that the phase shifter (126) is a voltage controlled phase shifter having a control input, the output of the phase detector (128) is supplied to an integrator (146), and the output of the integrator (146) is supplied to the control input of the voltage controlled phase shifter (126).

Inventors: BIANCHINI MICHAEL J, CHIESA JOHN A, MICHALIK RICHARD A, MISTLER JOANNE, BIANCHINI, MICHAEL J., CHIESA, JOHN A., MICHALIK, RICHARD A., MISTLER, JOANNE
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1992
Main classification: H03L7
IPC codes: H03L7, H03B5