| For applications such as radar,wireless communication,and radio telescopes,the phase noise of the local oscillator signal determines the signal-to-noise ratio of the system.Therefore,the development of low phase noise microwave sources has important application value.In order to develop a microwave source with low phase noise,it is often used to increase the Q value of the oscillator to suppress the phase noise of the oscillator near the carrier frequency.However,when the Q value increases to a certain extent,it will become difficult to continue to increase the Q value.The phase noise of the oscillator can be effectively reduced by adopting the method of phase noise measurement and feedback suppression.Therefore,this paper studies the measurement method of oscillator phase noise and its suppression method.The details are as follows:(1)Theoretical research on phase noise measurement.The commonly used phase noise measurement methods are listed,and the principles,measurement errors and dynamic response characteristics of different phase noise measurement methods are analyzed.(2)A phase noise measurement method based on digitally controlled phase locked delay demodulation is proposed.The method uses a broadband phase locked loop to track the rapid frequency changes of the measured signal,and combines the delay link to eliminate the influence of frequency drift on the measurement results.Accurate phase noise measurement of a microwave signal with fast frequency drift is achieved without degrading the measurement accuracy within the phase locked bandwidth.(3)Based on the proposed numerically controlled phase locked delay demodulation method,a phase noise measurement system is built.The measured signal is mixed with a low phase noise reference signal to obtain an IF,and then the FPGA is used to perform frequency tracking and drift compensation on the IF signal.A low residual phase noise frequency multiplier is designed,and a nonlinear transmission line(NLTL)and a step recovery diode(SRD)are used to multiply the frequency of the 100 MHz crystal oscillator to 6GHz through two-stage frequency multiplying.The residual phase noise of the frequency multiplying link is less than-170 d Bc/Hz(normalized to 100MHz).Using the 6GHz low-phase-noise frequency multiplied source as the reference clock,the reference source is generated by direct digital frequency synthesis(DDS),which is used to down-convert the measured signal to the IF,so that the FPGA-based phase noise measurement can be realized.Then,the phase noise measurement results of the phase noise measurement system and a commercial instrument were compared when measuring a 1.01 GHz voltage-controlled oscillator(VCO)with rapid frequency drift as the device under test.The results show that the phase noise measurement system can directly and accurately measure the phase noise of the signal with rapid frequency drift without stabilizing the frequency of the measured signal.(4)The phase stability of the spurious mode of an optoelectronic oscillator(OEO)is studied and analyzed,and a digital low-IF spurious phase measurement system is built.The phase stability of spurious modes is theoretically analyzed,and the effects of OEO mode spacing drift and optical power variation on the phase of spurious modes of OEO through intensity modulation-phase modulation(AM-PM)conversion are studied.A digital low-IF spurious phase measurement system is built,which is not affected by the frequency drift of the measured signal,the tracking error of the digital phase-locked loop and the steady-state error.Compared with FFT direct sampling measurement,the digital low-IF spurious phase measurement system can achieve lower spectral leakage(-120 d B),and shorter measurement time interval(41μs).The experimental study of single-ring OEOs constructed with 200 m,1km,and 4km fibers was carried out,and it was found that with the increase of fiber length,the stability of the optoelectronic oscillator spurious relative to the main mode phase also increased.Within 5ms,the maximum mean square values of phase instability are 100°(200m),15°(1km),and 3.5°(4km).For OEOs with the same fiber length(1km)but with different optical powers,the phase stabilities of the spurious mode are basically the same.This study shows that the digital low-IF spurious phase measurement system proposed in this paper can measure the phase of the OEO spurious mode more quickly and accurately than FFT sampling,which provides a theoretical and experimental basis for stabilizing the phase of the OEO spurious mode.(5)A delay line discriminator based on 4-1 multiplexer is proposed.Based on a sixport coupler and a 4-1 multiplexer,the proposed discriminator can extract phase noise without a phase-locked loop.Since there is no need to use a phase-locked loop,accurate measurement of phase noise near the carrier frequency can be achieved without limitation of the phase-locked bandwidth.At the same time,the structure of the frequency discriminator without the phase-locked loop can be greatly simplified,and the system stability will not be reduced due to the loss of the phase-locked loop.Compared with the previously reported analog multiplier-type discriminator,the multiplexer based delay line frequency discriminator can achieve larger input power detection range,lower noise,and lower power consumption.Using this frequency discriminator,the phase noise of the 9.95 GHz dielectric oscillator(DRO)was measured,and the error of the measured phase noise was ±1.5d B.In summary,a switching delay line discriminator is proposed for the first time,which provides a low noise,low power and large input power detection range solution for phase noise detection and suppression.(6)The feedback suppression of dielectric oscillator(DRO)phase noise is realized by using the proposed switched frequency discriminator.The phase domain theoretical model of oscillator phase noise suppression is established,and on this basis,the phase noise suppression ratio of the oscillator and the influence of the frequency discriminator noise on the suppression effect are analyzed.And the factors that affect the stability of the feedback system are studied,including loop gain,delay,poles and zeros.The loop filter is analyzed and optimized.According to the research results,the number of zeros and poles of the filter that can achieve the best phase noise feedback suppression effect is determined.With a 2km fiber as the delay,the switched frequency discriminator achieves a high-sensitivity phase noise detection effect.Based on the high-sensitivity switched frequency discriminator,the 10.7GHz DRO achieves 30 d B near-carrier phase noise suppression,and the suppressed phase noise reaches-87.6d Bc/Hz @1k Hz.Because the phase-locked loop is not used,the low-frequency offset(<50Hz)phase noise suppression effect is not limited by the phase-locked bandwidth.This research provides an effective optimization tool for oscillator phase noise suppression,and also verifies the effectiveness of the proposed switched frequency discriminator for oscillator phase noise feedback suppression. |
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