Design Of Push-push Oscillator Based On SIW Differential Bandpass Filter

Microwave frequency source plays a very important role in communication system,which is widely used in radar and wireless system,and oscillator is the core of frequency source.Due to the advantages of low phase noise and high output frequency,push-push oscillator has attracted more and more attention.This article firstly introduces the design principle of the push-push oscillator,then the push-push oscillator with low phase noise and high harmonic suppression are designed and manufactured that are based on a substrate integrate waveguide(SIW)differential band-pass filter with a high complex quality factor(Q_sc)as the shared resonant network of the push-push oscillator.The differential characteristics of this differential filter can make the two loops of the push-push oscillator have better symmetry and ensure the output signals of the two loops are orthogonal which can obtain better odd harmonics suppression and a improved even harmonics output after the two signals combined.Based on this differential structure,push-push oscillators with low phase noise and high harmonic suppression are fabricated.While achieving a push-push voltage controlled oscillator(VCO)based on a tunable SIW differential filter which has the oscillation frequency tunable in a wide frequency band.The measured data show that the push-push oscillator based on the SIW differential filter proposed in this paper has good phase noise performance.Based on the SIW differential filter and peak of Q_sc principle,this article designs and achieves three types of push-push oscillators with good phase noise performance:1.Firstly,a push-push oscillator based on a SIW rectangular dual-mode differential filter is proposed.According to the frequency index of the oscillator,the peak value of the filter's Q_sc is designed at the corresponding operating frequency.The final output frequency of the push-push oscillator is 13.42 GHz with the phase noise of-134.94dBc/Hz@1MHz and the FOM of-201.48dBc/Hz,which has a lower FOM value compared with other published oscillators.2.Secondly,aiming at the problem that the actual oscillation frequency of the push-push oscillator is deviated from the original design frequency due to factors such as physical processing errors and inconsistent transistor parameters,which causes the oscillator oscillation frequency not at the peak ofQ_sc frequency,resulting in degraded phase performance,a phase-tunable push-push VCO is propesed.The phase of the oscillator loop can be adjusted by adding a phase shifter in the circuit which can leads the final oscillation frequency to be tuned back to the frequency corresponding to the peak of the Q_sc.The output frequency of the oscillator is 13.66GHz with the phase noise of-136.1dBc/Hz@1MHz and the FOM of-202.81dBc/Hz,which has the phase noise performance further improved compared with the first design poposed before.In addition,a 180°hybrid ring is designed to replace the traditional Wilkinson power divider,which enables the fundamental wave and the second harmonic of the push-push oscillator to be implemented simultaneously.This design can be utilized in the harmonic radar systems,expanding the value of push-push oscillator futherly.3.Finally,a push-push VCO is achieved by utilizing a SIW differential filter with tunable center frequency which is used as the common frequency selection network of the push-push VCO.The output frequency of the oscillator is 13.48GHz?13.63GHz with a tunable range of about 150MHz and a phase noise about-126.87dBc/Hz@1MHz at 13.63GHz,which has a good phase noise performance compared with other published VCO.Simultaneously,a SIW power combiner is designed for the problem of the low fundamental wave suppression.The measured data indicate that the fundamental wave suppression of the oscillator is significantly improved with this SIW power combiner,reaching-32dBc at13.63GHz.