Research On Low Phase Noise Voltage Controlled Oscillator At S-band

The frequency synthesizer is an important part of virtually any radio-frequency(RF) and microwave test-and-measurement and communication system. The voltage controlled oscillator(VCO) is the key component of frequency synthesizer. The RF and microwave industry feels constant pressure to deliver wide frequency coverage, high linearity, low phase noise VCO designs.Firstly, this thesis introduced the history and current state of VCO and a broadband VCO which is extended from a feedback oscillator using a tunable open-loop filter as a feedback resonant network.Secondly, this thesis introduced the design procedure of tunable open-loop filter and the oscillator. According to the phase noise model of oscillator, to evaluate the effect of group delay and insertion loss of the filter on the oscillator phase noise, a phase noise figure of merit(PNFOM) was introduced. An open loop band pass filter whose center frequency is 2GHz is designed and simulated, and further extended it to tunable filter,the center is 1.8~2.3GHz. An oscillator is designed and simulated using this filter as feedback loop and extended to VCO. After test, the VCO shows a good performance, the output frequency is 1.61~2.10 GHz, the phase noise reached-113~-104 d Bc/Hz@100k Hz,and the linearity is very high.Furthermore, this thesis designed a frequency synthesizer to lock the VCO. The frequency synthesizer based on DDS drive mixing PLL, it has the advantage of small step size, low phase noise and high spurious suppression. After final test, the output frequency range of the frequency synthesizer is 1620~1838MHz, the step is 11 Hz,the phase noise is better than-115 d Bc/Hz@10k Hz, spurious suppression is better than-47 d Bc.Finally, this thesis discussed the problems occurred in the designing and testing of the frequency synthesizer and gave some solutions. Especially focus on problem that the low noise amplifier MGA-68561 has serious deterioration on OCXO phase noise. By experimental test, the three low noise amplifier designed based on silicon transistors deteriorate OCXO phase noise less than 2 d B. After replacing the low noise amplifier, the phase noise of frequency synthesizer improved a lot.