Coupling microstrip line bandpass filter applied to hybrid frequency synthesizers

Abstract

This paper designs a parallel coupled microstrip bandpass filter at the ends of the hybrid frequency synthesizer of PLL+DDS mixing in the outside of the feedback loop systems. This filter is used to suppress the frequency synthesizer′s spurs and harmonic in the output spectrum. The band pass filter is designed by the parallel coupled microstrip line. The measuring results show the center frequency is 2 350 MHz and the bandwidth is 50 MHz. And the maximum loss is -5 dB in the passband. In the external bandpass, the loss of 2 200 MHz is -47.7 dB, while at the 2 285 MHz，the loss is -29.0 dB. By measuring the filter and the output signals of the hybrid frequency synthesizer, it is proved the effectivity of the filter to suppress spurs and harmonic in the output spectrum of the frequency synthesizer.

Keywords

• parallel coupled microstrip bandpass filter
• hybrid frequency synthesizers
• spurs
• harmonic
• loss