Low noise fine frequency step synthesizer

Abstract

A frequency synthesizer (10) and method that achieves low phase noise and provides synthesized frequency signals in fine frequency step intervals. The synthesizer (10) comprises an L-band low phase noise frequency synthesizer with fine frequency step increments. The synthesizer (10) employs half-integer digital frequency dividing (25), VCO frequency offsetting, local oscillator harmonic mixing (34) and two phase locked loop circuits (12,13). The synthesizer (10) comprises a reference oscillator (11) for providing a reference frequency signal, and two phase locked loops (12,13). The first loop (12) is the fine loop and generates the frequency step size. The second loop (13) reduces the phase noise, reduces the frequency step size and translates to the desired freqeuncy. The output from a first VCO (21) is divided by a predetermined fixed number (31) to reduce the frequency step size and to reduce the phase noise. The second loop (13) has a second VCO (32) whose output, at L-band, for example, is divided by two (33), and then mixed (34) with the third harmonic of the reference frequency signal to generate an offset frequency signal that is locked to the low phase noise fine frequency step signal of the first loop (12). The output frequency is adjusted by changing the digital programmable half-integer frequency divider (25). The low phase noise and fine frequency step frequency synthesizer (10) provides a major improvement for Doppler radar and communication receivers.