It appears the high harmonic output is generated by the last MMIC amp. Not sure why unless it is sensitive to the impedance seen by the input looking back into the filter. There is a simple low pass filter after the oscillator buffer. That filter should have been after the MMIC amp but the amp was an after thought when I designed the PC board. The harmonics go away when I remove the last output capacitor from the filter. I then have the second harmonic down 35 DB. I attempted to replace the first and last capacitor with trimmers to see if I could improve the situation. Not really, so I left it as it was minus the last capacitor.
I adjusted the oscillator transistor bias resistors to improve the output while maintaining the same 35 db harmonic attenuation. The oscillator output level is now zero DBm.
That still puzzles me as to why I am getting the harmonics. The MMIC is not being overdriven. The MMIC is an Agilent ABA-53563 I got from Mouser Electronics. The 1-DB compression point is +14 DBm output. I appear to have around -15 DBm going into the MMIC so with a 21.5 DB gain that is below the 1-DB compression point. I checked my spectrum analyzer calibration against my signal generator. The spectrum analyzer reading agree with the settings of the generator.
I will leave the oscillator as it is and get back to testing the frequency calibration and oscillator heater. Something tells me I will be redesigning the PC board moving the filter after the MMIC.
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