Strange Spectrum Analyzer readings

Discussion in 'Wireless & RF Design' started by blkfrd, May 10, 2012.

  1. blkfrd

    Thread Starter New Member

    May 10, 2012
    I am developing a non-regenerative RF repeater and am getting Spectrum Analyzer readings I don't believe. I have filters that should be rejecting the harmonics of my in band CW test signal.

    For example: My 70 MHz bandpass IF 8 KHz BW is rejecting the 140 MHz harmonic by 15 dB??? I checked the filter response on the network analyzer and it should be rejecting the 140 MHz signal by -80 dB.

    I see similiar things after other bandpass filters in the system.

    I did a test and used a 302.75 MHz 6 pole 9 MHz wide bandpass filter connected between the sig gen and the spectrum analyzer. I set the sig gen fundamental to 303 MHz and the sig gen has a harmonic at 606 MHz. I combined a separate 607 MHz signal from a 2nd sig gen and then looked at the spectrum analyzer.

    I tried two different spectrum analyzers and they are showing almost the same thing. I am keeping the Spectrum Analyzer reference level above the fundamental level so as to not saturate the device.

    Here is what I see:

    The 303 MHz signal got attenuated by 2 dB (the insertion loss of the filter...good)

    The 606 MHz harmonic got attenuated by 12 dB What the...?!?!?!

    The 607 MHz signal got attenuated by -95 dB (expected due to rejection of BP filter)

    I put a 10 dB attenuator in and the 606 MHz harmonic got attenuated by DOUBLE or 20 dB. The 607 MHz signal got attenuated by 10 dB.

    What do I think: The harmonics level is bogus. The impedance mismatch caused by the filter outside its pass band is creating havoc with the spectrum analyzer and/or sig gen and the harmonics are being re-generated. The double attenuation is because of reflections.

    Solution: I need to filter out the fundamental to measure the true harmonics accurately

    What do you think? Thanks

  2. blkfrd

    Thread Starter New Member

    May 10, 2012
    I used a bandpass filter to filter out the fundamental and the unexpected harmonic seen on the Spec An disappeared.

    So...the Spec An and/or the sig gen was generating the harmonic from the fundamental apparently. I would think that many folks would get this misleading measurement from a Spec An when a filter (non constant impedance device) is attached to it.
  3. vk6zgo

    Active Member

    Jul 21, 2012
    It could have nothing to do with matching.
    If you put too high a level into the Spectrum Analyser,it could create harmonics from the input circuitry being driven into non-linearity.