Strange Spectrum Analyzer readings

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

  1. blkfrd

    Thread Starter New Member

    May 10, 2012
    3
    0
    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

    Tracy
     
  2. blkfrd

    Thread Starter New Member

    May 10, 2012
    3
    0
    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
    677
    85
    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.
     
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