Harmonics in signal generator higher bands. How to reduce them?

Discussion in 'General Technical Questions and Answers' started by SV3ORA, Jul 4, 2018.

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  1. N1OOQ

    N1OOQ Ham Member QRZ Page

    Not sure how much more help we can give with the harmonics without being there to probe it. It has some quirks... The 2N2222 pass transistor is interesting, but there doesn't seem to be any decoupling of the power in the osc/amp stages. I can't figure out if the right hand side of the differential pair is bias, feedback or both. One would expect a lot of feedback if the idea is to reduce distortion.
     
  2. K1ZJH

    K1ZJH Ham Member QRZ Page

    -30dBc harmonic levels seem reasonable for what it is.

    in the oscillator, which causes waveform distortion and thus harmonics...
    shielding will not eliminate harmonics passing through the following broadband RF stages.

    Why are the harmonics a problem? Most service grade signal generators used harmonics to operate on the highest range. Does the design incorporate a feedback loop to control oscillator
    feedback (I didn't read the entire missive.) If so, perhaps playing with some values might
    reduce the harmonics.

    Pete
     
    1 person likes this.
  3. SV3ORA

    SV3ORA Ham Member QRZ Page

    Harmonics are initially a problem, because I intend to use the generator not only for alignment of receivers but also as a test local oscillator for direct conversion receivers. This means that the mixer will respond not only to the main carrier of the LO but also on the harmonics, introducing noise or interference from these frequencies.

    Above 15MHz the first harmonic will start to get reduced if a suitable LPF is connected at the output.

    But I am still struggle in understanding which of the two outputs is the 50 ohm so as to connect the filter into it?
     
  4. K1ZJH

    K1ZJH Ham Member QRZ Page

    The output marked 50 ohms is the 50 ohm output. Put the filter after the 4.7uF tantalum capacitor. Both the AUX and 50 ohm outputs will then be filtered.
     
    1 person likes this.
  5. SV3ORA

    SV3ORA Ham Member QRZ Page

    That's a nice idea thanks.
    Note that the markings are mine in the schematic presented on the page. If you download the article you will see that he has not marked the attenuated port. If you see the PCB that he provides, you can notice that the non-marked port goes to a frequency counter.
    However the 560/56 combination gives a 50R output impedance that is why I am confused.
     

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