Need to Measure Transmitter Spurs, w/o Spectrum Analyzer

Discussion in 'General Technical Questions and Answers' started by KK5JY, Oct 19, 2017.

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  1. SM0AOM

    SM0AOM Ham Member QRZ Page

    You are entirely correct.

    Parasitic capacitances and inductances will modify the properties of especially high-value resistors in quite dramatic and sometimes somewhat unpredictable ways.

    A 20 dB power attenuator having a reasonable flat response up to about 80 or 100 MHz is practicable to home-brew, but you must pay close attention to lead-lengths and general construction.

    In my opinion, this is an opportunity to learn about measurements, instrumentation and associated topics, which is something every "real amateur" should have a working knowledge about.

    KK5JY likes this.
  2. WA7PRC

    WA7PRC Ham Member QRZ Page

    Off-the-shelf fixed and stepped PCB-mounted attenuators are available that have nearly zero lead length (and associated very high upper frequency limits). The step attenuators typically are programmed using a BCD input. Analog Devices and Mini-Circuits (and others I can't recall) have them available. The downside is, they can't handle a lot of drive. So, you start with a fixed attenuator something like one of these:
    Add a suitable heatsink, I/O interface, and you're in business. :)

    KK5JY likes this.
  3. KK5JY

    KK5JY Ham Member QRZ Page

    Sorry, I wasn't trying to make your Monday even worse. :rolleyes:
  4. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Building your own will certainly be an experiment in "real" passive components vs. "ideal" components. You could build one with through hole resistors, and one with SMD parts soldered directly together and compare their performance.

    As far as using it for calibrated measurement maybe not so much...
  5. KK5JY

    KK5JY Ham Member QRZ Page

    Well, this has already been a good learning experience. ;) Just the steps I had to go through to get here were all good experience. Some of the bone-headed things I tried to get a transmitter working were too embarrassing to report here, but they were educational, nonetheless. :cool:

    I don't think I need traceable accuracy. Reasonable consistency is probably more important, but the margin of error I can tolerate is probably substantial. E.g., if the worst-case spur reads 60dB below the carrier, that's fine as long as the margin of error of the device is less than +/- 15dB over the frequency range between the carrier and that spur.
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    I agree. If you do not have a rectum analyzer to test it you are sol. :rolleyes:
  7. KK5JY

    KK5JY Ham Member QRZ Page

    Sorry I missed this step earlier...

    I can only speak for the SDR, which I believe is the main item for which the DR really matters.

    According to SDRPlay, the RSP-1 has a 67dB SFDR in the A/D stage:

    According to this page:

    ...the RTL-SDR's 8-bit D/A has about 50dB dynamic range, but that paper makes some claims about improving the SFDR through narrow filters and decimation. I'll have to do some reading + thinking about that claim.
  8. AI3V

    AI3V Ham Member QRZ Page

    I'd be real cautious about believing the sdr play technical sheet.

    They plot the forward gain s(2,1) of a PASSIVE bandpass filter as having gain!!

    They are also curiously silent about the spectral purity of the lo.

    Please don't confuse sfdr of the a/d converter as sfdr of the entire receiver, they are 2 quite different parameters.

  9. K0OKS

    K0OKS Premium Subscriber QRZ Page

    In my experience these are the best lessons... the ones I will never forget, especially ones that emit showers of sparks.
    KK5JY likes this.
  10. N1OOQ

    N1OOQ Ham Member QRZ Page

    Regardless of the naysayers, you can build attenuators that work ok up through HF. Just don't go crazy. -10 dB is easy, -20 is doable, -40 is difficult without careful shielding. Likewise, VHF and up is difficult.
    KA9JLM, WA7PRC and KK5JY like this.

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