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. KK5JY

    KK5JY Ham Member QRZ Page

    I'm currently working on a home-brew HF transmitter. I have everything working like I want, but before I take it OTA, I want to measure the spurious products that it generates. I don't have a (real) spectrum scope or any real LMR-grade bench gear. So I'm looking for a way to make sure the transmitter is within Part 97 specs for spurs.

    My first thought was to use an SDR, and measure the carrier level shown in the waterfall, then move to each of the spurs and measure them similarly. The SDR software I'm using (GQRX) shows the waterfall in dB, so this seemed like a good (if slow) way to do a quick verification.

    So I tied the SDR to the transmitter through a "T" connector and a big attenuator; the other end of the "T" is a 50-ohm load resistor. This setup allowed me to use the SDR to watch the transmitter output without smoking the receiver. But even with the SDR in a completely metal case, it is picking up energy from pathways other than just the antenna cable. I'm guessing through the USB wires and just general proximity to the transmitter. So I don't completely trust the signal levels I'm reading to be an accurate representation of what will actually make it to the antenna if I take it OTA.

    Do any of you have words of wisdom on how to measure the carrier level and each of the spurs, without having to find somebody with an HP spectrum scope?
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    OSU RF engineering lab might have a lab-grade RF spectrum analyzer they'd let you use if you bring your rig to their lab?
    KK5JY likes this.
  3. WA3QGD

    WA3QGD Ham Member QRZ Page

    simple,just crank up the alc and mic gain usually the guys drivin the Cadalac radios can tell you how wall to wall and tree top tall you are in short order
  4. AF7XT

    AF7XT Ham Member QRZ Page

    At a first go I single tone into my favorite 5gal bucket (big cantenna) I sweep it with everything I have and stop to verify it's coming from the item under test.
    SDR's . Don't know what to tell you here but the sub $50 dongles and clones are easily overloaded by, well, anything. Baofeng much? I'm sure you get the idea.
    You can selectively go hunting down IMD3,5,7,9,11,13, off a known tone or do the math for the harmonics and a simple carrier.
    The more important thing is first a solid power supply HV or HA
    Avoid the compression threshold
    Never been a fan of ALC, I prefer careful stage gain.
    Low pass or band pass filters. Some tuners have a low pass function ie Pi-L (Drake mn2700b)
    Sample directly to the vertical plates of an old 'scope. Again will probably introduce its own distortion but will at least make zero crossing and flat top distortion obvious.

    Save for a spectrum analyzer That's what I got for ya :)
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    IMD is a difficult measurement, but "spurious outputs" include harmonics and mixing products that are there even with a carrier (no modulation), so they're not IMD.

    Actually, last I checked Part 97 has no IMD limits or restrictions at all.
  6. KK5JY

    KK5JY Ham Member QRZ Page

    That's a good point. I would expect such things to be in-band, so maybe they don't care as long as we only trash our own bands?

    I was mostly just trying to comply with 97.307(d) -- "the mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency below 30 MHz must be at least 43 dB below the mean power of the fundamental emission..."
  7. WA3QGD

    WA3QGD Ham Member QRZ Page

    Mom just beat on the floor if i got into her programme at 4 Merv Griffin I believe cycle 20 Conflicted youth.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, I'd want to comply with that also.

    That's spurious emissions (harmonic and non-harmonic) that are there without modulation. IMD only occurs with modulation and is really close to the carrier frequency; it takes a sophisticated setup to really measure that properly. But you can see continuously generated spurs a lot easier and don't have to modulate the rig.
  9. K7JEM

    K7JEM Ham Member QRZ Page

    I think the main thing you need to figure out is what the dynamic range of your setup actually is. You need to make sure the fundamental is well below whatever the max input signal is, and that the dynamic range exceeds 60 dB or so.

    Any overload will create apparent responses that are not actually there.
    AI3V and KK5JY like this.
  10. KK5JY

    KK5JY Ham Member QRZ Page

    This is a good point. I need to look at the receivers I have to see if any of them can capture the kind of range I need, and if so, where the best "zero" is for that.

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