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. G0HZU

    G0HZU QRZ Member

    Yes, I've bought some old school high end analysers for very little money in recent years. They often have very high performance. However, these analysers do tend to be quite big and heavy and produce lots of fan noise. So they often demand a sturdy workbench and lots of space. Best suited to a regular user who can put up with the size/power/noise and who can repair them if/when they go wrong.

    I had a quick go at comparing the harmonic levels on an old HP8566B display vs the soundcard method by looking at a non linear device at 14MHz. I posted up a quick and dirty youtube comparison video. The soundcard mixer LO is stepped in 14MHz steps to display the fundamental, then the second harmonic and then the third etc. The analyser in the background is a frozen display of the same device. Both mixers are driven well below -30dBm at RF.

    You can see that the measurements agree. K6BRN will probably be along to patronise me again and tell me this must just be a fluke and this would never be used in an official certification lab and that I have obviously never 'worked in the field' before ;)

    Obviously, it would take ages to sweep through this range manually, but if you just want to look at harmonics the system can step through them fairly quickly. Any ham with a decent sig gen and a spare Minicircuits mixer could lash this setup together in an hour or less. It's better than nothing and better than a scope and at least shows harmonic distortion to reasonable levels of accuracy if a genuine spectrum analyser is not available.

    14MHz 0dBc
    28MHz -45dBc
    42MHz -19dBc
    56MHz -53dBc
    70MHz -46dBc
    84MHz <-60dBc
    98MHz -38dBc
    112MHz <-60dBc
    126MHz -45dBc
    AI3V and WA7PRC like this.
  2. SM0AOM

    SM0AOM Ham Member QRZ Page

    The second-generation spectrum analysers such as the HP141 systems can be bought as cheap as
    $100 if you keep your eyes open.

    Analysers that once were very expensive are now quite reasonably priced, my R&S FSH4 cost the equivalent
    of $1000 as a barter involving a professional fee, and two of my HP141 systems were actually free for the pickup
    when a professional lab moved. The freight and storage costs were considered higher than the value of the instruments.

    KO4LZ and WA7PRC like this.
  3. N1OOQ

    N1OOQ Ham Member QRZ Page

    Yes, not too difficult if you work in an electronics related field. Just casually put the word out that you'll take anything that's being tossed.
  4. KK5JY

    KK5JY Ham Member QRZ Page

    Well, that and live close enough to civilization that such things exist in your state. ;)
  5. KK5JY

    KK5JY Ham Member QRZ Page

    So here's an interesting question:

    If I build a single Pi attenuator stage, that has a specific design gain (let's say -6dB or -10dB for example), should the actual effective attenuation of that single stage go up (more attenuation) or down (less attenuation) as the frequency of the input signal goes up?

    My intuition tells me that the measured attenuation at a higher frequency will be less than that at a lower frequency, due to capacitive coupling across the components increasing with increasing frequency. Am I correct?
  6. W5DXP

    W5DXP Ham Member QRZ Page

    A CLC pi-net antenna tuner is a low pass filter. A CLC T-net antenna tuner is a high pass tuner.
  7. KK5JY

    KK5JY Ham Member QRZ Page

    I was actually thinking about a resistive Pi circuit:
  8. WA4SIX

    WA4SIX Ham Member QRZ Page

    THe attenuator should be as flat as possible, frequency wise.

  9. AI3V

    AI3V Ham Member QRZ Page

    Go to eBay.

    Type in 75 ohm f connector attenuator for catv.

    Notice that they cost about 2 dollars each.

    Buy some.

    Measure your toy transmitter with your toy sdr.

    Learn a bit about overload, dynamic range, and spurious outputs of cheap local oscillators like in your tx and sdr.

    It's a hobby, not the transmitter used by Apollo 11 while walking around the moon.

    Or, go get a second mortgage on your house, and buy a lab full of gear, and a screen room to test your toy.

  10. AI3V

    AI3V Ham Member QRZ Page

    Attenuators are made by the hundreds of thousands on automatic machinery.

    You cannot build a better one at home.

    KK5JY likes this.

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