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

    You make a very good point. But those two receivers are what I have to work with. So it's either the SDRPlay (at whatever resolution it has), or the 8-bit RTL-SDR.
  2. AI3V

    AI3V Ham Member QRZ Page

    Let's see the pictures of the attenuators you built, and the technical measurment you made on them.

  3. N1OOQ

    N1OOQ Ham Member QRZ Page

    Nothing at all special here, the step attenuator is similar to one seen in "Solid State Design", W7ZOI & W1FB.

    attenuators cropped.jpg

    The 27 MHz data is funky, because I was trying to get more range out of the lab signal generator by a diode doubler. It sorta worked, but not the cleanest output.
    KK5JY and KA9JLM like this.
  4. AI3V

    AI3V Ham Member QRZ Page

    Nice work, but of course, with the measurements you have ( to a hundreth of a DB!) useless for testing even a HF tx.

    Again, for a couple bucks a piece you can get attenuators that are good to a GHz or so......

    This thread started with the op asking how to test a tx for Spurs, not how to build a attenuator that he could not trust, or how to build a selective level meter, or just who has the best dumpster to dive for 40 year old spectrum analyzers :)

  5. N1OOQ

    N1OOQ Ham Member QRZ Page

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  6. KK5JY

    KK5JY Ham Member QRZ Page

    Those are very cool. :cool:

    My first attempt at making an attenuator failed miserably. It definitely had some attenuation, but the SWR even at 80m was 2.5:1. :oops:

    Did you take pictures of the insides of those little boxes before you bolted them all together? The fixed, attenuators, that is?

    I'm going to take another swipe at making my own inline attenuator, and then I may have to punt and head over to Mouser with my credit card...
  7. VK2TIL

    VK2TIL Ham Member QRZ Page

    It's difficult to make switched attenuators that have good performance above HF; even getting to 30 MHz requires some care.

    Commercial units use special switches, not the common toggle or slide type; these special types are integrated with the PCB.

    They are not available to we ordinary folk.

    NA5N wrote an excellent article on the tricks required;

    I built one in similar style;


    Before assembly;


    The switches are surface-mounted, as are the resistors; pairs of resistors, each measured, give accurate values;


    Inter-stage shields were used but the performance was unacceptable;


    A considerable amount of copper foil was added;


    The foil wraps right around the PCB and the switch bodies and is soldered to each of the inter-stage shields; it was quite a job;


    Performance is, I think, about as good as is possible with home-brew techniques;


    Performance deteriorates as more sections are switched-in;


    If I did it again, I would build about 4 sections in different cases and connect them as required; this might give better performance.

    Now this was a lot of work and you may not wish to go to this extent; there are a few kits available, eg;

    These would probably work in the HF range.
    AI3V, N1OOQ, KK5JY and 2 others like this.
  8. KK5JY

    KK5JY Ham Member QRZ Page

    Wow, thanks for all the information. I have already started reading the links. :cool:
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

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  10. KK5JY

    KK5JY Ham Member QRZ Page

    This is what I probably need to do. A salvaged attenuator should be absolutely fine for the one-off measurements I want to make. I think nothing of swiping the card for a used vacuum variable cap for a loop project, and this is an even more one-off item. :cool:
    WA7PRC likes this.

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