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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

    That is really cool. I'm amazed that they can get that kind of attenuation in a single IC. I tried using a digital potentiometer as a variable attenuator, and even at low RF, the leakage was incredible. Apparently that IC is "special." :cool:
  2. KE0ZU

    KE0ZU Ham Member QRZ Page

    It takes considerable time and effort making attenuators that perform well through VHF and beyond. I just bought commercial/lab attenuators instead.

    The fifty watters do double duty as dummy loads and attenuators, when using the tx audio, or spectrum analyzers. Obviously the "ON" time must be kept short for higher power levels, and in those instances I also use a small fan.

    The 355 step attenuators are a great help some times.

    Last edited: Oct 28, 2017
    WA7PRC likes this.
  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    Nice... Thank you for the info. Just what I need for a project.

  4. W6RZ

    W6RZ Premium Subscriber QRZ Page

    Be aware that's it's only good for a 1/4 watt of dissipation.
    KK5JY likes this.
  5. W6RZ

    W6RZ Premium Subscriber QRZ Page

    IC based attenuators are sweet. On the Analog Devices AD9361, the on chip transmit attenuator has a range of 89.75 dB in 0.25 dB steps.
    KK5JY likes this.
  6. VK2TIL

    VK2TIL Ham Member QRZ Page

    It takes considerable time and effort making attenuators that perform well through VHF and beyond. I just bought commercial/lab attenuators instead.

    I agree; I built my switched attenuator "just because I wanted to" but I also have commercial units.

    This was assembled from three Telonics-Berkeley units; I put them in the case;


    The 10dB/ & the 1dB/ units were found here in VK at very reasonable prices and it was initially a "two-knob" build; I later added the 0.1dB/ unit which was found on ebay at a reasonable price.

    Ebay prices are now, several years afterwards, no longer reasonable; I'm glad I got them when I did.

    This one, using an attenuator salvaged from a Wavetek sweep generator and housed by me, probably gets the most use; I put it inline with my spectrum analyser to ensure no expensive blow-ups;


    In addition, there are probably some dozens of fixed attenuators; I also have an attenuator "chip" that I will put on a heatsink some day to make a 100W unit.

    Be aware that (the SV1AFN device is) only good for a 1/4 watt of dissipation.

    True; it may not suit everyone.

    High power can be reduced prior to a low-power unit with a suitable fixed attenuator, power tap or coupler.

    I've thought about building the deservedly-popular W7ZOI power tap but, with my other tools for reducing power, I don't really need it.

    I have several commercial couplers but I also use home-made couplers, eg;



    Performance is quite good; coupling is still 32dB at 150 MHz.

    KO4LZ and WA7PRC like this.
  7. SM0AOM

    SM0AOM Ham Member QRZ Page

    They work very well at the 5 W level. The temperature rise after dissipating 4.5 W for a whole day as level setting attenuators was about 20 C.
    It was impossible to detect any attenuation drift after operation.
    KK5JY likes this.
  8. KO4LZ

    KO4LZ XML Subscriber QRZ Page

    Mini-Circuits is definitely quality parts -- I work in RF test and measurement and the vast majority of the connectors, splitters, pads, etc. I use on the job are from Mini-Circuits.

    I have one of their 40 dB attenuators that I use when testing high power (high-hundreds of watts) devices and I trust it completely not to fail and fry the front end of the six-figure spectrum analyzer I use for measurements:)

    Best quality/price ratio out there in my opinion.
    KK5JY likes this.
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    How about power to weight ratio ? :rolleyes:
  10. KK5JY

    KK5JY Ham Member QRZ Page

    I was also surprised that the Elecraft attenuator was only specified for 2W of power. I looked at the parts list, and all of those resistors are rated for either 1W or 2W. When I did the calculation on all of the Pi sections, with all sections in-line, there wasn't a single resistor that exceeded 1W of dissipation with 5W of input, and the closest was 0.77W, IIRC. It did require all sections to be engaged, but in that configuration, the step-down was gradual enough to keep all of the resistors in-range of their ratings.

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