Tuesday, June 19, 2018

Comparing SDRPlay's spectrum analyzer function with calibrated HP8563A

With identical attenuators on each analyzer's input - and with the KX2 set for a 5-watt output, I re-ran the test comparing SDRplay's spectrum analyzer program with an HP 8563A. That program, intended for SDRplay' sline of receivers, can be downloaded here.

A previous comparison was made using the KX2's minimal output of 100 mW and with the rig being incorrectly keyed (misinterpretation of the XMIT/TUNE button) to produce a steady carrier.

The results below are an accurate portrayal of how each device responds to the KX2 on 3.5 MHz. The HP is calibrated annually and was last calibrated in Feb 2018 (four months ago) and it, of course, is displaying accurate results.

(For those interested, I also looked at the KX2's other bands and posted those plots here. Given the 10 MHz span of the SDRplay's Spectrum analyzer program, it is not possible to look at bands higher than 40 meters).
HP8563A spectrum analyzer, 80m: second harmonic is 52.63 dB below carrier

SDRplay's spec-an function, 80m: second harmonic is indicated (falsely) as being 41.9 dB below carrier



  1. Two readers have emailed me about the OVERLOAD (above, in red) indication on the RSPduo's software.

    Unfortunately, even though it shows Overload, this was the best-case scenario for the RSPduo. I tried different input levels from the KX2 and different dB offset and reference level settings on the software. Either the 2nd harmonic wouldn't show up at all or it only displayed when the fundamental (or the software) showed Overload.

    I will keep tinkering with it and make other measurements as well. For now, the 10 MHz BW is quite limiting - wider BW settings will be enabled in the next version of the program.

  2. Hi from SDRplay. Just to say that the Spectrum Analyser software is still in its "Alpha" stage and has a long way to go. we are in touch with the developer Steve, and will work with him to get the best out of the hardware...

    73, Jon, G4ABQ, SDRplay

  3. Hi John
    It definitely looks as if the Duo is being overloaded. One thing to point out is that currently the Ref-dBm level and Input offset controls only effect the display and have no effect on the Duo's (or any other RSP's) gain or signal attenuation. In the actual beta release, these controls will control the device's gain so that operation will be more in line with what you would expect from a professional analyser.

    When testing with a sig gen, the displayed levels tracked to within 1 or 2 dB's from the noise floor to around -15dBm, where overload started to occur. There is a bug in the overload detection code, the overload flag is not always cleared once the input level is reduced.

    This is only an alpha release, and was primarily released to get feedback on operation and control before committing to final coding. Wider sweeps and many other issues have been addressed and should be rectified in the beta release.

    More information may be found at: https://www.sdrplay.com/community/viewforum.php?f=12&sid=82481c1e0642da2d2318393950ca2515

    Best regards


  4. Hi John

    The following link may be of interest to you. I've done some further testing showing the results obtained using a signal generator. The link shows the results, along with a comparison with the levels shown using SDRuno. Hope this is of help to you.

    73's - Steve


  5. Tnx for the comments Steve - sorry for taking so long to post them but I only now found a page of comments waiting for posting that I wasn't notified of (via Blogger) for some reason.

    I'm looking forward to further developments of the spec-an software. In the meantime I'm impressed anew everytime I operate the Duo as a digi-mode receiver. Quite an amazing performance:dollar ratio.

    73 - John