SDRuno Fine Tuning Behaviour

SDRPlay Independent Community Forum Forums SDRUno SDRuno Fine Tuning Behaviour

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    Paul White

      Hi. I’ve been looking into using SDRuno to measure small carrier offsets of MW signals.

      My first observation is that the smallest VFO tuning step is 1 Hz, and that both Main and Aux SP windows at maximum zoom cannot show better than about that resolution.

      So, my FIRST question is whether there is an operational mode I’m missing here. And (if not) whether SDRplay plans to add higher resolution in future. My aim is to estimate offsets to the nearest hundredth of 1 Hz.

      My second observation is that changing decimation affects the measured carrier frequency. An example on 693 kHz of Main SP Offset (with ZIF) is:

      Offset(Hz) at SR:
      Dec 2M 4M 8M
      === == == ===
      1 28 60 100
      2 13 ?
      4 6 15
      8 3 7
      16 2 3
      32 1 2

      Here, the offset is roughly in inverse proportion to decimation.

      My SECOND question is why that behaviour occurs?

      My third observation is that when Main SP is tuned to the carrier peak, Aux SP show the carrier at *minus* the VFO/Main offset.

      So the THIRD question is why the IF passband is not centred on the current VFO setting? Perhaps on a 1 kHz or 100 Hz boundary?

      As far as I can imagine, radio frequency calibration should not be a factor in any of these. Thanks for your time.


        I don’t have an answer for you but I’m glad to see you posting questions here. just fyi, there is a facebook group rsplay but to be honest I don’t like working with facebook. Besides the privacy stuff its difficult to sift thru the postings to see if there is anything relevant so….I hope activity picks up here and maybe the oem engineers will join the discussions and attract those FB members.

        To address your question. You might try reading some of the docs especially the one about the API. I read some of it and it is over my head without doing a lot of studying. I suspect that the LO in there is not tuneable at that fine of a level that you are wanting as I can’t imagine that the original design would have that kind of requirement. You may get lucky but I doubt it. There could be some clever hack that would do what you want.

        Paul White

          Thanks, DIY, you obviously have a better grasp of this than me, so I’m not getting my hopes up – in general there’s a lot you can do in software to compensate, but that’s really clever stuff!
          I know the SDRplay chaps/chapesses are very busy, but they do get around to a lot of questions eventually :).
          Probably ought to be looking at Linrad or similar for carrier offset precision.


            Since I am old hardware EE kinda guy I tend to think of solutions that don’t involve software. So if you were willing to canabalize an sdr by breaking into a LO signal or just the base oscillator , set up a mixer, then mix audio frequencies with the original signal you could achieve a slight shift based on the frequency of that audio signal you apply to that mixer. Better yet, get your on VCO that has tight controls and use it instead of the LO. I am assuming that you can get to it. Some of those cheap dongles might be more forgiving of the hacking although you’d need an upconverter.

            You might find some other ideas by visiting this site where you might find links to devices that these guys use to participate in these frequency tests.


            Just to satisfy my curiosity why are you wanting to do this? Are you thinking that some of these MW stations are operating outside established spec limits? So then you could get with them to help them make corrections?

            Paul White

              Hi, and thanks for the response.
              I can see your point about hardware approaches to the problem, but I’m not in that league :).
              Also, GPSDO is not an option until I can afford to upgrade from my RSP1A.
              My reason for trying to measure MW offsets is that they are often characteristic for a particular station and can therefore help identification when audio is not conclusive (this is a MW DXers specialist interest).
              As for the other parts to my question, these are to clear up doubts how RSP/SDRuno operate and whether there are any problems here.
              Happy New Year.


                Thanks for the response. You are indeed involved in a specialized sport that I didn’t even know existed until recently. I had always thought that shortwave was the only place for listeners. I started swling in 1964. I guess I didn’t read the right magazines back then.

                For now I’m sticking with HDSDR. i’ve tried UNO a bit but I seem to end up in a spot where the mouse wheel and spectrum stop responding consistently. I must be invoking some config change and not realizing it.

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