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Re: Softrocks receiver out-of-band response

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  • vbifyz
    What do you mean by tuning HDSDR ? Unless you are demodulating the exact LO frequency (LO and Tune frequencies are exactly the same), the shown Tune frequency
    Message 1 of 3 , Mar 17, 2013
      What do you mean by "tuning HDSDR"?
      Unless you are demodulating the exact LO frequency (LO and Tune frequencies are exactly the same), the shown Tune frequency is not 1/3 of the signal frequency when you are receiving on the 3rd LO harmonic. The offset between LO and Tune is still 1x, and it sounds to me that you expect 3x.

      73, Mike

      --- In softrock40@yahoogroups.com, "warrenallgyer" <allgyer@...> wrote:
      >
      > With a weekend free I decided to try to document the performance of my 10/12/15 RXTX on receive at frequencies below 21 MHz. In particular I wanted to understand the response of the receiver at the third harmonic of the LO.
      >
      > For example, I have found that if I listen at 1/3 of the intended frequency of the build... tuning HDSDR in the 40 meter band with the 10/12/15, that I would hear images of strong signals on 15 meters in the 40 meter band. They were easily identified because they moved 3 KHz on the display for every KHz of change in the LO.
      >
      > So I set up and calibrated the 10/12/15 and the signal generator. I injected a +73 dBm signal at 30 MHz and then tuned the Softrocks to exactly 1/3 of that frequency, 10 MHz in this example. There was nothing there. But, in tuning around a bit, I found a strong spurious signal at 10.013883. I moved the generator down to 27 MHz and looked at 9.0. Again nothing. But again a spur at 9.013137. These spurs were 15-20 dB below the on frequency level... very strong.
      >
      > I continued this at 1 MHz intervals all the way down to 5 MHz, always turning around 1/3 the LO. In every case there was a strong spur approximately 13 KHz above the exact 1/3 frequency.
      >
      > I am puzzled. I have checked and re-checked the calibration of my receiver and of my signal generator. They are very consistent at +- 2Hz referenced to GPS. I have no combination I can think of that should make a spur that is consistently 13 KHz above 1/3 of the LO.
      >
      > I know it is a little bit esoteric.... and I clearly could be doing something really stupid here but any ideas would be welcome.
      >
      > Warren Allgyer - W8TOD
      >
    • warrenallgyer
      Good point Mike. Let me clarify. I have gone back and checked the relationship of the LO versus tune frequency and now it makes sense. Signal Generator:
      Message 2 of 3 , Mar 17, 2013
        Good point Mike. Let me clarify. I have gone back and checked the relationship of the LO versus tune frequency and now it makes sense.

        Signal Generator: 21.000000
        LO: 7.001
        Spur: 7.004 at -82 dBm

        Signal Generator: 21.000000
        LO: 7.002
        Spur: 7.008 at -82 dBm

        Signal Generator: 21.000000
        LO: 7.003
        Spur: 7.012 at -82 dBm

        So the 21.0 MHz generator is mixing with the 3rd harmonic of the LO at 7.001 with a harmonic at 21.003, audio product at 3KHz which, added to the LO, displays the spur at 7.004.

        I left everything set as it was and plugged in the RX II HF and the same spur is way down in the noise at -124 dBm. The difference, of course, it the RX II LF, when tuned to 7 MHz is severely attenuating the 21 MHz generator signal.

        So, in response to earlier posts about general coverage with an RXTX: Operation significantly below the design band will expose you to spurious signals that may show up on you band display only 12-15 dB lower than they actually are at their real frequency.

        Warren Allgyer - W8TOD

        --- In softrock40@yahoogroups.com, "vbifyz" <3ym3ym@...> wrote:
        >
        > What do you mean by "tuning HDSDR"?
        > Unless you are demodulating the exact LO frequency (LO and Tune frequencies are exactly the same), the shown Tune frequency is not 1/3 of the signal frequency when you are receiving on the 3rd LO harmonic. The offset between LO and Tune is still 1x, and it sounds to me that you expect 3x.
        >
        > 73, Mike
        >
        > --- In softrock40@yahoogroups.com, "warrenallgyer" <allgyer@> wrote:
        > >
        > > With a weekend free I decided to try to document the performance of my 10/12/15 RXTX on receive at frequencies below 21 MHz. In particular I wanted to understand the response of the receiver at the third harmonic of the LO.
        > >
        > > For example, I have found that if I listen at 1/3 of the intended frequency of the build... tuning HDSDR in the 40 meter band with the 10/12/15, that I would hear images of strong signals on 15 meters in the 40 meter band. They were easily identified because they moved 3 KHz on the display for every KHz of change in the LO.
        > >
        > > So I set up and calibrated the 10/12/15 and the signal generator. I injected a +73 dBm signal at 30 MHz and then tuned the Softrocks to exactly 1/3 of that frequency, 10 MHz in this example. There was nothing there. But, in tuning around a bit, I found a strong spurious signal at 10.013883. I moved the generator down to 27 MHz and looked at 9.0. Again nothing. But again a spur at 9.013137. These spurs were 15-20 dB below the on frequency level... very strong.
        > >
        > > I continued this at 1 MHz intervals all the way down to 5 MHz, always turning around 1/3 the LO. In every case there was a strong spur approximately 13 KHz above the exact 1/3 frequency.
        > >
        > > I am puzzled. I have checked and re-checked the calibration of my receiver and of my signal generator. They are very consistent at +- 2Hz referenced to GPS. I have no combination I can think of that should make a spur that is consistently 13 KHz above 1/3 of the LO.
        > >
        > > I know it is a little bit esoteric.... and I clearly could be doing something really stupid here but any ideas would be welcome.
        > >
        > > Warren Allgyer - W8TOD
        > >
        >
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