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Re: [softrock40] Increasing the usable dynamic range of the RXTX 40/30/20

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  • Jasmine Strong
    Jeez, get over yourself. -J.
    Message 1 of 39 , Feb 24, 2013
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      Jeez, get over yourself.


      On 24 Feb 2013, at 06:37, Jose Bonanca <jabct1aos@...> wrote:


      I didn't bother to read all of it, of course....

      On 24 February 2013 14:33, warrenallgyer <allgyer@...> wrote:


      If you don't appreciate detailed information then I suggest you don't bother to read it. Those of us that do would like to encourage all the detail a contributor is willing to provide us.

      Warren Allgyer -W8TOD

      --- In softrock40@yahoogroups.com, Jose Bonanca wrote:
      > Did you think you were writing a book.?
      > 73
      > On 24 February 2013 14:08, gerhard972 wrote:
      > > **
      > >
      > >
      > > I have been following the recent discussions on SoftRock dynamic range and
      > > intermodulation with great interest. Here is my personal experience.

      Jose (Ct1aos)

    • warrenallgyer
      I had an interesting day back in the lab after 10 days on the road. I won t run through the grueling details but I now have a 20 dB audio attenuator cable with
      Message 39 of 39 , Mar 2, 2013
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        I had an interesting day back in the lab after 10 days on the road. I won't run through the grueling details but I now have a 20 dB audio attenuator cable with 3.5 mm ends and I also built a hybrid RF combiner that allows me to combine or split two 50 ohm sources with 3 dB loss in each one. Without any trimming i got about 25 dB isolation between the ports up to 30 MHz. It will be a handy tool.

        My attenuator cable idea works for utilizing the unused noise floor in either a 16 bit or a 24 bit card. That is the good news. The bad news is there seems to be little point. On the high end, as Alan suggested, I found that by minimizing the PC input gain I was within a couple dB of the clip points on the op amps anyway. So, even though I was able to "raise the roof" on the sound card, I only gained 2-3 dB before the op amps clipped. This interested me so I ran through the 4 RXTX that I have here in Beijing to check at what level the op amps went into clipping:

        Radio Band OP Amp Saturate
        dBm (WSPR Center Freq)
        RXTX 160 160 -14

        RXTX 80/40 80 -15
        40 -17

        RXTX 30 -11
        30/20/17 20 -10
        17 -15

        RXTX 15 -19
        15/12/10 12 -18
        10 -17
        The answer is "anywhere from -10 to -19 dBm". So even if I could get the sound card range shifted up I was still limited by the op amps.

        The other problem with "raising the floor" is it is ugly down there! With my attenuator cord in place I was able to move the Realtek and the iMic card floors up to match the antenna noise. With the 24 bit X-Fi I was able to get it up within 10 dB of the noise. But with the iMic and especially with the X-Fi I found that the noise, spikes, and spurs were so prevalent in the bottom 10-30 dB of the card that I really did not want to see them. In real life the antenna noise hides a multiple of these digital sins. The X-Fi useable noise floor, once you get above the noise and spikes, is not a significant improvement over that of iMic and not as good as the internal 16 bit Realtek.

        So, again in real life, I am bounded on the top end by the op amps, let's say -17 dBm on 40 meters, and by the noise floor on the bottom end, normally about -110 dBm. That gives me 93 dB of useable dynamic range which is perfectly accommodated by a 16 bit card.

        I do confess to having an E-MU 0204 on the way by courier from the US. It should be here next week. It will be interesting to see if the lower 20 dB will be useable and I am justifying the purchase on the basis that it will be a lab tool rather than operational.

        Closing the book on this one for now…..

        Warren Allgyer - W8TOD
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