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

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  • warrenallgyer
    I have been staring at this problem too long and I think the simplest answer has been sitting right in front of me all along. A simple 20 dB or so attenuator
    Message 1 of 39 , Feb 26, 2013
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      I have been staring at this problem too long and I think the simplest answer has been sitting right in front of me all along.

      A simple 20 dB or so attenuator in both channels of the 3.5 mm cord from the Line In to the sound card will maximize the dynamic range and reduce the IMD3 products by 60 dB.... all in one fell swoop.

      Stay with me here... what do we know?

      1) IMD3 products and, ultimately, overload are caused by the audio card, not by the receiver itself. I know Gerhard is not yet convinced but just accept it for the moment for the sake of this discussion.

      2) Even 16 bit cards, with the audio input gain turned all the way down, show 8 to 10 dB of antenna noise in most cases. This is wasted dynamic range.

      3) Moving to a 24 bit card adds 12-15 dB of dynamic range BUT not on the top end. It extends the noise floor downward which does no one any good. It is completely masked by the antenna noise and the card still overloads at approximately the same level as the 16 bit card.

      Now, we add 20 dB of attenuation in the audio Line In path. We adjust the input audio gain until we just see the noise floor rise 2-3 dB with the antenna connected.

      Voila! By definition we have maximized the dynamic range of the receiver. If this is done using a 16 bit card the gain will be higher and the overload point will be reached sooner. If you move to a 24 bit card the input gain will be lower and the margin will accrue on the top end.

      And, since the IMD3 products increase in level at a 3X rate compared to the overloading signal, reducing that signal by 20 dB should give a theoretical 60 dB improvement in IMD3 response.

      I cannot find a flaw in this reasoning but I am 72 hours away from access to my workbench to try it out.

      What are your thoughts?

      Warren Allgyer - W8TOD

      --- In softrock40@yahoogroups.com, "warrenallgyer" <allgyer@...> wrote:
      >
      > Gerhard
      >
      > You have done a lot of very good work and it is really interesting to read this approach to a problem that I have been dealing with as well. A couple of observations and considerations:
      >
      > 400 mv at the antenna terminal would mean the incoming signal is arriving nominally at about -4dBm. Dave is correct; this will not cause the mixer to overload.
      >
      > I have to disagree with Dave a bit on his second comment. In my experience this level is not high enough to make the op amps go into clipping. I am guessing, as was the case with me, that it was the sound card that was going into overload. The regularly spaced spikes on the spectrum display are a symptom of this.
      >
      > I was actually able to cure the problem in a much simpler way. Like you, I was showing plenty of antenna noise rise. By reducing the audio input gain to the one step above the minimum on the Windows audio control panel I was able to solve the audio card overload and still have 3 dB or so of noise rise on 20 meters.
      >
      > I do not often see -4 dBm incoming although it has happened. My SW broadcast interference is in the 7200 - 7400 band and regularly hits -15 to -10 dBm. A little bit less than yours.
      >
      > Thanks for the great information and please ignore the moron who wants the "USA Today" version rather than the Wall Street Journal.
      >
      > Warren Allgyer - W8TOD
      >
      >
      >
      > --- In softrock40@yahoogroups.com, "gerhard972" <gerhard.vaneerden@> wrote:
      > >
      > > I have been following the recent discussions on SoftRock dynamic range and intermodulation with great interest. Here is my personal experience.
      >
    • 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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