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Re: [softrock40] Requirements for the opamps in the softrock RX/TX

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  • Marco
    ... Hello Heikki and group, I am one of those considering the SoftRock a nice test bench for new ideas and for the optimization of SDR frontends. Possibly my
    Message 1 of 2 , Sep 25, 2006
      At 14.15 25/09/2006, you wrote:

      Dear group,

      It is obvious that most involved with this group are aiming at
      constructing their softrock equipment from the kits available. There
      are, however, a group of us, including me, who would like to take the
      advantage of the great ideas presented, but still carry on with their
      own design including the component selection, board layout etc. Not
      too familiar with the current developments in the op amp technology
      and finding the softrock schematics with strange types (LM 741 I know,
      hihi),I would like to know their criteria for selection. For the rx
      part, low noise is one factor for sure, also 5 V supply voltage is
      compatible with the rest of the board. What about GBW-product, slewing
      rate etc. , are these any critical since the opamps are in the audio
      section ?

      Before buying the parts proposed I want to search my (not so small)
      junk box but as far as the opamps are concerned I would need some
      advice on what to look for .

      73 de Heikki (OH2LZI)

      Hello Heikki and group,

      I am one of those considering the SoftRock a nice test bench for new ideas and for the optimization of SDR frontends.
      Possibly my considerations will be well known to professional designers; but in general we (OMs interested in the technical side of things) have a lot to learn.

      I spent the past weekend trying to characterize an M-Audio Audiophile 192 alone and connected to my SoftRock V6. Here are some results.

      The A-192 declares a flat frequency response up to 80kHz while sampling at 192kHz, 24 bits.
      The measured response is flat up to over 90dB. The antialiasing filter starts cutting here, so not to see aliasing signals in band I suggest that it is useable up to 86kHz.

      Unfortunately, the noise floor is not flat. From 0 to 3kHz it decreases approx like 1/f, then it's flat up to 25kHz. From there up it increases by approx 5dB per 10kHz, rising by 30dB at 85kHz. I have no idea of the reason for this noise increment; the two channels are different, the R being approx 3dB less noisy than L. Anyone may explain this noise behaviour?

      Returning to dynamic range, the saturation level is exactly 4Vrms as declared. I used an unbalanced input, connecting the "-" input to ground.
      Considering the good zone of the noise floor (from 2 up to 25kHz) the dynamic range is in the order of 120dB in 2.4kHz. That's better than the declared 113dB in "A" weighting filter, that has 1.74kHz equivalent noise bandwidth (if I remember well).

      The input sensitivity corresponds to a -94dBm (50Ohm) noise floor. So we have to set our front-end output dynamic range from -94 to +26dBm.
      By instance, if we want a -120dBm noise floor at the antenna, the frontend should have 26dB voltage gain. I am deliberately ignoring the noise figure issue here.

      Now, my modified SoftRock has approx  20dB of voltage gain, so it is quite "centered" in the range. The measured noise floor is -113dBm in 2.4kHz, and is dominated by the front-end noise; by powering down the SoftRock the noise decreases by 2dB. The saturation happens at -5dBm, so the blocking dynamic range is 108dB.

      I leave to you all considerations about the post-mixer opamps. In my opinion they should be powered at least from +/- 6V or +/-9V  rails to use all the available headroom on 24 bit audio boards, and be able of 26-30dB gain in a 100kHz band and 120dB of dynamic range. Not trivial... :-) how deep is your junk box?

      A picture of the noise floor of the Audiophile 192 (at 96khz sampling rate here) is available from www.spin-it.com/download/IK1ODO_A192_96kHz_2ch.jpg
      73 - Marco IK1ODO
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