Loading ...
Sorry, an error occurred while loading the content.

20100Re: General Coverage SDR?

Expand Messages
  • Terry
    Apr 3 9:55 PM
    • 0 Attachment
      --- In softrock40@yahoogroups.com, "adibene" <i2phd@...> wrote:
      >
      > --- In softrock40@yahoogroups.com, "Terry" <wb4jfi@> wrote:
      > >
      > > If I understand, HF should have a dynamic range of 130dB or so.
      That
      > > translates to 22 bits, I believe. If appropriately dither is
      > > applied, and subsampling to demod audio for SSB or CW, the
      current
      > > crop of 14 to 16 bit A/D devices should just barely get us there.
      > >
      > > Static crashes and other anomolies that go beyond this 130dB
      range
      > > will still overload the A/D, although only at the peak excursions.
      >
      > Not sure, but 130 dB look to me a bit too many. No analog radio on
      > the market, AFAIK, has such a big dynamic range, but they have no
      > problems to cope with HF signals.
      >
      > 73 Alberto i2PHD
      >

      Hey Alberto. I've heard that number a few times, including Joseph
      Mitola's book (Software Radio Architecture). He suggests that 130dB
      is the dynamic range for HF-RF. He suggests an HF-IF (in the .2-
      10MHz range) sould be 72-120dB. Most other places that I've heard it
      probably reference back to him, although I think I saw it derived in
      at least one other independent book somewhere.

      Most radios are narrow-band, with filters, AGCs, switchable amps &
      attenuators, and other gadgets to translate the level of the
      currently received signal into a narrower dynamic range that the
      radio can handle.

      I think he is suggesting that the 130dB is the minimum for directly
      feeding an A/D converter without overloading, but still reaching the
      HF noise floor.

      Obviously, we can use the same range-limiting (or maybe it's better
      to say range-adjusting) tools if we are using an A/D receiver
      approach. Sticking a preamp and/or attenuator inline will help, as
      long as it can be switched out (or the gain adjusted with an AGC
      loop). Applying filters to remove large unwanted signals (such as
      the US AM broadcast band) is another possibility. I know hams have a
      hard time (with normal radios) trying to work 160M with nearby AM
      broadcast stations, and often resort to filtering.

      But, to a SDR "purist", sticking filters, preamps, attenuators, etc,
      may all be considered band-aids to mask the real issue - not enough
      dynamic range. And, they all potentially hamper optimum reception in
      one manner or another. Like in a traditional superhet, these band-
      aids may be necessary for a while yet.

      I forget where to find the noise floor on the various ham bands.
      But, I'm totally guessing that it's around -128dBm on some HF band.
      To need 130dB of dynamic range with that small a MDS signal means the
      MAXIMUM signal would be over 0dBm! That's a ton of signal. But, I
      hear some people who live near AM broadcast (US), or SW broadcast
      (europe) transmitters may get that strong of a signal. Running
      multiple stations nearby (field day, etc), may also create this
      problem.

      There was a QEX article in Sept/Oct 2002 (The DX Prowess of HF
      Receivers) that indicated the BDR of a K2 is 133dB (20kHz) and 126dB
      (for 5kHz).

      Feeding an antenna directly into an A/D input may be folly for a
      while yet, especially during electrical storms. I don't think
      anybody plans to have an A/D with that high a dynamic range!!

      Terry
    • Show all 36 messages in this topic