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Re: General Coverage SDR?

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  • Bill Meahan K8QN
    ... Correct. Using a QSD is only *one* of many ways to implement a SDR! The holy grail is a DC-to-daylight, high-dynamic range A/D converter that connects
    Message 1 of 36 , Apr 2, 2008
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      --- In softrock40@yahoogroups.com, k5nwa <k5nwa@...> wrote:
      >
      > At 08:59 AM 4/2/2008, you wrote:

      > Most commercial radios do not use the QSD circuitry the notable
      > exception being FlexRadio, the Persus, SDR-IQ, etc use a base band
      > converter, that is a very high speed A/D connected to the antenna
      > with a pre-amp/filter system in front. Many other radios use a
      > Superhet front end and a low IF that then gets digitized.
      >


      Correct. Using a QSD is only *one* of many ways to implement a SDR!
      The "holy grail" is a DC-to-daylight, high-dynamic range A/D converter
      that connects directly to the RF input and provides a digital output
      stream that is processed to recover the desired signal information and
      doing all necessary filtering in software. The technology isn't there
      -- yet.

      Until then, dual mixers with the LO's fed in quadrature are the usual
      commercial approach with QSD's finding increased use in
      home-experimenter/amateur circles. Analog Devices makes an interesting
      chip (AD8333) that generates the necessary quadrature from a 4X LO
      providing dual I/Q outputs. That's where I'm headed when I get the time.

      73 de K8QN
    • Terry
      ... That ... current ... range ... Hey Alberto. I ve heard that number a few times, including Joseph Mitola s book (Software Radio Architecture). He suggests
      Message 36 of 36 , Apr 3, 2008
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        --- 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
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