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Re: Software Defined Superhet

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  • Bob Hillard
    The first device in the received signal s path determines the system s noise figure. If it s a DBM, then the noise figure can t be any better than 6 dB.
    Message 1 of 8 , Oct 31, 2005
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      The first device in the received signal's path determines the system's
      noise figure. If it's a DBM, then the noise figure can't be any better
      than 6 dB. Putting a low noise preamp in front of the DBM fixes this.

      The transformer at the input to the SDR will have no effect on the
      system noise figure.

      Bob WA6UFQ

      --- In softrock40@yahoogroups.com, "Stan" <ak0b@s...> wrote:
      >
      > --- In softrock40@yahoogroups.com, KD5NWA <KD5NWA@c...> wrote:
      > >
      > > If you are going to put a pre-amplifier in front of the QSD that I
      > > would suggest a better pre-amplifier than a MMic chip. > >
      > > >Yahoo! Groups Links
      > > >
      >
      > I would 'not' use a pre-amplifier stage.
      >
      > In the QEX article a gain of 6 was obtain from the input transformer
      > which should be enough to overcome the losses in the BPF. Need to run
      > the AADE program to verify.
      >
      > The rule of thumb for the design of an RF stage is give it just enough
      > gain to overcome the tuned circuit losses. T1-4 does this plus
      > provides a degree of isolation.
      >
      > I would use high Q inductors (not the store purchased ones) and high
      > quality RF capactors in the BPF.
      >
      > Sometimes less is better. At least in this case this appears to be
      > true.
      >
      > Stan AK0B
      >
    • Stan
      ... system s ... better ... this. ... I seem to remember reading some place the QSD mixer had a noise figure in the area of 12 db ? Would this not be
      Message 2 of 8 , Nov 1, 2005
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        --- In softrock40@yahoogroups.com, "Bob Hillard" <rhillard@a...>
        wrote:
        >
        > The first device in the received signal's path determines the
        system's
        > noise figure. If it's a DBM, then the noise figure can't be any
        better
        > than 6 dB. Putting a low noise preamp in front of the DBM fixes
        this.
        >
        >

        I seem to remember reading some place the QSD mixer had a noise
        figure
        in the area of 12 db ? Would this not be acceptable except maybe
        above 28 mhz since the terrestial noise figure would be >15 db?

        It would be interesting to see a gain/noise chart by stages of the
        better commerical receivers when compared to the SDR-1000
        technologies.

        Does anyone know the maximum usable gain of a receiver(?) aren't we
        getting very close to that number in current HF technology?

        Are we not close to the point where the LO phase noise and front end
        IMD are the current limiting factors in a receiver design?

        I would be interested in reading others response in these area. It
        is an area in which I know very little.
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