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Software Defined Superhet

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  • Bob Hillard
    Consider using the SR40 as an IF strip , preceeding it with a MMIC preamp, some bandpass filtering, and a Diode Balanced Mixer using an AMQRP DDS module as
    Message 1 of 8 , Oct 28, 2005
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      Consider using the SR40 as an 'IF strip', preceeding it with a MMIC
      preamp, some bandpass filtering, and a Diode Balanced Mixer
      using an AMQRP DDS module as the LO source.

      I have done that very thing with excellent results.

      I've converted my SR40 to 30 meters, and use 10.138 mHz as the IF
      center frequency. I've located the LO above the incoming signal in
      order to reduce image frequencies.

      The output of the DDS module ranges from 11.9 mHz on 160 meters to
      31.6 mHz on 15 meters. By replacing the DDS module with the newer 60
      mHz model when it becomes available, coverage will be increased to
      include the 12, 10 and 6 meter bands.

      With the LO above the incoming signal, the radio operates on the lower
      sideband. However this can be corrected by software for bands that
      require USB operation.

      Band switching is accomplished by programming my DDS VFO controller
      (http://home.att.net/~wa6ufq/ddscontroller.html) so that each of the
      eight programmable configurations represent a band. This gives me a
      bandswitching VFO with wrap-around, up/down frequency control or
      direct entry, variable VFO steps, sweep and scan functions, and 20
      memory cells per band.

      Presently I am switching bandpass filters manually, but that will
      change with the addition of a band pass filter selection circuit
      similar to that found in the 'Software Defined Radio for the Masses'
      article. The bandpass filters are also similar to those found in the
      QEX article. One of the filters, the one I use most oftenly, covers
      the 15, 17, and 20 meter bands.

      By using this scheme, the SR40 circuitry is not taxed by trying to
      stretch it's operation past the limits of its design since it is
      always operating on 30 meters. However the radio itself can operate
      from 160M to 6M, with the exception of 30 meters (the IF frequency).
      Likewise, since the DDS frequency does not have to be divided by four,
      the useable range of the DDS module is extended to include all bands
      through 15 meters.

      What's next? Add a QSE module, a xmit DBM, some driver amps and WALLA,
      an all band transceiver. Then add an HC908 controller to take care of
      housekeeping, and a DSP module for operation independent of a
      computer; a nice winter project.

      I'll upload a block diagram of the radio.

      Bob WA6UFQ
    • windy10605@juno.com
      Excellent idea, Bob !! That solves a lot of problems caused by pushing the existing parts to their limits. Cliff was talking about doing that at one of the
      Message 2 of 8 , Oct 29, 2005
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        Excellent idea, Bob !! That solves a lot of problems caused by "pushing" the existing parts to their limits. Cliff was talking about doing that at one of the breakfasts but I don't think he's built his SR-40 yet.

        73 Kees K5BCQ
      • Phil Covington
        Hi, This is the path that I started down over a year ago. I was going to basically duplicate the front end and 1st mixer of the K2 along with most of the K2 s
        Message 3 of 8 , Oct 29, 2005
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          Hi,

          This is the path that I started down over a year ago. I was going to
          basically duplicate the front end and 1st mixer of the K2 along with
          most of the K2's PLL. The K2 uses a DAC tuned PLL reference to fill
          in the 5 Khz steps of the PLL. Using the software NCO I could "fill
          in" the PLL steps in software instead of using the DAC tuned reference
          osc. The K2 uses an approx. 5 MHz IF so I was going to have a 20 Mhz
          xtal osc into a divide by four counter to drive the QSD. This
          arrangement would give coverage of 160 - 10m. Unfortuanetly, the more
          you get into a design like this you begin to see that a lot of the
          performance advantages of the QSD are now compromised.

          73 de Phil N8VB


          --- In softrock40@yahoogroups.com, "Bob Hillard" <rhillard@a...> wrote:
          >
          > Consider using the SR40 as an 'IF strip', preceeding it with a MMIC
          > preamp, some bandpass filtering, and a Diode Balanced Mixer
          > using an AMQRP DDS module as the LO source.
          >
          > I have done that very thing with excellent results.
          >
          > I've converted my SR40 to 30 meters, and use 10.138 mHz as the IF
          > center frequency. I've located the LO above the incoming signal in
          > order to reduce image frequencies.
          >
          > The output of the DDS module ranges from 11.9 mHz on 160 meters to
          > 31.6 mHz on 15 meters. By replacing the DDS module with the newer 60
          > mHz model when it becomes available, coverage will be increased to
          > include the 12, 10 and 6 meter bands.
          >
          > With the LO above the incoming signal, the radio operates on the lower
          > sideband. However this can be corrected by software for bands that
          > require USB operation.
          >
          > Band switching is accomplished by programming my DDS VFO controller
          > (http://home.att.net/~wa6ufq/ddscontroller.html) so that each of the
          > eight programmable configurations represent a band. This gives me a
          > bandswitching VFO with wrap-around, up/down frequency control or
          > direct entry, variable VFO steps, sweep and scan functions, and 20
          > memory cells per band.
          >
          > Presently I am switching bandpass filters manually, but that will
          > change with the addition of a band pass filter selection circuit
          > similar to that found in the 'Software Defined Radio for the Masses'
          > article. The bandpass filters are also similar to those found in the
          > QEX article. One of the filters, the one I use most oftenly, covers
          > the 15, 17, and 20 meter bands.
          >
          > By using this scheme, the SR40 circuitry is not taxed by trying to
          > stretch it's operation past the limits of its design since it is
          > always operating on 30 meters. However the radio itself can operate
          > from 160M to 6M, with the exception of 30 meters (the IF frequency).
          > Likewise, since the DDS frequency does not have to be divided by four,
          > the useable range of the DDS module is extended to include all bands
          > through 15 meters.
          >
          > What's next? Add a QSE module, a xmit DBM, some driver amps and WALLA,
          > an all band transceiver. Then add an HC908 controller to take care of
          > housekeeping, and a DSP module for operation independent of a
          > computer; a nice winter project.
          >
          > I'll upload a block diagram of the radio.
          >
          > Bob WA6UFQ
          >
        • jabauzit
          ... ............. That is very good news. A while back I posted a message (No. 484) suggesting the same but did not get any feedback. I suppose 6m is not a
          Message 4 of 8 , Oct 29, 2005
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            --- In softrock40@yahoogroups.com, "Bob Hillard" <rhillard@a...>
            wrote:
            >
            > Consider using the SR40 as an 'IF strip', preceeding it with a MMIC
            > preamp, some bandpass filtering, and a Diode Balanced Mixer
            > using an AMQRP DDS module as the LO source.
            >
            > I have done that very thing with excellent results.
            >
            .............

            That is very good news. A while back I posted a message (No. 484)
            suggesting the same but did not get any feedback. I suppose 6m is not
            a high priority to the readers of this group.

            It would be very interesting if you could post some results regarding
            your experiments.

            Why didn't you use 9MHz as IF frequency? You could also have the 30m
            band?

            Jean-Claude Abauzit, PJ2BVU
          • KD5NWA
            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. They don t have the best IP3 figures
            Message 5 of 8 , Oct 31, 2005
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              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. They don't have the best IP3 figures in the world and can have a lot of noise.

              A Norton amplifier will be low noise and can have a very high IP3, for best results a Norton amplifier configured in a push-pull configuration will add even harmonic cancellation to boot.

              At 09:12 PM 10/28/2005, Bob Hillard wrote:
              Consider using the SR40 as an 'IF strip', preceeding it with a MMIC
              preamp, some bandpass filtering, and a Diode Balanced Mixer
              using an AMQRP DDS module as the LO source.

              I have done that very thing with excellent results.

              I've converted my SR40 to 30 meters, and use 10.138 mHz as the IF
              center frequency. I've located the LO above the incoming signal in
              order to reduce image frequencies.

              The output of the DDS module ranges from 11.9 mHz on 160 meters to
              31.6 mHz on 15 meters. By replacing the DDS module with the newer 60
              mHz model when it becomes available, coverage will be increased to
              include the 12, 10 and 6 meter bands.

              With the LO above the incoming signal, the radio operates on the lower
              sideband. However this can be corrected by software for bands that
              require USB operation.

              Band switching is accomplished by programming my DDS VFO controller
              ( http://home.att.net/~wa6ufq/ddscontroller.html) so that each of the
              eight programmable configurations represent a band. This gives me a
              bandswitching VFO with wrap-around, up/down frequency control or
              direct entry, variable VFO steps, sweep and scan functions, and 20
              memory cells per band.

              Presently I am switching bandpass filters manually, but that will
              change with the addition of a band pass filter selection circuit
              similar to that found in the 'Software Defined Radio for the Masses'
              article. The bandpass filters are also similar to those found in the
              QEX article. One of the filters, the one I use most oftenly, covers
              the 15, 17, and 20 meter bands.

              By using this scheme, the SR40 circuitry is not taxed by trying to
              stretch it's operation past the limits of its design since it is
              always operating on 30 meters. However the radio itself can operate
              from 160M to 6M, with the exception of 30 meters (the IF frequency).
              Likewise, since the DDS frequency does not have to be divided by four,
              the useable range of the DDS module is extended to include all bands
              through 15 meters.
               
              What's next? Add a QSE module, a xmit DBM, some driver amps and WALLA,
              an all band transceiver. Then add an HC908 controller to take care of
              housekeeping, and a DSP module for operation independent of a
              computer; a nice winter project.

              I'll upload a block diagram of the radio.

              Bob WA6UFQ





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              Cecil Bayona
              KD5NWA
              www.qrpradio.com

              I fail to see why doing the same thing over and over and getting the same results every time is insanity: I've almost proved it isn't; only a few more tests now and I'm sure results will differ this time ...

            • Stan
              ... 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
              Message 6 of 8 , Oct 31, 2005
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                --- 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
              • 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 7 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 8 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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