ssb demodulation

Discussion in 'Wireless & RF Design' started by PRS, Feb 5, 2010.

  1. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    As I understand it -- and I may be wrong -- in order to demodulate ssb you mix a sine wave 3 to 5 kHz away from the last IF amplifier to get an audio signal output.

    I have a superheterodyne receiver with a 455 kHz stage. From there I am using an active mixer as shown. So far I've gotten only noise out of it. Where am I going wrong?
     
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  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    How STRONG a signal are you injecting into the L.O. port? It should be at least 10 db stronger than the i.f. signal.

    eric
     
  3. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    One thng that may work better as well, is to CENTER your local oscillator right at 455 kc. The IF bandwidth is probably around 10 KHz or so, so you can actually select the sideband by tweaking the main tuning. Using a 5kc offset (or so) is more applicable to a more "hard core" SSB optimized I.F. This is standard practice in "passband tuining" receivers. However for a sloppy 455 kc if, I'd center the BFO right there.

    eric
     
  4. KL7AJ

    AAC Fanatic!

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  5. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    I'm not sure what the power level is, but the signal into the emitter is provided by an HP 3310A function generator capable of 30 volts peak to peak and having a 50 ohm source. I haven't measured the input impedance but I'm guessing it's rather small and so requires a strong signal to drive it. But this should be no problem for the function generator. ?
     
  6. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    I'll try this once I am satisfied everything is working properly. I'll test this active mixer tomorrow for its AC response. The only thing I'm sure of now is that the DC voltages came out as per calculation.
     
  7. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    I have tried this supposed ssb demodulation scheme for several days, using various voltages and changing the bfo frequency. It just doesn't work. Is this approach wrong in concept? Shall I try the same type of thing with a dual gate MOSFET?
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Here is an example of a SSB demodulator with a dual gate mosfet:
    http://www.dl5neg.de/thesis/ssb_demo.htm

    The example is made for 8Mhz , but this can be changed to your MF frequency.

    Greetings,
    Bertus
     
  9. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Thanks. I copied a circuit using a dual gate MOSFET and it didn't work. I think the problem was that I used an Enhancement type transistor instead of a Depleation type and the circuit I copied used self-bias. Back to the drawing board. Next time -- after some study, of course -- I may get it to work. ;)
     
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