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please help me with a filter circuit with approximately 4Kz bandwidth.

Discussion in 'General Electronics' started by abhilsah j, Jun 13, 2004.

  1. abhilsah j

    abhilsah j Guest

    I am trying to develop an intercom that sends out audio signals of 4Kz
    bandwidth (three different channels through the same cable/wire.
    (Prior to making that, I am actually trying to simulate that on a
    computer so as to reduce the cost). I have double side band
    suppressed carrier (DSSC) band modulator circuit (MC/LC 1496
    modulator), a summer circuit and single side band suppressed carrier
    (SSSC) demodulator circuit. But I don’t have the filter circuit
    that converts double side band signal to single side band and also I
    also don’t have the circuit for filtering out different channels
    from the wire. (Since it’s in a simulator, I have quite wide
    range of carrier frequencies by simply changing the parameters).
    Would you please help me with a filter circuit (4Kz) bandwidth and
    please tell me if i am missing anything.

    thank you.
     
  2. Jim Thompson

    Jim Thompson Guest

    SSB Mixer = Carrier at 0° mixed with modulation at 0°
    ADDED to Carrier at 90° mixed with modulation at 90°

    Sign of sum picks sideband.

    (I just designed one for a WiFi chip with selectable sidebands at
    594MHz or 462MHz;-)

    ...Jim Thompson
     
  3. Tim Wescott

    Tim Wescott Guest

    You don't need to convert the DSB signals to SSB: Just demodulate one
    sideband of the DSB as SSB and ignore the other.

    With a SSB demodulator you can just filter the demodulated signal from 0
    to 4kHz, this will reject the other signals on the wire.
     
  4. Joerg

    Joerg Guest

    Jim's scheme is one way to do it. However, performing a 90 degree phase shift on an audio signal that ranges from a few hundred Hertz to 4KHz is not trivial. The ARRL handbook, at least older ones, show how to do that with a few dozen R and C. It is called the "phase method" of generating SSB. For a good sideband rejection of 40dB or so you need 1% or 2% parts and with caps that gets expensive. The other option would be a DSP but that becomes a bit elaborate here.

    You could also directly filter the DSB signals using switched capacitor filters to pick out a sideband. One steep low pass plus a steep highpass per channel. Gets expensive as well, though.

    Why not just take three carrier frequencies and do an FM modulation? The receivers would just lock their PLL onto whichever carrier you select and the audio comes right back.

    Regards, Joerg
     
  5. **** Post for FREE via your newsreader at post.usenet.com ****

    Hi Joerg, you are 'analog', I just found some nice way to make SSB digital,
    remember the Weaver method? Never used is as an amateur, but look at this site:
    http://www.microtelecom.it/ssbdex/ssbdex-e.htm
    And 70 dB or better sideband surpression without any adjustment is not bad at all!
    JP

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  6. Joerg

    Joerg Guest

    Hi Jan,

    Yes, with a DSP or in this case PGA you can do it. I am pretty certain
    analog is out by now for that kind of stuff. This article is nicely
    documented. I like the Italian in the drawings. Everytime I get a
    schematic from there and it says "alimentazione" for power supply I am
    getting hungry :).

    I'd still do FM or some digital scheme here because it's so much
    simpler. And usually better.

    Regards, Joerg
     
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