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Cascading two 50 ohm RF blocks...

Discussion in 'Electronic Basics' started by billcalley, Jan 15, 2006.

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  1. billcalley

    billcalley Guest

    Something has me really stumped: Since cascading two 50 ohm BP
    filters can lead to big trouble*, wouldn't similar problems occur when
    connecting a 50 ohm filter to the input of, let's say, a narrowband
    LNA? Or to the output of a PA? Would the narrowband LNA (or PA)
    disrupt the filter's response, since the filter will now no longer see
    a wideband 50 ohm termination at one of its ports? I know I'm missing
    something here, but I just can't figure out what...

    Thanks,

    Bill

    * Since the BP filters themselves are expecting to see only a very
    wideband 50 ohm termination at both in/out ports in order to function
    as designed -- but when two filters are cascaded the passbands of these
    50 ohm filters are the *only* frequencies that actually and truly see
    50 ohms; the stopbands of each filter will now not see 50 ohms at all,
    as these stopbands are now each looking at the very low return losses
    inherent each other filter's reflective stopbands.
     
  2. jgreimer

    jgreimer Guest

    Yes, that's why it's a good idea to isolate each component (filter, LNA, PA)
    with a resistive pad of 2 or 3 dB. We once had a problem with a 3 W
    exiciter feeding a 40W PA. Both were in spec. individually but oscillated
    out of band when connected together. The problem was that the input to the
    PA, while 50 ohms in band, was essentially open out of band. It reflected
    enough energy to cause the exicter to oscillate. The solution was to pad
    the front end of the PA so the exciter would see 50 ohms at all frequencies
    and then add enough gain to complensate for the loss of the pad.

    The exception would be the output of the PA. Our PAs were required to be
    stable into both a short and an open attached to the other side of a
    stretchline at all phase angles. We never had a problem putting filters or
    cavities on the outputs of the PAs.
     
  3. billcalley

    billcalley Guest

    Thanks jgreimer for that info!

    Bill
     
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