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Some RF Help Please

Discussion in 'Electronic Design' started by AJ, Apr 22, 2007.

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  1. AJ

    AJ Guest

    Hi,

    In 2000 I purchased an AV transmitter through Silicon Chip but it has never
    been much use to me because the range is less than 5 meters. I pulled it
    out of my box the other day with the hope of improving it. When I first
    turned it on and tried to tune it I found that the signal was found all over
    the place from VHFL to the upper UHF and the range hadn't magically improved
    over its dust collecting years.

    It consists of an RF modulator operating on channel 0 or 1. Its output has
    an 82R pulldown to ground and is then coupled to an uPC1688 MMIC Amplifier
    via a 1nF which is coupled to a second stage with the same configuration via
    another 1nF. The input to the second amp also has an 82R to ground
    connected via a 1nF. The output of this amp then connects to the antenna
    via yet another 1nF. The modulator has an output of 80dBuV and the Amps
    have a power gain of 21dB each which should be plenty of power to get
    better range so the problem must be with the coupling. As far as I can tell
    the uPC1688 has a Zin of 50R while the Modulator has a Zout of 75R. They
    have used a 82R on the output so I was thinking the first step might be to
    replace this with a pi matching network to transform the 75R to 50R and this
    might also help block some of the harmonics that might be appearing on all
    the other channels. I could the do the same thing between the output of the
    second amp and the antenna if it helps. I have been looking into pi
    matching networks and have found some on line calculators but they all ask
    for the "Q" and I have no idea as to what that would be in this circuit or
    how to figure it out, isn't it X/R??? Am I even on the right track with a pi
    matching network between stages? I was also thinking that if the uPC1688's
    do have a Zin and Zout of 50R, is there any point to having the 82R to
    ground via the 1nF on the input to the second amp?

    All thoughts and suggestions greatly appreciated.

    Regards,


    AJ
     
  2. budgie

    budgie Guest

    If the signal is "all over the place" then the only way the coupling could be
    the problem is if it is causing instability. First step would be to fix the
    instability before getting concerned about any perceived loss of output power
    resulting from impedance mismatching.
     
  3. Bob

    Bob Guest

    Sounds likes it's faulty and it needs repairing not redesigning.
    Some part of it oscillating ?
    Perhaps it is off frequency and you you get somthing because it
    is overloading your TV at short range?

    Bob
     
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