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Too much ripple on a UHF amplifier using JFET transistor

Discussion in 'Radio and Wireless' started by JohnJohnCat, Jan 18, 2019.

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  1. JohnJohnCat


    Jan 10, 2019
    Hello all.

    I have implemented for first time an amplifier. It seems to work quite well, under my specification.
    But I am facing too much ripple.

    It is the first (of two) stage of a 25dBm power amplifier with 787MHz cut-off frequency.
    The gain seems to work as expected but still pending to measure the output level and distorsion.
    Anyhow it introduces (I think) too much ripple that I would like to cancel before continuing the project.

    Attached the PCB detail and the frequency response from a scalar network analyzer.
    Note the copper short-circuit was not there for the measurement.

    Do you have any advise for me?

    Thank you.

    Attached Files:

    Last edited by a moderator: Jan 18, 2019
  2. davenn

    davenn Moderator

    Sep 5, 2009

    The ripple would probably be coming from the PSU
    Have you tried the amplifier on batteries ?
    duke37 likes this.
  3. JohnJohnCat


    Jan 10, 2019
    Hello Davenn.

    I think, as you say, it comes from the PSU. But not because of it; Just because a radiated interference couples on the DC cables.
    I say this because even having the PSU off (but connected), the intererence was there. At the moment of disconnecting the DC bananas the interference disapears. I think it could be the reason.

    In attachment you can see the spectrum analyzer RF output. It is also present in the RF input, but atenuated.

    I will check now how to remove it and hope the S21 paramenter becomes interesting.

    Thank you!!
    Best regards.

    Attached Files:

    davenn likes this.
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