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Too much ripple on a UHF amplifier using JFET transistor

JohnJohnCat

Jan 10, 2019
6
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Jan 10, 2019
Messages
6
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.
Br.
 

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davenn

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Sep 5, 2009
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Anyhow it introduces (I think) too much ripple that I would like to cancel before continuing the project.


The ripple would probably be coming from the PSU
Have you tried the amplifier on batteries ?
 

JohnJohnCat

Jan 10, 2019
6
Joined
Jan 10, 2019
Messages
6
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.
 

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