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Issues with Return Loss at Return Port

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ElectronicBrain

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Hi,

I have designed a PCB for a radar system that is supposed to function at 5.8GHz. The board has been manufactured and populated; however, when I measure the return loss at the return port, I only get 1 or 2dB. Part of the reason appears to be that the capacitors that I used on the RF line (that are used to match the input of the low-noise amplifiers) are too large. However, replacing them with smaller components (without changing the footprint size) has not produced a much better return loss.

As well, I am aware that the SMA center pin pad is much larger than the line; this has been resolved (at least, according to CST Studio) by removing the ground plane and substrate from underneath the center pin pad.

Are there any issues with this section that I am missing that would cause such a terrible return loss?

Thanks.
 

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hi,
Amazing ! you mean -1 or -2 dB for the RL , I suppose ? If you have a doubt because of the width of the line, you can use a lambda/2 line long and so no matter the width. So, if your amplifier is matched to 50 ohms, the impedance continuity is conserved. Perhaps :
- I guess that your amplifier is not perfectly matched ?
- Your VNA is not well calibrated ?
- Your sma connector has problem ?
- ...
keep me informed.
bye
 

i agree with eraste. if your line length is less then lambda/2, the characteristic impedance of the line becomes insignificant. RL of -1dB or -2dB is very bad!

if you use large capacitor shunt to ground close to PA output , it will affect the S11 of output match. you are right in here! try to generate alternative matching network without cap shunt to ground close to PA output.

i dont think SMA pad size affects your S11!
 

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