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The flatness of broadband IQ demodulated signals - RF Cafe Forums

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chan
Post subject: The flatness of broadband IQ demodulated signals Posted: Tue Jul 29, 2008 7:52 am

Captain

Joined: Thu Sep 22, 2005 11:09 pm
Posts: 6
hello,

I have a PCB board with poor gain flatness.
How to improve the flatness of broadband IQ demodulated signals?
and no additional group delay deteriorated.

change the coupled capacitance between amplifiers? the bias inductor?
what about a High pass filter added to the circuit?
what's the most effective way?

Thanks!

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IR
Post subject: Posted: Tue Jul 29, 2008 4:23 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello chan,

While it is hard to give a simple answer out of the sleeve, you have to provide some more information about your circuit:

1. Do you use broadband amplifiers in your chain?
2. Are the amplifiers already matched internally or do you use matching networks?
3. Do you use fixed attenuators (pads)?

I also suggest to isolate each gain stage and check its flatness separately, by this method you might be able to find the problematic stage(s).

matching for flatness does not mean that you will have the best return loss, have you designed your gain stages for optimal flatness or for optimal return loss?


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chan
Post subject: Posted: Thu Jul 31, 2008 11:01 am

Captain

Joined: Thu Sep 22, 2005 11:09 pm
Posts: 6

Thanks for your reply,the amplifiers in my circuit are broadband.(SGA 6489,already matched internally)

The circuit was designed by others,the transmission line operating at L band is not 50 Ohm, but 80~95 Ohm.
In this situation, is it possible to improve the flatness?

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IR
Post subject: Posted: Thu Jul 31, 2008 4:36 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
So now you and I know the problem.
The solution is not easy. I would make a new layout.

You can try to match the line impedance to the amplifier's impedance by using LC matching networks, but this again would require layout change if you have a great number of gain blocks along the chain.


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chan
Post subject: Posted: Sun Aug 03, 2008 6:48 am

Captain

Joined: Thu Sep 22, 2005 11:09 pm
Posts: 6
Thanks. I will have a try.

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