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Better driver, worse system performance??? - RF Cafe Forums
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lna
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Post subject: Better driver, worse system performance???
Posted: Thu Jun 03, 2010 6:49 pm
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Joined: Mon May 04, 2009
6:47 pm Posts: 22 |
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I'm looking for help for a PA design.
My project is a balanced 1W(average) OFDM PA
at 600MHz, droven by a half watt class A driver.
The original design is very close to system requirements,
except marginal fail EVM, which I think caused by
the driver. So I found a drop in replacement driver
and evaluated it. With new match circuits, the new
driver alone performancs very good--2dB higher P1dB
and 1~2dB better EVM.
What confused me is
that system EVM drop by 2~6dB!!
I don't
think the driver change would impact the final stage
because there is a 50ohm splitter between them.
Any idea???? Please.
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IR |
Post subject: Re: Better driver, worse system performance???
Posted: Tue Jun 08, 2010 3:06 pm
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Joined: Mon Jun 27, 2005
2:02 pm Posts: 373 Location: Germany
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I think that the problem is not within the driver.
The new driver which you tested proves that
point.
If the problem is linearity - which
seems to be based on the description you showed,
then the problem lies within the balanced amplifier.
The significant stage for cascaded IP3 calculation
is the last stage of the Tx chain.
(Same
as the first stage for cascaded NF calculation).
Try to focus on the balanced amplifier and
optimize it.
In many data sheets of power
transistors, there are measured results for different
signals.
--> Have you looked on what are
the expected performance of the transistor (within
the balanced amplifier) for the type of signal you
are using (i.e. modulation type, filter applied,
clipping etc).
It might be the case that
you need to consider a final stage with more output
power.
_________________ Best regards,
-
IR
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Posted 11/12/2012
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