How to measure return loss for LNA's designed for -130dBm - RF Cafe
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Post subject: How to measure return loss for LNA's designed for -130dBm
Posted: Sat Aug 11, 2012 6:26 pm
Joined: Sat Aug 11, 2012
I'm trying to find a method or
instrument to measure return loss for a GPS front-end
consist of an LNA that's dynamically controlled
with AGC. The front-end of the GPS IC is
for around -130dBm and L1 (1.575420Ghz)
network Analyzer I'm using outputs a signal at -60dBm.
That's the lowest it can go without
being overwhelmed by noise. The problem is S11 impedance
on the smith chart display
appears almost as
a short. The GPS receiver works ok. To me it seems
obvious what's happening
here. The network analyzer's
relatively large signal is forcing the LNA into
Also the strong signal is
forcing the the AGC to switch the receiver into
its low gain mode.
The receiver upon start up
monitors the signal strength and chooses either
low or high gain
modes. I want to measure the
return loss in high gain mode.
So with all
that said, anyone have a suggestion as to how I
can measure return loss with using
of -130dBm (up to -110dBm would probably be ok too).
Are there any vector network analyzers out
there that can do this? Is a special test set needed?
How about using a typical spectrum analyzer. I just
need return loss, not necessarily the phase information.
Any help on this appreciated!
Post subject: Re: How to measure return loss for LNA's designed for
Posted: Wed Aug 15, 2012 12:29 pm
Joined: Sat Nov 08, 2008
Location: Ramona, CA
There are no Vector Analyzers that have the low
end dynamic range sensitivity to make this measurement.
I suggest an Agilent PXA that can measure signals
down to -174dBm with special processing and calibration
to calibrate out ambient thermal noise. Seeing as
you want to measure the LNA at it's input operating
point of -130dBm, you need to measure down to -150dBm
plus your coupler loss to make the 20dB return loss
measurement. I suggest using a coupler instead of
a bridge. Also you will have to measure this in
a shieled RF box to remove all unwanted noise and
signals that might interfere with the signal being
measured. R&S may also have a Spec An than can
measure down that low. Good luck with it.
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