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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 5:50 pm
I'm trying to find a method or
instrument to measure return loss for a GPS
consist of an LNA that's
dynamically controlled with AGC. The front-end
of the GPS IC is
designed for around -130dBm
and L1 (1.575420Ghz)
The network Analyzer
I'm using outputs a signal at -60dBm. That's the
lowest it can go without
the signal being
overwhelmed by noise. The problem is S11
impedance on the smith chart display
almost as a short. The GPS receiver works ok. To
me it seems obvious what's happening
The network analyzer's relatively large signal
is forcing the LNA into compression.??
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
So with all that said, anyone
have a suggestion as to how I can measure return
loss with using
a signal of -130dBm (up to
-110dBm would probably be ok too).
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
Any help on this
Post subject: Re: How to measure return
loss for LNA's designed for -130dB
Posted: Wed Aug 15, 2012 12:29 pm
Joined: Sat Nov 08,
2008 11:35 pm
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.