How to measure return loss for LNA's designed for -130dBm - RF Cafe Forums

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AJP2012

Post subject: How to measure return loss for LNA's designed for -130dBm

Posted: Sat Aug 11, 2012 6:26 pm

Lieutenant

Joined: Sat Aug 11, 2012 5:50 pm
Posts: 1

Hi there!

I'm trying to find a method or instrument to measure return loss for a GPS front-end which
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
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 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 gain mode.

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).

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!

best regards,
AJP2012

SunshineDesign

Post subject: Re: How to measure return loss for LNA's designed for -130dB

Posted: Wed Aug 15, 2012 12:29 pm

Colonel

Joined: Sat Nov 08, 2008 11:35 pm
Posts: 30
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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Posted 11/12/2012

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