How to measure the BW of a LNA

[Robismyname]

My question is two fold.

1) I was asked to measure the BW of a LNA (RF2472G). Is this done with a VNA. I have an obsolete model in my lab Agilent/HP 8720ES.

2) Exactly how would you measure the BW of the LNA in general? Using the VNA? Is this a S11, S12, S22, S21 parameter? please help.

 

[yungman]

I believe it's the -3db points on both side of the S21. This is power wave, it's the BW of the two -3db power point. I am not familiar with the VNA you have, but all you have to do is to plot the S21 in the rectangular form.

 

[the_emi_guy]

As youngman stated, bandwidth would be the S21 measurement. However, be aware that RF LNAs generally do not use the 3dB bandwidth as a metric as you would with an op amp. Their frequency range is an indication of the range over which they are usable, and this is generally well beyond its 3dB rolloff point.
For example, your device's advertised range is DC - 6GHz. Its gain at 6GHz is around 10dB and the low freq gain is over 20dB.

 

[yungman]

True, but usually you are measuring the whole circuit including the input and output matching. Or else you just read off the S-parameter file and get the answer, no need for VNA.

In my experience, we always limit the low end in the input/output matching network so you don't get more gain in the frequency range out of the frequency of interest. There's no reason for passing the lower frequency as you don't want to have unwanted frequency driving the LNA into saturation and affect the gain at the frequency of interest. At least that's what I did, making it a bandpass as much as possible.

 

[the_emi_guy]

You are right, if this amp is to be used in a narrowband application it will be part of an overall circuit that will have a well defined 3dB bandwidth governed by some external network. (I actually use these parts in broadband applications where I need gain from MHz to GHz). Maybe the OP can chime in, it sounded like he was trying to characterize the part by itself (and, as you mentioned, that data is freely given out by manufacturer).