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Measurements >> RF Measurements >> LNA voltage gain measurement method
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Message started by wccheng on Feb 15^th, 2007, 6:20am

Title: LNA voltage gain measurement method
Post by wccheng on Feb 15^th, 2007, 6:20am

Dear all,

I know I could use Network analyzer to measure power gain (S21) of the LNA. If I want to measure the voltage gain of the LNA, how could I do it? Actually, I am using AC analysis in the simulation to see the voltage gain of the LNA. Is it correct?

Thanks

wccheng

Title: Re: LNA voltage gain measurement method
Post by mg777 on Feb 18^th, 2007, 11:07pm

You can easily convert from S to Y or Z, and obtain the voltage gain for given source and load impedances. If you are new to a network analyzer you may want to consult Agilent's app notes on de-embedding the signal connections for board as well as on-wafer measurements.

In simulation AC analysis should be adequate, but expecting the simulation to match measurement depends on how good your modeling is. Especially the supply bypass.

M.G.Rajan
www.eecalc.com

Title: Re: LNA voltage gain measurement method
Post by aaron_do on Feb 19^th, 2007, 7:29am

Unfortunately, using a network analyzer also means you have a 50 ohm load. As far as a i know you need to design a buffer with known gain, high input impedance and 50 ohm output impedance. At RF that's pretty tough to do.

-Aaron

Title: Re: LNA voltage gain measurement method
Post by mg777 on Feb 20^th, 2007, 10:20am

If the cal is good the 50 Ohm load can be de-embedded, provided the amplifier is stable for the VNA set-up. That's the measurement part of it - the design is a different issue.

What is the sensitivity to an s22 cal error of δ? Assuming δ is real, then Z_o is off by a fraction 2δ. The corresponding error in voltage gain would be (1 + 2.δ)(1 - 2.δ.Z_o/R_L). If the s22 cal is good for 30 dB (δ=0.03) the error in voltage gain for R_L=250Ω is below 2%. A 30 dB on-wafer cal can be achieved out to 10 GHz even with an archaic SOLT substrate once the cables have warmed up.

M.G.Rajan
www.eecalc.com

Title: Re: LNA voltage gain measurement method
Post by aaron_do on Feb 20^th, 2007, 10:21pm

That was an excellent first pass analysis. For the actual voltage gain you'd have to find Zout using S22 and use the resistor divider concept to get the voltage gain.

I'm wondering though if you want to do noise measurements, how well de-embedded will the 50 ohm output port be after cal?