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Power Amplifier Measurements with Network Analyzer - RF Cafe Forums

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Post subject: Power Amplifier Measurements with Network Analyzer Posted: Mon Oct 06, 2008 12:35 pm


Joined: Mon Apr 10, 2006 12:43 pm

Posts: 25

Good day.

I measured a 50 W power amplifier using a network analyzer. The test set-up I used was

Port 1 -> 10 dB Attenuator -> 50 W PA -> 50 dB Attenuator -> Port 2

I got a value of -1.68 dB for S21 at 915 MHz. I set the power output of the network analyzer at -15 dBm (so the input power to the PA would be at -25 dBm).

At network analyzer power settings of - 10 dBm and -20 dBm I measured -0.6 dB and -2.9 dB for S21 respectively.

What would S21 for just the PA be?

According to the data sheet for the PA it has a small signal gain of 48 dB.

Thank you




Post subject: Posted: Tue Oct 07, 2008 11:54 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello Michael,

The results you obtain certainly don't match with the value stated in the PA specification. According to the results you get, the gain is 10dB higher.

Your setup is OK. To verify the small signal gain, I would do the following:

Connect signal generator to the PA's input (Which allows you to drive the PA with variable power levels) and connect the PA's output through 50dB (can also be lower) attenuator to a power meter. Then you can have an additional result to compare with the VNA result.

In modern VNA's, there is a calibration feature called ''Through Update'', which allows to calibrate the VNA (keeping the same frequency settings) in case you change the power level from the VNA. This reduces the failure caused by changing the VNA's internal attenuator for obtaining different power level.



Post subject: Posted: Thu Oct 09, 2008 2:55 pm

I think you need to clarify how you calibrated your VNA and the Class of amplifier that you are measuring.

If you calibrated the VNA this way:

Port 1 -> 10 dB Attenuator -> 50 dB Attenuator -> Port 2

And if then you are reading:

-2.9dB with Pin=-30

-1.68dB with Pin=-25

-0.6dB with Pin=-20

It is because you have a class C amplifier, you will need to drive the amplifier with more power to meas its gain.

If you want a linear amplifier, AB or A perhaps you have the bias point of the amplifier too low, so it is responding as a class C amplifier, remember that for linear operation, the bias point is selected by looking at the drain current and not to the gate voltage. If you have a too low bias drain current, then increase the gate voltage to find the correct drain current. And then you will have small signal gain (remember that a class C amplifier has no gain at small signal).



Post subject: Posted: Mon Oct 27, 2008 9:41 am


Joined: Mon Apr 10, 2006 12:43 pm

Posts: 25

Good day.

Thank you both for your responses. Unfortunately, I have not been able to get an amplifier to retake the measurements.

Perhaps someone could provide me with a generic answer for my problem -

1) I have a high power linear (Class A) amplifier (P1dB is 50 Watts)

2) In order to protect the network analyzer I need to put a 10 dB attenuator between Port 1 and the amplifier. Also I need to put a 50 dB attenuator between the amplifier and Port 2.

3) I measure S21 of this set-up.

4) What do I need to do in order to get just S21 of the amplifier?

Thank you for your time and help.




Post subject: Posted: Mon Nov 10, 2008 12:10 am


Joined: Tue Mar 15, 2005 11:43 pm

Posts: 65

Location: Singapore


If you want just S21 of the power amplifier only, there are 2 ways that I would like to suggest:

1-you can do a thru response cal of the VNA first, including both attenuators 10dB at input and 50dB at output, with averaging feature on.

2- If the response somehow looks to "noisy" you can take out both attenuators and do a recal with thru response again and re-measure it with the attenuators.

In your calculation, you need to add your S21 value from VNA by 60dB. I think it's your S21 gain.

Or if you want to verify, you can opt to use signal source and power meter. But be careful on the power meter dynamic range.

I hope it helps.


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