Post subject: Power Amplifier Measurements with Network
Analyzer Posted: Mon Oct 06, 2008 12:35 pm
Joined: Mon Apr 10, 2006 12:43 pm
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.
Post subject: Posted: Tue Oct 07, 2008 11:54
Joined: Mon Jun 27, 2005 2:02 pm
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.
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.
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).
subject: Posted: Mon Oct 27, 2008 9:41 am
Joined: Mon Apr 10, 2006 12:43 pm
Thank you both for your responses. Unfortunately, I have not been
able to get an amplifier to retake the measurements.
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
If you want just
S21 of the power amplifier only, there are 2 ways that I would like
1-you can do a thru response cal of the VNA first,
including both attenuators 10dB at input and 50dB at output, with averaging
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.