How to measure the output impedance of an active device?

measure output impedance pa network analyzer

I have vector network analyzer, which can measure input impedance.


I just put device on port 1, and measure the S11, then can get Zin from S11.


For there is no output power from the device in port, and the measured power is just the reflect power, which is a portion of port1 output from the analyzer.


But some device can out put power, such as PA, LNA, and etc.

How to measure the output impedance of them?

Thank U!

measure the output impedance s parameter

Usually the VNA is only sensitive to signal generated by the the VNA itself. This is due the fact the VNA working is coherent.
Opposite, the SNA is blinded by any other signals.

calculate output impedance

Hmm I doubt you can measure the input impedance from S11 only. You have to measure the 4 parameters of the two port network. After you do so, you can find the input and output impedances given the source and load impedances.

actv-2 output impedance

sergio mariotti said:

Usually the VNA is only sensitive to signal generated by the the VNA itself. This is due the fact the VNA working is coherent.
Opposite, the SNA is blinded by any other signals.

Click to expand...

Do U mean I can just test the S11 when the PA output port is conected to VNA or SNA port1 ?

If only the PA output power is not too high to damage the VNA or SNA.

Added after 15 minutes:

e.Horus said:

Hmm I doubt you can measure the input impedance from S11 only. You have to measure the 4 parameters of the two port network. After you do so, you can find the input and output impedances given the source and load impedances.

Click to expand...

I think S12 is so little to be considered as 0.

measuring impedance of high power sources

He mean s22 contents what you need.

using automatic tuners to measure impedance

stober said:

He mean s22 contents what you need.

Click to expand...

Thank U.

VNA port1----> PA in-------->PA out------>VNA port2 (My VNA only has 2 ports)

Then, the S22 is just I want to transform to get Zout.

Am I right?

Added after 6 minutes:

But, some times, the DUT (device under test) is not PA or LNA, which frequency from input to output R the same.

When the freq R not the same between input and output, what should I do ?

What I can do is just :

DUT out-------->VNA port1

So S11 is I want to transform to Zout.

how to calculate impedance power meter

Hi,

You can use a slotted line tuner and a power meter at the output of your frequency translating DUT, and tune it for maximum output power. You have to do this at each frequency point which is of interest. After that you can connect the tuner to the VNA and measure it's impedance ( for each mechanical position@frequency ). Your DUT impedance will be the conjugate of the tuner impedance. This is the way how the Zin and Zout of high power devices are measured. The mechanical repeteability of the tuners has to be high, and these kind of tuners are not cheap at all. Usually the measurements are made with so called automated tuners, with full automatic setup. You can check the links below (Maury and Focus are manufacturing these kind of tuners), there are a lot of aplication notes and technical articles:

hxxp://www.focus-microwaves.com/Literature/Literature.htm (you have to register for password)
hxxp://www.maurymw.com/support/appnoteslibrary/appnoteslib.htm

rgds,
Al

measure output impedance higher power

Thank U.


I have never see this equipment.


If only I have it!

input and output impedance

I am not sure I am in the discussion.

Look at the smith chart on the analyzer when S11 is on
then you read off the complex impedence, it has a real and imiginary component

output impedance s11

If the output power can damage your VNA, how can U read S11?

how to measure the output impedance of a network

Hi,

For a passive device you can calculate your input and output impedance using the measured S parameters (from your network analyzer). To calculate the input impedance you will need your input and output reflection coefficients. For a properly matched output your input reflected coefficient will be S11, and Zin will be:
Gammain=S11

Zin=Zo(1+Gammain)/(1-Gammain)

If your device is not properly matched at the output then you will need to take into account the contribution of the output reflected coefficient that is, your Gammain is not longer equal to S11. you well need to calculated a new Gammain as follow:

Gammain=S11+ [(S12*S21gammaout)/(1-S11*Gammaout)]

For an active device (small signal applications ONLY) you can calculate your input impedance as follow:

- Only the device under (with out input and output matching networks): You only need your measured S parameters and calculated your input and output impedance as above.
- Device under test with input and output matching networks: You need to calculated the input reflected coefficient that your source see, for example: if you want to math your input impedance to a 50 ohms of your system you will need and STUB to math the complex part and a transmition line to match the real part of it. then your input impedance that the source see will be: the impedance of your transmition line +- j *impedance of your Stub (open or shorted).

For a power device you can not measure your input and output impedance using your vector network analyzer. You need an equipment that measure the S parameters in GREAT signal. Another method is to used load pull and an spectrum analyzer.

I hope this information help you.

Added after 8 minutes:

Yellotown,

As I mentioned above,
The VNA that you are using if for SMALL SIGNAL applications (Small Signal S parameters). If you measure the S parameter in great signal you will need a NON LINEAR VNA or used LOAD PULL is more cheap.

greatings.

no output power

U R great !

measuring s22 with active output

For more information you can consult the Guillermo Gonzalez booK. "Microwave transistors Amplifiers"

regards.