how to measure the complex impedance of a RF source!

[zhonghua_li]

complex impedance

dear all:
I want do conjugate matching to the source.so I must get the complex impedance of the RF source!
thanks for you help!

 

[vfone]

measuring complex impedance

If the output power of the source is in the VNA input power range, you can use this device to measure the impedance. Just be careful to the input power at VNA input.

 

[FvM]

how to measure impedance

If the source is a PA with non-linear output stage, it has to be operated at nominal output level for the measurement. If the source's matched impedance is very different from VNA (50 ohm), an additional systematic error would be caused.

 

[zhonghua_li]

how to measure complex impedance

can not measure the impedance use a VNA.
because, the method of the VNA isutput a signal,and then measure the reflectiong of the load

 

[RF-OM]

measuring impedance at rf

I think that it is easy to measure with VNA. What is important it is right calibration. If PA output signal is out of VNA input power range use an attenuator of corresponding power range and calibrate VNA with this attenuator. In this case reducing of the accuracy will be minimized and depends only on changing the VNA directivity. Save the S-parameter file if you need many points and or you are planning to simulate this amplifier later. Do not use too much points for this measurements, usually 100 or 200 is more than enough.

 

[FvM]

impedance measurement with vna

can not measure the impedance use a VNA

When you operate the source in CW mode, you'll get interferences in VNA measurements for a small band around the carrier frequency. Outside of this band, the results should be correct.

Of course the VNA must not be overloaded, an attenuator may be necessary as suggested. The technique may not work with some instruments, but is generally used.

Alternatively, you can manually adjust a matching network (starting from an estimation of source impedance) to achieve power matching. As the matching network can contain resistors in this case, the adjustment is rather easy.

 

[BigBoss]

measure complex impedance

A sliding tuner that operates in the frequency of interest will supply complex conjugate of the source impedance ( when you connect a directional coupler between them..) while reflected wave is zero..
That's the impedance which are you looking for..

 

[biff44]

measurement source impedance

The argument here seems to be skirting the basic question: do you need large-signal or small-signal measurements?

It depends. If it is a low power source, with buffer amp after the oscillator, probably a small-signal measurement suffices. Just be careful, as stated, to not overload the VNA front end! Obviously, just putting a 10+ db attenuator between your source and the VNA is not the way to go if you are trying to make accurate s11 measurements.

If you are measuring an oscillator without a buffer amp (very sensitive to load pull), or an output amp that is putting out a lot of power (somewhat sensitive to load pull), using a small-signal measurement will give big errors. It would get you in the ballpark, however, so that you could tune up the result. A formal large signal meaurement, such as a load pull measurement, would be better in this case.

In SOME types of oscillators (GUNN DIODE, for instance), I do not think you could even make the measurement at all in small signal, as the oscillator might jump into the wrong mode during the measurment, and work in a different mode in the final matched circuit!

 

[toonafishy]

small signal impedance rf power

Load pulling is the way to go.

 

[zhonghua_li]

how to measure rf impedance

I think the "Load pulling" is the only way.

 

[yashiro32]

measure with vna

What is "Load Pulling" ?