Measuring LDMOS complex impedance with VNA

[PaulHolland]

Hi,

I have a LDMOS transistor and would like to know its complex impedance in a frequency range not specified by the vendor. I would like to measure it so that I can design a wideband matching network. How do I measure the complex impedance ?.

I wanted to do the following but don't know if this is the correct way:

I wanted to build a simple narrow band matching network to 50 Ohm for input and output. Match (tune) for optimum IP3 (that is needed due to complex modulation form OFDM). Than remove the LDMOS, open and close and cal. the VNA. Then put the LDMOS back and measure the complex input and output directly is a working circuit driven at the proper operating point and bias. And this for all freq. values ???... Does this work ???..

The Working Freq. is between 175 MHz and 450 MHz !!..

Thanks,

Paul.

 

[nandopg]

The problem with your set up is that the VNA doesn't have enough level to drive the LDMOS into the right power level so that you will have to use a driver between the DUT and the VNA for properly drive the DUT.
What happens is when you tune the fixture's narrow band network, you will be matching the DUT to the driver's output impedance which is not a standard 50Ohms. You can overcame this problem attaching an isolator between the driver output and the test fixture input so that you can guarantee the DUT is being matched to 50Ohms.
Another correction I would make is that once you have the fixture's input and output network tuned, you have to take the DUT off the fixture and the VNA should to be connected in place of the DUT. You can use a small piece of coaxial cable to connect the VNA to the fixture's input and output (as seen by the DUT), perform a shift in the phase plane to account for this small piece of coaxial and then measure the impedance as seen by the DUT's input and output. These are the conjugate of the impedances you will use to synthesize the final input matching and output transform network.
Yes, this approach works very well.

NandoPG

 

[PaulHolland]

Hi NadoPG,

Why do I have to drive the test fixture then with the VNA... I could also use a spectum analyzer and power meter and two signal generators to measure IP3...

BTW: I can calibrate the VNA in circuit.. make a short and open etc... What about that. I make a LDMOS enclosure with two SMA connectors (very short wires etc...)

BTW:.. How do I have to do this for a microwave transistor at 5 GHz... small cables in place of the DUT are not really an option ...

thanks for the tips..

PAul.

 

[nandopg]

1- I had assumed that the +10dBm output level from the VNA it is not enough to drive the LDMOS under test. If it is the case you will have to use a driver to the LDMOS. If that is not the case you are right you can drive the DUT using only the VNA and the isolator is not necessary.

2- The port phase extension to account for small launchers is effective for any frequency. However as you move to higher frequencies it is preferable to use mechanical tuners at input and output of the test fixture. You might want to have a look at https://www.focus-microwaves.com/ to see some manual tuners options.

3- Also for higher frequencies it is mandatory that you have a perfect characterization of the test fixture for de-embedding purpose.

4- Also you might want to look over the links below, where some app notes regarding loadpull, test fixtures and device characterization were posted.





NandoPG