How to measure the s-parameter of the common-gate LNA using VNA?

[chunlee]

Hi,

I want to measure the s-parameter of the common-gate LNA using VNA. the input port is at source of the mosfet and the output is at the drain of the mosfet.

my question is: can i measure the s-parameter of the LNA directly without connect a large Cin between input port of the LNA and VNA? VNA will draw DC current from LNA or not?

thanks.....

 

[pit]

Re: VNA measurement

Hello!

If you have a DC offset at your signals you will have to use a DC blocker (capacitor) between your circuit and your VNA. Bias tees are perfectly for that (if they include a capacitor), but of course you can use an appropiate capacitor, too.

Otherwise the 50 Ohm in the VNA will short the DC offset to ground.


CU

 

[chunlee]

Re: VNA measurement

thanks, pit.

Can u pls explain what DC offset means? I don't do any RF measurement before, so i have a lot of questions about RF measurement.

thanks again.

 

[rfmw]

Re: VNA measurement

What he meant is that if you have a DC voltage present at your I/O ports, which you surely do for DC operating point of the transistor.

Looking at my VNA ports, it says that it tolerates +-40V of DC voltage, so a capacitor must be already inside the VNA. What about yours?

 

[pit]

Re: VNA measurement

Can u pls explain what DC offset means? I don't do any RF measurement before, so i have a lot of questions about RF measurement.

DC offset means that you have a DC voltage with a RF voltage superimposed to it. This is the case when you omit coupling capacitors at your transistor stage as the bias and supply voltage are superimposed to the RF.

If your measuring equipment tolerates a DC offset you don't have to do anything than to stay inside the DC offset specs of your equipment. You can measure with a resistor meter between center pin a ground, if you see a clear 50 Ohm resistance your equipment is definitely not capable of handling DC offsets. Otherwise a look into the manual can clarify things regarding DC offsets at the connections.

CU

 

[chunlee]

Re: VNA measurement

Hi pit and rfmw,

The E8362B analyzer has two 50 ohm, 3.5 mm (m) test ports.

I found the above sentence from E8362B VNA datasheet from Agilent. Is it that measure for LNA measurement, i need connect a large capacitor between the input port of LNA and VNA?

Beside that, still have other solutions? I'll do on-wafer measurement.

Thanks...


Best regards,
Chun Lee

 

[pit]

Re: VNA measurement

The E8362B analyzer has two 50 ohm, 3.5 mm (m) test ports.

That doesn't help much, you will have to check for DC offset capability of the ports. Check user manual for details.

Beside that, still have other solutions? I'll do on-wafer measurement.

Apart from a blocking capacitor you can use a bias tee with an internal DC blocking capacitor. These devices are ready to use and very common in DC biased RF measurement setups. They are normally used to superimpose a DC supply voltage to an RF voltage, but you can also use a bias tee to just remove the DC offset.

Look here for readily available devices (bias tees and DC blocks):

**broken link removed**

There are lot of other manufacturers available, check for connector compatibility. "3.5 mm male" could refer to SMA style connectors (possibly K connectors, they are mechanically compatible but with better performance).


CU

 

[sergio mariotti]

Re: VNA measurement

Dear Chun Lee,
is the VNA operator manual available?
I don't know that new Agilent VNA, but historically Agilent VNA (eg 8510) is DC isolated and the two bias T are enclosed into spar test set.

If you are in doubt, eventually before buy expensive bias T, look at the DC resistance of the ports.