Hi Huriye,

Unfortunately I don't have an easy answer for you. I've attempted noise figure evaluations in a number of ways, and generally like to use a VNA for my test instrument when I can. The technique that R&S proposes in this app note can work, but there are a lot of caveats there. Most people prefer the noise source approach and consider that the "standard" for measurement, although I've heard of cases where that approach can fail too. I think the challenge for the most part with the VNA method is to get your signal conditioning into the range where what you're evaluating is the capability of your amplifier and not the other components used to do the test. That's not always easy and I suspect that's what's happening here.

Since I don't do this regularly myself, and it's far too complicated of a process to know for sure which way you went and what might have gone wrong, the best I can do is suggest you look at Section 1.6 "Validity of the Noise Calibration" in that app note and see if anything there helps point you towards what's going wrong. Beyond that, I'd give your local R&S Field Application Engineer a call and have them come show you how to make it work. Worst case they can probably guide you through it remotely.

Best of luck whichever way you go!

Michael

Hi Michael Foegelle ,

Thank you for getting back to me and your time.

Huriye, the "negative NF" problem can occur if the gain on the LNA is not constant, with noise source being on or off. See this interesting article:

http://ko4bb.com/Test_Equipment/Noise_Source/

Most very low noise amplifiers do not have their lowest noise figure when their input is perfectly matched to 50 ohms, or at their point of maximum gain. So it's not unusual for a good LNA to have an input VSWR somewhat different from 1.0:1. Since the attenuation due to component losses of a matching network could actually cause more loss than would be gained by a better match, you simply have to live with it.

Also, the gain of the LNA is strongly affected by the impedance presented at it's input.

The calculations performed above assume that the gain of the LNA is constant when the noise source is turned ON and OFF.

That's a big assumption, because if the gain changes by X dB, that will affect the noise power output by the same number, and that will affect the calculated NF by the same X dB.

This has caused some people to obtain negative noise figure at the end of the calculation. This occurs when the gain of the LNA increases so much when the noise source is off that the noise power output of the LNA is actually greater when the noise source is off.

Therefore it is essential to make sure that the impedance that drives the LNA is constant when the noise source is on or off.

This is usually obtained by placing sufficient attenuation between the noise source and the LNA. Of course, that only works if the resulting noise is above that necessary to be above the LNA's intrinsic noise.

Could this describe your situation?

Hi Albert Manfredi ,

First of all thank you for your time and answer.

Unfortunately, this situation does not depict my case for the NF measurement of my LNA. The setup in your attachment utilizes the Noise Source before the LNA so that the LNA's both input impedance and gain could alter with the Noise Source' state (ON or OFF). However, in my test setup, I am not using Noise Source.

Basically, I am using the setup in given R&S Vector Network Analyzer Application Note (setup option 1), also please find the setup in the attachment. I am not inserting any external attenuator or noise source before my LNA (DUT).

Huriye.

It might have something to do with the input match of your IC. Your link to app note did not work for me, though. Can you attach the note itself?

Hi Boris Aleiner ,

First of all thank you for consideration.

The note is attached at the question part also (at the very end of the question line), kindly find it.

Huriye,

The MAX2678 has a lot of gain. I would check that it is not being driven close to compression. Ensure the input signal to the DUT is at least 10 dB below the P1dB input level, and that the signal out of the DUT is appropriate for the VNA input range. If not, you may need to use an alternate setup described in the application note.

Huriye,

Typically noise parameters are measured with a help of a noise diode (see attached). I don't know how R&S incorporated it in their VNA - probably it is explained in their app note, but as Michael Foegelle pointed out - one has to be careful following their recommendations. Looks like the best bet for you is to consult their applications team.

Boris

You are using the so called cold noise measurement technique. This type of measurement using measured s parameter data and noise power values obtained from source pulling about a reflection coefficient with a Gamma of 0.5 or so. Be certain that the device is stable and check to see if the s data returned during measurement makes sense. Is the device matched? As a sanity check that your cal is good, measure a simple item prior to your LNA DUT. A 3 db pad should return ~ a 3 dB NF.

Practically from old school ; If your frequency of operation enables you to use a isolator after noise diode some of this phenomenum may go away. If your application note does not use a noise diode , I would still use a isolator if possible. You can characterize isolator loss separately to account for its loss. If you are using tuners you need two identical tuners to characterize position of two tuners in same position and cut insertion loss in half . Other old school measurements use Y factor and hot/cold load techniques .

In Theory some of the newer techniques used by Keysight PNA ;If you do not use a noise diode you need to measure Power density in a specified bandwidth with and without DUT. The bandwidth is the IF Bandwidth of your VNA used for measurement . Also you will need to know Gain of DUT , expected NF and temperture Tat time of test . his is not easy as you will to create error term measurements and Matrix math.

The cold noise method does not use the classic noise diode to generate a specific noise power. The technique utilizing the excellent property of a well calibrated VNA and measured s parameters will provide the required data needed to solve the noise matrix. However, the key here is well calibrated! Also, depending on the "expected" NF of the DUT, be certain that the noise contribution of the VNA receiver is SMALL. It is key that the noise power provided by the DUT as a function of source pull is always large in comparison to that of the VNA receiver. If not, you must engage a low noise pre amp in front of the VNA. If second stage noise contributions are large or interstage mismatch losses between DUT and VNA are large, cascade noise analysis will lead to negative first stage NF. Of course , that ancillary pre amp will become part of your calibrated system. Again, a sanity check is always helpful. Find a commercial amplifier whose pedigree papers provide a noise figure and gain that ideal would be in line with what you are trying to measure! That way problems that may be associated with your measurement system can for the moment be set aside.