How to mesure S11 and S22 of a PA or LNA with a VNA ? - Page 1
Forum section: electronics / rf-microwave

author: jujun
author_rank: Regular Contributor
timestamp: August 20, 2019, 09:24:21 am
content: Hello,I wonder how it is possible to make a measurement with a VNA on the input and output of a power amplifier or a LNA ?I want to know the S11 and S22 but I dont want to burn anything.For the input I guess that It's the easiest, nothing special to do ?If I use an attenuator on the output, I will only see the near 50ohm of the attenuator I guess.Sorry for this stupid question

author: erikka
author_rank: Regular Contributor
timestamp: August 20, 2019, 12:35:04 pm
content: Should be done in two steps.First attenuator at the input and then attenuator at the output.For the S11 and S22 the attenuator at the other side of the amplifier has no impact as the VNA is supposed to be 50 ohms anyway.Don't measure amplifiers without attenuator as this may destroy you VNA (depending on the amplification and output power)

author: radiolistener
author_rank: Super Contributor
timestamp: August 20, 2019, 01:35:00 pm
content: erikka, could you please explain more details, for example - how to measure 2 kW power amplifier with VNA?Did you mean to calibrate VNA through 80 dB attenuator and then connect it to PA through the same attenuator?

author: erikka
author_rank: Regular Contributor
timestamp: August 20, 2019, 03:07:07 pm
content: Assuming the amplifiers is linear then no need to measure impedances at full power.First calibrate the VNA as usual with the measurement plane at the end of the two cables that will be used to connect to the amplifier.Assuming the amplifier amplifies 25dB, take a 30dB attenuator (of sufficient power handling, depends on the port 1 output level of the VNA) and connect it to the output of the amplifier.Measure the input impedance (S11)Connect the 30dB attenuator to the input and measure the output impedance (S22)Or if you have a VNA with port 1 signal level setting, set the port 1 signal to -30dB and calibrate and measure without attenuators.

author: radiolistener
author_rank: Super Contributor
timestamp: August 20, 2019, 03:21:39 pm
content: Quote from: erikka on August 20, 2019, 03:07:07 pmLet's say, we have PA, which needs 6 W (38 dBm) on the input with 25 dB gain.With 6 W on the input, the output will be 2000 W, amplitude on the output is about 450 V.VNA output power is not enough to drive PA input.How to connect it to VNA?

author: erikka
author_rank: Regular Contributor
timestamp: August 20, 2019, 03:38:12 pm
content: The 6 Watt input power is only needed to get the 2000 Watt output.Under the assumption of the amplifier being lineair there is no reason to operate the test at 2000 Watt output so you can reduce the input power and connect directly to VNA.But be careful you do not get any switching glitches when the amplifier is connected to the VNAIf the assumption is wrong (!) you need a different measurement approach.

author: radiolistener
author_rank: Super Contributor
timestamp: August 20, 2019, 03:45:46 pm
content: Quote from: erikka on August 20, 2019, 03:38:12 pmbut I'm scared to connect the sensitive input of VNA to a high voltage output of PA. There is a high risk do destroy VNA...For example, there is possible some random pulse or unexpected voltage transition from VNA, so PA output will make barbecue from VNA...

author: erikka
author_rank: Regular Contributor
timestamp: August 20, 2019, 03:54:11 pm
content: Nice emoticon!!Agree with your concern.What do you want to measure? Transferred output power? There are other posts on this forum

author: radiolistener
author_rank: Super Contributor
timestamp: August 20, 2019, 04:18:34 pm
content: Actually I never thought about use VNA for PA measurement. I know there are other more safe ways to measure PA output response and impedance with no use VNA. But topic starter asked interesting question. Is it possible at all (to use VNA for PA measurements)?

author: Odysseus
author_rank: Regular Contributor
timestamp: August 21, 2019, 02:00:13 am
content: You need a VNA that supports connections to an external high power directional coupler.This is combined with a high power test amplifier to create a load-pull setup.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: August 22, 2019, 06:11:54 am
content: Quote from: erikka on August 20, 2019, 03:38:12 pmIs the entire point of a good VNA not to be able to measure the output impedance at different power levels - if you want to use your amplifier in CW mode for a constant-envelope signal at 2 kW, your assumption of 'it is linear' could be very much invalid. In fact, I would expect a amplifier capable of putting out 2 kW to be in an almost 'small-signal' regime when you feed it with just a few dBm (or less) of input power from a VNA with additional attenuator.For these kind of measurements, as Odysseus has suggested, you need a VNA with external high-power directional coupler, and a test-amplifier. However, this won't create a load-pull setup: a load-pull requires you to also send in a signal from the output port (if you do a electrical load-pull) - something I don't think is even done at these powers. If you want a load-pull, you would really want a passive load-pull setup here to deal with these powers (otherwise you need to generate a similar power signal as the DUT creates!).Quote from: radiolistener on August 20, 2019, 04:18:34 pmA VNA is the right tool for the job, especially when it comes to measuring in/output impedance. The only thing you have to watch out for is that the power levels don't go too high. Depending on how fancy your VNA is you can even do power-sweeps right from the VNA without any external components, and check for non-linearity (the top-of-the-line keysights and R&S models can do this).If you have such a fancy VNA, the trick is simply to calibrate the power levels of the VNA - usually this is done with an external power meter, check manual of your VNA - and then measure as you would (and use the power-sweep functions). In this case, with the high output power levels, you need external couplers because no VNA can deal with this much power. You can use external amplifiers and 'calibrate' them out because the VNA can tell from the reference signal what the provided signal power is.

author: virtualparticles
author_rank: Regular Contributor
timestamp: September 30, 2019, 02:23:11 pm
content: Most of the answers I see here are incorrect. One cannot simply add attenuators to the input and output of the VNA. If you were to put a 20 dB attenuator on Port 2 you would not be able to calibrate on the other side of it. The stimulus sees 20 dB attenuation and the reflection another 20 so there is no signal left to calibrate. One could do a response calibration and simply remove the effect of the attenuator in order to measure gain but this is not a full vector measurement and is unworthy of a VNA.VNA's can make measurements from one port to another and see down to 125 dB or so but the residual directivity limits one-port reflection measurements to 35 dB or so with any accuracy. (See video by me,https://youtu.be/P_Ox9tWXktE). 40 dB of two-way attenuation prevents one-port measurements."Hot" S22 measurement requires an external dual directional coupler and Direct Receiver Access (DRA) on the VNA. The Coupled ports can be down 40 dB preventing overload of the DRA inputs. Calibration is still difficult since a decent open, short and load that can take the full power of the PA must be attached to the output of the coupler. In addition, the port 2 stimulus must still be applied to the output of the coupler. A very good isolator might be used to prevent the output power of the PA from frying the Port 2 input. Some attenuation might be used as well, perhaps 10 dB, but this will increase the noise of the measurement.The input of the PA may be measured with the stimulus power turned down so the output power of the PA is correct.If S22 or S12 is not required, it would be much simpler to perform a 2-port one-path measurement to obtain vector measurement S11 and S21. Attenuation on the output of the PA is acceptable for this form of calibration since the stimulus on the other side is not used.However you do it, these measurements are tricky and are a balancing act to obtain accurate measurements without letting the smoke out of one of the VNA ports.Cheers!Brian

author: Roland_W
author_rank: Contributor
timestamp: October 08, 2019, 03:42:45 pm
content: Hi All,You could find test setup, and theory about PA measurment using the VNA in document :Basics of RF Amplifi er Measurements with the E5072A ENA Series Network Analyzerhttps://cdn.testequity.com/documents/pdf/RF-Measurements-E5072A-ENA.pdfBest RegardsRoland