RF: TX Output impedance characteristics (DUT) with a VNA?   - Page 1
Forum section: electronics / beginners

author: jordi
author_rank: Contributor
timestamp: July 21, 2020, 02:28:25 am
content: Hello everyone!First of all I'm not an expert in RF, but I understand a few things. I'm trying to impendace match (50ohms) the output of a Silabs 4042 at 915Mhz. I'm using the reference design AN435:https://www.silabs.com/documents/public/application-notes/AN435.pdfEverything suggested on the AN435 has been applied successfully and the radio works. But I noticed a lower output than expected (~14dBm rather than 20dBm or at least 19dBm). I already carefully calibrated/zeroed my spectrum analyzer and compared with other modules with a defined power output and readings are accurate, so we can remove that from the education (Yes I'm sending a CWM).I know that my PCB or some of the components I'm using are screwing things out, so what I wanted to do, for the pure sake of science is, how can I measure the impedance characteristics of Silab4042 - TX_pin output if I where to connect it directly to VNA (with a proper attenuator)? The problem is that I can't attach an input signal like you will normally do on a VNA. Is there any trick I can apply there? The SI4042 is using a class E amplier (switching power amplifier) internal and there is no way I can "intercept the input".I read something interesting on the AN435, but I'm still not sure if it helps in some way:QuoteAnother reason why I find it important is because AN435 states that the suggested values are for +20dBm, if you were to use a lower output power, the proposed suggested values might not apply the same. So I might need new components to properly match the lower output power. The reason is that I'm using an amplifier and the maximum input power is around ~10dBm (which at the moment works OK). I could just set it to "20dBm" and use an atennuator as I'm doing now, but, it will no help me understand how things work.So again, it is possible to somehow use a VNA to test the output of the TX_pin, figure out the state of the signal, and from there have an starting point of the changes needed to reach 50ohms or at least reach decent power? Seems like using the RLOAD trick and play with the values would be the only way, I'm not sure.How would you normally test any device that only has an output?Thanks!

author: bob91343
author_rank: Super Contributor
timestamp: July 21, 2020, 04:26:30 am
content: First you have to realize that the output impedance of a transmitter is not equal to its intended load impedance. The intended load impedance is just the design parameter that insures proper output power. The transmitter source impedance is probably a lot lower.You can measure it if you can vary the load impedance slightly. If you change from, say, 50 Ohms to 55 Ohms and measure the change in output voltage, you can compute the source impedance. This should be done without readjusting the transmitter. Simply use Thevenin's theorem/law and that will shake out the source impedance with the formula.If your 50 Ohm load isn't accurate (at these frequencies that's somewhat expected) then your power measurement is off.And expecting 1 dB accuracy with this gear is a bit optimistic. Even though that's only about a 10% error in voltage.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 21, 2020, 07:54:23 am
content: There is a few ways to approach this.Usually for amplifiers to figure out the optimal load, you do something called a load-pull. This can either be done in a passive way, by moving around shorts or stubs on open transmission lines, in front of the VNA, or through active methods (because you can make a output 'see' a load by feeding back a signal at a given phase offset and amplitude).Thing with these switching low frequency outputs (low frequency with respect to what CMOS is capable of) is that they usually act as a voltage/current source into 50 ohms, and not like a classic linear PA that is matched to the ideal load-pull curve. They are 'intended' to have abysmal S22 and S11, because nobody cares - people care about how much output power comes out.Have you been able to verify that your 50 ohm lines on PCB are actually 50 ohm? and what kind of loss are you seeing in your PCB and cables? Enough decoupling capacitance close by? The switching nature of those PAs might draw a lot of peak current and if your supply network can't supply that, you will have a sag in output power.

author: jordi
author_rank: Contributor
timestamp: November 22, 2021, 08:12:13 pm
content: Sorry for the late reply! Thanks a lot for the help. Well, has been a long way, I was able to reach almost +21dBm, the main problem: The PCB had parts incorrectly placed.