VNA usage at other impedances? - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: January 08, 2021, 03:16:42 am
content: Can I use a VNA like the Nano VNA as if it was looking at, say a 75 ohm transmission line on a basis of 75 ohms as being the target impedance, and if so, how? If I provide it a 75 ohm terminator as its "load" is it then likely to interpret some values correctly, I'd imagine the SWR reading at least to be close at low frequencies.Another perhaps better approach maybe is to use a good balun or unun at the beginning to provide a match to the desired impedance and then set the output there as the calibration plane? (and then maybe have another one at the other end if that was needed?Anyway, I am just speculating, I don't know.I'd like to be able to use it with both unbalanced and balanced transmission lines at various impedances sometimes.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: January 08, 2021, 04:56:50 am
content: You have several options to do measurements in a 75 ohm environment with a 50 ohm analyzer, depending on your requirements:1. Simply calibrate your analyzer to 50 ohm, measure your S matrix, and then convert it to a reference impedance of 75 ohm. Most newer analyzers should be able to do that in firmware. If not, you can do the transformation offline with your favorite programming language or math package (MATLAB, Octave, Python, etc.). This works well for linear DUTs such as filters, couplers, cables, etc.2. If you need to measure in an environment with 75 ohm connectors you can use mechanical adapters together with a 75 ohm cal kit. That was actually demonstrated in the video posted by Joe. The calibration will ensure that the reflection from the transition between the 50 ohm port and the 75 ohm environment is eliminated mathematically.3. If you have to ensure that the test port physically has 75 ohm impedance you can use a minimum loss pad to convert it from 50 ohm to 75 ohm. These pads will also convert the connector mechanically. Then use a 75 ohm calibration kit to calibrate to your 75 ohm connectors. To physically convert the port impedance may be important if you are characterizing a nonlinear DUT such as an amplifier or a semiconductor, which may behave differently when presented different impedances at the input and output. This difference cannot be accounted for mathematically in a simple way.It is usually not practical to use a balun to convert from 50 to 75 ohms, better use a broadband resistive minimum loss pad if you need it. The loss in dynamic range is not a problem in most measurements as modern lab grade analyzers have dynamic ranges in excess of 100 dB.Actually in most cases option 1 is the best for linear DUTs. A transition from 50 to 75 ohms (or vice versa) will lead to a mismatch loss of about 0.18 dB, that is 0.35 dB of loss in dynamic range in a \$S_{11}\$ or \$S_{21}\$ measurement. Two baluns will likely have a bigger insertion loss.Edit:By "balun" I actually meant an unun for impedance transformation. A balun could be used to transform the test port to balanced. However, it is better and more convenient to combine two ports for this purpose as was was pointed out by virtualparticles below.

author: virtualparticles
author_rank: Regular Contributor
timestamp: January 08, 2021, 03:13:23 pm
content: Quote from: cdev on January 08, 2021, 03:16:42 amA 4-port VNA can make a balunless balanced measurement. For a balanced transmission line, one port is applied to each wire, a series of unbalanced measurements are made and the balanced results are calculated and displayed. This works well for passive balanced measurements but will not work for a balanced amplifier circuit.https://coppermountaintech.com/video-multiport-balanced-measurements-with-a-vna/Good Luck!

author: cdev
author_rank: Super Contributor
timestamp: January 08, 2021, 04:39:05 pm
content: Quote from: joeqsmith on January 08, 2021, 03:43:03 amThank you, that was just perfect!I recently got an old ATSC tuner and am trying to see if I can receive anything near as many channels as we had before the HDTV changeover (when we got a huge number of channels and perfect analog TV without cable. After the changeover it declined to just a few.Its still fewer RF "channels" than before but because they are digital they are containers for a bunch of channels within channels so the total number of channels appears to be higher already. But the reception of many of them definitely can be improved substantially.So the VNA is likely going to be as useful in the 75 ohm context as it is otherwise.. Cool!Thank you!