FPC1500 RF input not 50R? - Page 1
Forum section: electronics / rf-microwave

author: Volcano
author_rank: Newbie
timestamp: December 18, 2023, 12:05:11 pm
content: Hello all,I've recently bought the FPC1500 fully loaded version and was excited to take the first measurements.First I tried TG measurement and 50R terminator.I was a bit surprised since I didn't get the expected result (instead around -9.5db I got -3.5db)After that, I've tried Smith chart measurement and I've got this:a) S11 measurement using 50R terminatorDot is on "1" as expectedb) S11 measurement of RF input portOf course, I understand the parasitic components of the cable and N-type to SMA adapter but should it be THAT bad?

author: pienari
author_rank: Regular Contributor
timestamp: December 18, 2023, 03:23:46 pm
content: I just looked tech specifications and in/out is 50ohm nominal.Have you calibrated unit?

author: Volcano
author_rank: Newbie
timestamp: December 18, 2023, 03:41:42 pm
content: I didn't calibrate it myself, but the device is new and was calibrated in the factory. The calibration was performed about 1 year ago and I know with age it can change a bit, but it looks a bit too high decalibration just only for age.

author: RFDx
author_rank: Regular Contributor
timestamp: December 18, 2023, 04:47:38 pm
content: Quote from: Volcano on December 18, 2023, 12:05:11 pmTransmission loss is 3.5dB and return loss would be 9.5dB.Quote from: Volcano on December 18, 2023, 03:41:42 pmMeant was the SOL-calibration you should do before your measurement.

author: pdenisowski
author_rank: Super Contributor
timestamp: December 18, 2023, 07:37:21 pm
content: You definitely should perform a measurement calibration (open-short-match) before making a VNA measurement. As some other posters have already mentioned, this is different from the instrument calibration (which is done every few years by a calibration center)https://www.rohde-schwarz.com/webhelp/fpc_html_usermanual_en/Content/3288351ddf664d2e.htm#d658222e5444If you're not familiar with calibration, I made a short introductory video about itHope some of that helps!

author: tggzzz
author_rank: Super Contributor
timestamp: December 18, 2023, 08:04:12 pm
content: Your measurements are almost certainly of the cable plus your UUT (whatever that is).You need to do a SOLT calibration with the specific cables; that will remove the effect of the cable and the measurement should then show just you UUT.Your "dB" measurements are meaningless without stating which parameters you are measuring, e.g. S11 or S21 etc.I'm not sure what you intend to achieve by having two 50ohms in parallel.

author: radiolistener
author_rank: Super Contributor
timestamp: December 18, 2023, 11:12:48 pm
content: Quote from: Volcano on December 18, 2023, 12:05:11 pmThis is rg316? Where did you get it? Some Chinese cables may have very different impedance, I remember one Chinese cable from aliexpress that has VSVR 1.3 at 30 MHz...Also T-tee connector with one unloaded output has pretty noticeable reflection at high frequency. Don't use it for VSVR measurements.

author: Volcano
author_rank: Newbie
timestamp: January 22, 2024, 11:08:49 pm
content: Sorry for a long time, I didn't reply.The SOL is the method to resolve that issue as you mentioned. Thank you guys.Quote from: RFDx on December 18, 2023, 04:47:38 pmThank you for your advice. I was simulating return loss and expected results as transmission loss stupid beginner mistake.I didn't perform SOL since back then I didn't have the equipment to do that.Quote from: radiolistener on December 18, 2023, 11:12:48 pmThe cable was RG-316 from TE Connectivity