How to Measure Frequency Response of a Filter? - Page 1
Forum section: electronics / beginners

author: fourierpwn
author_rank: Regular Contributor
timestamp: March 25, 2017, 11:01:27 am
content: For something as trivial as the title suggests, I have not been able to find suitable material online for such a measurement (maybe it's assumed to be common knowledge??)I am wanting to measure the frequency response (S21, S11) of a BPF. I have VNA's, spectrum analysers, dynamic signal analysers available to me.I believe the traditional method is to use a (calibrated; SOTL [Short, Open, Thru, Load]) VNA, but then due to my limited use of VNA's I am unsure as to how I proceed.Is it as simple as plugging one end in to Port 1, the in Port 2 and then using the menu to select S21, S11 etc? A VNA that I have available has 4 ports; RF Out, R, A, and B. How would I do this measure on such equipment?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 25, 2017, 11:43:43 am
content: I think if you are going to use equipment such as a VNA, it is advised to read over the user manual first. In any measurement it is always a good idea to familiarize one's self with the general block structure, performance, and theory of operation of a device, especially something as complex and advanced as a VNA. Another good thing that could work (if you are doing this at a University or Company lab) is to ask one of the people more familiar with the device to help you out the first time. They can also show you certain tricks or things to watch out for, far better than anyone else can over the internet.On a more direct note:What measurement you do will also depend on what frequency band you are working in, and what specifications are important. If all you really care about if amplitude of S21/S12, and the frequencies you are working at are not too high, it could be simpler to use a spectrum analyzer with a tracking generator or a source of white noise in your band. As you go up in frequency, it will be important to calibrate out the cable losses (in the end, this is part of the SOTL calibration you do with a VNA).If you also care about reflection, going to a VNA is the right choice, this is, after all, what they are built for.

author: fourierpwn
author_rank: Regular Contributor
timestamp: March 25, 2017, 11:54:41 am
content: FYI, center freq. is 9 GHz (maybe 9.25 GHz, I can't recall off the top of my head), -3 dB BW is 500 MHz

author: eb4fbz
author_rank: Regular Contributor
timestamp: March 25, 2017, 01:00:42 pm
content: VNA with proper calibration is the only way.

author: G0HZU
author_rank: Super Contributor
timestamp: March 25, 2017, 01:30:52 pm
content: If you just want to measure return loss and frequency response then a VNA isn't the only way. There are other traditional/basic methods that allow you to measure a filter up at 9GHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2017, 02:01:33 pm
content: Obviously, you need some source of AC, and a detector.So, it can be done with as little as an RF generator, RF voltmeter, and preferably, a return loss bridge to measure that.Beyond that, any means of automating the measurement helps. A sweep generator; spectrum analyzer; tracking generator; vector voltmeter; or, all together, a VNA.Tim

author: fourierpwn
author_rank: Regular Contributor
timestamp: March 26, 2017, 06:13:18 am
content: Quote from: TheUnnamedNewbie on March 25, 2017, 11:43:43 amYes, but for instruments such as Keysight's PNA-X, the user manual does not contain this type of information.While I appreciate the replies to this thread, it seems that you have all misinterpreted my question and have answered 'What equipment can measure...' instead of 'How to measure...'.