50 Ohm load - Page 1
Forum section: electronics / rf-microwave

author: Yaa
author_rank: Newbie
timestamp: June 27, 2023, 06:50:38 am
content: Hi all,Currently I'm doing a project about 50 ohm load terminators measurement. The 50 ohm load been used multiple times and I need to check the measurement whether it still in good condition or not. I've been measured the load using vector analyzer but don't understand the pass/fail criteria. How do I know if the 50 ohm load still in good condition?I read multiple post regarding to the 50 ohm load but still dont get it the pass fail criteria. They stated the VSWR, return loss etc.My question is :1) If i measure the 50 ohm load using handheld vector analyzer, do I need to calculate the return loss or just direct take the reading?2) What is the pass fail criteria for 50 ohm load? does the impedance has range?3) Does the frequency range impact the impedance?4) while measuring the 50 ohm load, which measurement i need to refer to? VSWR or smith chart?My goals is I need to make sure the 50 ohm load terminator is in good condition.Here I attached the result of the measurement by using handheld vector analyzer.

author: radiolistener
author_rank: Super Contributor
timestamp: June 27, 2023, 07:14:37 am
content: Quote from: Yaa on June 27, 2023, 06:50:38 amyou can show RL through VNA menu, but if your VNA doesn't have it you can calculate itQuote from: Yaa on June 27, 2023, 06:50:38 amit depends on your circuit pass/fail criteria for which you're planning to use that dummy load.For example if your circuit needs VSWR < 1.1 then pass/fail criteria is VSWR < 1.1.Quote from: Yaa on June 27, 2023, 06:50:38 amyes, every frequency has it's own impedanceAlso note that VSWR is not the same within some frequency bandwidth. This is a curve.Quote from: Yaa on June 27, 2023, 06:50:38 amit depends on your device requirements, for which you're planning to use that dummy loadQuote from: Yaa on June 27, 2023, 06:50:38 amit depends on your own requirements for dummy load terminator.If you're just want to load some amplifier output, there is no need for precise impedance match. For such purposes you can assume that terminator is good when it's VSWR < 1.1 and assume that terminator is acceptable when VSWR < 1.5.The terminator VSWR should not exceed 2.618 which is a threshold level for 3 dB bandwidth.But if you're want to use terminator for VNA calibration or precise impedance measurement, then your terminator needs much better precision, for example at least VSWR < 1.01 or better.

author: radiolistener
author_rank: Super Contributor
timestamp: June 27, 2023, 07:29:25 am
content: Quote from: Yaa on June 27, 2023, 06:50:38 amHere is calculated VSWR for your screenshot:150 MHz: VSWR = 1.06852, RL = 29.597 dB250 MHz: VSWR = 1.09486, RL = 26.881 dB350 MHz: VSWR = 1.10721, RL = 25.870 dB450 MHz: VSWR = 1.12577, RL = 24.559 dB750 MHz: VSWR = 1.27965, RL = 18.225 dB850 MHz: VSWR = 1.30801, RL = 17.493 dBAt a glance, your dummy load terminator is not acceptable for precise measurements, but it can be used as a usual RF terminator up to about 300 MHz. Above 300 MHz it has noticeable VSWR, but still can be used for applications where impedance accuracy is not criticalBut it all depends on your actual requirements for terminator impedance accuracy.

author: tkamiya
author_rank: Super Contributor
timestamp: August 01, 2023, 06:37:17 am
content: I'm thinking pass/fail criteria highly depends on what OP is doing. If it is going to be a reference for highly precise measurement, VSWR of 1.3 is not acceptable, but if it's just some casual measurement, I have no problem with this.Also, what frequency are you going to do your measurement? Any terminator/dummy load/registive reference has fmax.