Hi Z versus 50 ohm impedance - Page 1
Forum section: electronics / beginners

author: Aspiringloser
author_rank: Newbie
timestamp: April 15, 2014, 01:01:11 am
content: Hello all! Apologies if my first post has been previously answered in another thread. I have a Siglent SDG 5082 function generator, Protek 6506 scope and a Siglent SHS 810 scope. I've encountered an issue that I don't understand. With the function generator set to the 50 ? output, if I generate for example a 1 kHz sinewave at 1 Vp-p, both of my scopes measure 1 V peak, not peak to peak. This is measured without being connected to any circuit, just the scope probes connected to the BNC to alligator clips on the output of the function generator. When I switch the function generator to the Hi Z output setting, my scopes then measure the same peak to peak voltage which is being generated. My instructor at school has touched on impedance matching with me in the past. I took my scopes and generator to school to demonstrate my issue and he had no answer for me. All of the function generators in the lab are older and my instructor isn't familiar with the newer stuff. Does anyone have an explanation for this or a link to an explanation somewhere online?Many thanks in advance,Paul N. Wise.

author: c4757p
author_rank: Super Contributor
timestamp: April 15, 2014, 01:04:28 am
content: Quote from: Aspiringloser on April 15, 2014, 01:01:11 amThat's just sad.The function generator has a 50 ohm output impedance internally - this never goes away. When it drives a 50 ohm load, the two form a voltage divider that cuts the signal in half. Therefore, if you tell it there's a 50 ohm load at the other end of the cable, it will compensate for that by doubling the signal amplitude. That's all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 15, 2014, 01:05:41 am
content: FYI...What voltage do you get from the function generator?If you answered "what it says", YOU'RE FLAT WRONG!That's just what it's programmed to generate. It's not measuring what it's actually putting out. It's *certainly* not measuring what's at the end of the cable!Always measure with the scope, in circuit! That way you know for sure.Tim

author: uprightsquire
author_rank: Contributor
timestamp: April 15, 2014, 01:08:51 am
content: Recent video by w2aew:http://youtu.be/tClE8s6RZdg

author: FrankenPC
author_rank: Unknown
timestamp: April 15, 2014, 01:17:01 am
content: Quote from: uprightsquire on April 15, 2014, 01:08:51 amOne of the best beginners video I've seen. The other best beginners video was his db discussion. The man is awesome.

author: Aspiringloser
author_rank: Newbie
timestamp: April 15, 2014, 01:20:18 am
content: Thank you for that link. It answered my question completely.

author: w2aew
author_rank: Super Contributor
timestamp: April 15, 2014, 02:54:50 am
content: Quote from: Aspiringloser on April 15, 2014, 01:20:18 amI'm glad my video helped you. Maybe you should show it to your instructor. It really is sad that he didn't know the answer.FYI, my YouTube channel is loaded with similar "beginner" videos...

author: VK5RC
author_rank: Unknown
timestamp: April 15, 2014, 05:30:22 am
content: While it is sad your instructor couldn't help, it is a great lesson to follow your real questions until you feel answered, it may take several different sources.

author: Aspiringloser
author_rank: Newbie
timestamp: April 15, 2014, 09:28:58 pm
content: I e-mailed the link to him and spoke with him about it this morning. It made sense to both of us after it was presented so clearly in the video. 50 ohm internal impedance connected to a perceived 50 ohm load gives you an equal voltage divider. The function generator outputs twice the peak to peak voltage so that the output voltage is maintained. If the 50 ohm load isn't there, then twice the voltage will be read at the output. Everything he explained to me about impedance matching was there, we just failed to put the pieces together. The concept is not what my instructor wasn't familiar with, but the newer design of my function generator. Another thing I noticed from the video: the Tektronix function generator in the video has a 50 ohm, Hi-Z, and selectable output impedance. My Siglent only has 50 ohm and Hi-Z.

author: w2aew
author_rank: Super Contributor
timestamp: April 15, 2014, 10:03:18 pm
content: Quote from: Aspiringloser on April 15, 2014, 09:28:58 pmPlease note that the generator's output impedance is NOT selectable - is is ALWAYS 50 ohms. It is the expected *load* impedance that is selectable. This tells the generator what load is present so that it can calculate what voltage to generate "internally" so that the specified output amplitude is correct at the load.

author: David Hess
author_rank: Super Contributor
timestamp: April 16, 2014, 01:49:20 am
content: Quote from: VK5RC on April 15, 2014, 05:30:22 amIt reminds me of the time when I was in high school and we were visiting a local university and I asked the chemistry professor there what boiling chips were made of. He had no idea but to be fair, it was really more of a physics question.

author: Aspiringloser
author_rank: Newbie
timestamp: April 16, 2014, 02:55:00 am
content: Please note that the generator's output impedance is NOT selectable - is is ALWAYS 50 ohms. It is the expected *load* impedance that is selectable. This tells the generator what load is present so that it can calculate what voltage to generate "internally" so that the specified output amplitude is correct at the load.This is what I meant, though I may not have worded it as such.

author: richieHH
author_rank: Newbie
timestamp: May 27, 2022, 07:37:10 pm
content: Part of the confusion is that the UI references output impedance as opposed to the "input impedance of the load". So the user gets confused as to what exactly they're modifying. Or at least I do.

author: Gyro
author_rank: Super Contributor
timestamp: May 27, 2022, 08:15:03 pm
content: Just a note that this a necro of an 8 year old thread.

author: strawberry
author_rank: Super Contributor
timestamp: May 28, 2022, 07:42:15 am
content: Quote from: w2aew on April 15, 2014, 10:03:18 pmNOT ALWAYS 50ohms : 50, 75, 600 ...