I'm trying to understand impedance matching. - Page 1
Forum section: electronics / beginners

author: ukzappa
author_rank: Newbie
timestamp: May 13, 2014, 09:53:33 pm
content: I have a Thurlby Thandar TG330 frequency generator set to output a signal at 4vpp. The output is 50ohms.When connected to my Rigol DS1052E (hacked to 100MHz) through a 50ohm straight-through terminator at the scope input, I only read 2vpp. Without the terminator, it measures 4vpp. Should I be using a terminator or not? I thought I needed it but now I am confused. Rigol is set to X1 probe.Also, things become more confusing when I connect a counter (with T-piece) between the TG330 and the Rigol. Is it correct to use a 50ohm through terminator on the T-piece into the counter as well as the scope input?

author: Nerull
author_rank: Frequent Contributor
timestamp: May 13, 2014, 10:15:55 pm
content: The 50-ohm output of the generator and the 50-ohm terminator are forming a 1:2 voltage divider. This is a normal side effect of proper termination.

author: Fank1
author_rank: Regular Contributor
timestamp: May 13, 2014, 10:18:37 pm
content: The output of you generator is specified "open circuit".This is from the data sheet.2mV to 20V peak-peak open circuit (1mV to10V peak-peak into 50) in four switch-selectable ranges with 20dB vernier controlwithin each range

author: max666
author_rank: Frequent Contributor
timestamp: May 13, 2014, 11:11:16 pm
content: If you don't use the 50 ohm terminator your signal might look like this:Instead of how it should look like with the proper termination (Note: I set the scope to x2 probe, so it shows me that same 5 V as before, but of course at the imput jack only half of that is actually measured, like already explained):The reason for this is that if you don't terminate properly, signals get reflected at junctions where the impedance doesn't match, in your case where the 50 ohm coax cable goes over to the 1 Mohm input impedance of the scope. So if you care for signal integrity you should use the 50 ohm terminator. And if you want to connect the counter as well, then terminate the coax cable at the end, connect a T-piece and then connect the scope and counter (assuming the counter is set to high impedance as well), with as short as possible connections to the termination point (a.k.a the T-piece).Note, you only terminate once, so if your counter has a 50 ohm input impedance then you can use that instead of the external termination.

author: w2aew
author_rank: Super Contributor
timestamp: May 13, 2014, 11:19:11 pm
content: Here's a video I did that describes this behavior, and why it is normal.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 14, 2014, 01:46:15 am
content: A lot of function generators have an option "High-Z" or "50 Ohm" which changes the display assuming the termination is high impedance or 50 ohm. If you did that, the display would say 2 V p-p. The behavior would be the same: 4 V into a high impedance scope input, 2 V with a 50 ohm terminator.

author: ukzappa
author_rank: Newbie
timestamp: May 14, 2014, 04:49:52 pm
content: Thank you all for your replies. Very helpful.Yes, now you mention a voltage divider, I can see exactly what is happening. While only a novice, I'm kicking myself for not spotting that!Unfortunately, I don't have the option to select X2 on the scope or select the output impedance on the TG330 (not the best equipment!). However, I can live with that. Now I understand why it is happening and that it is to be expected I can use a terminator in the knowledge that it is doing its job correctly.I'm simply trying to get as faithful a representation of a signal as I can. I do realise that my TG330 is responsible for the rapid degradation of a square wave as frequency increases but wanted to eliminate ringing etc caused by incorrect matching.Thanks Alan for pointing me to your video. I don't know how I managed to miss this one (I think I have most of them on disk for reference.). In fact I should point out that it has largely been your excellent videos that have rekindled my interest in electronics (so this is all your fault!).

author: retrolefty
author_rank: Super Contributor
timestamp: May 14, 2014, 05:06:56 pm
content: I certainly can't explain it mathematically, but I've always relied on a old Ham Radio ARRL manuals statement (from memory only) that impedance matching is the state where the maximum possible POWER is transferred between a specific source and a specific load.

author: w2aew
author_rank: Super Contributor
timestamp: May 14, 2014, 06:58:29 pm
content: Quote from: ukzappa on May 14, 2014, 04:49:52 pmWell, I've been blamed for worse things! I'm very glad that my videos inspired you!