Why do I get wildly different readings on my scope in 1x and 10x mode at high hz - Page 1
Forum section: electronics / testgear

author: clientsoup
author_rank: Newbie
timestamp: July 21, 2015, 01:39:06 am
content: Two captures from my DS1054Z (upped to 100mhz):http://imgur.com/a/pCqD3I'm measuring the 64mhz output of the main clock of a microcontroller (HSI, so not very accurate). It is being powered with 5V.In the first screenshot, I have my probe & scope in 10x mode: note the VPP of 7.5v. How is this possible? Also notice that it's going negative, down to -0.7v.In the second screenshot, probe & scope are at 1x. I get a much more sane-sounding VPP of 2.8v, but this is still not what I would expect - it also doesn't reach down to 0v, only goes as low at 1.5v. The wave is also a much different shape.The frequency is measured correctly in both cases.What's going on here? The probe (RP2200) is rated for 150mhz bandwidth.

author: c4757p
author_rank: Super Contributor
timestamp: July 21, 2015, 01:43:31 am
content: Quote from: clientsoup on July 21, 2015, 01:39:06 amOnly in 10X mode.

author: clientsoup
author_rank: Newbie
timestamp: July 21, 2015, 01:51:50 am
content: Ah yes, I remember watching this video - I will revisit it.So, theactualvalue is the 7.5v VPP?

author: dadler
author_rank: Unknown
timestamp: July 21, 2015, 01:55:03 am
content: Try using the little ground spring (came with your probes) instead of the ground clip. You are seeing the inductance of the group clip wiring inducing ringing.Here is the full-differential-equation educational explanation:http://youtu.be/-gRXU-O1FY4

author: henas
author_rank: Contributor
timestamp: July 21, 2015, 01:56:38 am
content: The rating is only for x10 probe. When probe is set x1 it has has very low bandwidth, a few Mhz. It will attenuate the signal. This is normal

author: clientsoup
author_rank: Newbie
timestamp: July 21, 2015, 02:39:44 am
content: Quote from: dadler on July 21, 2015, 01:55:03 amYou're right on the money! Using the ground clip, saw the base voltage ~100mv, peak of ~2.6V, and 64.1mhz. The math behind this is a bit beyond me, but I'm studying up on my probe types now so I can better understand the why.Thank you!

author: w2aew
author_rank: Super Contributor
timestamp: July 21, 2015, 04:38:49 am
content: Quote from: clientsoup on July 21, 2015, 01:51:50 amAlso, make *sure* that you have properly compensated the probe, or the amplitude WILL be wrong!

author: w2aew
author_rank: Super Contributor
timestamp: July 21, 2015, 04:41:12 am
content: Quote from: clientsoup on July 21, 2015, 02:39:44 amI've got videos on that too:...and how you can make your own high performance probe socket:...and here's one on the importance of probe compensation and the affect on amplitude accuracy:

author: tggzzz
author_rank: Super Contributor
timestamp: July 21, 2015, 11:04:09 am
content: Quote from: clientsoup on July 21, 2015, 02:39:44 amHave a look at the references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/Good to see someone thinking about what they are doing, trying different methods of making the same measurement, and using that as a guide to what they could benefit from understanding. Too often people take an oscilloscope trace as being The Real Truth