Scope Transmission Lines and Impedance Matching - Page 1
Forum section: electronics / beginners

author: sy
author_rank: Regular Contributor
timestamp: February 03, 2024, 11:46:03 pm
content: I have a SDS2104x+ scope and was previously using BNC-alligator clips for testing. They are similar to the ones OP posted in this post:https://www.eevblog.com/forum/testgear/of-what-quality-does-a-function-generator-cable-needs-to-be/However, after reading the post I am not sure if these cables are suitable anymore, especially if I wanna maximize usage of the in-built 50Mhz generator?Not sure if I am measuring the cable correctly but I am getting 0.3Ohm across the cable. I usually see those 50Ohm and 75coax cables used for transmission lines and wonder if I should be using those instead for all cables used with my scope for impedance matching?

author: tggzzz
author_rank: Super Contributor
timestamp: February 03, 2024, 11:54:42 pm
content: The 0.3ohms is completely irrelevant in this context.You will benefit from understanding the basics of transmission line theory, in particular when you do/don't have to consider a connection as a transmission line. Hint: length and frequency.FFI see textbooks and application notes. They explain them much better than any quick informal post on a forum.As for using your scope, it looks like you will benefit from understanding why so many classes of probe exist, and their relative advantages and disadvantages. FFI , see the links athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: radiolistener
author_rank: Super Contributor
timestamp: February 04, 2024, 01:46:34 am
content: you can use it for DC measurements and for AC up to 100-300 kHz. But for higher frequency it's better to use probes, because above 0.3-1 MHz alligator clips start to affect AC waveform. Especially for non sine waveform, because they contains high frequency harmonics

author: MarkT
author_rank: Frequent Contributor
timestamp: February 04, 2024, 08:47:56 pm
content: And remember a 10x probe is used in 10x mode normally - using 1x mode is rarely done as the bandwidth is reduced and the circuit loading is much greater. Scope probes are not simple transmission lines, they are very clever bits of technology.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 05, 2024, 01:17:31 pm
content: You cant compare a bit of coax with crockclips to an X1 scope probe. The latter is not regular coax. See

author: MathWizard
author_rank: Super Contributor
timestamp: February 05, 2024, 11:59:02 pm
content: Just lately I got some brand name cables for my scopes/sig.gen., I haven't compared it at high freq yet tho, to those low freq. cheap ones. Actually i just tried repairing a cheap one, and it seemed to have aluminum wires welded to the alligator clip. The wires really did not want to re-wet with solder.IDK much about transmission lines yet either, but the BNC cable with alligator clips should be some 50 ohm coax cable. So it's an inner conductor surrounded by an insulator and then an outer conductor. Scope probes are something a bit different, with a lot more DC resistance iirc, I should watch Dave's video again.And for good coax, for some length, say 20ft (maybe it's way more), and upto 1GHz for something like RG-58, you might only loose 1dB of power to the cable.And it's normally used for power matching, so the output impedance and input impedance between circuit's should be complex conjugates, and the real part of each should be 50 ohms. And then the coax in between them, is almost lossless, and the power is shared equally between the source and the load. But maybe at 1GHz and some length of good cable, you would have a bit under 1 half the power at the load, as a little does get lost in the cable.