Matching Probe Bandwidth to Scope? - Page 1
Forum section: electronics / testgear

author: JP16A60
author_rank: Unknown
timestamp: May 06, 2016, 10:42:03 pm
content: Greetings!I've got a Tek 2225 (60MHz) that I'm using with properly compensated (i.e., adjusted) cheap, Chinese 100MHz probes. Am I OK with this setup, or should I ideally purchase a set of cheap, Chinese 60MHz probes?I've also acquired a couple of Tek modular scopes (SC501--5MHz, and SC502--15MHz) that I haven't unboxed yet. OK with 100MHz or 60MHzI'm a total n00b electronics hobbyist, so I'm not building nuclear-critical stuff here, but I'm annoyingly perfectionist, so accuracy (i.e., an ideal setup) at the cost of an extra few bucks is not a huge concern.BTW, I bought the 2225 based upon Dave's YouTube videos (Episodes #196 and #208), and I have absolutely no regrets--this is great scope for a beginner. Thanks, Dave!JP

author: mmagin
author_rank: Frequent Contributor
timestamp: May 06, 2016, 10:50:39 pm
content: A good passive 100 MHz probe, properly adjusted should be flat from DC to 100 MHz. That includes the DC to 60 MHz that you care about.

author: JP16A60
author_rank: Unknown
timestamp: May 07, 2016, 01:37:03 am
content: Awesome.I assume that a move to 250MHz or 300MHz probes would be prudent if I pick up something beyond 100MHz?JP

author: helius
author_rank: Super Contributor
timestamp: May 07, 2016, 02:05:49 am
content: The bandwidth is limited by the effective rise time of the probe+scope system. The rise time of the probe and that of the scope combine together to yield the effective rise time. A 100 MHz probe will have poorer performance than a 150 MHz probe onanyscope, all else being equal. But that effect is not so significant for scopes < 100 MHzYou can perfectly well use 100 MHz probes on a 500 MHz scope, but the rise time contributed by the probe will dominate in that case. You should not expect to see 500 MHz system bandwidth, of course. But even a 500 MHz probe will limit a 500 MHz scope's effective bandwidth to a measurable extent. You will see better performance with a faster probe.

author: JP16A60
author_rank: Unknown
timestamp: May 07, 2016, 02:36:47 am
content: Thanks, Helius.I think I've got the general ideal now!JP

author: helius
author_rank: Super Contributor
timestamp: May 07, 2016, 02:45:41 am
content: The other important aspect to probe selection is capacitance. Scope inputs have a capacitance that can distort signals. For a probe to be matched to a scope, it needs to have itscompensation networkadjusted to compensate for this capacitance: the probe should be adjustable over a range that covers the capacitance of the scope.

author: TimFox
author_rank: Super Contributor
timestamp: May 07, 2016, 03:31:52 am
content: As a reasonable approximation, the rise times of cascaded systems (such as a probe and an oscilloscope) add in quadrature: the total rise time squared will equal the sum of the squares of the individual rise times (approximately).Therefore, if the bandwidth of the probe is really 50 MHz and the bandwidth of the oscilloscope is really 100 MHz, then the final bandwidth should be about 45 MHz, dominated by the probe. If the bandwidth of the probe is 200 MHz on a 100 MHz 'scope, then the final bandwidth should be about 89 MHz, dominated by the 'scope.I suspect that the actual bandwidth of a quality probe is actually wider than the labeled bandwidth, to obtain close to the labeled bandwidth on a similar-bandwidth oscilloscope.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 07, 2016, 04:06:55 am
content: Congratulations on the new scope and probes.Not all the cheap probes from China are poor. I bought some 100MHz 10X probes from Hantek. There were two 100MHz in a pack for under $10/probe. I was not expecting much but the ones they sent me work well and I have been very happy with them.Quote from: JP16A60 on May 06, 2016, 10:42:03 pm

author: tggzzz
author_rank: Super Contributor
timestamp: May 07, 2016, 10:37:26 am
content: Don't forget there's no such thing as a 10 or 1 Mohm probe. At 100MHz the impedance will be more like 80ohm (20pF capacitance) - and theywillload your circuit correspondingly.Also don't forget the inductance of a ground lead: 6" => ~150nH, and work out the resonant frequency wiith 20pF.FFIhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: Fungus
author_rank: Super Contributor
timestamp: May 07, 2016, 10:55:30 am
content: Quote from: joeqsmith on May 07, 2016, 04:06:55 am100Mhz isn't really a problem. It's when you go about 300MHz-ish that things start to get interesting (ie. expensive).To Original Poster: No problem. You'll be fine so long as the probes are better than the scope.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 07, 2016, 12:30:38 pm
content: Quote from: Fungus on May 07, 2016, 10:55:30 am

author: Fungus
author_rank: Super Contributor
timestamp: May 07, 2016, 04:53:54 pm
content: Quote from: joeqsmith on May 07, 2016, 12:30:38 pmSo the $100 probe behaved the same as the $17 probe? That's good news for cheapskates like me.But...I was right about the frequency! Both your probes started attenuating badly at about 350MHz (watch the video at the 10:15 mark). The 300-350MHz frequency range is where probesstartgetting expensive.(How expensive can they get? Anybody know...? Shall I start a "Show us your probes!" thread?)PS: You've got some very pretty hardware in your collection.(green face)Quote from: joeqsmith on May 07, 2016, 12:30:38 pm

author: nctnico
author_rank: Super Contributor
timestamp: May 07, 2016, 05:08:39 pm
content: Quote from: JP16A60 on May 07, 2016, 01:37:03 amOver 100MHz the probe tip capacitance will have a large influence on the signal you are probing and also the grounding of the probe will become an issue. My rule of thumb is that you need to switch to lo-Z or active probes (both with extremely low input capacitances) for measuring a signal over 100MHz.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 07, 2016, 05:19:02 pm
content: Thanks. Most of my equipment is 10+ years old. Some is dating back to the 50s. Most of it was damaged when I bought it. I did the the repairs and alignments myself. It all works and does what I need it to.To answer your question, sort of. The cheap probes came two in a pack, so about $9/ea with shipping! $17 would be a high end probe!Other than that detail, yes these seem to do as good a job as my LeCroy branded probes. I have thought about buying another package down the road just to see if the quality has changed because I can't believe they could sell these for so little.You are right about the attenuation of the 10X probes I have. For higher frequencies I will switch to resistive probes. I have posted on this site about some of my attempts at rolling my own. I bought these many years ago.Quote from: Fungus on May 07, 2016, 04:53:54 pm

author: Fungus
author_rank: Super Contributor
timestamp: May 07, 2016, 05:49:46 pm
content: Quote from: joeqsmith on May 07, 2016, 05:19:02 pmIt's old but it looks like it would have beenreallyexpensive stuff in its day.

author: Zero999
author_rank: Super Contributor
timestamp: May 07, 2016, 10:10:28 pm
content: The probe's impedance can also be important. The lower the impedance, the more loading the probe will have on the circuit being measured. Above 20MHz to 30MHz a low-Z probe will have a higher impedance than a typical high-Z probe.

author: rickv14623
author_rank: Frequent Contributor
timestamp: May 09, 2016, 02:24:42 pm
content: Might want to check this out as well: