using 100 mhz probes on 200 mhz scope any issues. - Page 1
Forum section: electronics / testgear

author: chinoy
author_rank: Regular Contributor
timestamp: October 19, 2023, 03:27:59 pm
content: So I figured out how to do the upgrade via firmware for my Hantek scope.But the one thing holding me back is my probes are marked 100 Mhz.What are the implications of upgrading my scope but not my probes.What is the key diff between a 100 Mhz scope probe and a 200 Mhz scope probe.Any inputs much appreciated.

author: Antonio90
author_rank: Frequent Contributor
timestamp: October 19, 2023, 03:59:52 pm
content: The total system bandwith will be limited by the weakest link.A 200MHz scope with 100MHz probes will have approx. 100MHz bandwith.You can always get 200MHz probes later down the line if you need them.

author: tggzzz
author_rank: Super Contributor
timestamp: October 19, 2023, 04:02:34 pm
content: Quote from: chinoy on October 19, 2023, 03:27:59 pm100MHz *10 10Mohm "high" impedance probes on a 200MHz scope will result in a system bandwith of 100MHz, give or take.With a probe tip capacitance of (say) 15pF, the load on your circuit will be ~100ohms at 100MHz and ~50ohms at 200MHz - i.e. nowhere near 10Mohms.

author: KungFuJosh
author_rank: Super Contributor
timestamp: October 19, 2023, 04:36:17 pm
content: Passive probes tend to perform well above their rated limit, unless they're junk.

author: TimFox
author_rank: Super Contributor
timestamp: October 19, 2023, 05:19:00 pm
content: A common question on this site is "What will this object do in a range of operation outside the manufacturer's specification?"In general, many useful units will function at frequency or power reasonably outside the specification range, but the user must measure his own units to find out.

author: srb1954
author_rank: Super Contributor
timestamp: October 19, 2023, 07:46:55 pm
content: Quote from: chinoy on October 19, 2023, 03:27:59 pmApart from not being able to fully utilise your scope's increased bandwidth there may be additional signal imperfections i.e. ringing or overshoot on your viewed waveform. Cheaper, lower bandwidth, probes often have simplified compensation circuits and rely on the scope's limited bandwidth to hide some of the signal imperfections produced by the probe.You will only be able to fully check this effect if you examine the O/P of a clean, low aberration, pulse generator with less than 1ns rise time to fully test the combined probe and scope performance. Always ensure that you use the shortest possible ground lead when making these measurements or, better still, use a BNC to probe adapter to connect the signal source ground directly to the ground barrel of the probe tip.

author: chinoy
author_rank: Regular Contributor
timestamp: October 24, 2023, 06:55:46 am
content: So looks like I made the right call to not just blindly upgrade to 200Mhz.Will keep a look out for some quality brand 2nd hand probes. And do the update once I have the right probes. Thanks for the inputs

author: tggzzz
author_rank: Super Contributor
timestamp: October 24, 2023, 08:11:28 am
content: Quote from: chinoy on October 24, 2023, 06:55:46 amBasic physics indicates that even new quality brand probeswillintroduce aberrations - especially at high frequencies.One problem is, as I previously noted, excessive loading of the signal being probed.Another particular culprit is the standard 6"/150mm ground lead on a *10 "high" impedance probe. That will cause the scope trace to show ringing (approz 100MHz) that is not present in the signal being probed. Example demonstrated athttps://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/You will probably benefit from reading some of the references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2023, 03:22:10 pm
content: Quote from: srb1954 on October 19, 2023, 07:46:55 pmWith good probes that is usually the deficiency. Probe "bandwidth" is more about what frequencies a probe will faithfully reproduce than -3dB bandwidth, and even a good 100 MHz probe and 100 MHz oscilloscope "pushed" with a very fast edge may start to show aberrations which would be unrealistic in typical applications.QuoteA fast reference level pulse generator is always useful to confirm the combined probe and oscilloscope response, and then compare it to the oscilloscope response by itself. An adequate one for operation up to 200 MHz can be built with some AC or LVC logic gates and good construction technique over a ground plane. A while ago I modified the Sync output on a function generator to produce fast and clean pulses by rewiring it and replacing the 74140 TTL driver with S logic, but it was not as fast as AC or LVC can be. Testing revealed that S logic produced the cleanest waveform in this particular case.TheLeo Bodnar fast pulse generatoris a good alternative if you want to just purchase something.Quote from: tggzzz on October 24, 2023, 08:11:28 amThere is always an effect, but it is not always shown on the oscilloscope. Probes may be calibrated to take the loading into affect, so the oscilloscope displays what the signal would look like without the probe loading. In the past I think HP did this, while Tektronix calibrated their systems to include the result of probe loading. If I want to know what is really going on, then I use a sampling oscilloscope.When working with a terminated source to produce 25 ohms, my old 100 MHz Tektronix oscilloscopes show identical results between a direct connection and a good 100 MHz *or faster* probe.QuoteA *short* ground lead is usually acceptable up to 200 MHz when probing for logic levels, but the displayed waveform will not represent what is really there.