should oscilloscope probes be identical? - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: July 30, 2017, 05:27:39 pm
content: Hi! I would like to buy a few more probes for my micsig scope in addition to what was included (because bundled probes are not very good).Question 1: should I replace probes on four channels altogether? I do differential measurements (using two probes and math function) on up to, say, 10MHz signals.Question 2: in my scope there is only one ADC, so measuring signals from two or more channels simultaneously is not possible. Does this mean scopes with only one ADC inherently have a phase lag between channels?PS Probes I'm going to by is P6100, my oscilloscope is Micsig TO1104 (4 channels, 100MHz, up to 1GSa).

author: wraper
author_rank: Unknown
timestamp: July 30, 2017, 05:37:24 pm
content: With different probes you will get different propagation delay. With fast signal measurements this might be an issue as channels won't match in time.Quote

author: wraper
author_rank: Unknown
timestamp: July 30, 2017, 05:45:21 pm
content: Quote from: exe on July 30, 2017, 05:27:39 pmIf scope is properly built, then no. Yes channels are sampled at different times and could also have different wire length from each channel causing different propagation delay. But if properly done, this should be fully compensated in firmware.

author: StillTrying
author_rank: Super Contributor
timestamp: July 30, 2017, 05:49:57 pm
content: Quote from: exe on July 30, 2017, 05:27:39 pmI think theysampleall channels at the same moment. Connect all channels/probes to the same fast signal and see if the dots in dot mode are all aligned in time, - all on the same 2ns or 4ns time.

author: exe
author_rank: Unknown
timestamp: July 30, 2017, 06:59:42 pm
content: Quote from: wraper on July 30, 2017, 05:45:21 pmGood point!Quote from: StillTrying on July 30, 2017, 05:49:57 pmThat's a bit difficult to measure accurate. Yes, I can see the phase difference. But at 100MHz I can "move" traces just by bending ground leads. Or by touching wires, etc. I think I need to buy two BNC cables and a T-adapter for more adequate measurements.Anyway, I see that for proper measurements I need to replace all probes together.PS how is BNC adapter called for probes? It looks like PP150 (not P6100 I mentioned earlier) come without BNC adapters.

author: alm
author_rank: Super Contributor
timestamp: July 30, 2017, 07:21:09 pm
content: The common name is probe tip to BNC adapter. Keep in mind that probe tips are not standardized, so it is not a generic part. You would either buy it from the probe vendor as optional accessory (not likely with cheap eBay special probes) or find one for a probe that happens to have the same dimensions as yours.

author: PA4TIM
author_rank: Super Contributor
timestamp: July 30, 2017, 07:25:57 pm
content: If you often have to measure differential, you better buy a diffprobe. I experimented a lot with differential measurements on powersupplies. I tried the way you do but that was not really usable. I have several diff plugins for my 547 and a 7A15 for my Tek 7704. That kinda worked if I used identical 1x probes (home made because I have no commercial ones and switchable probes are crap.Then I could borrow a 25MHz Fluke from a friend. That did the job well. Bought one from Pico (25 MHz 20 and 200x) and that performed just like the Fluke (I think most are rebranded) I now use it like 6 months or so and almost every day. It is now my most used probe for trouble shooting. Dave sells a nice 75MHz versionAs a test try to probe the same source (high amplitude squarewave). If the commonmode rejection is good, you will see a flat trace. If not you see a squarewave. Then calculate the ratio of that against the real source amplitude.FredEdit, Forget my advise.I find that Pico a cheap diffprobe. But they are not even close in price. I now see the P6100's are not Tektronix but super cheap Chinese crap.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2017, 08:12:49 pm
content: Quote from: exe on July 30, 2017, 05:27:39 pmYou should use identical probes for each differential measurement but except for that and with a bandwidth of only 10 MHz, probe matching will not matter.QuoteOne ADC may still implement simultaneous sampling across multiple channels but even if it does not, there should be no phase difference. The oscilloscope knows the time relationship between samples taken on different channels and should align them on the display and with the trigger.

author: DeepBlu
author_rank: Newbie
timestamp: November 20, 2017, 07:57:55 pm
content: They can build what ever probe you wish. Need to use special low resistance low Z wire. short as possible, depends on freq or something std if your not designing to go to Mars.Probe Master OSC Probes USAhttp://probemaster.com/No affillation, just happened on it.

author: danadak
author_rank: Super Contributor
timestamp: November 20, 2017, 11:10:57 pm
content: If you look at your scopes specs you will see that its CMR vs Fand channel to channel isolation and linearity affect accuracyin differential measurements.Modern DSOs have probe calibration routines, actually totalsignal path routines, that correct for signal path issues.Regards, Dana.

author: macboy
author_rank: Super Contributor
timestamp: November 21, 2017, 03:12:21 pm
content: Quote from: exe on July 30, 2017, 05:27:39 pmThose generic P6100 probes are rubbish. They have a high tip capacitance (18 pF), which becomes around 88 Ohms at 100 MHz. Good passive 10x 10 MOhm probes have a tip capacitance of around 10 pF or less. This reduces probe loading at higher frequencies so that it affects the circuit less, and what you see is closer to reality.Also consider that if the probe is rated to "100 MHz" you can assume that it will be -3 dB down or so at 100 Mhz. Probe response affects the overall bandwidth of the measurement system. If your scope is also -3 dB at 100 MHz then probe+scope is -6 dB down. To maximize the useful bandwidth of your scope+probe combination, use a probe that isfasterthan the scope. The 150 MHz or 250 MHz versions of these probes are not that much more. Go for that. Or splurge on some used Tektronix probes like P6133. Those are rated as 150 MHz but I have measured them to about 500 MHz (no really) and the tip capacitance is a meager 10.8 pF, quite good.

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2017, 05:44:52 pm
content: Quote from: macboy on November 21, 2017, 03:12:21 pmThey are acceptable up to 100 MHz but quality is low and there are counterfeits of these probes out there which do not meet their 100 MHz specification. My only complaints are that the grabber hook is stamped making it annoyingly thick and the x1/x10 switch on several of mine has become intermittent leaving the probe in x10 mode.I have had much better results with these probes made byTexas. They OEM these probes for Agilent/Keysight and others.https://www.ebay.com/itm/One-new-250MHZ-Oscilloscope-clip-probe-w-readout-pin-X10-Tektronix-HP-RIGOL/280107318452https://www.ebay.com/itm/Two-new-250MHZ-Oscilloscope-clip-probes-w-readout-pin/280107317229They are part of the same series as the Keysight N2862B and N2863B probes which are also worth looking for on Ebay:https://www.keysight.com/en/pd-1938439-pn-N2862B/passive-probe-101-150-mhz-12-m?nid=-32557.964557&cc=US&lc=enghttps://www.keysight.com/en/pd-1938449-pn-N2863B/passive-probe-101-300-mhz-12-m?nid=-32557.964559&cc=US&lc=eng

author: majki
author_rank: Contributor
timestamp: January 18, 2018, 10:39:45 pm
content: I just purchased a pair of P6100 for my old analog scope.I tested the probes with Rigol DS1052E (untouched 50MHz BW), see the attached picture.It is a simple 5MHz square wave signal with 1:10 mode.- blue trace with the original Rigol probe RP2200 (150MHz)- yellow one is the P6100 (100MHz)Surprisingly the Rigol one has an overshoot after the rising edge which should not be there.

author: glarsson
author_rank: Frequent Contributor
timestamp: January 18, 2018, 11:11:03 pm
content: Are the probe compensation adjusted correctly?

author: David Hess
author_rank: Super Contributor
timestamp: January 19, 2018, 12:30:26 am
content: How are the probes connected to the signal source?What you are seeing may be the result of the faster probe picking up more of the overshoot from a long ground lead.

author: james_s
author_rank: Super Contributor
timestamp: January 19, 2018, 12:30:54 am
content: The P6100 probes are not spectacular, but they're not all garbage either. If it's only a 10MHz scope then the choice of probe hardly matters.

author: David Hess
author_rank: Super Contributor
timestamp: January 19, 2018, 01:30:18 am
content: Here is a photograph of the performance of one of my P6100 probes on a 100MHz Tektronix 2247A using a 500ps reference level pulse output. The first edge is the raw reference edge terminated at the oscilloscope input and the second delayed signal is the same through the P6100 probe using a coaxial connection to the probe tip.All of the aberration observed is from the oscilloscope and test setup; none of it is from the reference level pulse generator.

author: danadak
author_rank: Super Contributor
timestamp: January 19, 2018, 02:13:36 am
content: If you read the Tek and Agilent ap noes on probing, for diff measurements they alldiscuss need for diff probe rather than using A - B setup on older scopes.Seems a shame they peddled this capability for years even though functionallyit was a broken technique, except at very low CMR levels.http://www.tek.com/dl/Probing%2520Techniques%2520for%2520Accurate%2520Voltage%2520Measurements%252051W_60161_1_HR_Letter.pdfhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0ahUKEwiM38bW-OLYAhWuk-AKHZ7XA4oQFghEMAQ&url=http%3A%2F%2Fweb.mit.edu%2F6.101%2Fwww%2Freference%2FABCprobes_s.pdf&usg=AOvVaw3jqTqjxTe5xhU8dQaIF70UNewer scopes, DSOs, have a signal path compensation utility, but the scopes I have withit do not spec or reveal how the algorithm actually works. I have tested, at a crude level,that capability, all << 10 Mhz, seems OK (this on 500 Mhz and 1 Ghz DSOs). But at 200Mhz, 500....how good is it ? Don't know.Diff probe is where its at. Search eevblog, some DIY approaches to save $$$$.Last thought, if waveform periodic, take a record, store it, then another, same probe,and do the subtraction.....maybe that's one way of beating the problem......? I havenever tried that. Clearly sources must not drift, change, loading over the time periodone is gathering the data. Note layout of probe cable could impact this approach, fromone measurement point to another. so just one more reason leading to safe approach,diff probe.Regards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: January 19, 2018, 02:38:37 am
content: Quote from: danadak on January 19, 2018, 02:13:36 amOld analog oscilloscopes and a couple of old and odd DSOs like the 2230/2232 can make better use of A - B for differential measurements than modern DSOs can. The secret to getting good common mode rejection is to use the vertical variable controls to match the attenuation of each channel with the probes attached. You can actually *see* a null in the noise when doing this. The compensation of the probes can also be trimmed for better low frequency AC common mode rejection. If the probes have high frequency compensation adjustments, then these can be used to further improve performance but this is tricky to do.Modern DSOs do not work quite as well doing this because the subtraction increases quantization noise.That solves the DC and low frequency performance problems but limitations remain. The common mode range is restricted by the vertical input range so high sensitivity and high common mode range is not possible at the same time.

author: danadak
author_rank: Super Contributor
timestamp: January 19, 2018, 12:03:57 pm
content: @ David, that's essentially how we all did use the A - B, but I alwayswondered about the frequency response matching not being properlyhandled by simple probe comp, re "lumps and bumps" in signal pathone probe to the other. But to your point at 10 Mhz or less where I wasdoing much of my work probably not an issue. And the scopes mostlyran out of CMR gas anyways in the few Mhz region.Regards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: January 19, 2018, 04:17:04 pm
content: Quote from: danadak on January 19, 2018, 12:03:57 pmI never saw an application note which discussed trimming the common mode rejection with the vertical variable controls and probe compensation adjustments. I only did it out of desperation once when I lacked a dedicated differential probe and it worked perfectly. The AC common mode rejection calibration only extends to a fraction of the probe bandwidth although it should be better with a higher bandwidth probe that has multiple adjustments.Some special probes had build in DC and AC common mode rejection adjustments but I think LeCroy is the only manufacturer of these now.Modern inexpensive high voltage differential probes have their own set of problems. Apparently a lot of them are built on the wrong substrates or just cheaply so their AC common mode rejection drifts significantly with time and without user accessible calibration, they cannot be corrected.In practice I am not convinced that dedicated high voltage differential probes perform better than separate probes except where a high common mode range is needed. The source impedance mismatch and lead dressing will screw up even perfect AC common mode rejection.

author: danadak
author_rank: Super Contributor
timestamp: January 19, 2018, 06:39:00 pm
content: This ap note discussed using variable G to match probes -http://acrayara.com.ar/biblioteca/public/Instrumental/Differential%20Oscilloscope%20Measurements%20-%20Tektronix%20(1996).pdfhttp://www.personal.psu.edu/users/f/0/f0o/articles/differential_measurements.pdfRegards, Dana.