Lower bandwidth probes for a higher bandwidth oscilloscope - bad? - Page 1
Forum section: electronics / beginners

author: fazeka
author_rank: Newbie
timestamp: December 30, 2019, 08:22:54 pm
content: Hi all,I have an older Tektronix 2445A 'scope that needs a new pair of probes. The only work I do is in the audio realm. Highest frequency I would test would be the bias frequency of an analog tape recorder (432 kHz).My friend has a set of Pomona 6493 probes he would consider selling me. I did some research and found that these are 60MHz. The Tek 2445A is a 150MHz scope.I've researched online, seems everyone suggests going with probes that are higher bandwidth.What's issues would I encounter (for audio work, if it matters) with using 60MHz probes with a 150MHz scope?Thanks!

author: jadew
author_rank: Frequent Contributor
timestamp: December 30, 2019, 08:26:27 pm
content: For audio work you can use coat hangers and it would still work.That said, why not get 150 MHz probes and be able to use your scope to its full potential when you need it? I think there are plenty of low cost options for that range.

author: fazeka
author_rank: Newbie
timestamp: December 30, 2019, 08:43:32 pm
content: Quote from: jadew on December 30, 2019, 08:26:27 pmQuote from: jadew on December 30, 2019, 08:26:27 pmI know I said friend "would consider selling me"... I should have said "he owes me"...

author: fazeka
author_rank: Newbie
timestamp: December 30, 2019, 08:45:10 pm
content: One thing:Is it going to be a problem if the new Pomonas have a compensation range of 15 pF to 40 pF whereas the original Tek probes had a compensation range of 10 pF to 25 pF?

author: tautech
author_rank: Super Contributor
timestamp: December 30, 2019, 08:47:22 pm
content: Quote from: fazeka on December 30, 2019, 08:45:10 pmNope.If anything a wider compensation range for the probes is more desirable.

author: fazeka
author_rank: Newbie
timestamp: December 30, 2019, 08:49:08 pm
content: Well, there is no overlap in the lower part of that spectrum, 10-15pF range... that's what I was wondering.

author: tautech
author_rank: Super Contributor
timestamp: December 30, 2019, 08:51:50 pm
content: Quote from: fazeka on December 30, 2019, 08:49:08 pmCheck the manuals for the Tek input capacitance.......

author: fazeka
author_rank: Newbie
timestamp: December 30, 2019, 08:57:24 pm
content: "Input Capacitance: 15 pF, +/-3 pF"

author: tautech
author_rank: Super Contributor
timestamp: December 30, 2019, 09:06:40 pm
content: Quote from: fazeka on December 30, 2019, 08:57:24 pmThen if you're really worried about sub-perfect probe compensation maybe the cheap (and good) P6100 probes might be a good solution.Two for ~$11 is hard to beat.https://www.ebay.com/itm/P6100-2pcs-High-precision-oscilloscope-probe-1X-10X-100MHz-crocodile-clamp-SZHKU/253535303506?epid=2100268847&hash=item3b07e1b752:g:aBMAAOSwjL5ZFFbO

author: james_s
author_rank: Super Contributor
timestamp: December 30, 2019, 09:07:44 pm
content: They will probably work just fine.Keep in mind you can get a pair of halfway decent 100MHz probes from China for under $15. Going beyond 100MHz of real, useful bandwidth starts to get expensive, you really hit a wall around 350-400 MHz.

author: fazeka
author_rank: Newbie
timestamp: December 30, 2019, 09:16:29 pm
content: Cool. Thanks guys!

author: tautech
author_rank: Super Contributor
timestamp: December 30, 2019, 09:21:13 pm
content: Quote from: james_s on December 30, 2019, 09:07:44 pmEven the branded Siglent PP510 100 MHz probes for just $10 ea have been proved to give suitable accurate waveform representation at 200 MHz where they didn't differ in amplitude by more than a dB.Probes typically don't roll off the cliff at higher frequencies like scope inputs do.

author: fazeka
author_rank: Newbie
timestamp: December 30, 2019, 09:25:23 pm
content: Quote from: tautech on December 30, 2019, 09:21:13 pmGuys, didn't realize there were some decent cheap scope probes nowadays... even what tautech suggested with the P6100... Yes my friend owes me but maybe I can play that card later for something else...What other "value" probes do you guys like?

author: tautech
author_rank: Super Contributor
timestamp: December 30, 2019, 09:30:16 pm
content: The 100:1 100 MHz P4100 probes are also great value for the money.....~$13 ea.

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2019, 01:43:56 am
content: The 60 MHz probes will work fine on a Tektronix 2445A. Lower frequency oscilloscopes, in this case 150 MHz, are pretty tame so as long as the probe compensation range includes the oscilloscope's input capacitance, they will work fine.

author: james_s
author_rank: Super Contributor
timestamp: December 31, 2019, 03:26:46 am
content: I forgot to mention, most Tek scopes need probes that have a "readout pin" on the BNC, otherwise the V/div will be off by a factor of 10 when using 10x probes. Some modestly priced probes do have this but many of the cheapest ones like the P6100's don't.

author: tautech
author_rank: Super Contributor
timestamp: December 31, 2019, 03:45:57 am
content: Quote from: james_s on December 31, 2019, 03:26:46 amTypically prices for readout probes are just obscene when it's just a resistor between the pin and ground that sets the input attenuation.So I just get these 300 MHz Pintek 100:1 probes from their factory and retail them for around the same price as here:https://www.ebay.com/itm/1pc-Oscilloscope-Probe-Kit-CP-3308R-100-1-300Mhz-1500V-100M-Readout-Pin-PINTEK/132996940150?hash=item1ef73c4176:g:H3EAAOSwFGxc-xgGOtherwise these 300 MHz 10:1 Siglent auto sense probes are not too badly priced and quite a nice compact probe:https://siglentna.com/product/sp2030a-auto-sense-300-mhz-oscilloscope-probes/

author: JustMeHere
author_rank: Frequent Contributor
timestamp: December 31, 2019, 05:47:55 am
content: Quote from: fazeka on December 30, 2019, 09:25:23 pmRigol puts 150 MHz probes in the 1054z box now.

author: fazeka
author_rank: Newbie
timestamp: December 31, 2019, 06:20:39 am
content: Quote from: james_s on December 31, 2019, 03:26:46 amDoesn't seem the Pomona 6493 has these...

author: tooki
author_rank: Super Contributor
timestamp: December 31, 2019, 01:00:06 pm
content: Quote from: fazeka on December 30, 2019, 08:22:54 pmFor sure, those probes will do fine for your application.What's odd to me is that the Pomona datasheet for that probe (attached below) lists the probe bandwidth as being 150MHz (-3dB), but a "scope bandwidth" of 60MHz. Can someone more knowledgeable explain what they mean here??

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2019, 05:05:40 pm
content: Quote from: tooki on December 31, 2019, 01:00:06 pmProbes are not specified by -3 dB bandwidth. The bandwidth number is more about what maximum bandwidth will the probe support.For instance sticking a 100 MHz probe on a 100 MHz oscilloscope does not result in a 70.7 MHz bandwidth; it results in a 100 MHz bandwidth.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2019, 05:52:48 pm
content: It perfectly OK to have a slower probe with a fast scope if one does not need the full BW. Probes tend to have a limited life time as the thin delicate wire in the special coax tends to brake if bend too often. So it is OK to have cheap (today like 150 MHz) probes for every day use and have expensive really high BW probes only if really needed. Automatic detection of the attenuation can still be very handy.With a modern scopes with multiple inputs one may want 2 to 4 probes anyway.

author: tooki
author_rank: Super Contributor
timestamp: December 31, 2019, 07:18:29 pm
content: Quote from: David Hess on December 31, 2019, 05:05:40 pmWell that's what I'd assumed. But look at the datasheet I attached above. It says 150MHz -3dB, yet it seems to be universally marketed as a 150MHz probe, not the 60MHz "scope bandwidth" the datasheet also mentions. Hence my request for someone more knowledgeable to clarify.

author: tautech
author_rank: Super Contributor
timestamp: December 31, 2019, 08:17:23 pm
content: Quote from: tooki on December 31, 2019, 07:18:29 pmAnd very badly advertised......specs apparently interpreted by some desk jockey !QuoteIt's just a 60 MHz probe, no more, no less !Sure if it's swept to 150 MHz it's response is -3dB but that's NOT how probes are specified !