Finding the correct Probes for old Osciloscope - Page 1
Forum section: electronics / testgear

author: trying to be clever
author_rank: Newbie
timestamp: May 03, 2022, 10:25:21 am
content: Hi everybodyI'm a newbie and electronics is just a new hobby, sorry for my ignorance.I just bought my first oscilloscope, a 15MHz analogue Trio CS-1560A II, but it came without probes.I've searched on the forum for probes recommended by people, and found the following post:Quote from: garrettm on March 20, 2020, 10:43:08 pmThis post was very helpful because now I know I should buy probes with more frequency than the frequency of my oscilloscope.But I don't know what trimmer caps should the probes have for HF compensation.On the instruction manual of my oscilloscope it says "Input Capacitance 22pF+-3pF", what's the correct compensation range of the probes that I should buy for my oscilloscope?Is there any equation I should use to get these values?Besides frequency and HF compensation, is there anything else I should look for when buying probes?Thank you.

author: tunk
author_rank: Super Contributor
timestamp: May 03, 2022, 10:43:30 am
content: You could try a cheap P6100 probe from ebay, aliexpress, etc.

author: alm
author_rank: Super Contributor
timestamp: May 03, 2022, 10:46:02 am
content: For the record the 1 kHz square wave adjustment is low frequency compensation, not high frequency.For 15 MHz, the only thing will matter is the input capacitance. I would completely ignore bandwidth and get 60 MHz or 100 MHz probes, whichever are cheapest. A probe is generally specified to work with a certain range of input capacitances, for example 20 - 30 pF. You should make sure the input impedance of your scope falls within the range as specified by the probe. Now these trimmers aren't known for there close tolerances, so you can often get away even if it's a few pF off, although this is obviously not guaranteed. This is for 10x probes. 1x probes don't require compensation, but are limited in bandwidth and have a much higher circuit loading, even at 15 MHz.

author: srb1954
author_rank: Super Contributor
timestamp: May 03, 2022, 10:52:48 am
content: Quote from: peido on May 03, 2022, 10:25:21 amSince your scope is not very high performance the choice of probes is not so critical. There would be no point in paying extra money for a high bandwidth probe. A 100Mhz or even a 50MHz rated probe would be adequate to match the performance of your scope.The nominal scope input capacitance for the probe will be defined in the probe specifications. Select a probe in the range 20pF to 25pF - this should have adequate range of adjustment to match your scope input. Do not go for a probe rated for 15pF or lower nominal input capacitance as this will probably not have sufficient range to to adjust to your scope input.The other things to look for when selecting a probe depend on your intended application. If you are working on valve equipment make sure that the probe voltage rating is adequate. Most general purpose probes are good for 400-500V so should be OK.Some smaller body probes are rated for lower voltages but are better for accessing miniaturized circuitry.

author: trying to be clever
author_rank: Newbie
timestamp: May 03, 2022, 11:05:29 am
content: Thank you allYou've been very helpful.About voltage rating, I'm probably going to use it mainly on low voltage circuits, but in the distant future I would also like to use it to repair some old CRTs I have.Is it better for me to buy a more expensive probe rated for high voltages of a known brand on reputable seller, or can I trust a cheap probe where the seller says it's rated for high voltage on ebay?

author: srb1954
author_rank: Super Contributor
timestamp: May 03, 2022, 12:27:02 pm
content: Quote from: trying to be clever on May 03, 2022, 11:05:29 amI would stick with a standard probe for general purpose use rather than selecting a higher voltage probe (> 500V). The higher voltage probes tend to be more specialised, more expensive and can be significantly larger than a general purpose probe making then awkward to use in compact circuits.One factor you should be aware of is that all 10Mscope probes have to be derated in their operating voltage for higher frequency signals. This derating can start start at frequencies as low as a few hundred kHz and the allowable peak voltage can drop as low as 10% of the nominal voltage rating towards the upper end of their frequency range. Higher quality probes usually depict this on a graph in their operating manual although cheaper manufacturers are often silent on this derating.

author: trying to be clever
author_rank: Newbie
timestamp: May 03, 2022, 12:48:57 pm
content: Thanks.To start, I'll buy a Uni-T P01 (bandwidth 25MHz, 10x input capacitance 18.5~22.5pF, input resistance 1x 1M\$\Omega\$+-2% 10x 10M\$\Omega\$+-2%, working voltage 1x 300Vpp 10x 600Vpp) that a local person is selling for a fair price, instead of waiting months for another probe to arrive from ebay.In the future I'll buy something more specialized if I need.

author: tggzzz
author_rank: Super Contributor
timestamp: May 03, 2022, 01:05:13 pm
content: The most important decision is whatclassof probe to buy for your use case - and when it won't be suitable for safety or performance reasons.I suggest you look at the safety and praxis information inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/One point to note: the 300V rating will be at DC and low frequencies. If you tried to probe 300Vac 25MHz, then there would be~350A~1A flowing through its 18pF tip capacitance!EDIT: due to a brain failure, it is ~350ohmswith 300V across it - so "merely" ~1A.Senility is the only reason, but that's not an excuse

author: trying to be clever
author_rank: Newbie
timestamp: May 03, 2022, 01:18:41 pm
content: Quote from: tggzzz on May 03, 2022, 01:05:13 pmThanks for the link, good information in there that I did not know aboutQuote from: tggzzz on May 03, 2022, 01:05:13 pmThank you for the warning.EDIT:The seller I've mentioned before also has two Uni-T P04 (bandwidth 100MHz, 10x input capacitance 14~18pF, input resistance 1x 1MΩ+-2% 10x 10MΩ+-2%, working voltage 1x 300Vpp 10x 600Vpp) that I don't know if I should buy, because the input capacitance range doesn't match the 22pF+-3pF of my oscilloscope. I'll probably don't buy them.

author: tggzzz
author_rank: Super Contributor
timestamp: May 03, 2022, 01:35:54 pm
content: You need to distinguish between a *10 "high" impedance probe'stip inputcapacitance (that loads your circuit) with the probe'scompensationcapacitancerange. The compensation capacitance range must encompass thescope'sinput capacitance.It is entirely reasonable to have a probe with a 12pF tip capacitance and a 15-25pF compensation capacitance range connected to a scope with input capacitance of 20pF.

author: trying to be clever
author_rank: Newbie
timestamp: May 03, 2022, 02:10:08 pm
content: Oh I see.So using the example of the Uni-T P01 and P04, the compensation range is 8pF to 35pF, and that's what I need to have in account.Thanks

author: Fungus
author_rank: Super Contributor
timestamp: May 03, 2022, 03:00:54 pm
content: For 15MHz I wouldn't worry about the probe specs. Just aboutanypiece of shielded wire will work.If you need something better in the distant future then buy it in the distant future. Your 'scope might have stopped working before then.https://www.eevblog.com/forum/reviews/cheapest-100mhz-oscilloscope-probes-hands-on-review/

author: 2N3055
author_rank: Super Contributor
timestamp: May 03, 2022, 03:19:40 pm
content: Quote from: tggzzz on May 03, 2022, 01:05:13 pmImpedance would be 350 Ω.. ??

author: David Hess
author_rank: Super Contributor
timestamp: May 03, 2022, 03:28:37 pm
content: That post does not apply to your situation, and is not correct for all instruments anyway. For instance Tektronix rated the bandwidth of their probes and oscilloscopes combined *at the probe tip*, so a 500 MHz probe combined with a 500 MHz oscilloscope resulted in a bandwidth of 500 MHz.A better way to think of probe bandwidth is the frequency up to which the probe will faithfully represent a signal. In your case with a 15 MHz instrument, any probe with the appropriate compensation range will work. 10x probes will specify a compensation range and 200 MHz and below 10x probes will almost all work with a 22 picofarad input. 1x probes do not require compensation but have higher capacitance, like 100 picofarads and higher, at the probe tip.At 15 MHz you could get away with a length of coaxial cable but a 1x probe will perform better.

author: tggzzz
author_rank: Super Contributor
timestamp: May 03, 2022, 03:59:23 pm
content: Quote from: 2N3055 on May 03, 2022, 03:19:40 pmSigh. Quite right. Brain failure; must be senilityPrevious post edited for posterity.Still, ~1A at 300V would be entertaining, for a short time.

author: alm
author_rank: Super Contributor
timestamp: May 03, 2022, 04:23:24 pm
content: Quote from: David Hess on May 03, 2022, 03:28:37 pmAs long as your device under test doesn't mind the capacitive load. I would prefer a 10x probe over a 1x probe for a signal with 15 MHz bandwidth unless I needed the sensitivity. I think 15 MHz is borderline for a 1x probe, never mind a simple piece of (shielded) wire.

author: David Hess
author_rank: Super Contributor
timestamp: May 03, 2022, 04:42:00 pm
content: Quote from: alm on May 03, 2022, 04:23:24 pmSome circuits can tolerate that level of capacitive loading like transistor outputs and drivers, but I agree that a 10x probe is preferred.

author: 2N3055
author_rank: Super Contributor
timestamp: May 03, 2022, 05:11:25 pm
content: Quote from: tggzzz on May 03, 2022, 03:59:23 pmNah, it happens to me too.. More like absent minded... You calculated 350 something and then brain shortcuted last step and attached the unit..Yep, 1A /300V through the probe would be fun nevertheless ... And after it carbonizes enough then 350A would come soon enough.... :-)

author: james_s
author_rank: Super Contributor
timestamp: May 03, 2022, 05:43:55 pm
content: Quote from: David Hess on May 03, 2022, 04:42:00 pmI have never once (intentionally) used a 1X probe. For specialized applications where it would make sense to use one I connect directly with a piece of coax.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: May 03, 2022, 05:54:01 pm
content: If you match the impedance then the next thing isaccessories.Almost always use a 10X probe.If you buy a new set look for the maximum ofHooks and grabbers etc....Makes it easier to use

author: tggzzz
author_rank: Super Contributor
timestamp: May 03, 2022, 06:07:53 pm
content: Quote from: james_s on May 03, 2022, 05:43:55 pmI usually use a *1 probe on a circuit: with AC coupling it enables low level PSU noise and ripple to be viewed.For high speed my preference is a Z0 resistive divider probe with a 50ohm load, and preferably without 20pF load!

author: trying to be clever
author_rank: Newbie
timestamp: May 03, 2022, 10:19:14 pm
content: Thank you all for the comments, it's very helpful and I've been learning.I ended up buying new a Uni-T P01 and a Uni-T P04 for a fair price to the local seller I've mentioned before. (He also offered me a a syringe with flux, which is nice.)In the future if I need something better then I'll upgrade. I'm happy with what I got so farQuote from: Fungus on May 03, 2022, 03:00:54 pmQuote from: David Hess on May 03, 2022, 03:28:37 pmI'll definitely try a coaxial cable or any other shielded wire, just for fun.

author: David Hess
author_rank: Super Contributor
timestamp: May 03, 2022, 10:55:59 pm
content: Quote from: james_s on May 03, 2022, 05:43:55 pmFor an oscilloscope, a 1x probe has some advantages over a piece of coaxial cable. The capacitance for a given length is lower, leading to better bandwidth, and the transient response is better.

author: Fungus
author_rank: Super Contributor
timestamp: May 04, 2022, 12:27:48 am
content: Quote from: alm on May 03, 2022, 04:23:24 pmA fixed 10x probe is the one to go for but I remember looking on Aliexpress/eBay a few months ago and couldn't find a single fixed 10x probe for sale.Every seller/manufacturer seems convinced that a switchable 1x/10x probe is what people want.

author: james_s
author_rank: Super Contributor
timestamp: May 04, 2022, 12:29:31 am
content: I just glue the switch in the 10X position when I'm using a cheap probe.

author: Fungus
author_rank: Super Contributor
timestamp: May 04, 2022, 12:36:37 am
content: My Micsig came with four fixed 10x probes and I'm very happy about that. Micsig say they're better quality than an equivalent priced switchable probe would be. I think more 'scope manufacturers should do the same, especially at the entry level where the switch will just confuse people.(I have a switchable probe somewhere in case I ever need one...)

author: james_s
author_rank: Super Contributor
timestamp: May 04, 2022, 05:18:34 pm
content: The switch is always stupid. If I needed a 1X probe I'd buy a 1X probe. It's a specialized enough application that it makes no sense to combine it, but the Chinese probe makers seem to be like the Chinese flashlight makers where someone comes up with a stupid idea (like all the dumb flashing modes) and everyone else blindly copies it.

author: alm
author_rank: Super Contributor
timestamp: May 04, 2022, 05:46:14 pm
content: I agree about the switch. It will sometimes switch to the other setting while you're not paying attention. I'm pretty sure switchable probes existed back when HP and Tektronix were the major scope brands and were manufacturing probes in the US, but they were much more of a niche item back then. Probably bought for specific applications like field service. Often their performance was worse than contemporary 1x probes (switchable Tek P6062 had 7 MHz bandwidth with a 6 ft cable, while 1x Tek P6101 had 15 MHz with a 2 m cable). Somehow by late nineties they became the default probes for many scopes with up to 200 MHz bandwidth.By the way, one thing to keep in mind about probe input capacitance: A higher input capacitance does not just increase circuit loading, it also makes the probe more susceptible to series inductance (ground lead or a wire between the probe tip and the circuit). Resonance frequency is inversely proportional to sqrt(L*C) after all.

author: trying to be clever
author_rank: Newbie
timestamp: May 05, 2022, 10:35:02 am
content: I always see switchable 1x 10x probes for sale that I didn't even know there were probes with just the 1x or the 10x.

author: Fungus
author_rank: Super Contributor
timestamp: May 05, 2022, 12:17:11 pm
content: Quote from: trying to be clever on May 05, 2022, 10:35:02 amThey're as rare as hen's teeth.Micsig sell their x10 probes separately:https://www.aliexpress.com/item/32757011342.html

author: james_s
author_rank: Super Contributor
timestamp: May 05, 2022, 06:11:52 pm
content: Quote from: trying to be clever on May 05, 2022, 10:35:02 amThe A-list brands have them. I've never seen a switchable Tek probe. I assume HP/Agilent, RS, etc are similar.

author: David Hess
author_rank: Super Contributor
timestamp: May 05, 2022, 06:22:46 pm
content: Quote from: alm on May 04, 2022, 05:46:14 pmIn 1977 some oscilloscopes from Tektronix came standard with fixed probes and some came with switchable probes. Fast instruments like the 350 MHz 485 only came with coaxial patch cables standard.