Correct Capacitance for a 1X Oscilloscope Probe - Page 1
Forum section: electronics / beginners

author: techie1234
author_rank: Regular Contributor
timestamp: July 31, 2016, 01:37:54 pm
content: Having recently bought an HP 1743A oscope which only came with one 1X probe, I am in the market to buy a second probe. I think most of the things I will be measuring for the time being will be pretty low frequency, so I’d like to get a second 1x probe for simplicity’s sake. After reading a bit on probe capacitance, I think I’m more confused than when I started. There’s seems to be a lot of talk about “matching the probe to the scope”. The scope has a 20pf capacitance, but I cannot find a 1X probe with that low of a capacitance. The probe I got with the scope (a Nicolet) isn’t even adjustable but seems to work fine and produce nice sharp square waves.So, my question is: what is the “target capacitance” when buying a 1X probe?a) Matching the capacitance of the oscope to the probe (and, if so, where are all these low-capacitance 1X probes)b) Just keeping the capacitance of the probe as low as possible (and, if so, what range is acceptable)c) It doesn’t really matter when measuring at low frequencies

author: tggzzz
author_rank: Super Contributor
timestamp: July 31, 2016, 02:13:02 pm
content: Look at all the Tektronix references listed inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/They also cover vital topics such as safety.

author: techie1234
author_rank: Regular Contributor
timestamp: July 31, 2016, 02:40:37 pm
content: I've read a number of different sources like those - and therein lies my confusion. For example, here's a statement from Tek's Probe's Fundamentals:Probes also present a capacitance to the input of theoscilloscope, and this probe capacitance should matchthat of the oscilloscope. For 10X and 100X probes, thiscapacitance is referred to as a compensation range,which is different than tip capacitance. For probe matching,the oscilloscope’s input capacitance should be within thecompensation range of the probe.The first sentence seems to suggest that a probe's capacitance should match the oscope's. Fair enough - but I cannot find a 1X probe that allows an adjustment into that range. My Nic probe doesn't even have the ability to adjust it. Or are they talking about just divider probes and 1X probes are a special case?

author: PA4TIM
author_rank: Super Contributor
timestamp: July 31, 2016, 02:51:34 pm
content: There is no need to adjust a 1x probe because they are only usable for low frequencies. I have around 20 probes, one of them is a 1X probe and I used it this year for the first time because a service manual asked for it in the adjusting procedure.

author: techie1234
author_rank: Regular Contributor
timestamp: July 31, 2016, 02:54:57 pm
content: Quote from: PA4TIM on July 31, 2016, 02:51:34 pmSo, in that case, is the goal then "Just keeping the capacitance of the probe as low as possible"? If so, what range of values is reasonably acceptable? I've seen some down around 40pf and others up around 90pf at 1x.

author: Ian.M
author_rank: Super Contributor
timestamp: July 31, 2016, 03:00:45 pm
content: Frankly, you'd be crazy not to get a x1/x10 switchable probe. Even at 10KHz, 50pF of cable + input capacitance gives an impedance of 320K, so it dominates over the scope's 1Meg input resistance. If you are probing any high impedance circuit, you *NEED* a x10 probe, even at the high end of the audio range.

author: Kilrah
author_rank: Unknown
timestamp: July 31, 2016, 03:06:57 pm
content: +1, get a common x1/x10 probe, you can find some pretty cheap and at least won't go "damn" the one time you'll have to measure higher freqs.

author: techie1234
author_rank: Regular Contributor
timestamp: July 31, 2016, 03:30:17 pm
content: re : 1x/10x probes: I had been thinking I would get a pair of 10x-only probes (since it seems that the selector switch can become problematic over time) after buying a 1x and then have a pair of each - but now perhaps I will just get 10x probes instead. The consensus appears to be that 1x is seldom used anyway - and I already have one - so no real need to buy more 1x hardware.However,the question remains: if one were to buy a 1x probe, is the goal simply to keep the capacitance as low as possible and, if so, what would be a reasonable target?

author: German_EE
author_rank: Super Contributor
timestamp: July 31, 2016, 04:29:44 pm
content: The two probes on my oscilloscope are a) permanently attached and b) permanently on x10. Yes, I effectively lose the bottom range but I gain a range at the top with a new maximum input voltage of 200V

author: rstofer
author_rank: Super Contributor
timestamp: July 31, 2016, 04:30:02 pm
content: Capacitance is probably a bad thing as it tends to modify the signal, regardless of the frequency. It's true that high values will corrupt lower frequencies and all that. But...I almost never use the x1 selection on my x1/x10 probes - why would I? I also don't want to spend any money on really good probes because I simply don't work beyond 50 MHz. So I buy the cheap ones (pretty high x1 capacitance):http://store.digilentinc.com/bnc-oscilloscope-x1-x10-probes-pair/These are marginally better (lower capacitance):http://www.mpja.com/200MHz-Oscilloscope-Probe/productinfo/15087+TEThis is a bit over the top but the x1 capacitance is really low - 6.5 pFhttp://www.mpja.com/250MHz-12KV-Oscilloscope-Probe/productinfo/18147+TEIf you actually plan to use the x1 range (and you probably shouldn't), shop around for the lowest possible value you can find. Capacitance is never a good thing.As to the switch problem? What switch problem? People have been using x1/x10 probes for generations without failure. I suppose in the race to the bottom for price, there might possibly be an issue but more likely than not, the probes are set at x10 and left that way.

author: StillTrying
author_rank: Super Contributor
timestamp: July 31, 2016, 04:34:41 pm
content: Quote from: techie1234 on July 31, 2016, 01:37:54 pmYou won't find a 20pF high-impeadance X1 probe.You should be looking for the probescompensation range, - which will match the scopes 20pF.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 31, 2016, 04:34:52 pm
content: I do like the Hantek PP150 probes. Looks like they still sell them for $8/ea. These are 80 to 150pf in 1X. Video where I compared them with a few other probes:

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 31, 2016, 04:48:34 pm
content: Quote from: techie1234 on July 31, 2016, 03:30:17 pmi dont own 1x probe but i have long 50ohm coax cable that can be made 1x probe easily. and no, the goal is not to keep capacitance low, 1x coax has several magnitudes worse the capacitance, the longer the worse. the 1/10X probe goals is to lower that capacitance effect and ability to compensation network circuit in the probe, so you get flatter signal respond at greater spectrum, the down side is that only 1/10X of the signal will reach the scope and need to be multiplied back for the display.Quote from: techie1234 on July 31, 2016, 03:30:17 pmand again if we talk about coax cable. it will be usefull for analog signal 50/75 ohm transmission line. when you have a roll of coax cable, you wont see much point getting a 1X "probe", because with coax cable, you can learn and make anything in probing industry.

author: PA4TIM
author_rank: Super Contributor
timestamp: July 31, 2016, 05:17:52 pm
content: 10 pF at 100 Hz is 160 Mohm, so who cares for capacitance at low frequency ? ;-)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 31, 2016, 06:14:50 pm
content: A proper 1x probe has considerable attenuation (over 10x) above a modest frequency, a few MHz.Beware if you're ever looking at pulses or switching or noise, you'll see a lot less than what's really there!Tim

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2016, 06:33:27 pm
content: Quote from: Mechatrommer on July 31, 2016, 04:48:34 pmA 1x probe is not just ordinary co-axial cable. It's lossy. If you measure the resistance between the probe tip and connector, you'll find it's in the region of 200 Ohms, rather than zero.Ordinary co-axial cable will act as a transmission line with an open circuit at one end, so will reflect the signal back to the source, which could cause high voltages to develop when the signal is at the resonant frequency. In a lossy 1x probe, all of the power is dissipated, before the signal is reflected back to the source.If you want to use ordinary co-axial cable as a 1x probe, then add a resistor at the signal end to absorb any reflections. The resistor should be greater than the characteristic impedance of the cable but not too high as it will lower the roll-off frequency.

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2016, 08:13:54 pm
content: techie1234, x1 passive probes do not have any input capacitance requirements for the oscilloscope and they do not include a compensation adjustment. They are effectively a direct connection from the probe tip to the oscilloscope input.x10 and x100 passive probes *do* have requirements for the oscilloscope input capacitance because their AC attenuation adjustment in the form of the compensation adjustment only works over a limited range.So for a switchable x1/x10 passive probe, the compensation adjustment is made in x10 mode where the input capacitance of the oscilloscope matters and the x1 probe setting ignores the compensation adjustment.

author: techie1234
author_rank: Regular Contributor
timestamp: July 31, 2016, 11:47:46 pm
content: Quote from: David Hess on July 31, 2016, 08:13:54 pmBingo - that's exactly what I was trying to understand. Many thanks!

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2016, 03:22:51 am
content: Quote from: techie1234 on July 31, 2016, 11:47:46 pmSomething which I forgot to include which may help in understanding is what exactly the x1/x10 switch does.A x10 passive probe has a 9 megohm resistor in series at the tip. This combined with 1 megohm input resistance of the oscilloscope divides the input voltage by 10. It *also* has the effect of dividing the capacitance (typically by about 5) which would otherwise be present at the tip down which is what makes x10 probes so much faster. The probe's compensation adjustment makes sure that the RC time constants of the 9 and 1 megohm resistances are equal so that the AC response is flat over the operating frequency range of the probe.The x1/x10 switch just shorts out the 9 megohm resistor. The extra circuits which produce a flat high frequency response for the x10 probe are still there but they do not affect a x1 probe because its bandwidth is so much lower; the extra tip capacitance swamps everything out. Since there is no input resistor, there is no time constant and no compensation adjustment.