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Old 24th Jul 2017, 4:55 pm   #1
Skywave
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Question Home-brew 100:1 'scope probe.

Occasionally I have a need for a 100:1 'scope probe. The max. voltage it will ever see at the tip will be about 50 v. RMS. Based on an idea I found on the 'Net, I came up with the following drawing. The 'scope it will be used with is a Tek. 2465; 1MΩ input resistance + 15 pF. A few Qs. have arisen about this probe, so I'm asking for advice here, please.

1. What would be a suitable choice of coax cable and a suitable length for it? I'm thinking in terms of about 4 feet. I anticipate that the capacitance of that cable will be the predominant factor determining the B/W of the probe. What -3 dB B/W am I likely to achieve?

2. Should the probe body have a metal or a plastic case? Metal will provide screening but also introduce capacitance.

Your thoughts and comments will be appreciated: thank you.

Al.
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Old 24th Jul 2017, 5:15 pm   #2
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Default Re: Home-brew 100:1 'scope probe.

Scope probe co-ax is quite special (low capacitance i.e. a very thin inner conductor), your best bet is to modify an existing cheap 10:1 probe. question, why are you bothering with only 50V to measure? The circuit supplied only reduces the input (to the 'scope) volts and doesn't decrease the load to the circuit. I do have a proper 100:1 probe, 100M ohm resistance and only a couple of pF, now that is useful.
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Old 24th Jul 2017, 5:40 pm   #3
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Default Re: Home-brew 100:1 'scope probe.

There's a cheap Philips one on ebay at the moment if that's of interest. Search for "Philips probe", UK only. One of the listed ones is a 100:1
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Old 24th Jul 2017, 6:47 pm   #4
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Arrow Re: Home-brew 100:1 'scope probe.

Yes, thanks Mr. B.: found it. The photo doesn't make it clear if the 'scope end of the probe has a male BNC fitted. I'll be very surprised if it isn't but Philips often do things that aren't 'standard'. Anyway, I've asked the seller about it. So maybe . . . .

Al.
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Old 24th Jul 2017, 6:57 pm   #5
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Arrow Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by merlinmaxwell View Post
Scope probe co-ax is quite special (low capacitance i.e. a very thin inner conductor). Your best bet is to modify an existing cheap 10:1 probe.
Been down that avenue with bits from a salvaged Tek. probe. Trouble with that approach is that the coax cable inner is not only extremely thin & difficult to see, it also won't 'take' solder!
Modifying a 'standard' 10:1 probe is not feasible either, since it will be necessary to gain access to the inside of the probe to replace the series resistor. The probes I have - and all the ones that I've ever used - have plastic bodies which are cannot be dismantled - least, not without destroying the outer case (and probably the innards!)

Quote:
Originally Posted by merlinmaxwell View Post
The circuit supplied only reduces the input (to the 'scope) volts and doesn't decrease the load to the circuit.
The load on the circuit, compared to a 10:1 probe will be less on account of the higher value series resistor. Yes, the load on the circuit will also be determined by the probe's input capacity and as I said, I suspect that the capacity of the cable will be the determining factor of that in the given circuit.

Al.
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Old 24th Jul 2017, 7:19 pm   #6
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Default Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by Skywave View Post
The load on the circuit, compared to a 10:1 probe will be less on account of the higher value series resistor.

Al.
Not necessarily. I've come across 100:1 probes where the higher ratio was done by having a shunt resistor at the scope end and a shunt capacitor for equalisation.

If you're interested not so much for the voltage scaling as for the higher impedance, then building your own with 75 ohm coax isn't so bad as the capacitor ratio is more favourable at 100:1 so the cox isn't as damaging to bandwidth.

If you don't want a lot of voltage range, but do want high Z then you may be in the market for an active probe?

David
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Old 24th Jul 2017, 7:35 pm   #7
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Default Re: Home-brew 100:1 'scope probe.

Wouldn't the simplest thing be to use a standard 10:1 probe with a 'T' piece at the 'scope end and a shunt 100k resistor and a capacitor as required.
O.K. the probe input resistance would be 9.09M instead of 10M but would this matter?

The problem with any non-lossy coaxial cable is resonances, e.g. a 2m cable will be a quarter wave at about 25 MHz (assuming approx 67% velocity factor)and so would look like a short circuit at the probe end if unterminated at the 'scope end. Add the 'scope input capacity and the resonance would occur at an even lower frequency. For this reason normal probes use resistive inner conductors to damp out resonances.

Jim

Last edited by jimmc101; 24th Jul 2017 at 7:40 pm. Reason: Clarification
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Old 24th Jul 2017, 8:02 pm   #8
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Thumbs up Re: Home-brew 100:1 'scope probe.

Jim: the idea of a T-piece + 100 kΩ is a new one to me: I can see the logic and that it would indeed give 100:1 (or very near). I also did not know about the merit of a resistive inner conductor to damp out resonances.

All very thought-provoking: thank you.

Al.
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Old 24th Jul 2017, 8:05 pm   #9
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Default Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by Radio Wrangler View Post
If you don't want a lot of voltage range, but do want high Z then you may be in the market for an active probe?
Nice idea, but ones that are ready-made and off-the-shelf are too expensive for me.

Al.
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Old 24th Jul 2017, 8:33 pm   #10
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Default Re: Home-brew 100:1 'scope probe.

You can knock your own active probe up with ~ 90MHz of bandwidth for a couple of quid. See Bob Pease active probe from Troubleshooting Analog Circuits:

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I built one but with a 2N3958.

Note: I blew it up about a week later. They're quite fragile!
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Old 24th Jul 2017, 10:19 pm   #11
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Default Re: Home-brew 100:1 'scope probe.

Few articles treat the cable of a 'scope probe as anything other than a lumped capacity even though it is a significant fraction of a wavelength for frequencies well within the range of most 'scopes.
Trawling the internet since my last post turned up two articles that are well worth reading (in my opinion)...

The secret World of Oscilloscope Probes by Doug Ford
and
An introduction to passive voltage probes by Neutronix Ltd

Jim
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Old 24th Jul 2017, 10:29 pm   #12
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Default Re: Home-brew 100:1 'scope probe.

Thank you, Mr. B. That is worthy of serious consideration. Perhaps the design would tolerate a NPN/PNP pair with higher fT to give an increased B/W.

Al.
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Old 24th Jul 2017, 10:47 pm   #13
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Default Re: Home-brew 100:1 'scope probe.

A good read is the Tektronix Circuit Concepts and Measurement Concepts series, the vast majority of which are here http://w140.com/tekwiki/wiki/Concepts_Series

The particular ones of relevance are Oscilloscope Probe Circuits (in particular) and Probe Measurements.

But in a nutshell, the resistive core of the wire is there to provide electrical damping to prevent ringing. Also the resistance wire is very thin, and that reduces cable capacitance by a factor of 3 as compared with RG58.

But - since you aren't going to be really electrically stressing this probe, why not pay 10 - 20 and buy a Chinese Tek ripoff?

Craig

Last edited by Craig Sawyers; 24th Jul 2017 at 10:55 pm.
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Old 24th Jul 2017, 11:05 pm   #14
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Arrow Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by merlinmaxwell View Post
Question: why are you bothering with only 50V to measure?
I have an idea which is currently in the "does this make sense?" stage. Briefly, the use of such a probe for use with 'something' that is not an oscilloscope. That "something" will not 'see' any voltages greater than 50 v. rms. (Well, that's the 'design intention'! ) When - and if - things do materialize, I'll explain further. But that won't be soon: my available time to pursue my hobby is very restricted these days. And the reasons for that are off-topic here . . .

Al.
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Old 24th Jul 2017, 11:11 pm   #15
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Thumbs down Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by Craig Sawyers View Post
But - since you aren't going to be really electrically stressing this probe, why not pay 10 - 20 and buy a Chinese Tek rip-off?
Tried that - twice. The performance on both counts was absolutely dreadful! Just two more examples of "What you get is what you pay for it".

Al.
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Old 24th Jul 2017, 11:21 pm   #16
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Default Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by Skywave View Post
Thank you, Mr. B. That is worthy of serious consideration. Perhaps the design would tolerate a NPN/PNP pair with higher fT to give an increased B/W.

Al.
Possibly. I've actually been looking at some of the new opamps you can get which may be usable for this as well. Linear's LTC6268 has 4GHz gain-bandwidth product, 0.45pF Cin and 20fA (yes femto-amp) bias current for just over 6. Only bad side is stability and +/200mV input before it'll blow up.
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Old 25th Jul 2017, 12:13 am   #17
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Default Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by Skywave View Post
Quote:
Originally Posted by Craig Sawyers View Post
But - since you aren't going to be really electrically stressing this probe, why not pay 10 - 20 and buy a Chinese Tek rip-off?
Tried that - twice. The performance on both counts was absolutely dreadful! Just two more examples of "What you get is what you pay for it".

Al.
Well, using a 50 or 75 ohm coax will probably do no better than 10 or 20MHz.

If you need something approaching the 300MHz bandwidth of your scope, the only sensibly priced pukka option is an old P6009, which is good for about 200MHz. The odd NOS one crops up (I have one myself). About $50 to $80 plus shipping from the US.

The only other traditional resistive divider probes are the P5100 and P5100A, but those are still stupid money.

But you might consider a Z0 probe. These can be made in a roll-your-own and good to 1GHz. Although the input resistance might seem weedy at 1k to 5k (depending on attenuation), it hardly changes at all with frequency. Whereas a conventional probe has a falling impedance with frequency - at 100MHz such a probe, even with only 2pF input will look like j800 ohms, whereas a Z0 probe still looks substantially resistive.

Roll your own plans here http://emcesd.com/1ghzprob.htm
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Old 25th Jul 2017, 4:02 am   #18
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Default Re: Home-brew 100:1 'scope probe.

If anybody is interested in the Bob Pease articles, they can be found here

http://www.introni.it/riviste_bob_pease.html

These are the EDN articles, not the book as mentioned above.

A quick scan shows part 8 deals with probes, there could be more in other parts.

Terry
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Old 25th Jul 2017, 8:43 am   #19
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Default Re: Home-brew 100:1 'scope probe.

That's a veritable goldmine of information there. Thanks for posting.
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Old 25th Jul 2017, 10:17 am   #20
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Question Re: Home-brew 100:1 'scope probe.

Quote:
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Roll your own plans here http://emcesd.com/1ghzprob.htm
Thanks - read with interest. But I am puzzled by what he has written in the paragraph titled 'Theory of Operation'. He states that the coaxial cable is terminated with a 25Ω load: the 50Ω termination (the four 200Ω resistors in parallel) and the 50Ω characteristic impedance (Zo) of the coaxial cable in parallel with those four 200Ω resistors. But that "25Ω" is not right, is it? The Zo of the cable does not present a load that terminates the cable itself! Moreover, the coax cable will need to 'see' 50Ω if reflections are to be avoided, so his "25Ω" termination is simply wrong - in theory and in practice.

Or have I missed something there?

Al.
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