UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 24th Jul 2017, 4:55 pm   #1
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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.
Attached Thumbnails
Click image for larger version

Name:	PROBE.jpg
Views:	349
Size:	39.1 KB
ID:	146729  
Skywave is offline  
Old 24th Jul 2017, 5:15 pm   #2
Guest
Guest
 
Posts: n/a
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.
 
Old 24th Jul 2017, 5:40 pm   #3
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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
MrBungle is offline  
Old 24th Jul 2017, 6:47 pm   #4
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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.
Skywave is offline  
Old 24th Jul 2017, 6:57 pm   #5
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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.
Skywave is offline  
Old 24th Jul 2017, 7:19 pm   #6
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
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
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 24th Jul 2017, 7:35 pm   #7
jimmc101
Heptode
 
jimmc101's Avatar
 
Join Date: Mar 2009
Location: Portsmouth, Hampshire, UK.
Posts: 674
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
jimmc101 is offline  
Old 24th Jul 2017, 8:02 pm   #8
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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.
Skywave is offline  
Old 24th Jul 2017, 8:05 pm   #9
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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.
Skywave is offline  
Old 24th Jul 2017, 8:33 pm   #10
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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:

Click image for larger version

Name:	kfix4.jpg
Views:	226
Size:	71.6 KB
ID:	146742

I built one but with a 2N3958.

Note: I blew it up about a week later. They're quite fragile!
MrBungle is offline  
Old 24th Jul 2017, 10:19 pm   #11
jimmc101
Heptode
 
jimmc101's Avatar
 
Join Date: Mar 2009
Location: Portsmouth, Hampshire, UK.
Posts: 674
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
jimmc101 is offline  
Old 24th Jul 2017, 10:29 pm   #12
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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.
Skywave is offline  
Old 24th Jul 2017, 10:47 pm   #13
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,942
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.
Craig Sawyers is offline  
Old 24th Jul 2017, 11:05 pm   #14
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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.
Skywave is offline  
Old 24th Jul 2017, 11:11 pm   #15
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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.
Skywave is offline  
Old 24th Jul 2017, 11:21 pm   #16
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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.
MrBungle is offline  
Old 25th Jul 2017, 12:13 am   #17
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,942
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
Craig Sawyers is offline  
Old 25th Jul 2017, 4:02 am   #18
Terry_VK5TM
Nonode
 
Terry_VK5TM's Avatar
 
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
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
Terry_VK5TM is offline  
Old 25th Jul 2017, 8:43 am   #19
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
Default Re: Home-brew 100:1 'scope probe.

That's a veritable goldmine of information there. Thanks for posting.
MrBungle is offline  
Old 25th Jul 2017, 10:17 am   #20
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Question Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by Craig Sawyers View Post
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.
Skywave is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 3:00 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.