New Chat: Question: Understanding a scope probe #chat-notice


TekScopes@groups.io Notification <noreply@...>
 

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I would like to better understand the components in a scope probe. But first I should clarify my beliefs about a VTVM. As I understand it, most VTVM's have a plain ordinary wire with a pointed probe tip. Any wire would do. The probe runs into the VTVM, where the first thing the signal goes through is an 11Mohm resistor, before going on to an amplifier to run the meter that reads out the voltage. The purpose of the amplifier being to amplify the signal by an amount that compensates for the gigantic drop in strength from the 11Mohm resistor, so that the meter reads actual voltage. The purpose being to sample a very tiny amount of "water running through a pipe" to give a pressure reading. Compared to a regular VOM which would require such a large amount of "water in a pipe", that it would actually cause a drop in "water pressure" just to run the meter, that the reading would be useless. Have I made a good analogy? If so, why does an oscilloscope probe utilize a capacitor in parallel with the 11Mohm resistor, when the VTVM doesn't? What is the purpose of that capacitor? Thank you.

By: henryfinley <tomsradio@...>

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Tom Lee
 

In a word, bandwidth. VTVM's don't need it, scopes do.

If you want the nitty-gritty, email me off list and I'll send you a writeup with more detail than anyone wants.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 5/18/2021 23:03, TekScopes@groups.io Notification wrote:

A new chat has been created:

New

I would like to better understand the components in a scope probe. But first I should clarify my beliefs about a VTVM. As I understand it, most VTVM's have a plain ordinary wire with a pointed probe tip. Any wire would do. The probe runs into the VTVM, where the first thing the signal goes through is an 11Mohm resistor, before going on to an amplifier to run the meter that reads out the voltage. The purpose of the amplifier being to amplify the signal by an amount that compensates for the gigantic drop in strength from the 11Mohm resistor, so that the meter reads actual voltage. The purpose being to sample a very tiny amount of "water running through a pipe" to give a pressure reading. Compared to a regular VOM which would require such a large amount of "water in a pipe", that it would actually cause a drop in "water pressure" just to run the meter, that the reading would be useless. Have I made a good analogy? If so, why does an oscilloscope probe utilize a capacitor in parallel with the 11Mohm resistor, when the VTVM doesn't? What is the purpose of that capacitor? Thank you.

*By:* henryfinley <tomsradio@gmx.com>

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Jean-Paul
 

Can we please use the normal message board, and NOT chats? A bit confusing.

To OP Henry:

VTVMs had various Zin and probes, some had different Rprobe in AC and DC inputs.

Best ref for scope probes in general:

Tektronix Circuit Concepts "Oscilloscope Probe Circuits Concepts" everything from passive/active/Zo, DC-many GHz.

https://w140.com/tekwiki/images/6/62/062-1146-00.pdf

Worthwhile to have the printed booklet.

Jon


Milan Trcka
 

Henry,
See a very nice expose and repair of "The BEST VTVM? You Decide! HP 410B Restoration and Alignment" - a real vacuum tube voltmeter. Rabbit hole starts here:
https://www.youtube.com/watch?v=FWjXgWlgl7U

M


-
 

That would suffix for a low frequency and low sensitivity voltmeter but
many voltmeters contain an active amplifier in the probe itself. You might
want to look up the HP 8405A Vector Voltmeter for details about how some of
the higher grade voltmeters operate. A couple of links <
https://users.monash.edu.au/~ralphk/hp8405a.html> and <
http://azurelectronics.com/HP%208405A%20Vector%20Voltmeter.htm> and <
https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1966-05.pdf>. The 8405 uses
two probes and displays both the voltage difference and the phase
difference between the two input signals.

On Wed, May 19, 2021 at 2:03 AM TekScopes@groups.io Notification <
noreply@groups.io> wrote:

A new chat has been created:
New

I would like to better understand the components in a scope probe. But
first I should clarify my beliefs about a VTVM. As I understand it, most
VTVM's have a plain ordinary wire with a pointed probe tip. Any wire would
do. The probe runs into the VTVM, where the first thing the signal goes
through is an 11Mohm resistor, before going on to an amplifier to run the
meter that reads out the voltage. The purpose of the amplifier being to
amplify the signal by an amount that compensates for the gigantic drop in
strength from the 11Mohm resistor, so that the meter reads actual voltage.
The purpose being to sample a very tiny amount of "water running through a
pipe" to give a pressure reading. Compared to a regular VOM which would
require such a large amount of "water in a pipe", that it would actually
cause a drop in "water pressure" just to run the meter, that the reading
would be useless. Have I made a good analogy? If so, why does an
oscilloscope probe utilize a capacitor in parallel with the 11Mohm
resistor, when the VTVM doesn't? What is the purpose of that capacitor?
Thank you.

*By:* henryfinley <tomsradio@gmx.com>

View/Join This Chat <https://groups.io/g/TekScopes/chat/8133>

Do not reply to this message to post to the chat. You can participate in
chats only through the group's website.