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x100 'scope probe

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George Herold
Guest

Wed Sep 26, 2018 12:45 am   



On Tuesday, September 25, 2018 at 5:46:40 PM UTC-4, John Larkin wrote:
Quote:
On Tue, 25 Sep 2018 08:07:40 -0700 (PDT), George Herold
gherold_at_teachspin.com> wrote:

On Tuesday, September 25, 2018 at 10:45:57 AM UTC-4, jurb...@gmail.com wrote:
You need a 90 meg if coming off an X 10 probe, 99 to the
BNC.I think.

Off the top of my head I think you'll be down to 1/10th the upper limit bandwidth, and on AC coupling it will take much longer to settle to the center is there's DC on the input. Also not that in most scope front ends that will NOT give you 10 X the max DC input.

The sell X 100 probes, a bit more money per everything else. I used one for certain things when faced with a scope that lacked 20V/div. Yeah, they gave you more at the low end but took away from the high end. No more looking at the collector of an HOT/LOPT with a X 10 probe.

Right I was going to stick it one the end of a x10 probe... I don't
really care if it's a x110, probe... mostly want to look at what might be
a high voltage without blowing up the 'scope.
(maybe it's a bad idea.)

George H.

Amazon has decent 100 MHz 100x scope probes for around $35.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com


Right, I was looking at cheap and now.
I should buy a x100 probe...

George H.

amdx
Guest

Thu Sep 27, 2018 10:45 pm   



On 9/25/2018 2:08 PM, amdx wrote:
Quote:
On 9/25/2018 9:34 AM, George Herold wrote:
I was thinking of making a DC (slow) x100 'scope probe by adding a
series 100 meg ohm resistor.  Any thing I should look out for?

George H.

 I built an attenuator to measure some high voltage spikes 25 or 30
years ago.
 I still have it, just checked the values to put on the picture in the
link below. I don't recall the attenuation, It might make a 10x be a 20x
or a 40x. The values make me think 20x but the cap has me not sure.
 I was able to calibrate it using the scope calibrator for a good
square wave, but the shield was important to allow that.
 I was always a little concerned for the scope probe coax arcing, but
at 800 volts, it never did.
https://www.dropbox.com/s/iwk8yzmkx766ls6/Probe%20Attenuator.jpg?dl=0
 Note: I put a T on the scope, then the probe off one side and the
attenuator off the other side.
 If I get some time I'll set it up and check the attenuation.

                      Mikek

I set the attenuator up on my bench scope, 300MHz scope with matching
probe, makes the 10X probe a 20X probe. I can also calibrate the probe
using the built in calibrator.


If making this is to much, how about an on board voltage divider.
10meg from your source to a 1.39meg to ground. Place your 10meg scope
probe at the junction. This is will display as a 10 to 1 ratio of the
initial HV. This assumes your HV source will drive an 11.22 Meg load
without appreciable sag and that your probe is 10meg at the frequency
of interest.
Mikek

amdx
Guest

Sat Sep 29, 2018 1:45 am   



On 9/25/2018 10:06 AM, Tom Gardner wrote:
Quote:
On 25/09/18 15:34, George Herold wrote:
I was thinking of making a DC (slow) x100 'scope probe by adding a
series 100 meg ohm resistor.  Any thing I should look out for?

George H.


Don't forget the point David Hess frequently makes...

If you are relying on the scope's 10Mohm input resistance
to form the lower leg of the potential divider, then consider
what happens if you turn the scope to AC input coupling. The
*entire* input voltage appears across the scope's AC coupling
capacitor.

That can and should be avoided by having a probe with the
potential divider made from two resistors, one for the upper
leg and the other is (in parallel with the scope/s input)
the lower leg.


I described this in a later post, but I adjusted the resistor divider
to make up for the loading of the probe, so it will be 100X with the
the probe attached.

Quote:

... how about an on board voltage divider.
10meg from your source to a 1.39meg to ground. Place your 10meg scope probe at the junction.
This is will display as a 10 to 1 ratio of the initial HV. This assumes your HV source will
drive an 11.22 Meg load without appreciable sag and that your probe is 10meg at the frequency
of interest.


Mikek

Phil Hobbs
Guest

Tue Oct 02, 2018 3:45 pm   



On 09/25/2018 07:14 PM, George Herold wrote:
Quote:
On Tuesday, September 25, 2018 at 5:53:50 PM UTC-4, John Larkin wrote:
On Tue, 25 Sep 2018 13:04:38 -0700 (PDT), George Herold
gherold_at_teachspin.com> wrote:

On Tuesday, September 25, 2018 at 3:03:26 PM UTC-4, jurb...@gmail.com wrote:
"Huh, I'm not seeing that at all. Don't I still have 1 meg to
ground inside the scope?"

Not the ones I've seen. Just as a typical 50-100 MHz scope (plugin) example, in

http://bama.edebris.com/manuals/tek/7a18/

on PDF page 59 is the input circuit. On the J 10/ R 10 side of C 10 there is nothing to ground, therefore the resistance is infinity. The voltage at J 10 will go to 100 % of the input, as it would with a 10 X probe anyway, but a 100 X is likely to see higher voltages.

I have never seen a scope front end that was not similar.
Huh. OK... I didn't look at the schematic, but my TEK1001 floats the input
at AC, (as you say.) The new Keysight reads 1.0 meg ohm at DC 1.0 meg ohm at
AC. I put in a DC voltage through a 1 meg resistor, and measured the
voltage at the 'scope input (AC and DC). It was 1/2 the applied voltage.
The same in both cases.

So there is at least one 'scope that is different.
(I never knew... thanks)

If DC is not needed a cap of 1/10th the value of C 10 or its equivalent would form a capacitive divider and with even miniscule leakage of C 10 or equivalent, the voltage will stay down. Its leakage is like to be much higher than that of the higher voltage cap you add externally to protect the scope's input cap.
Right, I've done that... I wanted DC
(well I don't need it any more.. voltage was not as high as I thought.)

George H.

My Rigol measures 1M input on a DVM (to 5 decimal places) on either AC
or DC coupling.

Set to 50 ohms, it measures 50.37 whether AC or DC coupled.

Good to know, I guess.
I didn't know my 'scope could be floating at AC coupling....
That's even better to know.
(shame on me.)


Some scopes, such as my TDS 784As, warn you that if you select AC
coupling with 50-ohm Zin, your lower cutoff frequency will be much
higher than you expect. Thus it seems that the AC coupling happens
before the input load, at least on the 50-ohm scale.

I have a couple of Pomona boxes with series capacitors plus protection
diodes on the output.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
https://hobbs-eo.com

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