Maker Pro
Maker Pro

Homebrew HV hiZ scope probe

R

Robert Baer

Jan 1, 1970
0
See similar title in a.b.s.electronic for PDF.
Almost took longer to document than to build..
 
M

Martin Riddle

Jan 1, 1970
0
Jan Panteltje said:
Nobody can get that group.

alt.binaries.schematics.electronic ?
I get that group, but I don't see the post.

Cheers
 
R

Robert Baer

Jan 1, 1970
0
TheGlimmerMan said:
* Bet it took a fair amount of time for that labeling in the image.
HV Probes are easy. A sealed dry AIR based (or gas) container for a
precision HV resistor string. Potted versions lose accuracy and are
harder to make calibration adjustments on.
Would adding heat shrink on the 1% resistors mess them up at high (up
to 20KV) voltage?
As an alternate, what about wrapping them with Kapton(TM)?
 
L

legg

Jan 1, 1970
0
Yep. For some reason I _never_ see any posts by Panteltje >:-}

...Jim Thompson

I believe its Baer's post that's gone astray.
RL
 
J

Jasen Betts

Jan 1, 1970
0
Nobody can get that group.

freenews.netfront.net carries ABSE but only the smaller posts (not
large binaries)

For the binaries Astraweb.com 180GB $25 never expires. Since march
ABSE has used aboiut 0.68GB, so at that rate I'll be putting it in
my will.
 
R

Robert Baer

Jan 1, 1970
0
legg said:
Somehow the post vaporized, so i put it up on my corporate site at
http://www.oil4lessllc.org/HV probes/ .
That probe was a "quick and dirty"; rise time seems to be better than
20nSec (my "pulse generator" is a HP3312A function generator).
However, the output step only goes up 70% then a slow rise to max
with a time constant of about 600nSec.
*
Am using SPICE to help design a 40KV probe and i seem to be homing
into a design with values.
The listing is below (and on that site).
The attenuation ratio is 2500 and is "protected" to ground like the
Tektronix probes.
Why 2500? Because it takes two 1Gohm resistors to withstand the 40KV
(target rating is 30KV with some "elbow room").
Note the resistance ratio is exactly 2500:1 and (now) the capacitance
ratio is close also.

Physical layout is to have a floating ring around each resistor,to
provide a controllable input coupling and capacitive divider (which is
the secret of a (theoretical) infinite risetime.
Now around this whole assembly will be a grounded shield (to isolate
input from external bazzzz fazzzz).
I think that five sections is a reasonable division of each resistor
for emulation of the actual distributed part.

Risetime seems to be infinite, but there is this slow "hump" that i
am fighting.
Any ideas as how to solve?

Version 4
SHEET 1 2064 680
WIRE -352 -16 -448 -16
WIRE -208 -16 -272 -16
WIRE -64 -16 -128 -16
WIRE 80 -16 16 -16
WIRE 224 -16 160 -16
WIRE 448 -16 304 -16
WIRE 592 -16 528 -16
WIRE 736 -16 672 -16
WIRE 880 -16 816 -16
WIRE 1024 -16 960 -16
WIRE 1280 -16 1104 -16
WIRE 1328 -16 1280 -16
WIRE 1520 -16 1328 -16
WIRE 1552 -16 1520 -16
WIRE -352 16 -352 -16
WIRE -272 16 -272 -16
WIRE -208 16 -208 -16
WIRE -128 16 -128 -16
WIRE -64 16 -64 -16
WIRE 16 16 16 -16
WIRE 80 16 80 -16
WIRE 160 16 160 -16
WIRE 224 16 224 -16
WIRE 304 16 304 -16
WIRE 448 16 448 -16
WIRE 528 16 528 -16
WIRE 592 16 592 -16
WIRE 672 16 672 -16
WIRE 736 16 736 -16
WIRE 816 16 816 -16
WIRE 880 16 880 -16
WIRE 960 16 960 -16
WIRE 1024 16 1024 -16
WIRE 1104 16 1104 -16
WIRE 1280 32 1280 -16
WIRE 1328 64 1328 -16
WIRE 1520 64 1520 -16
WIRE 1568 64 1520 64
WIRE 1568 80 1568 64
WIRE -352 112 -352 80
WIRE -272 112 -272 80
WIRE -272 112 -352 112
WIRE -208 112 -208 80
WIRE -208 112 -272 112
WIRE -128 112 -128 80
WIRE -128 112 -208 112
WIRE -64 112 -64 80
WIRE -64 112 -128 112
WIRE -32 112 -64 112
WIRE 16 112 16 80
WIRE 16 112 -32 112
WIRE 80 112 80 80
WIRE 80 112 16 112
WIRE 160 112 160 80
WIRE 160 112 80 112
WIRE 224 112 224 80
WIRE 224 112 160 112
WIRE 304 112 304 80
WIRE 304 112 224 112
WIRE 448 112 448 80
WIRE 528 112 528 80
WIRE 528 112 448 112
WIRE 592 112 592 80
WIRE 592 112 528 112
WIRE 672 112 672 80
WIRE 672 112 592 112
WIRE 736 112 736 80
WIRE 736 112 672 112
WIRE 784 112 736 112
WIRE 816 112 816 80
WIRE 816 112 784 112
WIRE 880 112 880 80
WIRE 880 112 816 112
WIRE 960 112 960 80
WIRE 960 112 880 112
WIRE 1024 112 1024 80
WIRE 1024 112 960 112
WIRE 1104 112 1104 80
WIRE 1104 112 1024 112
WIRE 1280 144 1280 96
WIRE 1568 144 1520 144
WIRE -448 160 -448 -16
WIRE -32 160 -32 112
WIRE 784 160 784 112
WIRE 1568 192 1568 144
WIRE 1328 224 1328 144
WIRE -448 272 -448 240
WIRE -32 272 -32 224
WIRE 784 272 784 224
FLAG 1328 224 0
FLAG 1568 192 0
FLAG 1552 -16 scope
FLAG 1280 144 0
FLAG -448 272 0
FLAG 784 272 0
FLAG -32 272 0
SYMBOL voltage -448 144 R0
WINDOW 0 8 7 Left 2
WINDOW 3 11 105 Left 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value PULSE(0 250 0 1p 1p 10 20 1)
SYMBOL cap 1296 96 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C31
SYMATTR Value 2478p
SYMBOL res 1312 48 R0
WINDOW 3 30 126 Left 2
SYMATTR Value 4.008Meg
SYMATTR InstName R31
SYMATTR SpiceLine tol=0.1 pwr=1
SYMBOL cap 1552 80 R0
SYMATTR InstName C32
SYMATTR Value 22p
SYMBOL res 1536 160 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R32
SYMATTR Value 1Meg
SYMBOL cap -368 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C1
SYMATTR Value {Cr}
SYMBOL cap -288 16 R0
SYMATTR InstName C2
SYMATTR Value {Cr}
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SYMATTR InstName R1
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SYMATTR Value {Cr}
SYMBOL cap -144 16 R0
SYMATTR InstName C4
SYMATTR Value {Cr}
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SYMATTR InstName R2
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SYMATTR Value {Cr}
SYMBOL cap 0 16 R0
SYMATTR InstName C6
SYMATTR Value {Cr}
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WINDOW 0 -24 71 VTop 2
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SYMATTR InstName R3
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SYMATTR InstName C7
SYMATTR Value {Cr}
SYMBOL cap 144 16 R0
SYMATTR InstName C8
SYMATTR Value {Cr}
SYMBOL res 64 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R4
SYMATTR Value {Rp}
SYMBOL cap 208 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C9
SYMATTR Value {Cr}
SYMBOL cap 288 16 R0
SYMATTR InstName C10
SYMATTR Value {Cr}
SYMBOL res 208 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R5
SYMATTR Value {Rp}
SYMBOL cap 432 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C11
SYMATTR Value {Cr}
SYMBOL cap 512 16 R0
SYMATTR InstName C12
SYMATTR Value {Cr}
SYMBOL res 432 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R6
SYMATTR Value {Rp}
SYMBOL cap 576 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C13
SYMATTR Value {Cr}
SYMBOL cap 656 16 R0
SYMATTR InstName C14
SYMATTR Value {Cr}
SYMBOL res 576 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
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SYMATTR Value {Rp}
SYMBOL cap 720 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C15
SYMATTR Value {Cr}
SYMBOL cap 800 16 R0
SYMATTR InstName C16
SYMATTR Value {Cr}
SYMBOL res 720 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
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SYMATTR Value {Rp}
SYMBOL cap 864 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C17
SYMATTR Value {Cr}
SYMBOL cap 944 16 R0
SYMATTR InstName C18
SYMATTR Value {Cr}
SYMBOL res 864 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R9
SYMATTR Value {Rp}
SYMBOL cap 1008 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C19
SYMATTR Value {Cr}
SYMBOL cap 1088 16 R0
SYMATTR InstName C20
SYMATTR Value {Cr}
SYMBOL res 1008 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R10
SYMATTR Value {Rp}
SYMBOL cap -48 160 R0
SYMATTR InstName C21
SYMATTR Value {Cs}
SYMBOL cap 768 160 R0
SYMATTR InstName C22
SYMATTR Value {Cs}
TEXT 264 256 Left 2 !.tran 0 10m 0 10n
TEXT 264 -208 Left 4 ;40KV 2E9 ohms HV scope probe
TEXT -144 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi net distributed form
TEXT 168 160 Left 2 !.PARAM Cr=10p, Cs=7.9p, Rp=200Meg
TEXT 672 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi net distributed form
 
L

legg

Jan 1, 1970
0
On Mon, 22 Oct 2012 22:04:28 -0800, Robert Baer

Somehow the post vaporized, so i put it up on my corporate site at
http://www.oil4lessllc.org/HV probes/ .
That probe was a "quick and dirty"; rise time seems to be better than
20nSec (my "pulse generator" is a HP3312A function generator).
However, the output step only goes up 70% then a slow rise to max
with a time constant of about 600nSec.
*
Am using SPICE to help design a 40KV probe and i seem to be homing
into a design with values.
The listing is below (and on that site).
The attenuation ratio is 2500 and is "protected" to ground like the
Tektronix probes.
Why 2500? Because it takes two 1Gohm resistors to withstand the 40KV
(target rating is 30KV with some "elbow room").
Note the resistance ratio is exactly 2500:1 and (now) the capacitance
ratio is close also.

Physical layout is to have a floating ring around each resistor,to
provide a controllable input coupling and capacitive divider (which is
the secret of a (theoretical) infinite risetime.
Now around this whole assembly will be a grounded shield (to isolate
input from external bazzzz fazzzz).
I think that five sections is a reasonable division of each resistor
for emulation of the actual distributed part.

Risetime seems to be infinite, but there is this slow "hump" that i
am fighting.
Any ideas as how to solve?

It's conventional to include a 'tip' resistor, that will absorb
contact (arc) surges safely.

The hardest thing about HV probe structure, is getting practical
values that are physically possible, to do the job. Look at physical
embodiments, make a few measurements, then go to the model.

Not much point in speculating over something that you can't build.
Sizing the capacitive divider is a real physical issue. Your Cr/Cs
ratio may be impractical.

As to risetime, an intentional RCseries, paralleling C22 position can
act on leading edges. Look at the current midpoint voltage (jn
C10/C11). Also, run an ac sweep.......the LF and HF gains will meet at
some point that will look like a can of worms, and is just as easily
manipulated, given physical constraints.

RL
 
L

legg

Jan 1, 1970
0
Somehow the post vaporized, so i put it up on my corporate site at
http://www.oil4lessllc.org/HV probes/ .
That probe was a "quick and dirty"; rise time seems to be better than
20nSec (my "pulse generator" is a HP3312A function generator).
However, the output step only goes up 70% then a slow rise to max
with a time constant of about 600nSec.
*
Am using SPICE to help design a 40KV probe and i seem to be homing
into a design with values.
The listing is below (and on that site).

Have you attempted a spice simulation of the part you've actually
built and tested? This would be more than just instructive and assist
in developing the newer iteration and an understanding of the spice
derivative.

RL
 
R

Robert Baer

Jan 1, 1970
0
legg said:
On Mon, 22 Oct 2012 22:04:28 -0800, Robert Baer



It's conventional to include a 'tip' resistor, that will absorb
contact (arc) surges safely.

The hardest thing about HV probe structure, is getting practical
values that are physically possible, to do the job. Look at physical
embodiments, make a few measurements, then go to the model.
* The only guides i have so far,are the Tektronix P6015 probe and the
"quick and dirty" 1Gohm probe i built.
Unfortunately, i have no way to making reasonable guesses related to
capacitances involved in their construction. The few P6015 drawings
leave a lot to be desired, and measurements on my Q&D probe would be
extremely difficult to do due to some of the very low values.
Not much point in speculating over something that you can't build.
Sizing the capacitive divider is a real physical issue. Your Cr/Cs
ratio may be impractical.
* But i have one working one, and the model for it.
And as far as sizing the capacitive divider, as long as there is a
simple capacitor from tip to output, and its ratio to a total
corresponding capacitance from there to ground, there is no problem;
that is simple - for a simple model.
As to risetime, an intentional RCseries, paralleling C22 position can
act on leading edges. Look at the current midpoint voltage (jn
C10/C11). Also, run an ac sweep.......the LF and HF gains will meet at
some point that will look like a can of worms, and is just as easily
manipulated, given physical constraints.

RL
* The Cs was tweaked for "best looks", and could be easily done
mechanically. The Cr is done via floating coaxial "shield" around the
resistor (see the aluminum foil wrap on the Q&D probe i built).
 
R

Robert Baer

Jan 1, 1970
0
Jon said:
Robert Baer wrote:


High-Ohm resistors have been reported to be non-linear, possibly that is
what you are getting. You may need some more poles of RC compensation
to try to fix this.

Jon
"non-linear" can mean many things..voltage coefficient, temperature
coefficient, and resistance per unit across the length (i assume that is
what you are referring to).
Where is that "report"? What date?
I do not think the resistance per unit length varies much, certainly
not so much as to give the waveform seen in the simulation.
That can be varied by quite a bit by changing Cs and/or Cr.
However, it might be a GOOD THING (as Martha Stewart would say) to
use ten pi pads instead of five for the resistor.
 
R

Robert Baer

Jan 1, 1970
0
legg said:
Have you attempted a spice simulation of the part you've actually
built and tested? This would be more than just instructive and assist
in developing the newer iteration and an understanding of the spice
derivative.

RL
Yes, but i started with a very simple C across the 1G resistor and
one to ground; it was plain that i needed (so to speak) more cable
capacitance,and better adjust-ability of that.
Made no refinements.
Basically, that probe was an exercise of "can it be done"?
And it has the noted deficiency of no ground return inside (hence the
big fat warnings).
 
R

Robert Baer

Jan 1, 1970
0
Fred said:
Robert Baer a écrit :

Wow, you're advertising the use of a mutilmeter (pun) probe for 7kV
usage...
Yep; cheap, small, does the job.
 
L

legg

Jan 1, 1970
0
Yes, but i started with a very simple C across the 1G resistor and
one to ground; it was plain that i needed (so to speak) more cable
capacitance,and better adjust-ability of that.
Made no refinements.
Basically, that probe was an exercise of "can it be done"?
And it has the noted deficiency of no ground return inside (hence the
big fat warnings).

Does the model act as the physical part scopes out?

RL
 
F

Fred Abse

Jan 1, 1970
0
Am using SPICE to help design a 40KV probe and i seem to be homing
into a design with values.
The listing is below (and on that site). The attenuation ratio is 2500
and is "protected" to ground like the
Tektronix probes.
Why 2500? Because it takes two 1Gohm resistors to withstand the 40KV
(target rating is 30KV with some "elbow room").
Note the resistance ratio is exactly 2500:1 and (now) the capacitance
ratio is close also.

Physical layout is to have a floating ring around each resistor,to
provide a controllable input coupling and capacitive divider (which is the
secret of a (theoretical) infinite risetime.
Now around this whole assembly will be a grounded shield (to isolate
input from external bazzzz fazzzz).
I think that five sections is a reasonable division of each resistor
for emulation of the actual distributed part.

Risetime seems to be infinite, but there is this slow "hump" that i
am fighting.
Any ideas as how to solve?

Takes forever to converge.

I softened the analysis a bit by making the pulse risetime 20ns, similar
to your HP3312A, but it's still excruciatingly slow.

How sure are you of the resistor model? Have you considered making it a
uniform RC line (U) model? I'll try that in a while, it might converge
faster.

There's a pole at around 67 Hz that needs addressing.

I added 3 feet of RG179, which i have a model for, which is similar to
RG175, for which I don't. The inevitable quarter-wave spikes appear at
around 46MHz, et seq. 60 ohms in series with each end of the coax tames
this, and we now have a 3dB rolloff around 70MHz, making risetime about
5ns. That's why resistive cable is used in commercial probes.

I'd do what Tektronix do, and do all compensation at the 'scope end.

Bear in mind that Tek's 40kV probes used to run their HV resistor in an
atmosphere of Fluorcarbon 114 vapor, which needed topping up from time to
time. I don't know what they use these days in HV probes, but I doubt it's
FC.

Try this, it's your circuit with 3 feet of coax. Do an .ac analysis, with
and without the 60 ohm resistors.

Version 4
SHEET 1 2064 680
WIRE -352 -16 -448 -16
WIRE -208 -16 -272 -16
WIRE -64 -16 -128 -16
WIRE 80 -16 16 -16
WIRE 224 -16 160 -16
WIRE 448 -16 304 -16
WIRE 592 -16 528 -16
WIRE 736 -16 672 -16
WIRE 880 -16 816 -16
WIRE 1024 -16 960 -16
WIRE 1280 -16 1104 -16
WIRE 1328 -16 1280 -16
WIRE 1408 -16 1328 -16
WIRE 1568 -16 1488 -16
WIRE 1760 -16 1664 -16
WIRE 1952 -16 1840 -16
WIRE 1984 -16 1952 -16
WIRE -352 16 -352 -16
WIRE -272 16 -272 -16
WIRE -208 16 -208 -16
WIRE -128 16 -128 -16
WIRE -64 16 -64 -16
WIRE 16 16 16 -16
WIRE 80 16 80 -16
WIRE 160 16 160 -16
WIRE 224 16 224 -16
WIRE 304 16 304 -16
WIRE 448 16 448 -16
WIRE 528 16 528 -16
WIRE 592 16 592 -16
WIRE 672 16 672 -16
WIRE 736 16 736 -16
WIRE 816 16 816 -16
WIRE 880 16 880 -16
WIRE 960 16 960 -16
WIRE 1024 16 1024 -16
WIRE 1104 16 1104 -16
WIRE 1568 16 1552 16
WIRE 1680 16 1664 16
WIRE 1280 32 1280 -16
WIRE 1328 64 1328 -16
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WIRE 2000 80 2000 64
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WIRE -128 112 -208 112
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WIRE -64 112 -128 112
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WIRE 16 112 16 80
WIRE 16 112 -32 112
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WIRE 1104 112 1024 112
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FLAG 1328 224 0
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FLAG 1984 -16 scope
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FLAG 1680 224 0
FLAG 1552 224 0
SYMBOL voltage -448 144 R0
WINDOW 0 8 7 Left 2
WINDOW 3 11 105 Left 2
WINDOW 123 11 123 Left 2
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SYMATTR InstName V2
SYMATTR Value PULSE(0 250 0 20n 20n 1m 2m 10)
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WINDOW 0 24 56 Left 2
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SYMATTR Value 4.008Meg
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SYMATTR SpiceLine tol=0.1 pwr=1
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SYMATTR InstName C8
SYMATTR Value {Cr}
SYMBOL res 64 0 R270
WINDOW 0 -24 71 VTop 2
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SYMBOL cap 208 16 R0
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SYMATTR InstName C9
SYMATTR Value {Cr}
SYMBOL cap 288 16 R0
SYMATTR InstName C10
SYMATTR Value {Cr}
SYMBOL res 208 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R5
SYMATTR Value {Rp}
SYMBOL cap 432 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C11
SYMATTR Value {Cr}
SYMBOL cap 512 16 R0
SYMATTR InstName C12
SYMATTR Value {Cr}
SYMBOL res 432 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R6
SYMATTR Value {Rp}
SYMBOL cap 576 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C13
SYMATTR Value {Cr}
SYMBOL cap 656 16 R0
SYMATTR InstName C14
SYMATTR Value {Cr}
SYMBOL res 576 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R7
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SYMBOL cap 720 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C15
SYMATTR Value {Cr}
SYMBOL cap 800 16 R0
SYMATTR InstName C16
SYMATTR Value {Cr}
SYMBOL res 720 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R8
SYMATTR Value {Rp}
SYMBOL cap 864 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C17
SYMATTR Value {Cr}
SYMBOL cap 944 16 R0
SYMATTR InstName C18
SYMATTR Value {Cr}
SYMBOL res 864 0 R270
WINDOW 0 -24 71 VTop 2
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SYMATTR Value {Rp}
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SYMATTR Value {Cr}
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SYMATTR InstName C20
SYMATTR Value {Cr}
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WINDOW 0 -24 71 VTop 2
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SYMATTR Value {Rp}
SYMBOL cap -48 160 R0
SYMATTR InstName C21
SYMATTR Value {Cs}
SYMBOL cap 768 160 R0
SYMATTR InstName C22
SYMATTR Value {Cs}
SYMBOL ltline 1616 0 R0
SYMATTR InstName O1
SYMATTR Value RG179
SYMBOL res 1504 0 M270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R11
SYMATTR Value {R}
SYMBOL res 1856 0 M270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R12
SYMATTR Value {R}
TEXT 264 256 Left 2 !.ac dec 1000 1 200meg
TEXT 264 -208 Left 4 ;40KV 2E9 ohms HV scope probe
TEXT -144 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi net distributed form
TEXT 168 160 Left 2 !.PARAM Cr=10p, Cs=7.9p, Rp=200Meg
TEXT 672 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi net distributed form
TEXT 272 384 Left 2 !.model RG179 LTRA (\n+ len=3\n+ L=.1u\n+ C=19.5p\n+ R=252.5e-3\n+)
TEXT 272 328 Left 2 !;.step param R 10 100 10m \n.param R = 60
TEXT 272 -176 Left 2 ;3 feet RG179 (similar to RG175) added to model\nBelden 83265 data used.
 
R

Robert Baer

Jan 1, 1970
0
Fred said:
Takes forever to converge.

I softened the analysis a bit by making the pulse risetime 20ns, similar
to your HP3312A, but it's still excruciatingly slow.

How sure are you of the resistor model? Have you considered making it a
uniform RC line (U) model? I'll try that in a while, it might converge
faster.

There's a pole at around 67 Hz that needs addressing.

I added 3 feet of RG179, which i have a model for, which is similar to
RG175, for which I don't. The inevitable quarter-wave spikes appear at
around 46MHz, et seq. 60 ohms in series with each end of the coax tames
this, and we now have a 3dB rolloff around 70MHz, making risetime about
5ns. That's why resistive cable is used in commercial probes.

I'd do what Tektronix do, and do all compensation at the 'scope end.

Bear in mind that Tek's 40kV probes used to run their HV resistor in an
atmosphere of Fluorcarbon 114 vapor, which needed topping up from time to
time. I don't know what they use these days in HV probes, but I doubt it's
FC.

Try this, it's your circuit with 3 feet of coax. Do an .ac analysis, with
and without the 60 ohm resistors.
** This is what i have, before i saw your response; not too bad
considering simple compensation and implicit coax of indeterminate (but
limited) length to scope end.
Also posted at: http://www.oil4lessllc.org/HV probes/ .

Version 4
SHEET 1 2064 680
WIRE 1488 -64 1488 -240
WIRE -352 -16 -448 -16
WIRE -208 -16 -272 -16
WIRE -64 -16 -128 -16
WIRE 80 -16 16 -16
WIRE 224 -16 160 -16
WIRE 448 -16 304 -16
WIRE 592 -16 528 -16
WIRE 736 -16 672 -16
WIRE 880 -16 816 -16
WIRE 1024 -16 960 -16
WIRE 1168 -16 1104 -16
WIRE 1200 -16 1168 -16
WIRE 1312 -16 1280 -16
WIRE 1440 -16 1312 -16
WIRE 1568 -16 1440 -16
WIRE 1600 -16 1568 -16
WIRE -352 16 -352 -16
WIRE -272 16 -272 -16
WIRE -208 16 -208 -16
WIRE -128 16 -128 -16
WIRE -64 16 -64 -16
WIRE 16 16 16 -16
WIRE 80 16 80 -16
WIRE 160 16 160 -16
WIRE 224 16 224 -16
WIRE 304 16 304 -16
WIRE 448 16 448 -16
WIRE 528 16 528 -16
WIRE 592 16 592 -16
WIRE 672 16 672 -16
WIRE 736 16 736 -16
WIRE 816 16 816 -16
WIRE 880 16 880 -16
WIRE 960 16 960 -16
WIRE 1024 16 1024 -16
WIRE 1104 16 1104 -16
WIRE 1440 64 1440 -16
WIRE 1568 64 1568 -16
WIRE 1616 64 1568 64
WIRE 1312 80 1312 -16
WIRE 1616 80 1616 64
WIRE 1168 96 1168 -16
WIRE -352 112 -352 80
WIRE -272 112 -272 80
WIRE -272 112 -352 112
WIRE -208 112 -208 80
WIRE -208 112 -272 112
WIRE -128 112 -128 80
WIRE -128 112 -208 112
WIRE -64 112 -64 80
WIRE -64 112 -128 112
WIRE -32 112 -64 112
WIRE 16 112 16 80
WIRE 16 112 -32 112
WIRE 80 112 80 80
WIRE 80 112 16 112
WIRE 160 112 160 80
WIRE 160 112 80 112
WIRE 224 112 224 80
WIRE 224 112 160 112
WIRE 304 112 304 80
WIRE 304 112 224 112
WIRE 448 112 448 80
WIRE 528 112 528 80
WIRE 528 112 448 112
WIRE 592 112 592 80
WIRE 592 112 528 112
WIRE 672 112 672 80
WIRE 672 112 592 112
WIRE 736 112 736 80
WIRE 736 112 672 112
WIRE 784 112 736 112
WIRE 816 112 816 80
WIRE 816 112 784 112
WIRE 880 112 880 80
WIRE 880 112 816 112
WIRE 960 112 960 80
WIRE 960 112 880 112
WIRE 1024 112 1024 80
WIRE 1024 112 960 112
WIRE 1104 112 1104 80
WIRE 1104 112 1024 112
WIRE -448 160 -448 -16
WIRE -32 160 -32 112
WIRE 784 160 784 112
WIRE 1568 176 1568 144
WIRE 1616 176 1616 144
WIRE 1616 176 1568 176
WIRE 1616 192 1616 176
WIRE 1312 208 1312 160
WIRE 1168 224 1168 160
WIRE 1440 224 1440 144
WIRE -448 272 -448 240
WIRE -32 272 -32 224
WIRE 784 272 784 224
WIRE 1312 336 1312 272
FLAG 1440 224 0
FLAG 1616 192 0
FLAG 1600 -16 scope
FLAG 1168 224 0
FLAG -448 272 0
FLAG 784 272 0
FLAG -32 272 0
FLAG 1312 336 0
SYMBOL voltage -448 144 R0
WINDOW 0 8 7 Left 2
WINDOW 3 11 105 Left 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value PULSE(0 250 0 1p 1p 10 20 1)
SYMBOL cap 1184 160 R180
WINDOW 0 -37 59 Left 2
WINDOW 3 28 4 Left 2
SYMATTR InstName C91
SYMATTR Value 1170p
SYMBOL res 1424 48 R0
WINDOW 3 -89 120 Left 2
WINDOW 0 -49 70 Left 2
SYMATTR Value 4.008Meg
SYMATTR InstName R33
SYMATTR SpiceLine tol=0.1 pwr=1
SYMBOL cap 1600 80 R0
SYMATTR InstName C32
SYMATTR Value 22p
SYMBOL res 1584 160 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R32
SYMATTR Value 1Meg
SYMBOL cap -368 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C1
SYMATTR Value {Cr}
SYMBOL cap -288 16 R0
SYMATTR InstName C2
SYMATTR Value {Cr}
SYMBOL res -368 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R1
SYMATTR Value {Rp}
SYMBOL cap -224 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C3
SYMATTR Value {Cr}
SYMBOL cap -144 16 R0
SYMATTR InstName C4
SYMATTR Value {Cr}
SYMBOL res -224 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R2
SYMATTR Value {Rp}
SYMBOL cap -80 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C5
SYMATTR Value {Cr}
SYMBOL cap 0 16 R0
SYMATTR InstName C6
SYMATTR Value {Cr}
SYMBOL res -80 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R3
SYMATTR Value {Rp}
SYMBOL cap 64 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C7
SYMATTR Value {Cr}
SYMBOL cap 144 16 R0
SYMATTR InstName C8
SYMATTR Value {Cr}
SYMBOL res 64 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R4
SYMATTR Value {Rp}
SYMBOL cap 208 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C9
SYMATTR Value {Cr}
SYMBOL cap 288 16 R0
SYMATTR InstName C10
SYMATTR Value {Cr}
SYMBOL res 208 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R5
SYMATTR Value {Rp}
SYMBOL cap 432 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C11
SYMATTR Value {Cr}
SYMBOL cap 512 16 R0
SYMATTR InstName C12
SYMATTR Value {Cr}
SYMBOL res 432 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R6
SYMATTR Value {Rp}
SYMBOL cap 576 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C13
SYMATTR Value {Cr}
SYMBOL cap 656 16 R0
SYMATTR InstName C14
SYMATTR Value {Cr}
SYMBOL res 576 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R7
SYMATTR Value {Rp}
SYMBOL cap 720 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C15
SYMATTR Value {Cr}
SYMBOL cap 800 16 R0
SYMATTR InstName C16
SYMATTR Value {Cr}
SYMBOL res 720 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R8
SYMATTR Value {Rp}
SYMBOL cap 864 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C17
SYMATTR Value {Cr}
SYMBOL cap 944 16 R0
SYMATTR InstName C18
SYMATTR Value {Cr}
SYMBOL res 864 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R9
SYMATTR Value {Rp}
SYMBOL cap 1008 16 R0
WINDOW 0 20 11 Left 2
SYMATTR InstName C19
SYMATTR Value {Cr}
SYMBOL cap 1088 16 R0
SYMATTR InstName C20
SYMATTR Value {Cr}
SYMBOL res 1008 0 R270
WINDOW 0 -24 71 VTop 2
WINDOW 3 68 60 VBottom 2
SYMATTR InstName R10
SYMATTR Value {Rp}
SYMBOL cap -48 160 R0
SYMATTR InstName C21
SYMATTR Value {Cs}
SYMBOL cap 768 160 R0
SYMATTR InstName C22
SYMATTR Value {Cs}
SYMBOL res 1184 0 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R91
SYMATTR Value 1200
SYMBOL cap 1328 272 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C92
SYMATTR Value 900p
SYMBOL res 1296 64 R0
WINDOW 3 -56 49 Left 2
WINDOW 0 -40 8 Left 2
SYMATTR Value 590K
SYMATTR InstName R92
TEXT 264 256 Left 2 !.tran 0 10m 0 10u
TEXT 264 -232 Left 4 ;40KV 2E9 ohms HV scope probe 2500:1
TEXT -144 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi pad distributed form
TEXT 168 160 Left 2 !.PARAM Cr=2.048p, Cs=0.1p, Rp=200Meg
TEXT 672 -120 Left 2 ;Ohmite MOX2-131007FE\nin pi pad distributed form
TEXT 1536 -152 Left 2 ;[SCOPE]
TEXT 1320 -152 Left 2 ;[PROBE]
TEXT 336 -184 Left 3 ;Tr~27nSec; 2% overshoot at 1mSec
 
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