Homebrew HV hiZ scope probe

[Robert Baer]

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.

I see you made Cs, capacitance from floating shields, a lot larger =
8pf instead of my wild guesstimate of 0.1pf.
Those floating shields allow defined and predictable capacitive
coupling across the resistors; the capacitance from them to "outer
space" ground seems to be undefinable: coax capacitance runs
(log(D/d))^-1 and with a theoretically infinite D (or very large D in
reality),the capacitance is rather close to zero.
I picked 0.1pf as an estimate to that "zero"; seems you picked a huge
8pf.
*
I did not know of that uniform RC line (U) model; it would be an
excellent choice; better than the pi-pad scheme.
Values can easily be calculated on basis of resistor diameter,
dielectric material and floating shield diameter.
Seems that an eXplicit shield around the floating shield makes for
more trouble.

 

[Fred Abse]

I see you made Cs, capacitance from floating shields, a lot larger =
8pf instead of my wild guesstimate of 0.1pf.

No, I refer you to your previous post,<[email protected]>:


"TEXT 168 160 Left 2 !.PARAM Cr=10p, Cs=7.9p, Rp=200Meg"

Cs=7.9pF

Exact copy of your published schematic, with the addition of 3 feet of
cable, and two resistors.

I wondered about that.

The same value occurs in the listing on your website.

I didn't pick it, you did ;-(

 

[Fred Abse]

** 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.

You can't ignore the effect of what is, in effect, an open-circuit quarter
wave line, at a frequency within the intended bandwidth of the probe.

3 feet of 0,66 velocity coax, terminated in a large resistance will
resonate at about 54MHz, and ring like a bell. That's why resistive cables
are used.

 

[Jeroen Belleman]

You can't ignore the effect of what is, in effect, an open-circuit quarter
wave line, at a frequency within the intended bandwidth of the probe.

3 feet of 0,66 velocity coax, terminated in a large resistance will
resonate at about 54MHz, and ring like a bell. That's why resistive cables
are used.

Do you have any idea where one can buy resistive cable?

Jeroen Belleman

 

[TheGlimmerMan]

Do you have any idea where one can buy resistive cable?

Jeroen Belleman

Modern automotive "spark plug wire". Has a "graphite core". Specific
R per foot value. Meant to be an emission suppressive passage for HV
pulses being fed to the plugs, something which generally causes a pretty
big magnetic spike to emanate.

There were folks in the '50s and '60s (hams) who even went so far as to
fully ground shield each of their spark plug wires to reduce the problem.
Then they (those 'they' people) came out with the better, resistive wire
and the problem sufficiently subsided..

May not be exactly what you seek, but that is the only version I am
aware of.

 

[Spehro Pefhany]

Do you have any idea where one can buy resistive cable?

Jeroen Belleman

Probably easiest to buy a scope probe and cut the ends off!

See: Patent number: 2883619
Issue date: Apr 21, 1959

 

[Jamie]

Modern automotive "spark plug wire". Has a "graphite core". Specific
R per foot value. Meant to be an emission suppressive passage for HV
pulses being fed to the plugs, something which generally causes a pretty
big magnetic spike to emanate.

There were folks in the '50s and '60s (hams) who even went so far as to
fully ground shield each of their spark plug wires to reduce the problem.
Then they (those 'they' people) came out with the better, resistive wire
and the problem sufficiently subsided..

May not be exactly what you seek, but that is the only version I am
aware of.

We had an old car here once that I decided to tackle a blower problem
in the heating and ventilation system. THe problem, no blower operation.

After tearing out the blower box under the glove compartment, which
was a big job getting to, I found there was nothing wrong with the
blower motor. The plug was too hard to get to so it wasn't an easy
option to simply test it before hand. So I went to the control switch,
that worked ok. So I then started ripping apart the harness, they had
put a resistor wire in the harness for the blower to reduce the brush
noise in the electrical system.

At that point, I really didn't care much for the car, it was getting
old, so I just hung a copper wire outside of the harness.. I can say
this, not only was that wire there to reduce noise, it also served as a
voltage drop to the blower, that thing was like a wind tunnel afterwards

Jamie

 

[Jamie]

Do you have any idea where one can buy resistive cable?

Jeroen Belleman

maybe you need this? copper clad conductor..

http://www.awcwire.com/ProductSpec.aspx?id=RG178B-Coaxial-Cable

Jamie

 

[Jamie]

This is my best birthday present.

https://dl.dropbox.com/u/53724080/Truckee/Birthday_Baer.jpg


grrrrrrrr.

how many pine trees had to be cut down to build that wall for you?

Jamie

 

[Fred Abse]

Do you have any idea where one can buy resistive cable?

I suspect that probe manufacturers get their resistive cables made to
order. That implies a few thousand feet to get a cable manufacturer
interested.

*Almost* the same effect can be got by putting a small (tens to hundreds
of ohms) in series with each end, if all you want is to damp resonances in
a probe.

You might try a few of the "usual suspect" cable manufacturers. There are
a few specialist cable companies in Germany who claim to make small
quantities of specials to order.

 

[Jeroen]

I suspect that probe manufacturers get their resistive cables made to
order. That implies a few thousand feet to get a cable manufacturer
interested.

It's the scope probe type cable I'm after indeed. I contacted Draka
to see if they could deliver any. They said they were willing to make
it to my specs, provided I'd buy at least 1km. I'd need a dozen or so
five-meter pieces. Oh well. Maybe Spehro was right: Just chop of the
unwanted ends off a scope probe.

*Almost* the same effect can be got by putting a small (tens to hundreds
of ohms) in series with each end, if all you want is to damp resonances in
a probe.

I tried that. Not good enough.

You might try a few of the "usual suspect" cable manufacturers. There are
a few specialist cable companies in Germany who claim to make small
quantities of specials to order.

Thanks, I'll shop around.

Jeroen Belleman

 

[Robert Baer]

I see you made Cs, capacitance from floating shields, a lot larger = 8pf
instead of my wild guesstimate of 0.1pf.
Those floating shields allow defined and predictable capacitive coupling
across the resistors; the capacitance from them to "outer space" ground
seems to be undefinable: coax capacitance runs (log(D/d))^-1 and with a
theoretically infinite D (or very large D in reality),the capacitance is
rather close to zero.
I picked 0.1pf as an estimate to that "zero"; seems you picked a huge 8pf.
*
I did not know of that uniform RC line (U) model; it would be an
excellent choice; better than the pi-pad scheme.
Values can easily be calculated on basis of resistor diameter,
dielectric material and floating shield diameter.
Seems that an eXplicit shield around the floating shield makes for more
trouble.

Oops..now i see an unreasonable Cr of 10pf; that would total to 50pf
on a resistor; VERY hard to implement.
Dimensions:

+++ +++
| |++++++++++++++++++++++++++| |
----| |----
| |++++++++++++++++++++++++++| |
+++ +++
^-------metal end caps-------^

Metal end caps 0.285 dia, body 0.265 dia, end cap width 0.180, and
end-to-end width 2.085 inches
Most plastics (for insulation and support of the floating shield)
have roughly 3 for dielectric constant, and a tube minimum diameter
would contact only the end caps at best; 1/16 wall thickness at smallest.

 

[Robert Baer]

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
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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)
SYMATTR Value2 AC 1
SYMATTR SpiceLine Rser=50
SYMBOL cap 1296 96 R180
WINDOW 0 24 56 Left 2
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SYMATTR InstName C31
SYMATTR Value 2478p
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SYMATTR Value 4.008Meg
SYMATTR InstName R31
SYMATTR SpiceLine tol=0.1 pwr=1
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SYMATTR Value 22p
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SYMATTR Value {Cr}
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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.

It has been a loooong time; Tektronix made two high voltage probes -
one with a significantly higher voltage rating.
The one i have _did_ have the liquid inside but that has slowly
leaked out over the ages; it is the P6015 rated at 20KV, 40KV peak.
I think that is the top end in voltage.

 

[Robert Baer]

You can't ignore the effect of what is, in effect, an open-circuit quarter
wave line, at a frequency within the intended bandwidth of the probe.

3 feet of 0,66 velocity coax, terminated in a large resistance will
resonate at about 54MHz, and ring like a bell. That's why resistive cables
are used.

Perhaps, but us mortals with a limited budget cannot order umpteen
thousand feet of coax with a resistive center.
The P6015 used ten feet of coax with resistive center (direct quote
from manual).
Cannot say what its resistance is..i read 657 ohms end-to-end on the
cable, and the schematic states 150 ohms 10% at probe end, implying the
coax center is about 500 ohms (~5 ohms per foot).

Since you specified 75 ohm cable, the R should then be 75 ohms
instead of 60 ohms (pickie).

 

[Robert Baer]

Do you have any idea where one can buy resistive cable?

Jeroen Belleman

Yes...all it takes is a lot of $$ for the special order.
Unless one is VERY fortunate to find it in a surplus house or willing
to rat out the cable from a P6015..

 

[Robert Baer]

Modern automotive "spark plug wire". Has a "graphite core". Specific
R per foot value. Meant to be an emission suppressive passage for HV
pulses being fed to the plugs, something which generally causes a pretty
big magnetic spike to emanate.

There were folks in the '50s and '60s (hams) who even went so far as to
fully ground shield each of their spark plug wires to reduce the problem.
Then they (those 'they' people) came out with the better, resistive wire
and the problem sufficiently subsided..

May not be exactly what you seek, but that is the only version I am
aware of.

Did a Baby Bird (GooGull) snoop..NGK resistive wires "utilize a
construction method known as "variable pitch" wire winding to create
resistance to radio frequency interference. NGK wires have a lower
resistance than conventional carbon core wires (8k ohm/meter vs. 16k
ohm/meter)."
So these "better" spark plug wires have a "mere" 2.4K/foot
resistance, which exceeds a desirable amount by about an order of magnitude.
Excellent suggestion, however..

 

[Robert Baer]

Probably easiest to buy a scope probe and cut the ends off!

See: Patent number: 2883619
Issue date: Apr 21, 1959

SACRILEGE!!!

 

[Robert Baer]

how many pine trees had to be cut down to build that wall for you?

Jamie

If you were a true tree huger, you _would_ "pine" for them..

 

[Robert Baer]

I suspect that probe manufacturers get their resistive cables made to
order. That implies a few thousand feet to get a cable manufacturer
interested.

*Almost* the same effect can be got by putting a small (tens to hundreds
of ohms) in series with each end, if all you want is to damp resonances in
a probe.

* Check.

 

[Robert Baer]

maybe you need this? copper clad conductor..

http://www.awcwire.com/ProductSpec.aspx?id=RG178B-Coaxial-Cable

Jamie

That is exactly what was used as a reference by Fred Abse in his
Spice listing (Belden 83265 is RG-178B/U 50 ohms) .. but data used was
for the RG179 which is 75 ohms but otherwise rather similar.