Homebrew HV hiZ scope probe

Thanks! I knew one of them could be soldered..so that is the wire to use if the right resistivity can be has with a "reasonable" wire gauge (for the sleeving and outer braid shield).

Have you forgotten the stated end-to-end capacitance per Ohmite data?

In the case of MOX-2-13XXXX it's 0.6pF. I suspect that this will be the dominant parameter.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

Tried the BabyBird (GooGull); most hits were Chinese sellers, many do not say how to buy, those that do say how much do not give resistivity of the wire (ohms per unit length). So still have zero idea if any of the wire that might be available is in the right resistance range. Nobody seems to have a range of wire sizes.

The sample was too long to get meaningful open and shorted measurements, but the TDR showed some unexpected results.

On a 1.2 meter length of cable. transit time was 9.5 nanoseconds, giving a velocity of 42.11% of c. That corresponds to a dielectric permittivity of 5.64, which is too high for any flexible dielectric I know of. That suggests that the inner conductor is a helix. Resistance is 186.66 ohms per meter. Inductance calculates (from rho at the sending end, and velocity), to be 1.07 uH per meter, and capacitance 58.6 pF per meter.

The following model corresponds quite closely with measured data:

.model scopecbl ltra (

• len=1.2

• R=186.666

• L=1.07E-006

• C=5.86E-011)

The following is a good approximation to what the TDR shows. Change the time (X) axis to "time/2" to show one-way time.

Version 4 SHEET 1 880 680 WIRE -160 128 -320 128 WIRE -16 128 -64 128 WIRE -320 272 -320 208 WIRE -160 272 -160 160 WIRE -160 272 -320 272 WIRE -64 272 -64 160 WIRE -64 272 -160 272 WIRE -16 272 -16 160 WIRE -16 272 -64 272 WIRE 32 272 -16 272 WIRE 80 272 80 160 WIRE 80 272 32 272 FLAG 32 272 0 SYMBOL ltline 32 144 R0 SYMATTR InstName O1 SYMATTR Value scopecbl SYMBOL voltage -320 112 R0 WINDOW 3 -159 -8 Left 2 WINDOW 123 24 132 Left 2 WINDOW 39 24 28 Left 2 SYMATTR Value PULSE(0 1 0 22p 22p 1u 2u 1) SYMATTR SpiceLine Rser=50 SYMATTR InstName V1 SYMBOL tline -112 144 R0 SYMATTR InstName T1 SYMATTR Value Td=1n Z0=50 TEXT -312 384 Left 2 !.tran 0 100n 0 1p TEXT -312 336 Left 2 !.opt plotwinsize=0 TEXT -40 336 Left 2 !.model scopecbl ltra (\n

• len=1.2

\n

• R=186.666

\n

• L=1.07E-006

\n

• C=5.86E-011)

TEXT -312 360 Left 2 !.plot v(n001) TEXT -176 72 Left 2 ;TDR Simulation

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

L=1.07E-006\n+ C=5.86E-011)

Speak of variable results.. Concerning the original Q&D probe where the scope must be DC coupled ONLY, i have determined that pickup of external signals (hum and related bazz-fazz), having the floating shield is a bit better than not having it at all. The negative of that is, getting pinkies close to that shield does severely compromise the risetime & undershoot of the probe.

Concerning the "development" of the two 1G internally terminated probe, each of those resistors seem to be too long and pickup becomes intolerable, and a floating shield might have to be referred to ground (maybe a resistor). It also looks like a multiple pi-pad model is not the best for modelling these resistors..

I wouldn't want fingers anywhere near an energized HV probe. The right place to put compensation adjustment it at the scope end, like most good commercial probes do.

Use a fixed shield, referred to ground, and work with whatever capacitance you're left with. Minimize capacitance to ground as much as possible, it's the end-to-end capacitance that *really* matters for compensation. According to Ohmite, that's 0.6pF. For a 1:1000 probe, that means 600pF at the scope input.

multiple pi-pad model is not the best for

I'd agree with that. You need to do some work to determine what they really look like.

As an aside about modeling, I discovered that LTSpice supports the TXL (Y) transmission line model, though it's undocumented. It runs faster than the LTRA model, and also supports full RLCG, whereas LTRA only supports RLC. G can be a parameter, which suggests that may be a way of modeling frequency-dependent dielectric loss.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

I could find no end-to-end capacitance spec for these resistors; i calculate 0.125pF end-to-end on the basis of endcap spacing and aluminum oxide insulator. See

for dimensions and calculations. The cylindrical capacitance info at the bottom of the second page is reference only for now.

MOX-2-12 series 0.6pF end to end.

Ohmite PDF catalog October 26 2011. "Power resistors, Component Selector

4000J"

Page 93 in PDF version, labeled 85. "Maxi-Mox".

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

Well,i snooped around their site and could find no way to get or find that catalog. No way to get that description, etc. Dead end. The best i could find, using MOX as a 3-character selector for part number, thengetting the PDF for the Max-Mox series,was a one page PDF named res_maximox.pdf; page labeled 85. Now i see (only since you pointed it out) the 0.60pF . On the basis of my calculations, i do not see how it can be that large. I guess i will have to make a rather sensitive capacitance measurement device, as DVMs tend to imply the low value i calculated.

I got the catalog from the Ohmite site the same time this thread started. Took a bit of digging.

Don't you have a proper LCR meter, or bridge? A DMM isn't really suitable for measuring parasitic capacitance.

A "real" LCR meter, or impedance analyzer, is a must-have for any serious work. You could use an old Q meter instead.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

I've just reread the PDF, and beg to disagree.

"Probe 1Gohm-3.jpg" shows the step still rising at about 1.8 microseconds, where it goes off screen.

Risetime is the time from 10% to 90% *of the final steady value*. Risetime is at least 1.2 microseconds, possibly more. It doesn't show the final value.

"Probe 1Gohm-3.jpg" doesn't contain enough information to calculate risetime. It just shows the initial overshoot and ringing, ie. the first half-centimeter of the previous photo,some of what you see will be due to the cable.

What I see is an under-compensated probe.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)