Gee, Dave . . . -632 ohms?? If you parallel it with a reaonator, does it oscillate?? ;-)
I don't remember the 10430A exactly, and I'm too lazy to look it up, but typically the cable is rather high impedance, so relatively low capacitance.? If it's one of the high frequency probes (e.g. 500MHz) the compensation at the output is a bit more complicated, but lower frequency probes are generally pretty simple.
The 1meg (rather than 10meg) input drives the cable with a lower impedance and helps raise the frequency response.? The probe end must be (900k? and a little capacitance in parallel) in series between the probe tip and the cable.? On most probes, that parallel capacitance is not adjustable.? There may also be a bit of intentional inductance in series. And there needs to be 111k shunt to 'ground', so that and the scope 1M parallel to 100k.? To get a passive probe that works well to ~500MHz, the scope input capacitance has to match the design of the probe, thus that 6-9pF spec for the probe.
I do have a 50 ohm probe; the kit includes a selection of coaxial series resistors that yield higher resistance at the probe tip, but obviously various attenuations that the user must take into account.? As you've said, the high impedance probes typically have several pF shunt capacitance; 10pF at 200MHz is -j79.6 ohms!? In other words, your "high impedance probe" isn't exactly that at RF.? My 50 ohm probe with a 10:1 450 ohm series resistor looks like 500 ohms with shunt capacitance in the "under a pF" region, but of course that depends a lot on how you connect it to the circuit.
I wonder if there were any scope probe articles in the Hewlett-Packard Journal.? If so, they might tell you a LOT more about scope probe design.
Cheers,
Tom
----------------
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On 11/6/2023 4:37 PM, W0LEV wrote:
Not sure I follow you there Dave. I did an OSL before the measurement. How
can any plot be beyond the perimeter given the right hand side perimeter
represents infinity?
Yes, the right hand side of the Smith Chart, especially on the central
horizontal line, represents extremely high impedance, and on the line, high
resistance. Most o'scope probes are high-Z. 1 MegOhm with a small
capacitance. They should measure on the extreme fight side of the chart.
If you happen to have a relatively rare 50-ohm probe or using a 50-ohm
feedthrough connector, your probe will measure on the extreme right side of
the Smith Chart.
Outside the chart simply indicates the VNA is incapable of calibrating or
measuring such a high impedance. An RF open, especially at the higher
frequencies, is far more difficult to make than a short or proper
termination.
BOTTOM LINE: Your VNA is incapable of measuring such a high impedance (or
resistance). Even the very expensive VNAs are incapable of measuring 1
MegOhm with any accuracy.
Your o'scope probe SHOULD measure at the extreme right and "almost" on the
central horizontal line depending on your measurement frequency. Yes,
but it isn't, it's outside the chart area, hence the question.
Your NANOVNA is incapable of calibrating and measuring such a high
impedance or resistance. I have an HP 8753C and I can assure you, even
that can not calibrate and accurately measure a 1 MegOhm carbon resistor at
any reasonable frequency (bottom end is 300 kHz).
I wouldn't expect the nano to measure 1Meg R accurately. That's well beyond
it's 50 Ohm reference. It's the reactive component I'm interested in and
that should be within range.
The reactive component should be somewhere between 5 and 40 pF. It is
adjustable and is there to compensate for the lead length between the
o'scope and the business end of the probe. I'd suggest attempting to
measure at a very low frequency like 500 kHz or 1 MHz.
Interesting: I just picked an HP 10430A 10:1 o'scope probe. On the
o'scope end of the probe, it is noted as 1 Meg, 6 to 9 pF input. I
calibrated my 8753C between 300 kHz and 30 MHz and made an S11 measurement
(actually Z11). At 300 kHz, it measures in excess of 5 kohms which is
unstable due to noise in the measurement and 4 pF. However, at 30 MHz it's
in the 100's of ohms, but similar reactance for the frequency. I'm puzzled.
In measuring a 4.7 kohm carbon resistor, the 300 kHz reading is nuts on,
but at 30 MHz, it's in the -632 ohms OUTSIDE the perimeter. Now I've got
to think about how these probes are designed. Mystery?
Dave - W?LEV
On Mon, Nov 6, 2023 at 9:52?PM KillingTime <bbdowns@...> wrote:
On Mon, Nov 6, 2023 at 08:55 PM, W0LEV wrote:
Of course the Smith Chart plot is outside the perimeter! A proper OSL
should show your o'scope probe right on the perimeter.
Not sure I follow you there Dave. I did an OSL before the measurement. How
can any plot be beyond the perimeter given the right hand side perimeter
represents infinity?
Your o'scope probe SHOULD measure at the extreme right and "almost" on
the
central horizontal line depending on your measurement frequency.
Yes, but it isn't, it's outside the chart area, hence the question.
I wouldn't expect the nano to measure 1Meg R accurately. That's well
beyond it's 50 Ohm reference. It's the reactive component I'm interested in
and that should be within range.
