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capacitance load of oscilloscope probes

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Blue
Guest

Tue Apr 06, 2010 8:13 pm   



I am looking for oscilloscope probes. There are a number of choices on
ebay. I am looking for passive 1x/10x probes for a 60 MHz scope with
input capacitance of 30 pF.

One of the choices is a probe with capacitance that can be changed from
15-40 pF. Its capacitance load is 90pF/20pF (I assume for the two
attenuations). Would this be okay with the scope? Any other pointers
what to keep in mind wrt to the capacity of the probes?

Thanks.

Jon Elson
Guest

Tue Apr 06, 2010 11:11 pm   



Blue wrote:
Quote:
I am looking for oscilloscope probes. There are a number of choices on
ebay. I am looking for passive 1x/10x probes for a 60 MHz scope with
input capacitance of 30 pF.

One of the choices is a probe with capacitance that can be changed from
15-40 pF. Its capacitance load is 90pF/20pF (I assume for the two
attenuations). Would this be okay with the scope? Any other pointers
what to keep in mind wrt to the capacity of the probes?
That sounds like that should work fine.


Jon

Blue
Guest

Wed Apr 07, 2010 12:20 am   



Jon Elson wrote:
Quote:
Blue wrote:
I am looking for oscilloscope probes. There are a number of choices on
ebay. I am looking for passive 1x/10x probes for a 60 MHz scope with
input capacitance of 30 pF.

One of the choices is a probe with capacitance that can be changed from
15-40 pF. Its capacitance load is 90pF/20pF (I assume for the two
attenuations). Would this be okay with the scope? Any other pointers
what to keep in mind wrt to the capacity of the probes?
That sounds like that should work fine.

Jon

Thanks. I was reading a bit more about probes and realized that the
variable capacitor on the probe is used to cancel the scope's input
capacitance. Hence the former should have the latter within its range.
However, that leaves us with the probe's own capacitance. In this case,
it is 90pF/20pF (for 1:1/1:10 attenuation). Are these values typical for
probes for 60 MHz scopes?

Jon Elson
Guest

Wed Apr 07, 2010 8:32 pm   



Blue wrote:

Quote:
Thanks. I was reading a bit more about probes and realized that the
variable capacitor on the probe is used to cancel the scope's input
capacitance. Hence the former should have the latter within its range.
However, that leaves us with the probe's own capacitance. In this case,
it is 90pF/20pF (for 1:1/1:10 attenuation). Are these values typical for
probes for 60 MHz scopes?
Yes, pretty typical. At 1:1 attenuation, the probe is basically just a

length of coax cable, and the capacitance of those is always pretty
high, tens of pF per foot for even the best. 20 pF for a standard X10
probe is about normal, some really high-end Tektronix probes with 400
MHz bandwidth that cost $400 each about 20 years ago, are 14 pF.

If you need lower capacitance, you have to go to amplified probes, and
those cost several thousand $.

Jon

Blue
Guest

Thu Apr 08, 2010 2:31 am   



Jon Elson wrote:
Quote:
Blue wrote:

Thanks. I was reading a bit more about probes and realized that the
variable capacitor on the probe is used to cancel the scope's input
capacitance. Hence the former should have the latter within its range.
However, that leaves us with the probe's own capacitance. In this case,
it is 90pF/20pF (for 1:1/1:10 attenuation). Are these values typical for
probes for 60 MHz scopes?
Yes, pretty typical. At 1:1 attenuation, the probe is basically just a
length of coax cable, and the capacitance of those is always pretty
high, tens of pF per foot for even the best. 20 pF for a standard X10
probe is about normal, some really high-end Tektronix probes with 400
MHz bandwidth that cost $400 each about 20 years ago, are 14 pF.

If you need lower capacitance, you have to go to amplified probes, and
those cost several thousand $.

Jon

Okay, thanks for the explanations.

One more point I just was wondering about. Is it typical that probes for
different bandwidths have same capacitance? e.g. should a 60 MHz probe
have the same capacitance as a 200 MHz probe? I am suspecting that
higher frequency probes are going to have lower capacitance else higher
freqs are going to be attenuated more. But I could be wrong, because
based on your explanation, at 1:1 all probes are going to have the same
capacity.

StepNfetchit
Guest

Thu Apr 08, 2010 3:18 am   



On Tue, 06 Apr 2010 15:13:09 -0400, Blue <blue_at_somewhere.org> wrote:

Quote:
I am looking for oscilloscope probes. There are a number of choices on
ebay. I am looking for passive 1x/10x probes for a 60 MHz scope with
input capacitance of 30 pF.

One of the choices is a probe with capacitance that can be changed from
15-40 pF. Its capacitance load is 90pF/20pF (I assume for the two
attenuations). Would this be okay with the scope? Any other pointers
what to keep in mind wrt to the capacity of the probes?

Thanks.

Many good o'scope and probe tech and learning pdf's at the following
including Tektronix ABC's of probes:
http://www.ko4bb.com/cgi-bin/manuals.pl?dir=Tektronix/Tektronix_-_Probes

Jon Elson
Guest

Thu Apr 22, 2010 11:39 pm   



Blue wrote:
Quote:

One more point I just was wondering about. Is it typical that probes for
different bandwidths have same capacitance? e.g. should a 60 MHz probe
have the same capacitance as a 200 MHz probe? I am suspecting that
higher frequency probes are going to have lower capacitance else higher
freqs are going to be attenuated more. But I could be wrong, because
based on your explanation, at 1:1 all probes are going to have the same
capacity.



You won't find high-frequency, high-impedance passive probes with 1:1
attenuation. It just can't be done. The higher-frequency probes with
10:1 attenuation generally cost much more, and they generally cut the
capacitance at the probe tip a bit, but there is a limit to how low you
can go on a passive probe. A Tek P6139A is a 500 MHz probe, and has an
amazing 8.0 pF of probe loading. I can't even imagine how they got it
that low, it is a REAL achievement. A FET probe can get down to a
couple pF.

Jon

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