From: firstname.lastname@example.org (Tom Bruhns)
Subject: Re: Matching oscilloscope probes
Date: 30 Oct 2002 11:08:08 -0800
NNTP-Posting-Date: 30 Oct 2002 19:08:08 GMT
"Phil Allison" wrote in message news:...
> "Richard" wrote in message
> > When buying after-market probes for an analog scope (e.g., Tek 465/475),
> > are they fairly generic / interchangable (assuming BNC), or do you need
> > to buy for your specific scope model?
> > From reading posts, I gather there's:
> > * a 50 / 1M Ohm rating that needs to be matched to the scope
> ** Scopes are virtually all 1 Mohm with 20 to 35 pF in parallel on the
> vertical inputs.
Errrk! A whole lot of them are rather lower capacitance than that.
The two I have handy here are nominally 13pF (100MHz bw) and 7pF
> > * a capacitance rating
> ** 10:1 probes ALL have an adjustment for this.
Yes, but the adjustment range is generally limited. Decent scope
probes are rated for the range of scope input capacitances they will
> > * a bandwidth rating
> ** Basically a scam - the rating is based on a 25 ohm source impedance.
> > * (anything missing here?)
> > Does the bandwidth rating just need to be greater than the signal being
> > probed, up to the rating of the scope? (i.e., a 100MHz probe would work
> > on a 200MHz scope for signals up to 100MHz, right?)
Generally right, but if one probe is -3dB at 100MHz and another more
conservatively rated one is -1dB at 100MHz, you might see rather
different results from the two in your circuit. Also, the length of
the probe lead and some other design factors matter, because they
affect the capacitance at the probe tip, and that capacitance will
load your circuit. Longer cables present more capacitance at the
probe tip, other things being equal.
There really is a difference in probes. I have some cheap
after-market ones with nominally the same bandwidth rating as some
good ones I also have, and there is a difference in the response, even
the flatness in the lower passband when properly compensated. I can
be a little hard on probes sometimes, so using cheap ones when I don't
need especially accurate readings makes sense for me. When I _really_
want to see what's going on, I use the good probes.
A probe worth its salt will be rated for the range of scope input
capacitances it will match, and it will be rated for the capacitance
(and resistance) it presents to the circuit under test. One with a
relative narrow range of scope input capacitances, but whose range
matches your scope, may well be better than a wide-range one.