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From: "Phil Allison"
References: <3DBF0409.D2B34DBF@azglobal.com> <firstname.lastname@example.org>
Subject: Re: Matching oscilloscope probes
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Thu, 31 Oct 2002 11:38:36 +1100
NNTP-Posting-Date: Thu, 31 Oct 2002 11:29:33 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Tom Bruhns" wrote in message
> "Phil Allison" wrote in message
> > "Richard" wrote in message
> > news:3DBF0409.D2B34DBF@azglobal.com...
> > > When buying after-market probes for an analog scope (e.g., Tek
> > > are they fairly generic / interchangable (assuming BNC), or do you
> > > 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
> > 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
> (500MHz bw).
** 500 MHz scopes are scarce in my part of the world. I checked several
handbooks for scopes in the 20 to 60 Mhz range before I made the post.
> > > * 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
> match properly.
** So what ? The range is enough to cope with nearly any scope
> > > * a bandwidth rating
> > ** Basically a scam - the rating is based on a 25 ohm source
> > > * (anything missing here?)
> > >
> > > Does the bandwidth rating just need to be greater than the signal
> > > probed, up to the rating of the scope? (i.e., a 100MHz probe would
> > > 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.
** Massive overstatement. All passive probes have severe limitations
when used on high frequency and fast rise time circuits.
A length of RF co-ax terminated in its characteristic impedance is
a predictable and true wide band probe with a known and consistant load from
DC to hundreds of MHz.
> 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.
** Take this with a large lump of salt.
. . .............. Phil
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