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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Making a fast scope probe
Date: Tue, 24 Dec 2002 06:39:07 GMT
NNTP-Posting-Date: Mon, 23 Dec 2002 22:39:07 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
> Terminate the 50 ohm coax at the scope end in 50
> Not 50 ohms in parallel with a bunch of C on a
> 1meg input,
> use the 50 ohm internal temination.
> Ground the loose end of the coax to the ground of
> your source
> with zero lead length. Put any value resistor
> from the source
> to the loose center conductor with zero lead
> The "gain" wil be 50/(r+50). Risetime will be as
> good as you're
> ever gonna get.
> The problem happens if the resistor has other than
> or the lead length is other than zero or the coax
> has loss or the
> scope has input C or ....
> That's why Tek gets big bux for scope probes.
> A 1/8W carbon composition 450 ohm resistor hanging
> on the end of a coax
> makes a pretty good 500 ohm 10x probe.
> Or you can buy the same thing from TEK in a nice
> plastic probe
> for BIG bux.
> Of course, your source has to be able to drive
> whatever resistance
> you use.
> Jim boy wrote:
> > When I connect a 250MHz scope probe on X10 to the inbuilt 1ns rise time 1Mhz
> > square wave in my oscilloscope with the ground lead floating, there is
> > ringing on the edges. This is down to the inductance of the loop formed by
> > the inner conductor of the probe and the ground of the scope resonating with
> > the input capacitance of the scope. Connecting the ground of the probe to
> > the scope ground via it's BNC eliminates post of the ringing. The loop is
> > much smaller since the outer conductor of the probe now transfers most of
> > the ground signal.
> > On the advice of a link that was posted here a few months back on how to
> > construct a "wide-band" probe to 1Ghz consisting of a 100R resistor on the
> > inner conductor of co-ax at the sensing end terminated in in it's
> > characteristic impedance I noted the following:-
> > 1. With the outer conductor only connected at the BNC, there should be
> > ringing but there wasn't. The rise time was poor
> > 2. Connecting the outer conductor at the sensing end to the BNC slightly
> > improved the rise time
> > 3. Removing the 50R characteristic impedance made the rise time
> > significantly worse with still no ringing seen.
> > My conclusion is that without the terminating resistance, you've merely got
> > a bog-standard x1 probe that is limited by it's ability to charge the input
> > capacitance of the scope. The terminating impedance improves things only
> > because it dominates the input capaciance at the expense of less signal at
> > the input. Reducing it to 10R improves the rise time further.
> > So what is a better solution? How about splitting the terminating impedance
> > as 49R in series with 1R across the input capacitance? I'll try this after
> > the new year in the hope of seeing ringing.
> > JB
> Bunch of stuff For Sale and Wanted at the link
> Tek 2465, TFT projector, ham radio, 30pS pulser
> Tektronix Concept Books...
There is also the 100X probe, which can be made with a 5.1K carbon
comp (ratio is a bit higher).
Closer to the Tek construction would be a 4.7K and the use of a coax
with resistive center to make up the difference.
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