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Subject: Re: Making a fast scope probe
Date: Mon, 23 Dec 2002 20:52:30 -0800
X-Mailer: Mozilla 4.78 [en] (Win98; U)
Terminate the 50 ohm coax at the scope end in 50
Not 50 ohms in parallel with a bunch of C on a
use the 50 ohm internal temination.
Ground the loose end of the coax to the ground of
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
for BIG bux.
Of course, your source has to be able to drive
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.
Bunch of stuff For Sale and Wanted at the link
Tek 2465, TFT projector, ham radio, 30pS pulser
Tektronix Concept Books...
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