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From: "Jim boy"
Subject: Making a fast scope probe
Date: Mon, 23 Dec 2002 23:51:31 -0000
Reply-To: "Jim boy"
NNTP-Posting-Date: 23 Dec 2002 23:44:26 GMT
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
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.
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