From: John Larkin
Subject: Re: Making a fast scope probe
Date: Wed, 25 Dec 2002 11:19:53 -0800
Organization: Posted via Supernews, http://www.supernews.com
X-Newsreader: Forte Agent 1.91/32.564
On Mon, 23 Dec 2002 23:51:31 -0000, "Jim boy"
>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.
This is an interesting widget:
This is an HP 54006A 6 GHz passive probe. It consists of, basicly, an
SMA female-female connector, a ground spike, and a Caddock
non-inductive resistor. The probe kit comes with a bunch of 450 and
950 ohm resistors for 10:1 and 20:1 probing. The resistors have short,
maybe 1/4 inch, clipped leads, and you plug one end of a resistor into
the SMA female, and the other end becomes the probe tip. There's a
plastic screw-on cover to hold the resistor in place, but that has no
electrical effect. It works into a 50-ohm scope, of course.
The input capacitance of this is so low that it doesn't really need a
ground connection. With no ground, it picks up a bit more ambient RF,
but it still doesn't ring.
I have one of these, and it's great. A homemade version, using the
same Caddock resistors, works just as well on my 20 GHz scopes.