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Subject: Re: Making a fast scope probe
Date: Tue, 24 Dec 2002 03:49:47 -0800
X-Mailer: Mozilla 4.78 [en] (Win98; U)
References: <3E07E80E.D26397D8@juno.com> <3E080167.CD03BEA0@earthlink.net>
Robert Baer wrote:
> mike wrote:
> > Terminate the 50 ohm coax at the scope end in 50
> > ohms.
> > 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
> > length.
> > 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
> > resistance
> > 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.
> > mike
> > Of course, your source has to be able to drive
> > whatever resistance
> > you use.
> > mike
> > 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
> > below.
> > Tek 2465, TFT projector, ham radio, 30pS pulser
> > Tektronix Concept Books...
> > http://www.geocities.com/SiliconValley/Monitor/4710/
> 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.
Resistive wire works well for high impedance
probes. Think it will just cause
you grief with a low-Z, high-frequency probe.
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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