The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Making a fast scope probe
References: <3E07E80E.D26397D8@juno.com> <3E080167.CD03BEA0@earthlink.net> <3E0849DB.1FFA87AA@juno.com>
Date: Wed, 25 Dec 2002 03:37:27 GMT
NNTP-Posting-Date: Tue, 24 Dec 2002 19:37:27 PST
Organization: EarthLink Inc. -- http://www.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...
I was under the impression that Tektronix used a resistive center for
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup