Re: [SI-LIST] : Resistive probe question

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From: Douglas C. Smith (doug@emcesd.com)
Date: Tue Jan 02 2001 - 10:21:04 PST


Hi Chris,

I can think of a few drawbacks. Since the signal that
arrives at the scope is reduced by the divider factor,
keeping interference out is a plus that the coax provides.
Also most good scopes have a 50 Ohm input resistance so you
will need to match to that at the end of the cable. If you
do that with resistors, you will be back to the same factor
as if you used 50 Ohm coax in the first place.

Depending on the tip resistor you use, in any case, you may
have to compensate for the parasitic capacitance of the tip
resistor. For a bandwidth of 1 GHz and a 1206 1000 Ohm tip
resistor the gain will be up by about 6 dB at the high end
without compensation. For values much above 1000 Ohms you
will need to use special high frequency resistors designed
with low capacitance. See my article on building a
compensated resistive probe at http://www.dsmith.org in the
section on "Technical Information and Downloads" which
contains other articles on this subject as well.

Doug

> "Christopher R. Johnson" wrote:
>
> I have a question about the "Make your own resistive
> probe" article at signalintegrity.com. Unfortunately Dr.
> Johnson is out of action for a while due to an injury.
> His staff suggested I post my question to this list.
>
> The article discusses making a resistive probe by just
> putting a series resistor at the end of a piece of coax
> to make a cheap low capacitance probe. The signal
> amplitude is reduced by the resistive divider formed by
> the series resistor and the cable impedance. The probe
> capacitance s essentially the capacitance across the
> series resistor.
>
> Is there any reason that you can't use twisted pair flat
> cable instead of coax? The required length is about 18
> inches. Twisted pair cable flat is available with 100 ohm
> single ended impedance. The higher impedance allows you
> to have less of a resistive load on the circuit for the
> same output signal level. The pairs can be separated such
> that they are only close together in the untwisted mass
> termination portion of the cable, thus reducing cross
> talk.
>
> Any thoughts would be appreciated.
>
>
> Chris Johnson
>
>

-- 
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