That rule of thumb is not crappy, but there is a caveat: an unterminated
cable appears as a nasty, predominantly capacitive impedance; for your
run-of-the-mill, polyethylene cable, capacitance is 30 pF/ft, what would
amount to 3000 pF for 30 meter piece. If your driver can handle that kind of
load, you are OK, but most drivers, without special measures, would
oscillate, if not fall flat on their face. Special measures are usually
adding a resistor between output and load, sometimes trying to compensate
phase, whatever does it take to survive.
So, when you transfer 1 MHz signal on a 30 meter cable you want see much of
reflection effects. Rule of thumb says significantly smaller that ?
wavelength; 1 MHz in air is 300 meters, in polyethylene around 600 meters
what yields 1/20 of wavelength; that is better than ? of ? wavelength.
However, cable driver might be a problem.
Regards
Miroslav Pokorni
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-----Original Message-----
From: F F [mailto:ferfons@...]
Sent: Friday, July 06, 2001 1:42 AM
To: TekScopes@...
Subject: Re: [TekScopes] Coax BNC Cable Termination
>From: "John Rehwinkel" <spam@...
<mailto:spam@...> >
>Reply-To: TekScopes@...
<mailto:TekScopes@...>
>To: TekScopes@...
<mailto:TekScopes@...>
>Subject: Re: [TekScopes] Coax BNC Cable
Termination
>Date: Thu, 5 Jul 2001 17:39:29 -0400
(snip)
1MHz signals
>should tolerate a 30 meter long mismatch, and so
on.
My humble twopence: Crappy rule of thumb! Few years ago
while integrating an ATE we had a 1MHz square signal we thought it wouldn't
need matching pretty much for the same reasons. Oh we were so bloody wrong
and it took a while to figure it out. We did have to match that particular
stretch of cable - 3 or 4 "lousy" meters of coax - down to 50 ohms (TDS420
alterative Zin) and through line drivers, keeping the signal below the
scope's max input power rating. That's how we got it sorted.
Fernando
Portsmouth, UK
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