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From: Miroslav Pokorni <mpokorni@...>
Reply-To: TekScopes@...
To: "'TekScopes@...'" <TekScopes@...>
Subject: RE: [TekScopes] Coax BNC Cable Termination
Date: Mon, 9 Jul 2001 17:24:55 -0700

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



-----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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Hello,
I am new in this group. I have bought an Tektronix 585A oscilloscope from the Swiss army. The condition is after 35 years storage in a air-conditioned fortress like ex works. It is a very good instrument with the finest beam I ever saw. It has only one disadvantage: It has no automatic position in the trigger system. When ever you use the instrument, you have to set the Stability knob to a free running position that you can see the basic line on the screen. After you can see the unsynchronised signal on the screen, you have to find a suitable trigger position by turning the Stability knob. Can anybody of the former Tektronix people tell me why Tektronix didn't take care for an AUTOMATIC TRIGGER position?

Many thanks for an answer

Urs

I believe that a question did come up somewhere concerning where one
could buy an in-line termination. Besides the obvious expensive
source (), I believe you can get them from Pomona
Electronics (the makers of cables and adaptors) and you might see if
Mini-Circuits carries them.

Phil (VA3UX) wrote:

The Telequipment name carried on until some time in the

early 70's until the name was dropped and the Tek name was put on the
instruments. Telequipment instruments with the Tektronix name had model
numbers that began with "T". Eventually, the whole product line was
dropped and that was the end of Telequipment. Of all the good things Tek
did, buying Telequipment was not one of them. In a small way, the scope
market would have been better off if Telequipment had just been left alone.

Phil

Well, I concur with all of the above, except the following. I don't ever remember
the "Tek" name appearing on any Telequipment scope. The T900 series was a separate
engineering effort based in Beaverton, OR and had nothing whatsoever to do with
Telequipment, as far as I know. It was a low-cost effort and that is where the
similarity to Telequipment ends, to the best of my recollection.

What really happened to Telequipment is that a decision was made by Tek management
to shut it down because customer satisfaction with the products was just too low.
What was saved in the cost of initial purchase was quickly spent in maintenance of
Telequipment scopes. When you save the cost of transistor sockets, for example,
you have to spend more to replace the transistor when it does go bad. The long
term economics of buying a low-cost scope is just not a good concept, in general,
especially for commercial customers.

Stan
w7ni@...





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