But the result disagrees with the manufacturer's specs for RG59.
There's an error somewhere in your measurement setup.
For example, it could be a time offset error (or even differing trigger
levels with a sinewave input) between the time interval counter start
and stop channels.
Bruce
Tom Duckworth wrote:
Magnus,
We've made this measurement using a 20 ps time interval counter and a
GPS disciplined Rubidium frequency standard as the time base; making
many concurrent measurements with no dead time between. The resultant
measurement was very close to the 1 ns/ft benchmark with RG-59 (BNC
connectors), 10 MHz source. So we felt ok with using the 1 ns/ft
estimate.
Tom
Tom Duckworth
[email protected]
----- Original Message ----- From: "Magnus Danielson"
<[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
Sent: Tuesday, January 05, 2010 7:26 PM
Subject: Re: [time-nuts] Newbie questions
Tom Duckworth wrote:
Jim,
We use a benchmark 1 ns per foot of coax (RG-59).
This sounds fast. The normal taxiometer is at 66% of speed of ligth
in vaccum, which for 1 ns is about 3 dm so for the RG-59 that would
be about 2 dm.
Some cables reach 78%, but RG-58 and RG-59 is down at normal 66%.
You could measure the delay by using a resistive splitter (50 ohms)
and two cables (say a 2 foot and a three foot, each terminated at
the far end with a 50 ohm pass through terminator). Drive the
splitter with your 10 MHz signal and measure, at the far end, using
an appropriate 2-channel scope or counter with the necessary
resolution, the difference in time delay between the two, which will
give you a pretty accurate delay per foot. Both cables should be the
same coax type.
Being a time-nut, using time-interval counters or TDR would be my
choice, but these tools/toys outnumbers the scopes...
Cheers,
Magnus
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