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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