[time-nuts] Newbie questions
Tom Duckworth
tomduck at comcast.net
Wed Jan 6 07:58:58 UTC 2010
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
tomduck at comcast.net
----- Original Message -----
From: "Magnus Danielson" <magnus at rubidium.dyndns.org>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
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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