[time-nuts] Newbie questions

[Tom Duckworth]

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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