[time-nuts] Newbie questions

[Bruce Griffiths]

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
> 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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>
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