[time-nuts] Testing frequency using NTP

David Forbes dforbes at dakotacom.net
Thu Oct 2 21:14:18 EDT 2008


At 5:32 PM -0700 10/2/08, Tom Van Baak wrote:
>>  Tom
>>
>>  The short term phase instabilities of the sound card LO that occur
>>  during the finite interval between the zero crossing times for one
>>  channel and the zero crossing time for the other channel can be
>>  significant.
>>  ...
>
>Bruce,
>
>I'm all ears for details about the phase stability of sounds cards.
>But you need to define "significant" here. Give me real numbers
>and I'll believe it.
>
>Bear in mind that we're talking about a PC-based application with
>a couple of milliseconds of expected NTP time jitter, and a modest
>goal of measuring frequency to ppm levels even if it takes hours
>to achieve that level.
>
>/tvb

Tom,

I expect that the answer is highly dependent on what sort of PLL they 
use on the sound card to generate the sampling clock from the 
oscillator. PLLs used on digital cards can be very lousy with regard 
to jitter.

I remember designing a VMEbus card about 10 years ago that used a 
little 20-pin IC Works PLL chip to double the clock. When I looked 
closely, I saw that that chip was responsible for generating a 
nanosecond of jitter on a 66 MHz clock. That is one crappy PLL!

But a nanosecond is much smaller than the millisecond NTP jitter, so 
you'd have to compare the sound card's clock jitter to that of the 
NTP server averaged over millions of 1 KHz clock input edges to see 
the effects of the PLL.

A sound card that uses a directly divided crystal oscillator, if such 
things exist, should be several orders of magnitude better.

Curiously, the icworks.com domain name is now available for purchase.
-- 

--David Forbes, Tucson, AZ
http://www.cathodecorner.com/



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