[time-nuts] Comparison of Logic Standards for Clock Distribution

[SAIDJACK at aol.com]

Hello Stephen,
 
to help with your question, here is my 2-cents from empirical  experience:
 
Fundamental Sine waves cause the lowest amount of noise since they do  
(should) not include any harmonics, and mixing products that can arise from  these 
harmonics. Thus sine waves are used extensively in T&M measurement  equipment 
to distribute 5 or 10MHz references over cable.

They also have the benefit of allowing the longest cable runs, since  digital 
signals are comprised of harmonics (particularly 3rd and 5th  harmonic) they 
are affected much more by cable dispersion, skin effect, and  other effects 
than a fundamental Sine wave is.
 
That said, LVDS is a much more modern standard than PECL or ECL. It was  
specifically designed for low-power (laptop and HDTV) signal transmission on  
really cheap cables between a graphics chip and an LCD panel such as an  HDTV. You 
may get cheaper, and better chips when using LVDS. PECL and ECL were  not 
specifically designed to drive long cables I believe, rather they are PCB  and 
backplane specific.
 
Attached are two phase-noise measurements of our new Jackson-Labs Fury  GPSDO 
which has both digital and analog 10MHz outputs. Both outputs drive about  
4-feet of Belden 316 cable, then go into a TSC5120A analyzer. You can see a much 
 more significant spur spectrum on the digital signal, there are essentially 
no  spurs at all on the Sine wave cable! 
 
I think this is not only caused in the digital driver chip, but also in the  
ADC circuits on the receiving side due to harmonics on the square wave all the 
 way up to 1GHz...
 
BTW: our FireFox GPSDO referenced signal generator has LVDS, PECL, CMOS,  and 
Sine wave outputs all driven in paralell.
 
The cable used is also very important, since cheap RG58 has very poor  
shielding, lot's of dispersion and a high loss-tangent etc...
 
hope this helps,
bye,
Said
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