[time-nuts] Motorola Oncore GPS Interface Board

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Nov 27 18:15:43 EST 2006


David Forbes wrote:
> Jason Rabel wrote:
>   
>> A good initial buffer chip for all the TTL signals would be an IDT QS3384
>> with 10 inputs/outputs.
>>
>>     
> Jason,
>
> The only problem with that part is that it's not a buffer! It's a 
> switch. It has no drive capability at all.
>
> There are useful buffer chips for this task, but their names escape me 
> just now. Perhaps the 74FCT245 or equivalent would work.
>
> Driving a full 5V TTL signal into a 50 ohm load is another matter - you 
> need a lot of DC power (1/2 watt) to do that. I was just using a Liner 
> Tech LT1010 for a similar job - it would get the job done, but folks 
> here might complain about the phase noise.
>
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>   
Phase noise manifests itself as jitter on the PPS signal. You would have 
to really work at it to significantly degrade the inherent jitter in the 
GPS derived PPS signal.
Consequently as long as the devices used are fast enough and not too 
noisy, phase noise will not be a significant problem unless your PCB 
layout is inadequate so that the PPS timing is modulated by some 
unrelated signal.

Bruce




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