[time-nuts] Motorola Oncore GPS Interface Board

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Nov 27 22:24:23 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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>   
If one uses a 5V source with a 50 ohm output impedance and terminates 
the 50 ohm cable in a high impedance a 5V swing is developed across the 
load and the source impedance damps out the reflection from the 
mismatched termination. This also has the advantage that power is only 
dissipated during signal transitions so that when using this technique 
to drive a 50 ohm cable terminated in a high impedance with a PPS signal 
which only has 2 transitions per second the power required to drive the 
load is very low whilst the waveform at the load is resonably clean (as 
long as the load (not cable) capacitance isn't too high.

The attached GIF file illustrates how this may be done using a 74AC540 
octal inverter.

Bruce
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