[time-nuts] Motorola Oncore GPS Interface Board
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Mon Nov 27 17:38:05 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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>
Use a 74FCT540/74ACT540 (or 74FCT541/&4ACT541 if you don't wish to
invert the signal), the layout is much simpler, inputs on one side,
outputs on the other.
If you just want to drive the input of a counter such as an HP5370A/B,
it is not necessary to drive 5V into 50 ohms, )0.8 -1V into 50 ohms is
more than sufficient.
Connecting the output of a 74ACT14 (or equivalent) inverter in series
with 270 ohms before driving a cable terminated in 50 ohms will suffice.
You can also combine the outputs of several inverters using resistors to
produce a 2V source with a 50 ohm source impedance.
Bruce
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