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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