N
Norm Dresner
- Jan 1, 1970
- 0
We need to drive a terminated by a load that's equivalent to 50 ohms with a
pulse created by the TTL-compatible output of a PC counter/timer board.
The hard part is that the pulse at the receiving end has to go from 0 to 5V
(more or less). Standard 50 ohm TTL drivers (74S140) are only spec'd to
produce a TTL-high of 2.5 volts and that's what I've observed; paralleling
74128 (75 ohm drivers) doesn't do much better. The basic specs are 0-5v
into 50 ohms with rise/fall times on the order of (but not much greater
than) 20 ns. The board we're using has a 5v supply output so we can use
that for any TTL(or equiv)-logic and a single voltage external supply to
provide the higher voltage needed to get rail-to rail 0-5v output. We have
on hand some 6v/1A regulated wall-warts which should be adequate.
My prototype used a 7407/7417 (HV OC inverter) to drive a 2N2219 transistor
operating from a 5.7v separate supply, and while this works, I think there's
got to be a neater way. I'm hoping to find a single IC that will accept a
TTL-compatible input and produce the necessary output, but so far I've
failed to find anything. Since we're only producing single-digit
quantities, cost is not a primary factor.
TIA
Norm
pulse created by the TTL-compatible output of a PC counter/timer board.
The hard part is that the pulse at the receiving end has to go from 0 to 5V
(more or less). Standard 50 ohm TTL drivers (74S140) are only spec'd to
produce a TTL-high of 2.5 volts and that's what I've observed; paralleling
74128 (75 ohm drivers) doesn't do much better. The basic specs are 0-5v
into 50 ohms with rise/fall times on the order of (but not much greater
than) 20 ns. The board we're using has a 5v supply output so we can use
that for any TTL(or equiv)-logic and a single voltage external supply to
provide the higher voltage needed to get rail-to rail 0-5v output. We have
on hand some 6v/1A regulated wall-warts which should be adequate.
My prototype used a 7407/7417 (HV OC inverter) to drive a 2N2219 transistor
operating from a 5.7v separate supply, and while this works, I think there's
got to be a neater way. I'm hoping to find a single IC that will accept a
TTL-compatible input and produce the necessary output, but so far I've
failed to find anything. Since we're only producing single-digit
quantities, cost is not a primary factor.
TIA
Norm