[time-nuts] 50 ohm drivers

Gerhard Hoffmann dk4xp at arcor.de
Sun Mar 4 00:32:50 EST 2018



Am 04.03.2018 um 03:17 schrieb David C. Partridge:
> Brice said:
>
>> . Some fast CMOS devices (esp clock drivers) have an output R close to 50
> ohms as they are intended to drive 50 ohm source terminated transmission
> lines.
>
> Any in particular that you'd recommend?   I need to drive a 50ohm line and a
> single gate inverter doesn't have the grunt to do so ...
>
Back in 80386 times, when AMD still cared about MSI, there were DRAM 
drivers with symmetrical output impedance for the Hi and Lo states. 
Today that is done by the PC chip set, with different levels.

Not very much later, I had a bus fight between a 74AS244 (saying LO!) 
and a prehistoric
Xilinx XC3020 FPGA (saying Hi!) . The AS244 was specc'ed at > 64 mA, 
used to be the
king of the hill. But the XC3020 _enforced_ a _valid HI_ . These CMOS 
thingies CAN drive
if you let them.

In my 10 MHz mod for the Lucent KS24361 I used 2 CMOS single gates with 
100 Ohms
each in series,
< 
https://www.flickr.com/photos/137684711@N07/32245910240/in/album-72157662535945536/lightbox/ 
    >

the falling edge looks like that:
< 
https://www.flickr.com/photos/137684711@N07/31781694064/in/album-72157662535945536/lightbox/ 
 >

For the rising edge follow the arrow to the right.

Remember, when you terminate at the load with 50 Ohms, too, you get only
half the voltage. A 5V driver will deliver only 2.5V, nice for 2V5 CMOS, 
74HCT,
TTL and friends; quite OK for 3V3 CMOS.
I do like coax cable that is terminated in 50 Ohms.  On both sides.

Fairchild has a 74LVC family that features 7V abs max ratings; at 6.4V
that should produce picture book 3V3 levels.

Now I use 3 gates with 150 Ohms each; whatever the gate's output 
impedance is,
it doesn't matter any more against the 150 Ohm. Watch out that a gate with
elevated Vcc may need a higher HI level itself.

At 6V it rises/falls even somewhat faster.

The 3 single gates do not occupy more space than a SO-14, including their
termination resistors.

regards, Gerhard




More information about the time-nuts mailing list