Re: [time-nuts] 50 ohm drivers

[Bob kb8tq]

Hi

We have gone over CMOS 50 ohm line driving a bunch of times. Check the archives 
for all
of the various opinions. A quick summary:

If you are driving CMOS, the output swing does not have to hit 99% of the 
supply. You
can do a pretty good job with gates in parallel and no source termination. 

If you want both source and load termination *and* want 5V p-p on the load, you 
will
need a 10V p-p source. Good luck if that source ever gets tied to a “normal” 
CMOS 
input. Something made from 2N3904’s and 2N3906’s is probably your best bet 
(along 
with dual supplies) if *really* want to do this.

Source termination (open load) is the low power approach. Cable reflections are 
minimized
and you don’t use a lot of power.

===

If all you are after is sine wave signals, a pair of gates running on 5V will 
give you 20 dbm 
without a lot of effort. There aren’t many applications in the timing world 
that need more
power than that. 

Bob

> On Mar 3, 2018, at 9:17 PM, David C. Partridge 
>  wrote:
> 
> 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 ...
> 
> Thanks
> David
> 
> 
> 
> 
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Re: [time-nuts] 50 ohm drivers

[Gerhard Hoffmann]

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


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Re: [time-nuts] 50 ohm drivers

[Bruce Griffiths]

These devices are only suitable for driving source terminated 50 ohm 
transmission lines not a 50 ohm load to ground (or even 1/2Vcc) to produce CMOS 
levels at the load.

If you are driving a low pass filter or similar intending to produce a sinewave 
output then its somewhat easier.

Even paralleling CMOS outputs won't produce quite a full CMOS swing across a 50 
ohm load.

The classical solution was to either double the swing and use both source and 
load termination or use the Thevenin equivalent using a switched current source.

If AC coupling were allowed push pull drive of a 1:1 RF transformer from a pair 
of complementary 25 ohm Zout CMOS drivers would produce a full amplitude swing 
across a 50 ohm load, however some dc biasing would be required at the load to 
achieve CMOS levels. 

Bruce

> 
> On 04 March 2018 at 15:17 "David C. Partridge" 
>  wrote:
> 
> 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 ...
> 
> Thanks
> David
> 
> ___
> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to 
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
> 
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[time-nuts] 50 ohm drivers

[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 ...

Thanks
David




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