[time-nuts] Re: RCB-F9T Adapter PCB with USB and 50 Ohm Timepulse SMA Connectors

[ghf at hoffmann-hochfrequenz.de]

Am 2022-08-23 21:37, schrieb Carsten Andrich via time-nuts:
> On 23.08.22 00:50, John Ackermann N8UR via time-nuts wrote:

>> The resistors may not do any one thing perfectly, but they provide 
>> something resembling a source termination as well as equalize current 
>> if the gates don't all trip at exactly the same time. Whatever the 
>> theory, it seems to provide a fairly robust output that can drive coax 
>> and deliver a useful output at the far end. As far as I've ever been 
>> able to measure, the 74AC delay/jitter doesn't have any noticeable 
>> detriment to the real world rise time.
>> 
>> As Bob said, this topic has been discussed a lot over the years and 
>> it's really tough to reach an optimum solution because there are so 
>> many different use cases.  The parallel AC04 gates have worked well 
>> for us.
> 
> Thanks for the suggestion. My general use case requires very low noise
> and high slew-rates. The line drivers shouldn't substantially
> deteriorate the performance of an LTC6957 (320 ps typ. output rise
> time). I've narrowed usable parts down to AD8000, TI BUF602, and TI
> BUF608. For now I've picked the BUF602, because it's hand solderable.

Beware that the LTC6957 might be home of some gremlins, too.
The last picture was taken on an unused output, just 30 mm wrap wire
and 100K/0.6pF/2.4GHz probe. I don't believe that the other output
is unaffected by that, PN or jitter-wise. At least with some load
there is nothing to see on this scope.

>> Note that a potential issue with modern hardware is 3.3V GPS systems 
>> trying to drive 5V logic inputs. 

There is no need for 5V logic inputs. Even in the 80s there was ACT and
TTL levels have nothing to do with 5V, other that it was the supply 
voltage.
Even connecting unused inputs to 5V was considered bad style without a 
current limiting resistor
for backwards biasing the open emitters.
Using 74AC for interfaces in a TTL world was simply wrong.
ACT is the very same process, just different w/l scaling for the input 
FETs.

>> That doesn't give you a whole lot of margin for termination loss. 
>> Fortunately, everything I need to plug in to these days seems to be 
>> happy with 3.3V levels.
> I've made the same observation. The RF ICs that I need to deliver
> 10/100 MHz and pulse signals to are even fine with 3.3V / 2 = 1.65V.

Single 74LVC gates (1G04, 1G125) behave as nearly ideal switches and 
they
do not inherit ground bounce problems from other outputs. Some 
manufacturers
spec Vcc abs max of 6.5 V, so Vcc = 6V should be OK if one is desperate.
Pics of the LVC driver use 50 Ohm scope input & 5.3 nsec of RG174-like 
coax.

Gerhard
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