[time-nuts] Practical considerations making a lab standard with an LTE lite

Said Jackson saidjack at aol.com
Tue Nov 25 11:19:45 EST 2014


Dave,

Exactly.

Sent From iPhone

> On Nov 25, 2014, at 7:34, Dave Martindale <dave.martindale at gmail.com> wrote:
> 
> The 20 MHz output should be OK, since it is series-terminated with 50 ohms
> at the source and the buffer can source enough current.  The driver sees a
> 100 ohm load (50 ohm resistor in series with 50 ohm coax impedance) for
> that 32 ns round trip time, so it will increase power dissipation (as you
> note).  But the load at the far end of the coax should see a clean edge,
> and the reflection should be absorbed when it returns to the source (due to
> the source terminator).  Just don't look at the signal half way along the
> coax.
> 
> The other outputs apparently don't have either the current drive or the
> source terminator, so a long piece of coax is likely to do unpleasant
> things to the edge.
> 
> In either case, if you want to run any of the signals 10 feet it's likely
> better to run a very short connection from the LTE-Lite to a proper 50 ohm
> line driver.  That gets the power dissipation off the board, and then you
> can use drivers that give you whatever output swing you want, and which can
> drive a 100 ohm load continuously so you can use parallel termination at
> the far end.
> 
> - Dave
> 
> On Tue, Nov 25, 2014 at 12:23 AM, Hal Murray <hmurray at megapathdsl.net>
> wrote:
> 
>> 
>> Said Jackson said:
>>> Correct, and thats why its all a bad trade off if you have to use 50 Ohms
>>> termination. Either more heat or more PN, and more circuitry.
>> 
>>> So driving 50 Ohms inputs is not optimal here, 1M inputs are much better
>> for
>>> this purpose.
>> 
>> That only works if you have a (very) short connection to the next stage.
>> 
>> Things get interesting if you have, say, 10 feet of unterminated coax.
>> 
>> 10 MHz is 100 ns, or 50 ns between transitions.  Coax is ballpark of 5/8 c
>> so
>> that's 16 ns one way or 32 ns round drip.  That's 60% of the heat as well
>> as
>> lots of nasty reflections.
>> 
>> (Somebody please check my numbers.)
>> 
>> 
>> --
>> These are my opinions.  I hate spam.
>> 
>> 
>> 
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