[time-nuts] Practical considerations making a lab standard with an LTE lite
SAIDJACK at aol.com
SAIDJACK at aol.com
Tue Nov 25 19:48:05 UTC 2014
That's why I said its up to the user to decide what they want their
trade-off to be.
For permanent installations I personally would not run the unbuffered 10MHz
output through more than about a foot of coax cable to the buffer.
The rise/fall time of the TCXO output is slow enough (typical spec is 4ns)
to make that a lumped system rather than a reflected system. You won't see
any reflections on a foot or less of cable.
For short-term phase noise measurements I have run that signal through 6
feet of coax no problem, but there are quite significant reflections at that
point so I would strongly advise against that. If I break the TCXO here on
my bench due to my own stupidity its a different situation than if the
customer has that happen in their setting..
bye,
Said
In a message dated 11/25/2014 09:34:11 Pacific Standard Time,
csteinmetz at yandex.com writes:
Said wrote:
>The increased current for the driver will cause heating near the
>crystal in both the CMOS driver and the 3.0V LDO as the LDO has to
>convert the excess voltage into heat. This may or may not affect the
crystal.
There would be next to no additional heating in the CMOS driver,
because there is very little voltage across it in either logic
state. And the additional power supply current is so small that the
increase in LDO dissipation will also be very low. At the extreme
worst, any such effects would be somewhere between imperceptible and
negligible. But on the other hand, if there is a possibility that a
passive filter can create a clean, 50 ohm sine wave output for free,
the potential up side is huge.
>Adding an external buffer is so simple that I just did not think it
>would be worth it..
An external buffer is a fine way to go, but it would need to be very
close to the driver chip -- which is why I suggested on Sunday
building it onto a breakout card that plugs directly onto the LTE
Lite's MMCX output connector. You really don't want to run a naked
CMOS output at 10MHz much farther than that, both for the corruption
it may suffer and also for the mischief that radiation and capacitive
coupling can cause to other nearby circuitry (the LTE Lite) as the
loop gets larger than that.
I'm not sure I see why a small additional source of heat is such a
dramatic concern with the 10MHz TCXO, but apparently not for the
20MHz TCXO, which by accounts has an actual buffer amp that must
create comparatively massive heating. A temperature difference isn't
a problem in and of itself -- only a changing temperature creates a
problem. Whatever the dissipation situation is, it should settle
into stasis if one takes the slightest care with the thermal design.
Best regards,
Charles
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