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

Charles Steinmetz csteinmetz at yandex.com
Tue Nov 25 12:31:55 EST 2014

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,


More information about the time-nuts mailing list