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

Joel Jameson w0kgw3 at gmail.com
Tue Dec 2 11:12:51 EST 2014

Just to give back to the group, here are the connectors I chose from
Digikey  to make the GPS MMCX antenna included with the kit detachable for
an enclosure:

1x  744-1715-ND      RF CABL MMCX ML STR / ML RA 6"


On Tue, Nov 25, 2014 at 1:48 PM, S. Jackson via time-nuts <
time-nuts at febo.com> wrote:

> 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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