[time-nuts] Buffer / distribution amplifier for TCXO

Henry Hallam henry at pericynthion.org
Wed Aug 4 17:46:07 UTC 2010

Dear time nuts,

I have built a GPS receiver based around the SE4120L front end IC [1].
 I used a KT3225 TCXO [2] at 16.3676MHz driving the front end through
a 10nF series capacitor as in the example circuit in [1].  Inside the
front end, this oscillator is multiplied up to form a local oscillator
at 1571.2896 MHz.  The 16.3676MHz signal is also divided to form a
4.0919MHz sampling clock.  Digital I and Q samples then go to a DSP
where the GPS signal processing is done in software.  My receiver
works nicely, getting it online was a boatload of fun and I'm hoping
to make it available soon along with open-source software as a GPS
experimenter's kit.

I'd like to clock multiple receivers from a single 16.3676MHz
oscillator, in order to combine measurements from multiple antennas.
The clocks must be at the same frequency, i.e. from the same source,
but it is not necessary that they have any particular phase
relationship as phase offsets are removed in the navigation

What sort of distribution amplifier should I use to split the output
of one TCXO into four front ends?  Do I need some kind of impedance
matching network?  How would I go about designing that?  This sort of
analog/RF design is unfamiliar territory for me, though I'd like to

The TCXO advertises a minimum output level of 0.8Vpp into (10kohm in
parallel with 10pF).  The front end requires a minimum oscillator
drive level of 0.2Vpp.  The front end datasheet lists "recommended
crystal parameters" including a load capacitance of 10pF (typ),
although I don't know whether or not that refers to the front end
input capacitance.

My guess is that phase noise performance is not particularly crucial,
at least by time-nuts standards.  I guess it would be nice if the
amplifier didn't make the phase noise "significantly" worse than it
already is from the cheap TCXO.

Many thanks,
Henry Hallam

[1] http://www.sige.com/support/download-form.html?dl=DST-00059_SE4120L_Datasheet_Rev_3p5_CYW_May-26-2009.pdf
[2] http://global.kyocera.com/prdct/electro/pdf/tcxo/172_e.pdf

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