[time-nuts] Buffer / distribution amplifier for TCXO

[Bruce Griffiths]

Henry Hallam wrote:
> Dear time nuts,
>
> Background:
> 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.
>
> Problem:
> 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
> processing.
>
> 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
> learn.
>
> 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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>    
The TCXO output waveform is presumably a clipped sinewave as required by 
the SE4120L?

In which case a linear distribution amplifier is probably required.

With only a ~3V supply available, options for the distribution amplifier 
topology are somewhat limited.
In principle you could use an emitter follower driving 4 other emitter 
followers with a resistor in series with the emitters of the output 
devices and the AC coupled loads to match the source to the 
interconnecting cable impedance to minimise reflections without 
requiring excessive dissipation in the emitter followers.
With the low voltage supply available, using an RF choke is series with 
the emitter follower's emitter to ground resistor will also be useful in 
achieving the required dynamic range.

Bruce