[time-nuts] Buffer / distribution amplifier for TCXO

[Bruce Griffiths]

The GPS receiver chip actually specifies that a clipped sinewave should 
be used.
Presumably this is necessary to limit the harmonic contents.
In which case low pass filtering the CMOS outputs may be necessary.
The 74AHC04 or equivalent may be a better choice as its ground and Vcc 
bounce is lower than that of a 74AC04.

Bruce

Bob Camp wrote:
> Hi
>
> I suspect you will find that the phase noise floor of the distribution 
> system does indeed matter.
>
> Likely the "easy way" to go:
>
> Square the TCXO up with a biased CMOS inverter (at least as fast as a 
> 74AC04). Run a seperate inverter to drive each of the receivers. A hex 
> inverter chip would do it all quite nicely. There should be plenty of 
> isolation and far more signal than is needed. Attenuating it at the 
> receiver with a pair of resistors should get all the levels to match 
> up. If you want to get fancy, transformer couple into each receiver 
> after attenuating.
>
> Bob
>
> --------------------------------------------------
> From: "Henry Hallam" <henry at pericynthion.org>
> Sent: Wednesday, August 04, 2010 1:46 PM
> To: "Discussion of precise time and frequency measurement" 
> <time-nuts at febo.com>
> Subject: [time-nuts] Buffer / distribution amplifier for TCXO
>
>> 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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>
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