[time-nuts] Buffer / distribution amplifier for TCXO

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Aug 5 00:21:50 UTC 2010


Is that also true for AHC devices which otherwise have similar 
characteristics (apart from ground bounce) to AC devices?

Bruce


Bob Camp wrote:
> Hi
>
> The phase noise floor of the HC is *much* higher than the floor of the AC gates. The main reason it specifies clipped sine is that's what the cheap TCXO's put out.
>
> Bob
>
>
> On Aug 4, 2010, at 6:42 PM, Bruce Griffiths wrote:
>
>    
>> 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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>>
>>
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>
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