[time-nuts] Buffer / distribution amplifier for TCXO

Bob Camp lists at rtty.us
Thu Aug 5 02:43:30 UTC 2010


Hi

It's a speed thing. The faster silicon based CMOS is , the quieter it seems to be. 

Bob


On Aug 4, 2010, at 8:21 PM, Bruce Griffiths wrote:

> 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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