[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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