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"<he...@pericynthion.org> >>>> Sent: Wednesday, August 04, 2010 1:46 PM >>>> To: "Discussion of precise time and frequency >>>> measurement"<time-nuts@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 >>>>> >>>>> _______________________________________________ >>>>> time-nuts mailing list -- time-nuts@febo.com >>>>> To unsubscribe, go to >>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>> and follow the instructions there. >>>>> >>>>> >>>>> >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.