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.