Hi The 5 MHz stuff was down at or below 1.5x10^-12 at one second by our measure. Others measured them a bit lower than that. We didn't do 100% testing at 10 sec, so I don't have a lot of data there. The ones 55 Hz higher often came at or above 4x10^-12.
Bob On Feb 6, 2010, at 9:40 PM, Bruce Griffiths wrote: > As a matter of interest just how bad were those OCXOs? > > e.g. what was the ballpark ADEV for 1s, 10s etc.? > > Bruce > > Bob Camp wrote: >> Hi >> >> Occasionally you also come across 5.000055 MHz OCXO's that have 5 MHz >> crystals in them. Then you discover just how much short term stability can >> degrade when they move the crystal 55 Hz. Same vendor crystal, same crystal >> spec., same oscillator circuit, not even close on short term stability.... >> >> Bob >> >> >> On Feb 6, 2010, at 9:02 PM, Bruce Griffiths wrote: >> >> >>> JPL resorted to using a commercial synthesiser set for an offset of 123Hz >>> (to minimise spurs and other artifacts) in their 100MHz N channel mixer >>> system. >>> >>> Occasionally one comes across 5.000055MHz OCXOs that use 10.000110MHz >>> crystals internally. >>> The resultant 55Hz (with 5MHz source) or 110Hz (with 10MHz source) beat >>> frequencies are lie between the hamonincs of either 50Hz or 60Hz line >>> frequencies. >>> >>> Bruce >>> >>> Bob Camp wrote: >>> >>>> Hi >>>> >>>> Any approach that includes building a low noise synthesizer is opening up >>>> a whole new set of issues. I would much prefer to do my building at audio. >>>> Audio parts are cheap, and performance is usually a lot easier to check >>>> than at RF. >>>> >>>> Bob >>>> >>>> >>>> On Feb 6, 2010, at 8:30 PM, Bruce Griffiths wrote: >>>> >>>> >>>> >>>>> Which just leaves the minor problem of the offset oscillator. >>>>> >>>>> One option is to use a phase truncation spur free output frequency from a >>>>> DDS. >>>>> If one is using the Costas receiver approach the beat frequency need not >>>>> be a nice round number like 1.0000KHz. >>>>> >>>>> Another method is to use a crystal whose frequency is offset a few kHz >>>>> from 10MHz. >>>>> >>>>> Yet another is the classical method of dividing 10MHz by 100 and >>>>> subtracting (using an LSB mixer) the resultant 100KHz from 10MHz to >>>>> produce 9.9MHz, then divide the 9.9MHz signal by 100 and add (using a USB >>>>> mixer) the resultant 99kHz signal to the 9.99Mhz signal to produce a >>>>> 9.999MHz output. >>>>> >>>>> Bruce >>>>> >>>>> John Miles wrote: >>>>> >>>>> >>>>>> A sound-card back end has always seemed like a pretty reasonable >>>>>> approach to >>>>>> me, if you're inclined to go the DMTD route. I wouldn't send a >>>>>> 'baseband' >>>>>> signal to the sound card, though -- I'd upconvert it to a few kHz to get >>>>>> away from the numerous bad things that sound cards do near DC. >>>>>> >>>>>> -- john, KE5FX >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>>> Hi >>>>>>> >>>>>>> My main concern with the low frequency pole in the sound card is >>>>>>> the quality of the R/C used. You can certainly model what ever >>>>>>> you have. If they used an aluminum electrolytic for the "C" it >>>>>>> may not be the same next time you check it .... >>>>>>> >>>>>>> On a 10 Hz system, a 1 Hz pole is probably not an issue. It might >>>>>>> get in the way with a 1 Hz beat note. >>>>>>> >>>>>>> Another thing I have only seen in passing: "Sigma Delta's have >>>>>>> poor low frequency noise characteristics". I haven't dug into it >>>>>>> to see if that's really true or not. If you buy your own ADC's, >>>>>>> you certainly would not be restricted to a Sigma Delta. >>>>>>> >>>>>>> Even with a cheap pre-built FPGA board, you could look into >>>>>>> higher sample rates than a conventional sound card. You would >>>>>>> drop back to 16 bits, but it might be worth it. >>>>>>> >>>>>>> Bob >>>>>>> >>>>>>> >>> >>> > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
