Hi Warren, actually, we need to differentiate between phase and frequency errors; Allan variance is typically a measure of phase drift over time. It's not quite the same as a spectral power plot on a spectrum analyzer, but it's related. Some things to keep in mind: * A 12 bit DAC may have an error of 1/2, 1, or even more than a couple of LSB noise/monotonicity error/etc. Every DAC has a minimum quantization error of 1/2 LSB. * On a 5V 12 bit DAC, you are going to switch the OCXO with a massive 1.22mV jolt when the dac LSB changes (if the DAC doesn't have any other errors). Most Euro-can OCXO's have about 8Hz/V frequency sensitivity, or even more than this, so 1.22mV could actually result in 9.76E-010, or almost 1ppb frequency change! On a Micro Crystal DIP-14 OCXO the change would be closer to 15Hz/V, so almost 2ppb steps per LSB. Not good. When controlling a Rubidium with only +/-0.2Hz overall control range this is a totally different story of course. * GPSDO's don't really have "tracking times", they rather just use low-pass filters with natural cutoff-frequencies around 100s to 1000s (1/100Hz or 1 /1000Hz). Meaning they measure the phase error, low-pass filter this phase error, and apply the result to the DAC (oversimplified of course). * The overall error of the system is a combination of the DAC quantization error (1/2 LSB) PLUS the error of the OCXO PLUS the error of the DAC voltage reference PLUS the inherent DAC errors. All noise sources add since they are typically stochastic, so to expect an Allan variance to be the same as just the DAC quantization noise won't work out, since we need to take all of the other noise sources into account too. Lastly, to measure Allan variance down to xE-011 per second, we can use Agilent 53132A counters, TSC5110A or TSC5120A measurement systems, CNT-90 counters, Wavecrest DTS series counters, or other 12-digit per-second resolution time intervall measurement systems. Most of these would work by measuring phase drift between two oscillators. So there is no easy answer to your problem, but yes - depending on your requirements a 12 bit DAC may just work out ok if you have a very good OCXO with less than +/-0.5Hz overall adjustment range (1E-07 at 10MHz). Hope that helps, bye, Said In a message dated 12/1/2008 21:48:01 Pacific Standard Time, [EMAIL PROTECTED] writes:
BTW I've head from others that a 2.44E-11 resolution Dac works OK for most applications. More is better of course. Maybe there is some additional filtering going on or maybe it is they just can't measure short time period noise very well. That comes out to about 10 ps or so of 1 sec phase noise.Not many instruments are going to be able to measure that! What is it you use using to measure the 2.44E-11 freq noise at 1 second? _______________________________________________ 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.
