Hi Warren, as Bruce mentioned, you will want much more resolution than 12 bits to properly correct for tempco, and more importantly aging. Having only 4096 steps for adjustment is definitely not enough, it will give you a huge offset of 2.44E-011 per step for your 1E-07 OCXO. That may not seem like much, but it actually is for a phase locked system (2.44E-011 is 24.4ns over 1000s). As an example, we have a unit running in Mexico that has an average EFC tracking range of less than +/-20 Microvolts over the period of a typical day, meaning the control voltage remains within a 40 microvolt window over the entire day. See the plots at: _http://resco.ucol.mx/Fury/gpsstat.htm_ (http://resco.ucol.mx/Fury/gpsstat.htm) If you only have 12 bits - 4096 steps, into a 5V control range, then this gives you a step size of 1.22mV, a huge difference of 61 to 1 between the maximum control window range on this particular OCXO, and your minimum 12 bit step size. Your system would be constantly switching between two steps, and generate a massive amount of phase drift. Also, it is fairly simple to cascade DACs to get say 24 bits resolution out of two inexpensive 16 bit DAC's etc. There was a recent Design Idea in EDN or Electronic Design (don't remember which one) where someone claimed 32 bit resolution out of two 16 bit dacs. That claim is ridiculous of course due to noise and matching etc, but you can probably get 20 - 24 bits resolution and accuracy out of a cascaded system with some care. bye, Said In a message dated 12/1/2008 14:58:47 Pacific Standard Time, [EMAIL PROTECTED] writes:
Doesn't this imply that a Dac with more than about 12 bits becomes an overkill at 100 seconds tracking when using a 1E-7 turning range OSC, and around 18 bits for the same Osc when tracking at 1000 Sec? WarrenS _______________________________________________ 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.
