From: [EMAIL PROTECTED] Subject: Re: [time-nuts] questions on uncompensated crystal oscillators Date: Wed, 5 Jul 2006 14:12:48 EDT Message-ID: <[EMAIL PROTECTED]>
> Hi John, Hi, > great idea to compensate for temperature using the varicaps in a > free-running system. Recall that AT-cut crystals have a third-degree temperature-to-frequency curve. > The basic design was for locking an MPEG video stream to a broadcaster's > 27MHz master clock using the MPEG time stamps in a digital PLL loop. Actually, when running in such an application you can autotrim very cheaply. The naive approach would be just to lock it (through a software loop) and then use holdover (i.e. just keep the last frequency correction prior to breaking the loo). A little more advanced approach would be to correlate the needed correction with that of the temperature. You can either do a lock-up table or better yeat, figure out the 4 unknowns in the third-degree equation f = a*T^3 + b*T^2 + c*T + d and then use that as a separate correction method. Whenever you are tracking, you also record the required total correction for a few different temperatures. When you have recent measures for four different temperatures, you can solve the equation. T may be any unit really, so it hasn't have to be scaled to match "real" scales. It should just be whatever the temperature measurement cranks out. > When free-running (eg playing back video from a hard disk etc), the > temperature compensation should work great to keep the 27MHz within specs. Indeed. By providing the temperature compensation in combination with tracking, the necessary tracking-dance will be lowered. Cheers, Magnus _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
