In a message dated 12/24/2006 15:05:52 Pacific Standard Time, [EMAIL PROTECTED] writes:
[EMAIL PROTECTED] wrote: > Hi Bruce, > > yes the heater circuit is problematic, and will affect stability a lot. > > For example, some OCXO's don't have separate (Kelvin sensed) ground pins for > the heater, and EFC voltage. > > The result is that any changes in the heater current will cause a voltage > change in the ground pin of the OCXO from the OCXO body to the PCB ground plane > (due to the finite resistance of the ground pin). > > This I2R loss is very small, but will superimpose onto the EFC voltage, so > it's as if this voltage is placed onto the DAC output. > > We have measured this change of voltage to be in the order of 10's of > microvolts for some OCXO's, and that translated into 10's of EFC DAC steps worst > case (during power-on for example when the heaters are running at maximum > current). > > Again the problem is not the steady-state heater current, but changes in > heater current due to thermal tracking, and loop stability of the heaters, this > in turn causing changes in EFC voltage. We are talking about 1E-012 or better > being desirable stability, so these marginal effects almost sudden become a > real issue. > > The best approach is to separate the ground return current of the heater > circuitry completely from the DAC ground circuitry. But that's not possible on > many OCXO's... > > Bye, > Said > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > Said I guess this means you have to model the effect of the changes in IR drop due to heater current changes and modify the EFC voltage to compensate. The complicating issue being that the value of R will vary with ambient temperature (copper has a resistance tempco of about 0.4%/K). Since the a compensation accuracy of around 1% or so will suffice, analog compensation circuitry could be used, however it would have to be adjusted to suit individual oscillators. Bruce Hi Bruce, well, there are two things going for us: the required offset for any given temperature typically does not vary over time (if we compensate for aging), and the GPS disciplining can give us an absolute reference of EFC versus temp required to get an exact frequency. These facts allow us to compensate without an underlying understanding of the exact parameters of the hardware (which change board-to-board). So for any given temperature, if we are well locked to the GPS, we can get the required absolute EFC setting for that particular temperature (again this only works if we compensate our data for aging). This temperature-to-EFC offset typically does not change much over time, and is well correlated, e.g. there is a nice curve of the requied EFC offset versus temperature for a given output frequency. The advantage of this method is that it also measures and compensates for all other tempco's such as DAC and reference tempco, PCB tempco, etc! The closer this compensation is to the perfect values, the better the OCXO will behave in the face of temperature changes. Of course the overall error will eventually be minimized by GPS Disciplining anyway. bye, Said _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
