[time-nuts] Effects of filter delay
SAIDJACK at aol.com
SAIDJACK at aol.com
Sun Dec 24 15:10:03 EST 2006
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
More information about the time-nuts
mailing list