[time-nuts] Effects of filter delay
SAIDJACK at aol.com
SAIDJACK at aol.com
Sun Dec 24 19:49:13 EST 2006
In a message dated 12/24/2006 15:05:52 Pacific Standard Time,
bruce.griffiths at xtra.co.nz writes:
SAIDJACK at aol.com 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
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>
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
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