[time-nuts] Fury Interface Board: 5MHz needed?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Nov 7 17:03:12 EST 2007


Tom Van Baak wrote:
>> George for a single ground pin OXCO you might want to consider
>> compensating  for the oven's internal ground bounce. The voltage drop
>> caused by a normal  heater current is around 1/4 to 1/2mV for an MTI
>> 260 oven.
>>     
>
>   
>> All I can add in regards to the Fury GPSDO is: the EFC control  voltage
>> ground, and heater ground MUST be Kelvin-sensed.
>>
>> Any ground loop error above about 10 - 50 microvolts would affect the
>> systems' accuracy on good double oven OCXO's.
>>     
>
> Hal, Said, and others,
>
> I need convincing that this single-ground-pin OCXO worry is
> valid.
>
> During the first few minutes of oven warm-up one can expect
> large variations in heater current. But during this time no one
> is concerned with EFC stability. So we ignore that case.
>
> After that, oven current is relatively constant by comparison.
> In a GPSDO the EFC is part of a closed loop. So even if there
> is a small voltage offset in the EFC control or return path due to
> heater current, it seems to me that it still has no effect on either
> the accuracy or the stability of the GPSDO. So we ignore that
> case too.
>
> Is the worry, then, about the minor ambient temperature related
> variations in steady-state oven current? Or something else?
>
> /tvb
>
>
> _
Tom

Even without frequency disciplining it doesn't appear to be a
significant issue (if we accept Peter Schmelcher's data) for OCXOs like
the MTI 260 and 230 series as this effect is swamped by the frequency
tempco due to other effects such as the finite change in oven
temperature due to the limited thermal gain etc.

However for an OCXO using a high thermal gain oven controller (E1938A)
or a double oven (OSA B8607) such effects may be significant if a single
ground pin is used.

For example if the resistance of the shared ground wiring were say 0.1
ohms and the current varied by 300mA over the temperature range the EFC
voltage would change by 30mV with a corresponding frequency change of
3E-10  which is comparable to the max frequency change (2E-10) over the
rated temperature range for a standard 8607 and 10x that for some 8607
options.
With a star ground system wthin the OCXO it may be possible to reduce
the shared ground resistance by a factor of 10 or so.

The conclusion being that the smaller the oscillator tempco the more
significant such effect are.
For most single oven OCXOs this effect isnt likely to be significant.

Bruce




More information about the time-nuts mailing list