[time-nuts] Pre-Release Docs and Schematics for the Fury Interface Board
SAIDJACK at aol.com
SAIDJACK at aol.com
Tue Oct 30 00:28:48 EDT 2007
In a message dated 10/29/2007 19:11:00 Pacific Daylight Time,
xaos at darksmile.net writes:
>The Fury Interface board is closer to reality!
>I think I have included everything in there. At least the things that
>I would like to have. However, I am sure that the members of time-nuts
>will add many suggestions.
Here are some thoughts from my side:
The LT1208 has a fairly significant Voffset-versus-temp sensitivity. Maybe a
part like the LTC6244HV with about 1/10 the offset drift would work better,
and reduce the thermal sensitivity of the overall system.
For example, the LT1208 has 25uV per Degree C drift, and with a standard MTI
OCXO which has about 40Hz deviation from 0V to 5V EFC this would be about
2E-011 drift per degree Celsius worst case just due to the opamp drift, a fairly
The LTC6244HV part only allows a power supply range of +-5V so you may want
to add low-noise 5V and -5V regulators for the opamps.
The feedback and reference resistors R1, R2, R5, R6 are fairly high value,
and thus will add noise to the EFC. Make sure to use metal film types here. To
reduce overall noise, you may want to add a simple RC lowpass filter to the
output of the opamp, say four X7R 22uF10V caps driven by a 4.7K metal film
Ground loop current coming from the OCXO heater through the OCXO ground pin
and the ground plane can also cause some EFC error. Make sure to route the
OCXO ground to the OCXO ground pin on the Fury, and then separate-out the EFC
ground through a star-type layout. The idea is to prevent the OCXO heater
ground current from flowing through the EFC circuitry and SMA cable, in which it
would cause voltage drop and thus thermal sensitivity.
Also, you may want to allow the OCXO to be powered from the Fury OCXO
footprints' power pins. The Fury can compensate for thermal sensitivity if this is
done. Alternatively, a thermistor may be connected to the OCXO, and driven by
the Fury's OCXO power pins (thermistor must handle 10.5V!). As long as the
current varies with temperature on the OCXO power pins, the Fury will learn,
and compensate thermal sensitivities in the system. BTW: the thermal tempco
measurement is disabled via software command by default on the SMA Fury's, and
can be enabled via SCPI command.
Hope this helps,
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