© 2012 Jackson Labs Technologies, Inc. 9
LC_XO GPSDO User Manual
2.4.1 “Help” and command overview
•
A listing of the available serial commands can be shown by typing "help?" when using TeraTerm
(recommended) or Hyperterm.
•
"*IDN?" can be used to see if the connection works. Both commands need to be followed by
pressing “Enter”.
2.4.2 Loop parameter adjustment
•
All loop parameters can be controlled via the serial port.
•
Loop parameters are optimized for the OCXO/TCXO on the board, and changing the factory
settings may result in the unit’s performance to deteriorate.
The commands to control the loop parameters are part of the servo? command. See also the SERVO
Subsystem section in Chapter 3 (Section 4.9 ).
The individual commands are:
EFC Scale: this is the proportional gain of the PID loop. Higher values will give quicker
convergence, and faster locking of the GPS time (lower loop time constant), lower values give less
noise. Values between 0.7 and 6.0 are typical.
EFC Damping: overall IIR filter time constant. Higher values increase loop time constant. Jackson
Labs Technologies, Inc. typically uses values between 10 to 50. Setting this value too high may cause
loop instability.
Phase compensation: this is the Integral part of the PID loop. This corrects phase offsets between the
LC_XO board 1PPS signal and the UTC 1PPS signal as generated by the GPS receiver. Set higher
values for tighter phase-following at the expense of frequency stability. Typical values range from 4
- 30, 25 being the default. Setting this value too high may cause loop instability.
A well-compensated unit will show performance similar to the plot in Figure 2.5 when experiencing
small perturbations
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