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5−
15
2
48
where 48 is the number of binary digits (bits) used by the DDFS and (F osc) is
nominally 10 MHz.
Input to the microcontroller is the relative phase of the DDFS output and the
input signal. Software computes phase deviation from initial reference and
rate of change of phase difference. These parameters are used to compute
an adjustment to the integer (N) which controls the DDFS. The
hardware/software combination constitutes a discrete implementation of a
second-order linear control loop (
Δ
T is 8 seconds). The step-response of
such a system is a function of the difference of two exponential terms which
are the time constants. There is a pair of time constants for each of ACQUIRE
1, ACQUIRE 2, and normal tracking. The primary (shorter) time constant is
the dominant factor in determining initial behavior.
The control loop of the 54522 is piece-wise linear, since normal and ACQUIRE
2 can alternate.
5.5217 Controls and Latches
The output phase is compared with the phase of the recovered input reference
clock by means of a pair of counters and latches, good signal or bad:
a. When the input signal is deemed good, the microcontroller tracks the input
phase and, approximately once per second, calculates a new 48-bit control
word for the DDFS based on the input/output phase difference and rate of
change. The 16 most significant bits (MSBs) of the control word are fixed
and the microcontroller calculates the 32 least significant bits (LSBs).
b. When all inputs are deemed bad, tracking ceases (HOLD mode). Any fault
which triggers the HOLD state saves the last known good output frequency
correction factor and uses it to determine the HOLD frequency.
5.5218 Controls and Indicators
The following controls and indicators appear on the front panel of the 54522
card:
a. The red FAIL LED indicates a fuse has blown or the power supply or
microcontroller has failed.
b. The green INPUT A/B LEDs indicate which of the two INPUT signals the
card is tracking.
c. The yellow HOLD LED indicates that the unit is in the HOLD mode.
d. The green ACQUIRE LED indicates that the unit is in the ACQUIRE mode.
e. The green LOCKED LED indicates that the unit is locked to an input.
f. The yellow FREERUN LED indicates that the unit is in the FREERUN
mode.
