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5−
39
2
48
where 48 is the number of 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 54593 is piece-wise linear, since normal and ACQUIRE
2 can alternate.
5.9216 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, every 7 seconds, calculates a new 48-bit control word for the
DDFS based on the input/output phase difference and rate of change. The
16 MSBs of the control word are fixed and the microcontroller calculates the
32 LSBs.
b. When the input is 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.
5.9217 Controls and Indicators
The following controls and indicators appear on the front panel of the 54593
card:
a. The red FAIL LED indicates an internal 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.
