CXR Larus 80-100-400
Issue 1, July 2006
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5−
22
5.6213 Framer
The 54523 communicates with a CS2180 Transceiver Chip for the T1 inputs or
a CS2181 Transceiver Chip for the E1 inputs. The framing circuit detects the
following framing errors:
a. Yellow Alarm, LOS (175 ±75 consecutive zeros)
b. BPVs
c. Invalid CRC6 codes (ESF framing only) or CRC4 codes (CRC4 framing
only)
An impaired signal on all inputs causes the microcontroller to cease tracking,
go into the HOLD mode, and indicate the type of impairment on the seven
segment display.
5.6214 Rubidium
Oscillator
The 50.255 MHz rubidium oscillator output is divided by five to provide a
10.051 MHz signal to a 48-bit DDFS.
5.6215 Direct Digital Frequency Synthesizer
The oscillator is coupled to the DDFS and acts as a reference for it. The
DDFS operates on the presumption that oscillator frequency is kept within a
reasonable degree of precision, e.g. within one part in 10
–9
. The DDFS uses a
digital process to convert the oscillator signal into a signal having a second
related frequency. The DDFS accomplishes this conversion by utilizing a
stored algorithm and a numerical input from the microcontroller.
The DDFS accepts an integer, N, from the microcontroller. The DDFS then
generates a new output frequency 'F out' from 'F osc' according to the
following formula:
(N) (F osc)
F out =
--------------
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