Motorola ASTRO Digital Spectra, Подробное руководство по обслуживанию

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September 5, 2008 6881076C25-E
4-8 Troubleshooting Procedures : ASTRO Spectra Procedures
At 450 MHz, there are 72,000 counts of 2.22 nanoseconds each per reference period. When
modulus control (MCT) is high, the VCO output is prescaled by 255 (see the diagram below). The
output frequency of the prescaler is 1.765 MHz which corresponds to a period, per-cycle, of 567
nanoseconds. The “A” counter runs long enough to count down 90 cycles which equals 51
microseconds. When MCT is low, the prescaled output equals 1.758 MHz which corresponds to a
period of 569 nanoseconds. The “B” counter counts 192 cycles which takes 109 microseconds. The
total time required for proper loop division is thus 160 microseconds (the reciprocal of 6.25 kHz).
4.1.4.3 Second VCO Checks
1. Check for 300 kHz reference frequency at U601, pin 31.
2. Check for 0.5 to 4.0-V phase detector output at U601, pin 30.
3. Check for -12 to -16 dBm at 109.2 MHz feedback (U601, pin 26).
4. Check the divide-by-N test point for a 700-mV p-p waveform at 300 kHz (the second VCO
frequency divided by 364). See the example below.
NOTE: The second VCO circuit is external to U601 and, while it does depend on U601 for proper
phase-locking, it should free-run, open-loop, at some frequency, if U601 fails. If the 8.8-V
super filter and the oscillator are “dead,” U601 is defective.
4.1.4.4 Troubleshooting the Back-End
Refer to "Chart C.1 RF Board Back-End," on page 5-3 .
COUNTER:
COUNTER RESET:
PRESCALER DIVIDES BY:
TIME (Microseconds):
LOOP DIV. TIME ( Sec):
A
90
255
51
A
90
255
51
A
90
255
51
B
192
256
109
B
192
256
109
B
192
256
109
LOW LOW LOW
HI HI HI
MODULUS
CONTROL:
160 160 160
109.2 MHz
364
= 300 kHz