Second Vco Checks; Troubleshooting The Back-End - Motorola ASTRO Digital Spectra Plus Service Manual

4-8
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).
MODULUS
CONTROL:
COUNTER:
COUNTER RESET:
PRESCALER DIVIDES BY:
TIME (Microseconds):
LOOP DIV. TIME ( Sec):

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
October 28, 2002
HI
LOW
A
B
90
192
255
256
51
109
160
109.2 MHz
= 300 kHz
364
Troubleshooting Procedures: ASTRO Spectra Procedures
HI
LOW
A
B
90
192
255
256
51
109
160
5-3.
HI
LOW
A B
90 192
255 256
51 109
160
6881076C25-D