Motorola ASTRO Digital Spectra, Подробное руководство по обслуживанию

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September 5, 2008 6881076C25-E
3-4 Theory of Operation: RF Board
The synthesizer generates a modulus control output which instructs the prescaler to divide by either
P or P + 1 (that is, 255 or 256). When modulus control is low, the prescaler is dividing by P + l (256)
and the A counter is running; when modulus control is high, the prescaler is dividing by P (255) and
the B counter is running. One complete cycle of loop division is repeated for each reference period.
Assume that the VCO is operating correctly at 403 MHz, and the reference frequency is 6.25 kHz.
The prescaler and loop divider work in tandem to divide the VCO frequency down to the reference
frequency. The waveforms in
Figure 3-3 depict what happens in a locked system. Notice in the
waveforms that the leading edge of F
r
goes high to turn on the constant current source Q607. The
ramp capacitor (C634) begins to charge through Q607 and R627, charging at a constant rate, while
the prescaler and loop divider are dividing the VCO frequency by N (64,480 in the example). At this
point, the loop divider generates a loop pulse (F
v
) which turns off the current source.
Figure 3-3. Loop Divider Waveforms
The voltage that was on C634 is sampled and held by the phase detector. This voltage is amplified
approximately 1.8 times and applied to the VCO varactors via the adaptive loop filter and the
steering line. This event is repeated at the reference rate so that frequency errors will always be
corrected.
NOTE: In VHF receive mode, for frequencies divisible only by 2.5 kHz (for example, 146.0025 MHz),
capacitor C670 will be switched in parallel with C634 by Q670. The reference frequency will
be 2.5 kHz instead of 5.0 kHz or 6.25 kHz. In transmit mode, the 2.5 kHz reference is not
used.
Assume that the VCO frequency tends to drift low. If this happens, the loop pulse will occur at some
later time. The current source still begins at the rising edge of F
r
but it stays on longer because the
leading edge of F
v
has been time delayed. Thus, C634 charges to a higher value and the steering
line drives the VCO to a higher frequency. The opposite case also applies.
3.1.2.3 Programmable Reference Divider
The reference frequency for 800 MHz is 6.25 kHz; for VHF and UHF, the typical reference frequency
is 5.0 kHz. In VHF radios, the reference frequency is 2.5 kHz for receive frequencies not evenly
divisible by 5.0 kHz or 6.25 kHz.
FR
FV
SAMPLE
AND HOLD
REFERENCE
FREQUENCY
LOOP
DIVIDER
RAMP
CAPACITOR
RAMP DISCHARGE
MAEPF-25183-O