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3.4 PLL Synthesizer Tuning System: Theory of Operation
As
shown
in the
Figure,
the
output
frequency
of the
FM/AM
local oscillator
is automatically
locked to a con-
stant frequency
by the PLL
network
which
is operating
under microprocessor control.
tn the
FM
mode,
part of the local oscillator
output
is
coupled to the FM
input terminat of the PLL block via a
buffer amplifier (for example, when the received frequency
is 98.1 MHz, the local oscillation frequency is 98.1 + 10.7
= 108.8 MHz).
Meanwhile,
the
microprocessor
accepts
frequency
data
input from
the keyboard
and
provides
the data of, say,
98.1 MHz to the display. It aiso provides frequency divid-
ing-ratio data to the PLL
biock. Since the reference fre-
quency
for the FM
mode
is 25 kHz, dividing ratio N is
determined as follows:
:
N = 108.8 MHz + 0.025 = 4352
When the 50 kHz stepping interval is selected
in the FM
mode, the FM frequency band is between 87.5 and 108.0
AMRF
FM RF
4.5MHz
11/3928—
1/4748}
AM
i
1 (4748)
MHz,
and
hence
the
local
frequency
band
is between
98.2 and 118.7 MHz. As a result, the dividing ratio is be-
tween 3928 and 4748.
/
Once dividing ration is determined, the local oscillation
frequency
is divided by N, and the resultant signal phase
is compared. with the reference
signal phase.
The refer-
ence
signal
is created
by dividing
the master
oscillator
output
of 4.5 MHz,
and its frequency
accuracy depends
on that of the quartz crystal element
used in the master
oscillator.
:
The frequency
divided
in the PLL block (108.8 MHz +
435 = 25 kHz} is phase-compared
with the reference fre-
quency of 25 kHz, and the phase difference between the
two signa{s is converted
by the PLL into a corresponding
pulse array, This pulse array is cougled to a low-pass filter,
where it is converted into a corresponding DC level, which
is then feed back to the local oscillator's contro} input to
control the faca! oscillator output frequency to a constant.
PHASE
COMPARATOR
Reference
frequency
divider
"Vay
FM:
25kHz
AM:
SkHz
ic}
FM: 25kHz
AM: 10kHz
|
11/3944—
4)4744
FM:
25kHz
AM:
10kHz
|
|
:
1/108—1/229 | 1/98
— 1/207 | 4/98—14/207
Receiving frequency
87.50 ~ 108.0 MHz
522 ~
1611
kHz
87.50 ~ 108.0 MHz
530 ~~ 1620 kHz
i. 87.9~ 107.9MHz_
[sar Ste |
200 kHz
Loca! oscillator frequency
MN1455LF
- Figure 7.
Number
of channels
98.2 ~ 118.7 MHz
972 ~ 2061
kHz
98,2
~ 118.7 MHz
980 ~ 2070 kHz -
93.6 ~ 118.6 MHz
10 kHz
530 ~
1620 kHz
980 ~
2070 kHz
r——:
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