DS8907 AM FM Digital Phase-Locked Loop Frequency Synthesizer
July 1986
DS8907 AM FM Digital
Phase-Locked Loop Frequency Synthesizer
General Description
The DS8907 is a PLL synthesizer designed specifically for
use in AM FM radios It contains the reference oscillator a
phase comparator a charge pump a 120 MHz ECL I
2
L
dual modulus programmable divider and an 18-bit shift reg-
ister latch for serial data entry The device is designed to
operate with a serial data controller generating the neces-
sary division codes for each frequency and logic state infor-
mation for radio function inputs outputs
The Colpitts reference oscillator for the PLL operates at
4 MHz A chain of dividers is used to generate a 500 kHz
clock signal for the external controller Additional dividers
generate a 25 kHz reference signal for FM and a 10 kHz
reference signal for AM One of these reference signals is
selected by the data from the controller for use by the
phase comparator
Data is transferred between the frequency synthesizer and
the controller via a 3 wire bus system This consists of a
data input line an enable line and a clock line When the
enable line is low data can be shifted from the controller
into the frequency synthesizer When the enable line is tran-
sitioned from low to high data entry is disabled and data
present in the shift register is latched
From the controller 20-bit data stream the first 2 bits ad-
dress the device permitting other devices to share the same
bus Of the remaining 18-bit data word the next 13 bits are
used for the PLL divide code The remaining 5 bits are con-
nected via latches to output pins These 5 bits can be used
to drive radio functions such as gain mute FM AM and
stereo only These outputs are open collector Bit 16 is used
internally to select the AM or FM local oscillator input and to
select between the 10 kHz and 25 kHz reference A high
level at bit 16 indicates FM and a low level indicates AM
The PLL consists of a 13-bit programmable I
2
L divider an
ECL phase comparator an ECL dual modulus (p p
a
1) pre-
scaler and a high speed charge pump The programma-
ble divider divides by (N
a
1) N being the number loaded
into the shift register (bits 1 鈥?13 after address) It is clocked
by the AM input via an ECL
d
prescaler or through a
d
prescaler from the FM input The AM input will work
at frequencies up to 15 MHz while the FM input works up to
120 MHz The AM band is tuned with a frequency resolution
of 10 kHz and the FM band is tuned with a resolution of
25 kHz The buffered AM and FM inputs are self biased and
can be driven directly by the VCO through a capacitor The
ECL phase comparator produces very accurate resolution
of the phase difference between the input signal and the
reference oscillator The high speed charge pump consists
of a switchable constant current source (
b
0 3 mA) and a
switchable constant current sink (
a
0 3 mA) If the VCO fre-
quency is low the charge pump will source current and sink
current if the VCO frequency is high When using an AFC
the charge pump output may be forced into TRI-STATE by
applying a low level to the charge pump enable input
A separate V
CCM
pin (typically drawing 1 5 mA) powers the
oscillator and reference chain to provide controller clocking
frequencies when the balance of the PLL is powered down
Features
Y
Y
Y
Y
Y
Y
Y
Uses inexpensive 4 MHz reference crystal
F
IN
capability greater than 120 MHz allows direct syn-
thesis at FM frequencies
FM resolution of 25 kHz allows usage of 10 7 MHz ce-
ramic filter distribution
Serial data entry for simplified control
50 Hz output for 鈥樷€榯ime-of-day鈥欌€?reference driven from
separate low power V
CCM
5-open collector buffered outputs for controlling various
radio functions
Separate AM and FM inputs AM input has 15 mV (typi-
cal) hysteresis
Connection Diagram
Dual-In-Line Package
Order Number DS8907N
See NS Package Number
N20A
TL F 7511 鈥?1
Top View
TRI-STATE is a registered trademark of National Semiconductor Corporation
C
1995 National Semiconductor Corporation
TL F 7511
RRD-B30M105 Printed in U S A
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