General Description
The DS8908B is a PLL synthesizer designed specifically for
use in AM/FM radios. It contains the reference oscillator, a
phase comparator, a charge pump, an operational amplifier,
a 120 MHz ECL/I2L dual modulus programmable divider,
and a 19-bit shift register/latch for serial data entry. The
device is designed to operate with a serial data controller
generating the necesary division codes for each frequency,
and logic state information for radio function inputs/outputs.
A 3.96 MHz pierce oscillator and divider chain generate a
1.98 MHz external controller clock, a 20 kHz, 10 kHz, 9 kHz,
and a 1 kHz reference signals, and a 50 Hz time-of-day
signal. The oscillator and divider chain are sourced by the
VCCM pin thus providing a low power controller clock drive
and time-of-day indication when the balance of the PLL is
powered down.
The 21-bit serial data steram is transferred between the frequency synthesizer and the controller via a 3-wire bus system comprised of a data line, a clock line, and an enable
line.
The first 2 bits in the serial data stream address the synthesizer thus permitting other devices such as display drivers to
share the same bus. The next 14 bits are used for the
PLL(Na1) divide code. The 15th bit is used internally to
select the AM or FM local oscillator input. A high level on
this bit enables the FM input and a low level enables the AM
input. The 16th and 17th bits are used to select one of the 4
reference frequencies. The 18th and 19th bits are connected via latches to open collector outputs. These outputs can
be used to drive radio functions such as gain, mute, AM,
FM, or charge pump current source levels.
The PLL consists of a 14-bit programmable I2L divider, an
ECL phase comparator, an ECL dual modulus (p/p a1) prescaler, a high speed charge pump, and an operational amplifier. The programmable divider divides by (Na1), N being
the number loaded into the shift register. The programmable
divider is clocked through a d-/8 prescaler by the AM input
or through a d$*/64 prescaler by the FM input. The AM input
will work at frequencies up to 15 MHz, while the FM input
works up to 120 MHz. The VCO can be tuned with a frequency resolution of either 1 kHz, 9 kHz, 10 kHz, or 20 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 and sink. The charge
pump can be programmed to deliver from 75 mA to 750 mA
of constant current by connection of an external resistor
from pin RPROGRAM to ground or the open collector bit outputs. Connection of programming resistors to the bit outputs
enables the controller to adjust the loop gain for the particular reference frequency selected. The charge pump will
source current if the VCO frequency is high and sink
current if the VCO frequency is low. The low noise operational amplifier provided has a high impedance JFET input
and a large output voltage range. The op amp’s negative
input is common with the charge pump output and its positive input is internally biased.

Features
- Uses inexpensive 3.96 MHz reference crystal
- FIN capability greater than 120 MHz allows direct synthesis at FM frequencies
- FM resolution of either 10 kHz or 20 kHz allows usage
of 10.7 MHz ceramic filter distribution
- Serial data entry for simplified control
- 50 Hz output for time-of-day reference driven from separate low power VCCM
- 2 open collector buffered outputs for controlling various
radio functions or loop gain
- Separate AM and FM inputs; AM input has 15 mV (typical) hysteresis
- Programmable charge pump current sources enable adjustment of system loop gain
- Operational amplifier provides high impedance load to
charge pump output and a wide voltage range for the
VCO input