April 1990
DS8911/DS8913 AM/FM/TV Sound
Up-Conversion Frequency Synthesizer
General Description
The DS8911 is a digital Phase-Locked Loop (PLL) frequen-
cy synthesizer intended for use as a Local Oscillator (LO) in
electronically tuned radios. The device is used in conjunc-
tion with a serial data controller, a loop filter, some varactor
diodes and several passive elements to provide the local
oscillator function for both AM and FM tuning.
frequency. The DS8911 derives the 450 kHz second IF by
mixing an 11.55 MHz first IF with a 12.00 MHz reference
frequency.
FM and WB (weather band) tuning is done using the con-
ventional down conversion approach. Here the VCO signal
is buffered to produce the LO signal and then mixed on chip
with the RF signal to obtain an IF frequency at the MIXER
output pins. This IF frequency is typically chosen to be 10.7
MHz although placement at 11.50 MHz can further enhance
AM mode performance and minimize IF circuitry.
The conventional superheterodyne AM receiver utilizes a
low IF or down conversion tuning approach whereby the IF
is chosen to be below the frequencies to be received. The
DS8911 PLL on the other hand, utilizes an up-conversion
technique in the AM mode whereby the first IF frequency is
chosen to be well above the RF frequency range to be
tuned. This approach eliminates the need for tuned circuits
in the AM frontend since the image, half IF, and other spuri-
ous responses occur far beyond the range of frequencies to
be tuned. Sufficient selectivity and second IF image protec-
tion is provided by a crystal filter at the output of the first
mixer.
The PLL provides phase comparator reference frequencies
of 10, 12.5, 25, and 100 kHz. The tuning resolutions result-
ing from these reference frequencies are determined by di-
viding the reference by the premix modulus. Table II shows
the tuning resolutions possible.
The DS8911 contains the following logic elements: a volt-
age controlled oscillator, a reference oscillator, a 14-bit pro-
grammable dual-modulus counter, a reference frequency di-
vider chain, a premix divider, a mixer, a phase comparator, a
charge pump, an operational amplifier, and control circuitry
for latched serial data entry.
A significant cost savings can be realized utilizing this up-
conversion approach to tuning. Removal of the AM tuned
circuits eliminates the cost of expensive matched varactor
diodes and reduces the amount of labor required for align-
ment down from 6 adjustments to 2. Additional cost savings
are realized because up-conversion enables both the AM
and FM bands to be tuned using a single Voltage Controlled
Oscillator (VCO) operating between 98 and 120 MHz. (The
2 to 1 LO tuning range found in conventional AM down con-
version radios is reduced to a 10% tuning range; 9.94 MHz
to 11.02 MHz).
The DS8913 includes all the above logic elements except
that it requires a 10 MHz reference frequency instead of
12 MHz.
Features
Y
Direct synthesis of LW, MW, SW, FM, and WB
frequencies
Y
Serial data entry for simplified processor control
Up-conversion AM tuning is accomplished by first dividing
the VCO signal down by a modulus 10 to obtain the LO
signal. This LO in turn is mixed on chip with the RF signal to
obtain a first IF at the MIXER output pins. This first IF after
crystal filtering is mixed (externally) with a reference fre-
quency provided by the PLL to obtain a 450 kHz second IF
Y
10, 12.5, 25, and 100 kHz reference frequencies
Y
8 possible tuning resolutions (see Table II)
Y
An op amp with high impedance inputs for loop filtering
Y
Programmable mixer with high dynamic range
TRI-STATEÉ is a registered trademark of National Semiconductor Corporation.
MICROWIRETM is a trademark of National Semiconductor Corporation.
C
1995 National Semiconductor Corporation
TL/F/7398
RRD-B30M105/Printed in U. S. A.
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