AN-543
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APPLICATION NOTE
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High Quality, All-Digital RF Frequency Modulation Generation with the ADSP-2181
DSP and the AD9850 Direct Digital Synthesizer
by Dean R. Becker
The following describes the implementation of a high
quality all-digital FM RF generator for use with audio fre-
quency input signals. The FM RF signal output is in the
broadcast FM band and can be monaural or stereo.
Recent developments in digital signal processing (DSP)
devices have made it possible to create high quality FM
modulation using all-digital circuitry. This modulator
does not require periodic recalibration, it does not drift
with temperature or power supply variations, and it is
very easy to reproduce since there are no analog adjust-
ments (other than the level of the input analog modulat-
ing signal). At the heart of this circuit is a Direct Digital
Synthesizer (DDS), such as the AD9850 Complete-DDS
(C-DDS) device. The AD9850 consists of a digital phase
accumulator, a phase/amplitude converter and a D/A
converter. It creates a linear ramp in the phase accumu-
lator with a frequency that is dependent on the phase
accumulator’s input value. The ramp is mapped to a
sampled sinusoidal signal by the phase-to-amplitude
converter. The sampled digital signal is then D/A con-
verted and filtered with a reconstruction filter to pro-
duce an analog waveform. All functions of the DDS
(except the analog reconstruction filter) are contained in
the AD9850 chip. The sampled signal is created at a
sample rate of 125 MHz, allowing high frequency RF out-
put. Prior to the release of the AD9850, very fast, inex-
pensive C-DDS devices did not exist. For a more detailed
explanation of the operation and specifications of a
C-DDS device, please refer to the theory of operation in
the AD9850 data sheet.
FM modulators have been with us since the time of
Major Edwin H. Armstrong (sometimes known as the
father of FM) and his classic 1936 paper on the subject.
He first put down on paper the fundamental concept of a
fixed center frequency that is varied per a constant de-
viation ratio that is independent of the frequency of the
modulating signal. FM became very popular because
much of the noise generated in nature exhibits AM char-
acteristics. The signal-to-noise ratio of an FM signal will
be found to exceed that of an AM signal due to the im-
provement ratio related to the deviation ratio. We are all
familiar with the wideband FM broadcast band and its
superior sound quality and noise immunity when com-
pared with AM broadcast.
Analog FM modulators typically employ an oscillator
with a varactor diode in the tuned circuit. The voltage to
the varactor is varied by the modulating signal, which
changes its capacitance, therefore changing the reso-
nant point of the oscillator’s tuned circuit, which shifts
the frequency. As noted above, constant center fre-
quency and deviation ratio are essential to quality FM.
Much work has been done to embellish the basic oscilla-
tor circuit to provide the needed stability. Even though
complex circuitry can be added to automatically com-
pensate for power supply and temperature variations,
these circuits still require periodic recalibration to deal
with component aging.
HARDWARE IMPLEMENTATION
The implementation of the FM RF generator consists of
an Analog Devices EZ-KIT Lite 16-bit DSP development
board, some I/O decode circuitry, and an evaluation
board for the DDS—the AD9850-FSPCB as shown in
Figure 1.
LINE IN
AD9850/FSPCB
DDS
AUDIO IN
L AND R
EZ-KIT LITE
AD9850
FM RF
AD1847
STEREO
CODEC
ADSP-2181
BUS
I/O DECODE
LOGIC
8-BIT
DATA
ADSP-2181
DSP P
OUTPUT
DAC
OUT
125MHz
XOSC
Figure 1. Block Diagram of FM RF Generator Implementation
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