Datasheet
AD9851
–10
–
IOUT and IOUTB are equally loaded with 100
. Two 100 k
resistors sample each output and average the two volt ag es. The
result is ltered with the 470 pF ca pac i tor and ap plied to one
comparator input as a dc switching threshold. The ltered DAC
sine wave output is applied to the other com par a tor input. The
comparator will tog gle with nearly 50% duty cycle as the sine
wave al ter nate ly traverses the center point threshold.
AD9851
DDS
REFERENCE
CLOCK
IF FREQUENCY
IN
FILTER
FILTER
TUNING
WORD
RF
FREQUENCY
OUT
Figure 3. Frequency/Phase-Agile Local Oscillator
for Frequency Mixing/Multiplying
FILTER
PHASE
COMPARATOR
DIVIDE-BY-N
LOOP
FILTER
VCO
AD9851
DDS
REFERENCE
CLOCK
TUNING
WORD
RF
FREQUENCY
OUT
Figure 4. Frequency/Phase-Agile Reference for PLL
PHASE
COMPARATOR
LOOP
FILTER
VCO
AD9851
DDS
REFERENCE
CLOCK
TUNING
WORD
RF
FREQUENCY
OUT
FILTER
REF CLK IN
PROGRAMMABLE
DIVIDE-BY-N FUNCTION
(WHERE N = 2
32
/ TUNING WORD)
Figure 5. Digitally Programmable Divide-by-N
Function in PLL
AD9851
DDS
FM RF
OUTPUT
ADSP-2181
DSP
PROCESSOR
ADSP-2181
BUS
INPUT/
OUTPUT
DECODE
LOGIC
8-BIT
DATA
BUS
AD1847
STEREO
CODEC
L & R
AUDIO IN
REF
OSC
DAC
OUT
AD9851/FSPCB
EVALUATION
BOARD
EZ-KIT LITE
DSP
Figure 6. High Quality, All Digital RF Frequency
Mod u la tion
High quality, all digital RF frequency modulation generation with
the ADSP-2181 DSP and the AD9851 DDS is well documented
in Analog Devices’ application note AN-543. It uses an image of
the DDS output as il lus trat ed in Figure 8.
I/Q MIXER
AND
LOW-PASS
FILTER
I
Q
AD9059
DUAL
8-BIT ADC
8
8
DIGITAL
DEMODULATOR
ADC ENCODE
AGC
32
CHIP/SYMBOL/PN
RATE DATA
ADC CLOCK FREQUENCY
LOCKED TO
Tx CHIP/SYMBOL/PN RATE
180MHz
OR 30MHz
REFERENCE
CLOCK
Rx
BASEBAND
DIGITAL DATA OUT
Rx
RF IN
VCA
AD9851
CLOCK
GENERATOR
Figure 1. Chip Rate Clock Generator Application in a Spread Spectrum Receiver
MICROPROCESSOR
OR
MICROCONTROLLER
DATA
BUS
180MHz OR 30MHz
REFERENCE
CLOCK
8-BIT PARALLEL DATA,
OR 1-BIT 40 SERIAL DATA,
RESET, W CLK AND FQ UD
IOUT
100k
100k
470pF
100
IOUTB
R
SET
3.9k
CMOS
OUTPUTS
QOUTBQOUT
LOW-PASS
FILTER
7TH ORDER ELLIPTICAL
70MHz LOW PASS
200 IMPEDANCE
200
VOLTAGE HERE = CENTER POINT
OF SINE WAVE (0.5V TYPICALLY)
USING PASSIVE AVERAGING CIRCUIT
0 TO 1V p-p
SINE WAVE
AD9851
200
Figure 2. Basic Clock Generator Con guration
REV. D