AD9851 Analog Devices, AD9851 Datasheet - Page 6

• Manufacturer Part Number: AD9851
• Description: CMOS 180 MHz DDS/DAC Synthesizer
• Manufacturer: Analog Devices

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