AD9851

–6–

REV. C

Both IOUT and IOUTB are equally loaded with 100

Ω. Two

100 k

Ω 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.

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

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

3.9k

CMOS

OUTPUTS

QOUTB

QOUT

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 Configuration

PHASE

COMPARATOR

LOOP

FILTER

VCO

AD9851

DDS

REFERENCE

CLOCK

TUNING

WORD

RF

FREQUENCY

OUT

FILTER

REF CLK IN

PROGRAMMABLE

"DIVIDE-BY-N" FUNCTION

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 Modulation

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.