TRF4400

SINGLECHIP 433MHz RF TRANSMITTER

SLWS113D −NOVEMBER 2000 − REVISED FEBRUARY 2005

8

POST OFFICE BOX 655303

direct digital synthesizer

general principles of DDS operation

In general, a direct digital synthesizer (DDS) is based on the principle of generating a sinewave signal in the

digital domain. Benefits include high precision, wide frequency range, a high degree of software

programmability, and extremely fast lock times.

Figure 4 shows a block diagram of a typical DDS. It generally consists of an accumulator, sine lookup table, a

digital-to-analog converter, and a low-pass filter. All digital blocks are clocked by the reference oscillator.

Synthesizer

Frequency Register

N-Bit Register

+

Sine

Lookup

Table

DAC

Low-Pass

Filter

Analog Output Signal

Load With Frequency Word

Figure 4. Typical DDS Block Diagram

frequency register whereby generating a digital ramp waveform. Each number in the N-bit output register is used

to select the corresponding sine wave value out of the sine lookup table. After the digital-to-analog conversion,

a low-pass filter is necessary to suppress unwanted spurious responses.

The analog output signal can be used as a reference input signal for a phase-locked loop (PLL). The PLL circuit

multiplies the reference frequency by a predefined factor.

TRF4400 direct digital synthesizer implementation

Figure 5 shows a block diagram of the DDS implemented in the TRF4400. It consists of a 24-bit accumulator

clocked by the reference oscillator along with control logic settings.

DDS Mode0

Frequency Setting

Mode0/1

Select

Logic

24-Bit

Register

DDS Frequency Register

Modulation

Control

Logic

22

22

24

8

+

+

24

11

11-Bit

DAC

Sine

Shaper

Low-pass

Filter

TX_DATA − (Terminal 14)

C − Word / MM Bit

(Modulation Mode Select)

to

PLL

FSK Frequency

Deviation Register

D − Word / DEV Bits

(FSK Deviation)

A − Word

MODE − (Terminal 11)

B − Word

DDS Mode1

Frequency Setting

Reference Frequency,

Figure 5. DDS Block Diagram as Implemented in the TRF4400