TRF6900
SINGLE-CHIP RF TRANSCEIVER
SLAS213C – SEPTEMBER 1999 – REVISED MAY 2000
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.
AblockdiagramofatypicalDDSisshowninFigure15. Itgenerallyconsistsofanaccumulator, sinelookuptable,
a digital-to-analog converter, and a low-pass filter. All digital blocks are clocked by the reference oscillator.
Synthesizer
Sine
Lookup
Table
Low-Pass
Filter
Analog Output Signal
+
DAC
N-Bit Register
Frequency Register
Load With Frequency Word
Figure 15. Typical DDS Block Diagram
N
The DDS constructs an analog sine waveform using an N-bit adder counting up from 0 to 2 in steps of the
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. The PLL circuit then
multiplies the reference frequency by a predefined factor.
TRF6900 direct digital synthesizer implementation
A block diagram of the DDS implemented in the TRF6900 is shown in Figure 16. It consists of a 24-bit
accumulator clocked by the reference oscillator along with control logic settings.
24
ƒ
to
PLL
DDS
11-Bit
DAC
Sine
Shaper
Low-pass
Filter
+
Reference Frequency, ƒ
24-Bit
Register
ref
11
DDS Frequency Register
24
MODE – (Terminal 17)
22
22
DDS Mode0
A – Word
B – Word
Frequency Setting
Mode0/1
Modulation
Control
Logic
Select
Logic
+
TX_DATA – (Terminal 19)
DDS Mode1
Frequency Setting
D – Word / DEV Bits
(FSK Deviation)
FSK Frequency
Deviation Register
C – Word / MM Bit
8
(Modulation Mode Select)
Figure 16. DDS Block Diagram as Implemented in the TRF6900
17
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