3. Principle summarize
3.1. Principle frame
3.2. Working principle of DDS
* To generate a voltage signal, the traditional analogue signal source adopts electronic components as
oscillator in different ways. So both frequency accuracy and stability are not high enough. Besides, it is of
the disadvantages of complicated technique, low resolution and inconvenient frequency setting and
realization of computer control. Direct Digital Synthesize (DDS) technique is a new developing method of
generating signals without any oscillator components, by which a series of data stream are generated
using digital synthesizing method and then a pre-established analogue signal is generated from digital-
analogue converter.
* To generate a sine signal, for example, the function of y=sinx should be digitally quantized first, and then
taking x as the address and y as the quantized data to store them into waveform memorizer. DDS uses
phase adding technique to control the address of waveform memorizer. Add a phase increment on the
present result of phase accumulator in each sampling clock period so as to change the output frequency
value by change phase increment. According to the address from the phase accumulator, take the
quantized data out from the wave memorizer and then convert it into analog voltage via digit-analog
converter and operation amplifier. Since the waveform data are discontinuous sampling, stair sine
waveform is output from DDS generator. The included high-level harmonic wave should be filtered by
lowpass filter so to output a continuous sine wave. With high accurate reference voltage source in digit-
analog converter, the output waveform is if high amplitude accuracy and stability.
* Amplitude controller is a digital-analogue converter. Based on the amplitude value preset by user, it
generates a corresponding analogue voltage and then multiplied by the output signal so to guarantee the
amplitude of output signal to be the preset value. Offset controller is a digit-analog converter. Based on
the offset value preset by user, it generates a corresponding analog voltage and then added with the
output signal so to guarantee the offset of output signal to be the preset value.
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Micro-
controller
MCU
Digital
synthesis
FPGA
Digital-analog
converter D/A
Amplitude
control
Offset control
Clock
20MHz
Keyboard,
display,
adjusting knob
Voltage
amplification
Power
amplification
Attenuator
20dB+40dB
SYNC output
Main output
Output
protection
Lowpass filter
Data
storage
FLASH
Programmable
interface
USB
Power
