4. Chapter 2 Principle introduction
This chapter describes the basic concept of signal shaping and internal operation of the generator, to help
users to learn more about the performance and specifications then operate the generator more smoothly.
4.1. Principle diagram of generator
4.2. DDS working principle
To generate a voltage signal, traditional analog function generator adopt electronic components to consist
oscillator by many different means, the signals generated are of poor frequency accuracy and stability, low
resolution, inconvenient to set frequency and programmed with computer, but request complex techniques.
Direct Digital Synthesis (DDS) technology is an up-to-date technique to generates signals, it need no
oscillator but digital synthesis technique to generate series of data-current which convert to analog signals
through digital-analog 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 of high amplitude accuracy and stability.
-9-
Micro-
controller
MCU
Digital
synthesis
FPGA
Digital-analog
converter D/A
Amplitude
control
Offset
control
Clock 20 MHz
Keyboard,
display,
adjusting knob
Voltage
amplification
Power
amplification
Attenuator
20dB+40dB
SYNC output
Output port
Output
protection
Lowpass filter
Data storage
FLASH
Programmable
interface USB
Power
