Design of Digital Controlled Signal Generator Based on DDS and MCU
Yinjun Chena, Zehuai Yuan
Faculty of Electronic Information and Mechanical Electrical Engineering, Zhaoqing University, Zhaoqing 526061, China
Keywords: DDS; MCU; Signal generator; Phase Accumulator; DAC
Abstract. Its advantage to use DDS chip is output signal frequency bigger, and precision higher, But users can't change the output signal waveforms. The MCU can produce the required arbitrary waveforms, but its program execution of the order limit the speed.So we use their Synergy to design the digital controlled signal generator. The System has the advantage of output good quality waveform, frequency of precision and stability ,and high frequency, empty, amplitude and phase is to step into the need.
Introduction
The digitally synthesized sine waveform (Direct Digital Synthesis, DDS) is a well-known method and has been applied to many embedded applications [1]. This technique can be used to create a positive digital sine waveform. Compared to other frequency composing method, Direct Digital Frequency Synthesis(DDS) has been the most popular trend in modern frequency synthetic technique for its excellent characteristics. The signal source that the DDS technology realizes can carry out accurate controlling on DDS frequency , extent , phase exporting wave form's etc. by numerical control circuit, the system making use of this method has many merits such as stability, reliably and accuracy.
The commercial DDS chip can only export a sine wave for the data in the ROM form already has been solidified. If needing to come into being any wave form, it may come true by the way that FPGA adopts DDS IP core or hardware describe language etc, however,the cost of This way cost is high; on the other hand, any wave form can be achieved by making use of micro controller unit (MCU) to carry out figure frequency combining and DA converting. The experiment and applying testing have shown that the numerical control signal source composed of STM32 micro controller and DDS chip can work well.
DDS and wave form programming pattern
The core of DDS system is phase accumulator carriage, and it is composed of a ADR and one
unit phase register. When any clock comes, the phase register increases by with the step length, phase register output and phase control word add together, and then the output is imported to sine inquiry form address.
The sine inquiring form includes the numeral extent information of one-period sine wave, each address corresponds to the phase dot of 0~360 degrees of the sine wave. The mapping digital signal drives DAC and outputs analog value. The output sine circle and frequency are
The phase register will return to the original state when the 2N/M fc clock is finished. Accordingly, the DDS system output a sine wave when the sine inquiring form finish a circle. The output sine circle and frequency is TO --output sine wave circle, unit: s; TC --external referenced clock circle, unit: s; M --accumulated step length of phase register, constant; f out --output sine wave frequency, unit: Hz; f c --external referenced clock frequency, unit: Hz; N --phase accumulator digit, constant.
The relationship among the frequency control word, and the output signal frequency and the referenced clock frequency are:
Frequency control word is directly proportional to the output signal frequency. In order to describe clearly, the sine wave form is as one vector turns around phase circle, the phase circle matches along with a cycle sine wave. Every sampling pots in wave form corresponds to a phase dot of the phase circle.
To synthesis the required frequency signal, it needs to accomplish the following steps 1. Controlling every sampling increment of phase and accumulating them (frequency control word K), output 2 pi cumulated phase (using phase accumulator).
2. Converting 2 pi accumulating phase into the corresponding sine amplitude, use ROM to store the corresponding phase-extent form of sine function in general. 3. Use DAC to change extent code into the signal simulating voltage.
4. The voltage signal that DAC exports is ladder wave form , the required simulation voltage out is achieved after LPF smoothing.
Numerical control DDS signal source system designs analysis
System uses the STM32 as control core and the AD9850 as generator. STM32 is 32-bit ARM-based micro controller with 128 K byte flash memory.
The two signal output of STM32 can be achieved by controlling AD9850 and DAC0832 output simultaneously. The system designs block diagram is shown in Fig. 1. One signal output can generate 30 MHz sine wave and rectangular wave by controlling AD9850, the other output generate any wave form with its frequency less than 10 KHz by numerical frequency mixture of DAC0832.
Fig. 1 System designs block diagram
Hardware design AD9850 module
AD9850 contains the DDS system and high-speed comparator. The AD9850 can realize the entire numerical frequency combining. The core of the programmable DDS is the phase accumulator, it is composed of a ADR and a N bit phase register, N is for 24 ~ 32.
After connecting to the accurate clock source and writing the frequency phase control word, AD9850 can generate the frequency-programmable and phase-programmable output of analog sine wave, which can be used as the direct frequency signal source or be transferred into rectangular wave through high-speed comparator.
With the 125 MHz clock, 32-bit frequency control word can carry out the output frequency resolution ratio of AD9850 with 0.0291 Hz[4]. DAC0832 module
The circuit exports the phase data sheet to DAC0832 from STM32 and gets corresponding wave form by DA converting. The step-by-step adjusting phase amounts can create arbitrary frequency, the PWM signal from the STM32 transfers into the corresponding voltage by low-pass filter, therefore, the referenced voltage of DAC0832 is controlled, furthermore, the output wave form extent is tuned appropriately. The digital to analog conversion circuit is shown in Fig.2
Fig. 2 DAC circuit
PWM converting DA circuit
The low-pass filtered PWM signal from STM32 is then stable using the voltage follower, which will yield a stable output voltage; the voltage can be adjusted by tuning the PWM dutyfactor. The system output three PWM signals, which controls AD9850 output extent, dutyfactor and the output extent of DAC0832, respectively,. Fig. 3 shows the PWM controls DA transferring circuit.
Fig. 3 PWM controlled DA converting circuit
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