Direct digital synthesis based - function generator with digital signal modulations
Zdenek Machacek*, Martin Gabzdyl**, Viktor Michna***
VSB – Technical University of Ostrava,FEECS,
Department of Measurement and Control, Ostrava,Czech Republic
(Tel: 420 597329337; e-mail: *zdenek.machacek@vsb.cz,** martin.gabzdyl@vsb.cz, ***viktor.michna@vsb.cz).
Abstract: This paper focuses on signal generation with digital modulations, especially on the possibility of using the direct digital synthesis method. The specific structure of the function generator is explained. The developed construction is designed as a laboratory device. Realization enables to operate this device in two ways, via the user interface in the PC through the USB bus or by the display and keypad located directly on the product. Furthermore, this paper deals with results of measurement digital modulation generating, signal distortion and the test measurement in frequency and time domain.
Keywords: signal processing, direct digital synthesis, microprocessor, digital modulation, measurement.
1. INTRODUCTION
Direct digital synthesis DDS is a method for signal generation. This method allows the digital controlled signal generation with derived frequency from the single frequency standard. Although the principle of Direct digital synthesis was known for many years, until recently, these districts didnrsquo;t play a dominant role in the signal generation due for limited parameters of digital electronics. Advance in technology of integrated circuits allows effective use of this method in practice.
Realization of specific function generator core is composed of logic circuits, ROM memory and digital to analog converter. The specific solution enables generating the basic signals - sinusoidal, triangle and rectangle signal. Furthermore, there is possibility to generating the chosen digital signal modulations. The paper describes the method DDS, the structure of specific developed generator, digital modulation, user interface, testing and measurement results.
2. PRINCIPLES OF DDS METHOD
The direct digital synthesizer should contain a phase accumulator, a conversion table, a digital to analog converter. The developed solution is described by DDS method structure (Fig. 1). The solution by DDS method solves problems of the almost analog signal generators, such as tuning, resolution, temperature dependence, speed stabilization, user comfort of parameters setting by digital interface. The DDS circuits generate frequencies from mHz to 400 MHz. There is the single frequency standard up to 1 GHz. Resolution is less than one degree of a phase. The only disadvantage of DDS circuits is needs to use an external analog filter to suppress unwanted frequency components of the output of a digital to analog converter.
The phase accumulator output is a linear growing digital word phi;. The growing rate is dependent on the size of the
control word M and a reference frequency fCLK. Every clock step with frequency fCLK, the output digital signal (digital word phi;) from the phase accumulator is summed with control word M in the feedback loop. The linear growing digital word phi; is transformed by the conversion table to the required signal course uOUT(phi;). The result of the signal course uOUT(phi;) is discrete in time and value domain. This discrete signal is converted by a D/A converter to a time-continuous signal uOUT(t). Size of the phase register is defined by n bits. A destination phase can take 2n values. The conversion table with data of one generated signal period is contained in the memory. The number of phase register overflowing per second corresponds to the output analog signal frequency (1). In practice, the phase accumulator output (24-48 bit digital word) is reduced to digital word phi; (13-19 bit digital word) for conversion table input. The reason for signal forming is a memory size reduction, non effect of the frequency resolution, small tolerable phase noise of the output analog signal (Sykora, 2005).
Fig. 1. Solution of DDS method.
The following equation (1) presents computation of a final frequency of generated output signal fOUT [Hz] from setting of these values: frequency of reference signal fCLK [Hz], control word M [-], number bits of the phase register n [-].
(1)
Usually, there is possibility to increment the digital word phi; by an offset number. In this case, the offset number corresponds with phase of the generated output signal. Some of D/A converters implemented in DDS circuits require an additional external reference voltage source. D/A convertor resolution is given by digital word size (within the range from 10 to 14 bits in DDS circuits). The analog output signal is biased by a quantization distortion.
3. DESCRIPTION OF FUNCTION GENERATOR
The described function generator is designed as a cheap compensation of professional devices mainly for education of the DDS method and for generated signal analyses. Designed solution of the function generator is primarily applied for sinus, triangle and rectangle signal generation. The developed equipment enables signal generating with variable frequency, amplitude, DC signal component setting. Furthermore compare to other solutions, there is a possibility to generate a several types of modulations. Implemented types of digital modulations (Machacek, 2009) are:
bull; Amplitude-shift keying ASK – generated by amplitude changing in the D/A converter circuit by the control of reference voltage source.
bull; Phase-shift keying PSK – generated by the offset number addition to the digital word phi;, which corresponds with the phase modulated signal.
bull; Frequency-
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