
Research Article


Direct Digital Frequency Synthesizer Simulation and Design by means of QuartusModelSim


A.A. Alsharef,
M.A. Mohd. Ali
and
H. Sanusi


ABSTRACT

A new full simulation, design and verification of a Direct
Digital Frequency Synthesizer (DDFS), utilizing only one quarter of a given
sinusoidal wave, are presented in this study. A reduction in the size of the
LUT is accomplished as the new design requires storing only a quarter of the
sine wave. The Register Transfer Level (RTL) and the Gate level is implemented
by the Quartus II. The Quartus II will then invoke the ModelSim Altera software
to simulate the output. The DDFS consists of three major models, mainly a Phase
Accumulator (PA), a Phase Register and a Look Up Table (LUT). All of the mentioned
models are realized by a Verilog code. The spurious free dynamic range is achieved
with a value of 73 dB using a 16 bit phase accumulator. The proposed design
is verified through the application of different input frequencies and obtained
results showed that output frequency is directly proportional to the tuning
input frequency. 




Received:
July 03, 2012; Accepted: September 29, 2012;
Published: November 01, 2012 

INTRODUCTION
Direct Digital Frequency Synthesis (DDFS) is a multi step process of generating
sinusoidal analogue waveforms. DDFS has a wide application in the modern communication
era such as radio receivers, mobile telephones, radiotelephones, walkietalkies,
CB radios, satellite receivers and none the less GPS systems (Grayver
and Daneshrad, 1998). The focus of this paper is on the design, analysis
and simulation of DDFS using QuartusModelSim. Traditional designs found in
literature of high bandwidth frequency synthesizers make use of a Phase LockedLoop
(PLL) approach. The PLL offers very good wide tuning bandwidth due to the use
of a programmable divider as compared to DDFS approach (Sunderland
et al., 1984). On the other hand, DDFS provides many significant
advantages such as fast settling time, subHertz frequency resolution, continuousphase
switching response and low phase noise (Hegazi et al.,
1998; Mortezapour and Lee, 1999). One key design
parameter of the DDFS is a Look Up Table (LUT). The response time, the power
consumption and the size of the DDFS are factors that depend on the size of
the LUT. In addition to that, the resolution and the size of DDFS are also dependable
on the size of the phase accumulator (Bramble, 1981).
In this method, the phase/amplitude of an analogue sine wave cycle is sampled
with an equal phase intervals to obtain a discrete sequence of a single sine
wave period thus quantifying its analog amplitude. Consequently, a single period
of a sine wave is converted into a binary sequence. This binary sequence represents
the quantized amplitude of the sine wave to be stored in a ROM. Therefore, the
content of the ROM matches the phase sampling for a single full cycle. The main
purpose of this study is to propose a new technique through the implementation
of a quarter of the input analogue waveform that results in the size reduction
of the LUT unit. The design and the implementation of DDFS systems supported
by VERILOG is simple and less demanding thus progressively became the trend
towards DDFS design (YiYuan and XueJun, 2011). Altera’s
Quartus II, an appropriate platform for VERILOG compilation and synthesis, offers
a strong simulation tool therefore, can extremely simplify the overall design
procedure.
This study begins by introducing the basic architecture of DDFS with a review of the available design methods. Followed by that, an extensive developmental procedure of the proposed design using the Quartus II environment is presented. BASIC ARCHITECTURE OF DIRECT DIGITAL SYNTHESIZERS
The general structure of any given Direct Digital Synthesizer (DDS) may seem
complex. DDS major components include the phase accumulator, the lookup table
and the phase resistor. In this brief description the last component, the phase
resistor, is the easiest part to start with as to simplify the flow of the proposed
scheme. The original task of DDS’s is to obtain an output signal in the
form of a sinusoidal wave given a specified reference frequency. Since the output
of the DDS is in a form of digital signals, a Digital to Analogue Converter
(DAC) is needed. This means that the structure of any DDS should contain a DAC.
The DAC output should pass through a Low Pass Filter (LPF), an antialiasing
filter, to suppress any reproduced images of the output spectrum. To obtain
an output sinusoidal signal, a sequence of input sampling sinusoidal signals
is applied to the DAC. Obtaining a sinusoidal signal through a digital system
is not direct and may need different orientation for implementation such as
a lookup table method. A Look up Table (coding table) is most often placed in
a ROM. A code which is fed to the address inputs of the ROM, is the argument
x of the sinusoidal function sin(x) while the obtained output code from the
ROM represents the calculated value of sin(x). Forming a linear timevarying
sequence of codes is simple and can be implemented via a binary counter. Therefore,
the simplest type of DDS is a binary counter that generates the address of the
ROM. The ROM would then comprise a table of one period of the sinusoidal function
in a form of digital codes. These codes are then sent to the DAC that regenerates
the analogue sinusoidal output signal then filtered by LPF to generate the main
output of the DDS as shown in Fig. 1.
To adjust the frequency of the DDS output there is a need for a frequency divider
with variable division factor that receives a clock signal from a reference
generator. This structure of DDS has some drawbacks. The main drawback is the
poor regenerative frequency process. Indeed, since the clock frequency undergoes
a division by an integer, steps of adjustments became variable, therefore, the
smaller the division ratio the greater the value of the step adjustment. This
step becomes unsatisfied at a small division factor. In addition to that, when
adjusting the output frequency, the sampling frequency will also change. This
complicates the filtration process of the output and leads to a suboptimal use
of highspeed DACs. Regardless of the weaknesses described above, the structure
can be developed by replacing the counter of the address memory with other digital
devices, called accumulators (Vankka and Halonen, 2001).
For each cycle reboot, the accumulator adds a specific value, called a tuning
word, with its feedback output as shows in Fig. 2. The content
of the registers increases linearly with time with a constant step size. If
the accumulator is used to generate the phase code it is called the phase accumulator.
The output code of the phase accumulator is the code of the instantaneous phase
of the output sinusoidal signal.
The tuning word represents a constant increment per cycle value added continuously
to the phase accumulator. The faster the phase varies in time, the greater is
the frequency of the generated signal.

Fig. 1: 
Simplest direct digital synthesizer, ROM: Read only memory,
DAC: Digital to analogue converter 

Fig. 2: 
Direct digital synthesizer based on the accumulator, ROM:
Read only memory, DAC: Digital to analog converter 
Therefore, the value of the phase increment is actually a frequency tuning
word. Indeed, if the phase increment is equal to unity, the accumulator is no
different from the a regular binary counter. But if the phase increment is equal
to, for example, two, then the code phase will change twice as fast. In this
case, the DAC code will come up with the same frequency but will not be a neighbor.
If the frequency of a generated signal is twice as large, the sampling rate
will stay unchanged. The phase accumulator is operated during a single stage
of overflow, within arithmetic modulo 2^{n}. This stage of overflow
corresponds to a sinusoidal wave with a period of 2π. In other words; the
frequency of the overflow of the phase accumulator is the frequency of the output
signal. This frequency is determined by the formula (Sharma
and Upadhyaya, 2010):
where, F_{out} is the output frequency, F_{clk} is the clock frequency, M is the code rate; and n is the bit of phase accumulator. In essence, the clock frequency is divided by a number which is determined by the code frequency and the phase accumulator word length. At the same time the frequency step is equal to:
From this formula, it may be concluded that if the word length n is increased,
the frequency step size will reduce with no special restrictions. For example,
if the bit accumulator is 32 bits and the clock frequency is 50 MHz, the frequency
resolution is in the order of 0.01 Hz. This means increasing the bit size (number
of bits) of the phase accumulator does not require increasing the bit address
of the ROM (Nicholas et al., 1988). The address
may use only the necessary number of significant bits of the phase code. To
reduce the size of the ROM, the sinusoidal symmetrical properties of the function
may be used (Bellaouar et al., 2000). Therefore,
only a quarter of the sinusoidal wave is needed to be stored in the ROM, As
a result, a more complicated logic of forming the address (address forming)
is required. Thus, in DDS the phase accumulator generates a sequence of codes
of the instantaneous phase signal that varies linearly. The change in the rate
of phase gives the frequency code. Further, by using a ROM, the linear change
of the phase is converted to an output varying sinusoidal signal. Samples are
then fed to the Digital to Analogue Converter (DAC), therefore the output of
the DAC is a step like sinusoidal signal. Afterwards the step like wave is filtered
by a LPF and its output represents the desired sinusoidal wave.
SYSTEM LEVEL SIMULATION
In order to carry out the system level modelling and simulation of . a the
DDFS a QuartsModelSim is utilized. This would help in visualizing the functionality
and the flow sequence from the input part to the output part. The model of DDS
consists of a phase register, a Phase Accumulator (PA) and a Look Up Table (LUT).
Figure 3 shows the RTL level schematic of the DDS.

Fig. 3: 
QuartsModelSim model of direct digital synthesizer 
Each model of them is built up using a Verilog then are assembled in the top
level model. Thus, it is possible to modify each model according to the predetermined
design requirements. The phase register is implemented using a 16 bit register.
The phase accumulator is implemented via a register along with an adder and
a feedback loop. The LUT is implemented using a ROM. The size of the memory
will decrease as only one quarter of the sinusoidal wave is stored as given
in Fig. 3. The model flow would be as follows. The content
of the phase register is added to the phase accumulator at each clock pulse.
The phase accumulator produces the phase value of the output sinusoidal wave.
The phase value of the PA provides the address for the LUT. The LUT contains
sample values of the quarter of the sinusoidal wave during one cycle overflow
of the phase accumulator. The data is generated by the MATLAB. The calculation
of a single full period sine wave would be achieved as follows. The full period
of the sine wave is divided into quarters with an index number given as nxσ/2,
where n = 14. The description would be given for the first . half halve of
the full period mainly for n = 1 and 2. For n = 1, the program will read the
contents of the ROM normally during the first quarter until the phase argument
is σ/2. The address bus is normally incremented during this part from 0
to the maximum value. During the second quarter where n = 2, the address of
the ROM is decremented down to the start point. Data are read from the ROM opposite
to the first stage. For the next half period, where n = 3 and 4, the same procedure
would be followed given that the contents of the ROM would be first multiplied
by 1. As a result, a full sine wave period is generated using the data of only
a quarter of the wave and utilizing a special address formatting procedure.
Such procedure has the advantage of saving the total ROM size down to 25% of
the actual needed size. The overflow rate of the phase accumulator depends on
the bit size of the phase accumulator and the frequency tuning word. The larger
the size of the frequency tuning word, the faster the PA overflows. The output
frequency of the direct digital synthesizer is directly proportional to the
frequency tuning word. Therefore, the larger the frequency tuning word, the
faster the PA overflows and the higher the output frequency.
RESULTS AND DISCUSSION Examples are given to clarify the above suggested method of the new designed DDS. Assume for example a reference frequency (clock) of 16.6 MHz with a tuning word of 10. The time reading of an oscilloscope output would read a full period of 0.863864x10^{5} psec (picosecond) which represents a frequency of 115758 Hz as shown in Fig. 4. By changing the tuning word to be 20 and keeping the same reference frequency will result in a time reading of the oscilloscope of 0.456932x10^{5} psec. This represents a frequency of 218850.94 Hz as shown in Fig. 5. Examples are also given to clarify the gate level simulation of the new designed DDS. Assume for example a reference frequency (clock) of 16.6 MHz with a tuning word of 10. The oscilloscope time reading of one full period output would be 0.863864x10^{5} psec which represents a frequency of 115758 Hz as shown in Fig. 6. By altering the tuning word to be 20 and keeping the same reference frequency, the time reading of the oscilloscope would be 0.456932x10^{5} psec. This represents a frequency of 218850.94 Hz as shown in Fig. 7. As a conclusion the output frequency is directly proportional to the input frequency tuning word.
Measurement results: A real time result is carried out by a DE2 board
Altera Cyclone^{®}II 2C35 FPGA. A summary of the devices usage
is given in Table 1. The result is measured using a logic
analyzer.

Fig. 4: 
RTL Output for tuning word is equal to 10 

Fig. 5: 
RTL Output for tuning word is equal to 20 

Fig. 6: 
GLS Output for tuning word is equal to 10 

Fig. 7: 
GLS Output for tuning word is equal to 20 
Table 1: 
Device utilization summary 


Fig. 8: 
Measuring power spectrum 
The clock is taken from the pattern generator of the logic analyzer. The data
are saved in a form of a text file in the logic analyzer. Using a MATLAB routine,
the power spectrum is plotted as shown in Fig. 8. The calculated
spurious free dynamic range is about 73 dB using a 16 bit phase accumulator
which is Adequate compared with other studies using a 32 bit phase accumulator
(Vankka et al., 1998; Wang
et al., 2010).
CONCLUSION
This study demonstrates a new design and the simulation of a direct digital
frequency synthesizer. The DDFS’s digital part includes a phase register,
a phase accumulator (PA) and a ROM. The design is created using Verilog HDL.
The RTL level modelling and simulation of a DDFS is implemented using QuartsModelSim. The phase register is achieved via a register. A phase accumulator is achieved by a register along with an adder and a feedback loop. The LUT is implemented by a Read Only Memory (ROM). In this proposed process, the size of the ROM is reduced. This is done by saving only a quarter of the sinusoidal signal in the LUT. The required address formatting for such design is also fully described. The spurious free dynamic range by the value of 73 dB using a 16 bit phase accumulator is obtained. The result of the simulation shows that the output frequency is directly proportional with the frequency tuning word.

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