Functional DDS generator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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This DDS function generator (version 2.0) of signals is assembled on the AVR microcontroller, has good functionality, has amplitude control, and is also assembled on a single-sided printed circuit board.
This generator is based on the algorithm Jesper DDS generator, the program has been upgraded to AVR-GCC C with assembly code inserts. The generator has two output signals: the first is DDS signals, the second is a high-speed (1..8 MHz) "rectangular" output, which can be used to revive an MK with incorrect fuses and for other purposes.
The high-speed signal HS (High Speed) is taken directly from the Atmega16 OC1A (PD5) microcontroller.
DDS signals are generated from other outputs of the MK through a resistive R2R matrix and through the LM358N chip, which allows you to adjust the amplitude (Amplitude) of the signal and the offset (Offset). Offset and amplitude are adjustable with two potentiometers. The offset can be adjusted in the range +5V..-5V, and the amplitude 0...10V. The frequency of DDS signals can be adjusted from 0 to 65534 Hz, which is more than enough for testing audio circuits and other amateur radio tasks.
Key Features of DDS Generator V2.0
- a simple circuit with common and inexpensive radio elements;
- one-sided printed circuit board;
- built-in power supply;
- separate high-speed output (HS) up to 8 MHz;
- DDS signals with variable amplitude and offset;
- DDS signals: sine, rectangle, saw and reverse saw, triangle, ECG signal and noise signal;
- 2×16 LCD screen;
- intuitive 5-button keyboard;
- steps for frequency adjustment: 1, 10, 100, 1000, 10000 Hz;
- remembering the last state after power on.
The block diagram below shows the logical structure of the function generator:
As you can see, the device requires several supply voltages: +5V, -12V, +12V. The +12V and -12V voltages are used to control the signal amplitude and offset. The power supply is designed using a transformer and several voltage regulator chips:
(click to enlarge)
The power supply is assembled on a separate board:
If you don’t want to assemble the power supply yourself, then you can use a regular ATX power supply from a computer, where all the necessary voltages are already present. ATX connector pinout.
LCD screen
All actions are displayed through the LCD screen. The generator is controlled by five keys
The up/down keys are used to move through the menu, the left/right keys are used to change the frequency value. When the central key is pressed, the generation of the selected signal starts. Pressing the key again stops the generator.
Generator menu system:
A separate value is provided for setting the frequency change step. This is convenient if you need to change the frequency over a wide range.
The noise generator does not have any settings. For it, the usual rand() function is used, continuously fed to the output of the DDS generator.
The HS high speed output has 4 frequency modes: 1, 2, 4 and 8 MHz.
Schematic diagram
The function generator circuit is simple and contains easily accessible elements:
- microcontroller AVR Atmega16, with external quartz at 16 MHz;
- standard HD44780-type LCD screen 2×16;
- R2R-matrix DAC from conventional resistors;
- operational amplifier LM358N (domestic analogue KR1040UD1);
- two potentiometers;
- five keys;
- several connectors.
(click to enlarge)
Plateau
The function generator is assembled in a plastic box
Test run
Software
As I said above, at the heart of my program I used the algorithm Jesper DDS generator. I added a few lines of assembler code to implement stop generation. The algorithm now contains 10 CPU cycles instead of 9.
void static inline Signal_OUT(const uint8_t *signal, uint8_t ad2, uint8_t ad1, uint8_t ad0){
asm volatile( "eor r18, r18 ;r18<-0″ "\n\t"
"eor r19, r19 ;r19<-0" "\n\t"
"1:" "\n\t"
"add r18, %0 ;1 cycle" "\n\t"
"adc r19, %1 ;1 cycle" "\n\t"
"adc %A3, %2 ;1cycle" "\n\t"
"lpm ;3 cycles" "\n\t"
"out %4, __tmp_reg__ ;1 cycle" "\n\t"
"sbis %5, 2 ;1 cycle if no skip" "\n\t"
"rjmp 1b ;2 cycles. Total 10 cycles" "\n\t"
:
:"r" (ad0),"r" (ad1),"r" (ad2),"e" (signal),"I" (_SFR_IO_ADDR(PORTA)), "I" (_SFR_IO_ADDR(SPCR))
:"r18", "r19"
);}
The table of DDS-signal forms is located in the flash memory of the MK, the address of which starts with 0xXX00. These sections are defined in the makefile, in their respective locations in memory:
#Define sections where to store signal tables
LDFLAGS += -Wl,-section-start=.MySection1=0x3A00
LDFLAGS += -Wl,-section-start=.MySection2=0x3B00
LDFLAGS += -Wl,-section-start=.MySection3=0x3C00
LDFLAGS += -Wl,-section-start=.MySection4=0x3D00
LDFLAGS += -Wl,-section-start=.MySection5=0x3E00
LDFLAGS += -Wl,-section-start=.MySection6=0x3F00
The LCD library can be found at winavr.scienceprog.com.
I do not want to go into a detailed description of the program code. The source code is well commented (albeit in English) and if you have any questions about it, you can always use our MK forum, forum.cxem.net.
The test is
I tested the generator with an oscilloscope and a frequency counter. All signals are well generated in the entire frequency range (1...65535 Hz). Amplitude and offset control works fine.
(click to enlarge)
In the next version of the generator, I think to implement a rising sinusoid signal.
The latest software version (WinAVR20071221), source code, EagleCAD and Proteus files can be скачать здесь.
Author: Koltykov A.V.; Publication: cxem.net
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