Direct Digital Synthesis DDS VFO || Different Parts, Functions and Working

A direct digital synthesis VFO or DDS VFO is a type of variable frequency oscillator, which employs digital frequency synthesizer or clock generator integrated circuits to produce sine waves of a specific frequency. It is run by the Arduino Atmega 328 microcontroller and displays the output through a 16 x 2 LCD display or a Nextion screen. Nowadays, it is widely used in digital communication devices such as radio receivers, transmitters, mobile phones, satellite receivers, set-top box, HDTV, radio control toys and games, WiFi routers, communication gadgets etc. It provides much better frequency stability, improved phase noise and output phase control than the analog VFOs. It also provides a good input interface and a pleasing visual display.

DDS VFO Working with 12 Mhz IF Shift

Parts of a DDS VFO
A DDS VFO consists of two main functional segments, A. The hardware part and B. The software part. The software part gives command to the hardware section to produce sine waves of a specific frequency and perform functions according to the input given. The hardware section works in tandem with the software part to accept the input, generate sine waves and display the readout onto the screen.

A. DDS VFO Hardware:

The hardware part of a DDS VFO consists of various devices and modules that carry out specific functions. Those devices and modules used in the construction of a DDS VFO are as follows:

1. DDS Chip:

The frequency generation of this VFO is done by the direct digital synthesizer chips available as the si5351, si570, AD9850, AD9851 etc. The chip used here is the si5351, an I2C configurable clock generator. It has three different clock outputs, clock 0, clock 1 and clock 2. This chip can generate frequencies from 8 khz to 160 MHz. It needs a supply voltage of 3.3 volts.

 

2. Microcontroller:

Arduino nano: It is a small and complete board based on the ATmega328 single chip microcontroller with a clock speed of 16 MHz. It has an operating voltage of 5 volts. The CPU is composed of the Microchip AVR(8bit). Arduino nano has 30 male input/output headers in a DIP-30 configuration. There are 14 digital I/O pins of which 6 pins provide PWM output and 8 analog input pins. It has 32 KB of flash memory and can be programmed using the Arduino software IDE. The board can be powered using a type-B mini-USB cable or a 9 volt battery. It provides serial data communication, available through the digital pins 0 (RX) and 1 (TX). The onboard LED flashes when the data is being transmitted.

3. Input Devices:

These consist of the rotary encoders, keypads, touchscreens etc. A rotary encoder is an electro-mechanical device, which converts the angular position or motion of a shaft or axle into digital output signals. It uses mechanical, optical or magnetic technology to encode the information. There are two pins on one side, which is used for switching and three pins on the other side, which is for varying the frequency.

 

 

4. Display Screen:

Screens such as the 16 x 2 LCD display or Nextion screen are used for display. The 16 x 2 LCD display has a total of 16 pins. The first pin is connected to the ground, the second pin is given +5 volts, and the third pin is contrast control, through which positive voltage is fed through a 10k preset. Pins 4, 6, 11, 12, 13 and 14 are connected to the Arduino. The 15th pin is given +5 volts and the 16th pin is connected to the ground, these pins are used for LCD backlight.

5. Power Supply:

The voltage for the Arduino nano is 5 volts and it is supplied by the 7805 IC voltage regulator. The direct digital synthesizer chip needs about 3.3 volts to operate, which is obtained from the 17th pin of the Arduino nano board.

B. DDS Software:

The software part of the DDS VFO consists of the Arduino IDE and the DDS VFO software. The latest version of Arduino IDE is installed on the computer. The Arduino Nano is connected to the USB port of the computer. All library files and driver files of the connected hardware are installed. Download a suitable DDS VFO software from the GitHub website. Open Arduino IDE, insert the downloaded DDS VFO program into the IDE. Do the compilation and make sure there are no errors present. Add the required library files if errors are still present on compilation. Edit the lines of code for changing or tweaking up the functions and parameters of the VFO. Upload the program into the Arduino nano. Now the operating frequency and all other parameters become displayed onto the LCD screen.

Working of a DDS VFO
The operation of a DDS VFO is controlled by the Arduino Atmega 328 microcontroller. Various programs and codes for running the VFO are written in the Arduino nano. Necessary commands, reference waveforms and various functions are also stored into the memory. The input devices such as the rotary encoder or keypad are used to feed data such as the VFO frequency, BFO frequency, step size and other functions to the microcontroller.

A crystal oscillator, frequency reference clock is used to create identical waveforms. It operates on fixed crystal frequencies of 25 or 27 MHz. A phase-locked loop section helps to stabilize the output frequency to the generated sine wave. It is multiplied and then divided by a specific number to get the required output frequency. There are three clock outputs available on the DDS chip, one is used for the VFO and another for the BFO.

 

 

The program executes the code and the frequency synthesizer chip generates the waveform according to the input given. The generated frequency waveform is obtained from the clock output. The frequency of operation, BFO frequency, intermediate frequency, step size, memory channels and other functional parameters are displayed onto a 16 x 2 LCD display or a 2.8 inch Nextion screen.

Advantages of the DDS VFO

1. A DDS VFO has many advantages over the analog VFO. These advantages are,
2. Frequency stability. It operates by phase-locked loop method and therefore the output has high frequency stability.
3. Frequency agility. The DDS output frequency is determined by the value stored in the frequency control register, which controls the phase accumulator step size, thereby increasing the frequency agility.
4. Reduced phase-noise and jitter. The superior phase-noise performance is due to the feed-forward system of the DDS.

Applications of DDS VFO

1. DDS VFOs are used in modern radio receivers and transmitters.
2. It is used in household devices and gadgets such as mobile phones, satellite receivers, set-top box, HDTV, radio control toys and games, WiFi routers, communication gadgets etc.
3. It is used for instrumentation purposes such as signal generators,  oscilloscopes, reflectometers, radars etc.