Frequency Division Multiplexing (FDM) with Arduino

Frequency Division Multiplexing (FDM) is a key technique in signal processing and telecommunications, allowing multiple signals to share the same medium without interference by assigning each signal a unique frequency band. While FDM is often associated with complex systems, you can create a basic FDM demonstration with an Arduino, making it a great project for learning the fundamentals of multiplexing. In this post, we’ll walk through the steps to set up an FDM demonstration using an Arduino and some basic electronic components.

What is Frequency Division Multiplexing (FDM)?

Frequency Division Multiplexing is a technique in which multiple signals are transmitted simultaneously over a single communication channel by assigning each signal a distinct frequency range. This way, several “channels” can coexist on the same transmission line, with minimal cross-interference, as long as each signal stays within its designated frequency band. Commonly used in radio broadcasting and telecommunications, FDM enables efficient use of bandwidth. See how to design AM receiver using Arduino.

Why Use Arduino for an FDM Demo?

While an Arduino is limited in terms of processing speed and output range, it’s still capable of generating simple signals at different frequencies, making it a great low-cost platform to demonstrate the basic concepts of FDM. In this project, we’ll use the Arduino to create two frequency signals, combine them, and then filter them out to simulate FDM.

Materials Needed

Arduino (e.g., Arduino Uno or similar)
Breadboard
Resistors and capacitors for creating bandpass filters
Connecting wires
Optional: Oscilloscope or frequency analyzer (for visualizing signals)

Step 1: Set Up Frequency Generators

The Arduino will generate two distinct frequency signals, each representing a separate data channel in this FDM demo. Here’s a basic example of how to use two digital pins to output different frequencies.

const int freq1Pin = 3;  // Channel 1 - 1 kHz
const int freq2Pin = 5;  // Channel 2 - 5 kHz

void setup() {
  pinMode(freq1Pin, OUTPUT);
  pinMode(freq2Pin, OUTPUT);
}

void loop() {
  // Generate 1 kHz signal on freq1Pin
  digitalWrite(freq1Pin, HIGH);
  delayMicroseconds(500); // 500 microseconds high for 1 kHz
  digitalWrite(freq1Pin, LOW);
  delayMicroseconds(500); // 500 microseconds low for 1 kHz
  
  // Generate 5 kHz signal on freq2Pin
  digitalWrite(freq2Pin, HIGH);
  delayMicroseconds(100); // 100 microseconds high for 5 kHz
  digitalWrite(freq2Pin, LOW);
  delayMicroseconds(100); // 100 microseconds low for 5 kHz
}

In this code, the digitalWrite with suitable delay is used to generates square waves at 1 kHz and 5 kHz on separate pins, simulating two different channels.

Step 2: Combine the Signals

To simulate combining signals for FDM, connect the two frequency signals to a summing point. You can do this with a basic circuit on a breadboard by connecting each signal through a resistor to a single output. This summed signal represents an FDM “transmission.”

If you want to see the combined signal, connect it to an oscilloscope. You should see a complex waveform representing both frequencies mixed together.

Step 3: Filter Each Frequency (Simulating Channel Separation)

The next step is to separate the signals on the receiving end. We’ll use bandpass filters to isolate each frequency, simulating how FDM allows receivers to tune into specific channels without interference.

Create two bandpass filters on your breadboard, one tuned to 1 kHz and the other to 2 kHz. This can be done with resistor-capacitor (RC) filter circuits. Here’s a simple configuration:

Filter for 1 kHz Channel: Use an RC circuit tuned to 1 kHz
Filter for 2 kHz Channel: Use an RC circuit tuned to 2 kHz

Each filter should pass its designated frequency while attenuating others.

Step 4: Display the Results

To verify that each filter isolates its target frequency:

Connect each filter output to a separate Arduino analog pin.
Write code to measure and display the frequencies using the Serial Monitor.
Optionally, connect LEDs to indicate when each frequency is detected, as a visual confirmation.

#include <LiquidCrystal.h>

// Initialize the LCD library with the numbers of the interface pins
LiquidCrystal lcd(7, 6, 5, 4, 3, 2);

const int freq1FilterPin = A0;  // BPF for 1 kHz
const int freq2FilterPin = A1;  // BPF for 5 kHz

void setup() {
  Serial.begin(9600);         // For debugging only
  lcd.begin(16, 2);

  // Display scrolling message
  scrollText("Frequency Division Multiplexing Demo", 1); // Adjust delay for scroll speed
  delay(100);
}

void loop() {
  int freq1 = measureFrequency(freq1FilterPin);  // Measure frequency on BPF centered at 1 kHz
  int freq2 = measureFrequency(freq2FilterPin);  // Measure frequency on BPF centered at 5 kHz

  // Print values to serial monitor
  Serial.print("CH1 Freq: ");
  Serial.print(freq1);
  Serial.println("");
  Serial.print("CH2 Freq: ");
  Serial.print(freq2);
  Serial.println("");

  // Print values to LCD
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("CH1 Freq: "); 
  lcd.print(freq1);
  lcd.print(" Hz");
  lcd.setCursor(0, 1);
  lcd.print("CH2 Freq: "); 
  lcd.print(freq2);
  lcd.print(" Hz");
  
  delay(500);
}

// Function to measure frequency on a given pin
int measureFrequency(int pin) {
  // Measure the time for one period of the waveform
  unsigned long highTime = pulseIn(pin, HIGH); // Time signal is HIGH
  unsigned long lowTime = pulseIn(pin, LOW);   // Time signal is LOW
  
  // Calculate the total period
  unsigned long period = highTime + lowTime;

  // Avoid division by zero and return frequency
 if (period > 0) {
    return 1000.0 / (period / 1000.0);  // Frequency in kHz
  } else {
    return 0; // If no signal is detected
  }
}

// Function to scroll text across the LCD
void scrollText(String message, int delayTime) {
  message = "   " + message + "   "; // Add padding spaces for smoother scrolling
  int len = message.length();
  for (int pos = 0; pos < len - 15; pos++) {
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print(message.substring(pos, pos + 16)); // Display 16 characters at a time
    delay(delayTime);
  }
}

Circuit Diagram

The following is the circuit diagram:

Conclusion

Congratulations! You've set up a simple FDM demonstration with Arduino. This project showcases how multiple signals can share the same medium without interfering, a principle behind many communication systems. While Arduino's capabilities are limited, this setup provides a basic understanding of FDM in a hands-on, accessible way. For a more advanced setup, consider using higher-frequency generators and more precise filters.

By experimenting with FDM using an Arduino, you gain insight into a fundamental concept in telecommunications that enables modern communication networks to transmit vast amounts of data efficiently. Happy experimenting!