Digital and Microwave Communication Engineering-1.5

 Frequency Division Multiplexing with practical examples, phase-locked loop.

Frequency Division Multiplexing (FDM): Concept, Examples & Applications: -

In today’s interconnected world, communication systems handle a massive amount of information simultaneously—voice, data, video, and signals from various sources. To ensure smooth transmission, engineers use multiplexing techniques. One of the oldest yet widely used methods is Frequency Division Multiplexing (FDM).

FDM works on a simple principle: when multiple signals need to be transmitted over a single communication channel, each signal is assigned a different frequency band. These bands are carefully spaced to prevent overlapping, ensuring that all signals travel together without interfering with each other.

How Frequency Division Multiplexing Works

Imagine a highway with multiple lanes. Each car (signal) travels in its own lane (frequency band) without disturbing the others. Similarly, FDM divides the available bandwidth into smaller frequency ranges, and each user or signal gets its own dedicated range.

At the transmitter end:

• Signals are modulated with different carrier frequencies.
• These modulated signals are combined into a single composite signal.

At the receiver end:

• A demultiplexer separates signals by tuning into their unique frequency bands.

Practical Examples of Frequency Division Multiplexing

1. Radio Broadcasting

When you tune your FM radio, you switch between frequencies like 91.1 MHz, 93.5 MHz, or 98.3 MHz. Each station is allocated a frequency band, and all of them broadcast simultaneously. This is a perfect example of FDM—multiple radio channels sharing the same air medium but using different frequency slots.

2. Cable Television

Cable TV operators use FDM to transmit dozens or even hundreds of channels through a single coaxial cable. Each TV channel is assigned its own frequency band, and your television tuner separates the required one. This allows you to watch Channel 10 while your neighbor watches Channel 20 using the same cable line.

3. Telephone Systems

In the past, traditional telephone systems used FDM to combine multiple voice calls over a single transmission line. For instance, in long-distance trunk lines, each voice call was modulated into a separate frequency band and then combined into one high-capacity channel. Although digital techniques like Time Division Multiplexing (TDM) have replaced this, FDM was the backbone of early telephony.

4. Satellite Communication

Satellites handle a variety of communication signals, including TV broadcasting, data services, and telephony. Since satellite bandwidth is expensive and limited, FDM allows multiple signals to be transmitted over the same transponder by assigning different frequency ranges.

Advantages of Frequency Division Multiplexing

Simple and Effective: Easy to implement with analog signals.

Simultaneous Transmission: Multiple users can transmit data at the same time without waiting for turns.

Continuous Availability: Unlike time-sharing methods, each channel has constant access to its frequency slot.

Limitations of Frequency Division Multiplexing

Bandwidth Requirement: Requires a large bandwidth to accommodate multiple channels.

Guard Bands Needed: To prevent interference, small unused bands (guard bands) are placed between channels, which reduces efficiency.

Noise Sensitivity: Analog systems using FDM are more prone to noise and crosstalk compared to digital methods.

Real-Life Analogy

Think of a shopping mall where each store plays music. If all stores played on the same loudspeaker system, it would be chaotic. Instead, each store uses its own speakers (frequency band). Customers can enjoy music inside without disturbing others—just like FDM ensures signals remain separated.

Conclusion

Frequency Division Multiplexing is a powerful and time-tested method that has shaped modern communication. From tuning into your favorite FM station to receiving hundreds of TV channels through one cable, FDM is working silently in the background. Although digital multiplexing techniques are now dominant, FDM continues to be a practical solution in broadcasting, satellite systems, and certain data communication networks.

By understanding FDM, we appreciate how engineers creatively use the frequency spectrum to allow millions of people to share the same medium without stepping on each other’s signals.

 

 

Phase Locked Loop (PLL): -

In today’s digital world, where precision and synchronization are critical, the Phase Locked Loop (PLL) plays an essential role. Whether it’s in your smartphone, television, radio, or computer, a PLL silently ensures that signals remain stable, synchronized, and noise-free.

What is a phase-locked loop

A phase-locked loop is an electronic control system that generates an output signal whose phase is locked to the phase of an input reference signal. Simply put, it continuously adjusts its own oscillator to “track” or “lock” onto another signal.

A PLL consists of three main blocks:

Phase Detector (PD): Compares the input signal with the output signal.

Low Pass Filter (LPF): Removes high-frequency noise from the detector output.

Voltage Controlled Oscillator (VCO): Adjusts the frequency based on the control voltage.

Some designs also include a frequency divider to achieve frequency synthesis.

How Does a PLL Work

Imagine tuning a radio station manually. You keep adjusting the knob until the sound is clear and in sync. A PLL does this automatically and far more precisely. When the input and output phases differ, the phase detector creates an error signal. This error is filtered and used to control the VCO. Over time, the output locks with the input in both phase and frequency.

Applications of PLL

PLLs are used almost everywhere in modern electronics. Some common applications include:

Communication Systems: Frequency modulation (FM) demodulation, carrier recovery, and synchronization in wireless systems.

Clock Generation: Used in microprocessors to multiply base clock frequencies for high-speed operation.

Data Transmission: Ensures accurate timing recovery in digital data streams.

Instrumentation: Frequency synthesizers in test equipment.

PLL chip used in microcontrollers or RF circuits

Conclusion

The phase-locked loop is a perfect blend of analog and digital electronics, providing stability, synchronization, and accuracy. From radio receivers to high-speed processors, PLLs are indispensable in bridging the gap between noisy real-world signals and the precise timing needs of digital systems.

As technology evolves, the importance of PLLs will only grow, powering the next generation of high-speed, low-noise communication and computing devices.

--------------------------------Merits and demerits of TDM and FDM Next Page --------------------------------