FDM

Combine multiple voice calls into one coaxial cable. Analog telephone networks heavily rely on this technique to transmit many signals simultaneously.

Core Concept

Frequency division multiplexing separates distinct communication channels in the frequency domain. It allows multiple users to share a single physical medium without overlapping. Engineers use this concept heavily in FM radio broadcasting.

The system shifts each baseband message to a unique frequency band using a subcarrier. We use local oscillators and mixers like the NE602 to perform this upconversion. The multiplexer then sums all the shifted signals together.

Guard bands sit between the active channels to prevent interference. These empty spaces ensure that simple analog filters can separate the channels at the receiver. The receiver filters out the desired band and demodulates it back to audio.

Key Equations

The total bandwidth required depends on the number of channels and guard bands. Btotal = N * Bsignal + (N-1) * Bguard calculates this requirement.

Here N is the number of channels. Bsignal is the bandwidth of one modulated channel. Bguard is the empty space placed between adjacent channels.

Given:\nNumber of channels (N) = 12\nSignal bandwidth (Bsignal) = 4 kHz\nGuard band (Bguard) = 1 kHz\n\nWhy this formula:\nTotal bandwidth is the sum of all signal bandwidths plus the guard bands between them.\n\nFormula:\nBtotal = N * Bsignal + (N-1) * Bguard\n\nSubstitution:\nBtotal = 12 * 4 + (12-1) * 1\n\nCalculation:\nBtotal = 48 + 11 * 1\nBtotal = 48 + 11\nBtotal = 59\n\nFinal Answer:\n59 kHz

Exam Tip: Guard bands waste valuable spectrum but strictly prevent adjacent channel interference. Questions often test your ability to subtract one from the total number of channels when counting guard bands.

Key Properties

• The CCITT hierarchy groups 12 voice channels into a standard 48 kHz basic group.
• FDM uses Single Sideband (SSB) modulation to conserve bandwidth.
• Pilot carriers are transmitted along with the signal to help receivers maintain synchronization.
• Crosstalk increases if filters have poor selectivity or if amplifiers exhibit non-linear behavior.
• The system requires complex analog filters at both the transmitter and receiver.

Quick Revision

• Channels are separated purely by frequency spacing.
• All users transmit continuously at the exact same time.
• SSB modulation is the standard choice for voice networks.
• Non-linear amplifiers create severe intermodulation distortion between channels.
• Guard bands act as safety buffers for filter roll-off.
• Exam trap: Forgetting to multiply the baseband signal by two if standard AM is used instead of SSB.