Communication Systems, Modulation and the Superheterodyne Receiver

By the end you should be able to: Draw the general block diagram of a communication system and of a fibre optic link, explain amplitude and frequency modulation qualitatively, and describe the AM and FM superheterodyne receivers.

The syllabus states no derivation required for AM and FM, and asks for block diagrams. This topic is accordingly qualitative — but the reasoning behind each block is what earns marks, not the boxes.

The general communication system

Information source → Transmitter → Channel → Receiver → Destination, with noise entering at the channel.

The transmitter contains a transducer (converting sound, images or data to an electrical signal), a modulator, and an amplifier. The receiver reverses this with an amplifier, a demodulator, and an output transducer.

Noise is drawn entering at the channel because that is where it is picked up and where it cannot be removed — everything downstream can only fail to make it worse.

Why modulate at all

Three independent reasons, and an exam answer should give all three:

Antenna size. An efficient antenna must be comparable to a quarter of a wavelength. A 1 kHz audio signal has a wavelength of 300 km, needing a 75 km antenna. Shift it onto a 1 MHz carrier and the wavelength is 300 m, so 75 m suffices.
Multiplexing. Every station transmitting its baseband audio directly would occupy the same band and interfere hopelessly. Placing each on a different carrier lets a receiver select one.
Range and penetration. Higher frequencies propagate and radiate far more effectively than audio frequencies.

Amplitude modulation

The amplitude of the carrier is varied in proportion to the instantaneous message, while frequency stays fixed.

m = \frac{V_{max} - V_{min}}{V_{max} + V_{min}}

is the modulation index. Exceeding $m = 1$ is overmodulation, which distorts the envelope and cannot be recovered.

AM produces a carrier plus two sidebands, occupying twice the message bandwidth. It is simple and cheap to demodulate — an envelope detector is a diode, a capacitor and a resistor — which is why AM broadcasting came first. Its weakness is that noise is itself amplitude variation, so AM has poor noise immunity.

Frequency modulation

The frequency of the carrier is varied in proportion to the message, while amplitude stays constant.

Because the information is carried in frequency, amplitude noise can be stripped off entirely by a limiter before demodulation. FM therefore has far better noise immunity, which is why it is used for high-quality broadcasting.

The cost is bandwidth: FM occupies much more of the spectrum than AM, and its receiver is more complex.

	AM	FM
Varies	amplitude	frequency
Noise immunity	poor	good
Bandwidth	narrow	wide
Receiver	simple	complex
Typical use	long-wave/medium-wave broadcast	VHF broadcast, two-way radio

The superheterodyne receiver

The problem: a receiver must select one station from many and amplify it enormously, and doing that at the incoming frequency would require filters and amplifiers that retune every time you change station.

The superheterodyne solution: convert every incoming station to one fixed intermediate frequency (IF), and do all the hard filtering and amplification there.

Block diagram:

RF amplifier → Mixer → IF amplifier → Detector → AF amplifier → Speaker, with a local oscillator feeding the mixer, and AGC feeding back from the detector.

RF amplifier — provides initial selectivity and improves the noise figure.
Local oscillator — tuned together with the RF stage (ganged), so that it always runs a fixed amount above the incoming frequency.
Mixer — multiplies the two, producing sum and difference frequencies. The difference is the IF: 455 kHz for AM, 10.7 MHz for FM.
IF amplifier — fixed-frequency, so it can be optimised once for gain and selectivity. This stage provides most of both.
Detector — an envelope detector for AM; a discriminator or ratio detector for FM, preceded by a limiter that strips amplitude variation.
AGC — automatic gain control, feeding a DC level back to reduce gain on strong stations, so volume stays roughly constant across the dial.

The insight worth stating: the selectivity and gain are provided at a frequency that never changes, so tuning only has to move the oscillator. That is why the superheterodyne displaced every earlier design and why essentially every radio receiver since 1930 uses it.

Fibre optic communication

Block diagram: Information → Transmitter (drive circuit + optical source) → Optical fibre → Optical receiver (photodetector + amplifier) → Destination, with repeaters on long links.

Source — a laser diode for long-haul, an LED for short links.
Fibre — a glass core of higher refractive index surrounded by cladding of lower index, so light is guided by total internal reflection.
Detector — a PIN photodiode or avalanche photodiode.

Advantages: enormous bandwidth; very low loss, allowing tens of kilometres between repeaters; complete immunity to electromagnetic interference; no crosstalk; small and light; and no electrical conduction, so it is inherently safe in hazardous environments and cannot be tapped without detection.

Disadvantages: fragile, difficult and expensive to splice, and it cannot carry power to remote equipment.