Basic circuits of radio equipment
Every radio uses a few small circuits again and again.
- Oscillator: makes a steady wave of one frequency. It uses an LC circuit (coil L and capacitor C) or a quartz crystal. The frequency of an LC circuit is f = 1 / (2π√(LC)).
- Amplifier: makes a small signal bigger using a transistor. A power amplifier is the last stage of a transmitter.
- Modulator: changes the carrier with the message. In AM the height of the carrier follows the sound; in FM its frequency follows the sound.
- Detector (demodulator): takes the message back out. In AM a diode and a small filter do this.
- Mixer: joins two frequencies to make their sum and difference.
- Filter: lets one band pass and blocks the rest.
Transmitters and receivers
Transmitter: microphone, then audio amplifier, then oscillator and modulator, then power amplifier, then antenna. The power amplifier decides how far the signal goes.
Simple receiver: antenna, tuner, detector, audio amplifier, speaker. It is easy but not very sharp, because the tuner alone must separate close stations.
Superheterodyne receiver: after the antenna and a first amplifier, a local oscillator and mixer change every station to the same fixed intermediate frequency (IF), for example 455 kHz for AM. Fixed filters at the IF are sharp and cheap, so selectivity is good. Local oscillator frequency = station frequency + IF.
Good receivers are judged by sensitivity (weakest signal heard), selectivity (separating nearby stations) and fidelity (sound close to the original).
Microwave communication equipment
Microwaves have frequencies from about 1 GHz to 30 GHz, so they have very short wavelengths (centimetres). They carry a lot of information, and small dishes can aim them in a thin beam. They travel in straight lines, so a link needs a clear line of sight between two dishes on towers or hills.
The distance of one hop is limited by the radio horizon, roughly d = 4.12 × (√h1 + √h2) km, with heights h1 and h2 in metres. Long routes use a chain of repeater stations, each receiving, amplifying and re-sending the signal on a new frequency. A microwave terminal has a modulator, a transmitter, a waveguide or coax feeder, a dish, and the same parts in reverse for receiving. Satellite earth stations and ship radars use the same ideas.
Distress and safety communication equipment
At sea, rescue depends on radio. The world system is called GMDSS. Key items:
- VHF radio: short-range voice. Channel 16 (156.8 MHz) is for distress and calling. The spoken distress call is "Mayday" three times.
- DSC (digital selective calling): press one red button and the set sends a digital alert with the ship's identity and position to all stations in range.
- EPIRB: an emergency beacon on 406 MHz. It floats free, switches on in water and sends the ship's identity and position by satellite.
- SART: a search-and-rescue transponder. When a rescue ship's radar hits it, it answers with a line of dots on the radar screen, so the lifeboat can be found.
- NAVTEX: prints weather and safety warnings from the coast.
Key formulas and definitions
- LC oscillator: f = 1 / (2π√(LC))
- Wavelength: λ = c / f, with c = 3 × 10⁸ m/s
- AM bandwidth = 2 × highest audio frequency
- Superheterodyne: f(local oscillator) = f(station) + f(IF)
- Image frequency = f(station) + 2 × f(IF)
- Radio-horizon hop: d ≈ 4.12 (√h1 + √h2) km, h in metres
Worked examples
1. A coil of 100 µH and a capacitor of 100 pF make an oscillator. Find its frequency.
LC = 10⁻⁴ × 10⁻¹⁰ = 10⁻¹⁴, so √(LC) = 10⁻⁷ s. f = 1 / (2π × 10⁻⁷) ≈ 1.59 × 10⁶ Hz = 1.59 MHz.
2. An AM station carries music up to 5 kHz. What bandwidth does it need?
AM makes two side bands, one above and one below the carrier. Bandwidth = 2 × 5 kHz = 10 kHz.
3. A superheterodyne radio with IF = 455 kHz is tuned to a station at 1000 kHz. Find the local oscillator frequency and the image frequency.
Local oscillator = 1000 + 455 = 1455 kHz. Image = 1000 + 2 × 455 = 1910 kHz. The image is a second frequency that would also give 455 kHz at the mixer, so a filter at the front must block it.
4. Two microwave towers are each 36 m tall. What is the longest clear hop?
d = 4.12 × (√36 + √36) = 4.12 × 12 ≈ 49.4 km.
Common mistakes
- Thinking the antenna makes the signal strong. The power amplifier does that; the antenna only sends it out.
- Mixing up modulator and detector. The modulator adds the message at the sender; the detector removes it at the receiver.
- Saying microwaves bend round hills. They travel straight and need a clear line of sight.
- Forgetting the local oscillator is ABOVE the station by the IF (f + IF), not below it, in a normal AM set.