3.3 Electromagnetic Spectrum – Digital Signalling
Learning Objectives
- Identify the seven regions of the electromagnetic (EM) spectrum, their order in wavelength/frequency and typical everyday uses.
- State that all EM waves travel at ≈ 3.0 × 10⁸ m s⁻¹ in vacuum.
- Explain the harmful effects of high‑energy EM radiation and list simple safety measures (Core 3.4).
- Describe the three main advantages of digital signalling over analogue signalling (Supplement 3.3):
- Higher data‑transmission rates
- Longer transmission range thanks to accurate signal regeneration
- Built‑in error detection and correction
- Connect digital signalling to the part of the spectrum that is actually used (radio‑frequency & microwave bands).
1. The Electromagnetic Spectrum (Core 3.3)
Electromagnetic waves cover a continuous range of frequencies. In the IGCSE syllabus the spectrum is divided into seven regions, listed from longest wavelength (lowest frequency) to shortest wavelength (highest frequency). All of them travel at the same speed in vacuum:
Speed of EM waves in vacuum ≈ 3.0 × 10⁸ m s⁻¹.
| Region | Wavelength (m) | Frequency (Hz) | Typical everyday uses |
|---|
| Radio | > 10 m | < 30 MHz | FM/AM broadcasting, walkie‑talkies, TV broadcasting, mobile‑phone base stations |
| Microwave | 0.01 – 10 m | 30 MHz – 300 GHz | Radar, satellite links, Wi‑Fi, microwave ovens, digital TV & radio (DAB) |
| Infra‑red (IR) | 7 µm – 10 µm | 3 × 10¹³ – 4 × 10¹⁴ | Remote controls, thermal cameras, fibre‑optic communication (light‑pulse carriers) |
| Visible | 400 nm – 700 nm | 4 × 10¹⁴ – 7.5 × 10¹⁴ | Human vision, lighting, displays |
| Ultraviolet (UV) | 10 nm – 400 nm | 7.5 × 10¹⁴ – 3 × 10¹⁶ | Sun‑burn, sterilisation, black lights |
| X‑rays | 0.01 nm – 10 nm | 3 × 10¹⁶ – 3 × 10¹⁹ | Medical imaging, security scanners |
| Gamma‑rays (γ‑rays) | < 0.01 nm | > 3 × 10¹⁹ | Radiotherapy, nuclear industry, astrophysics |
2. Harmful Effects of High‑Energy EM Radiation (Core 3.4)
- Microwaves & Radio waves: Low‑energy; can cause heating if the power is very high (e.g., microwave ovens).
Safety: Keep a safe distance from high‑power transmitters. - Infra‑red: Intense sources can cause skin burns.
Safety: Avoid looking directly at powerful IR emitters. - Ultraviolet: Damages DNA → sunburn, premature ageing, increased skin‑cancer risk.
Safety: Use sunscreen, sunglasses and protective clothing. - X‑rays & Gamma‑rays: Ionising radiation; can break chemical bonds, leading to cell damage and cancer.
Safety: Limit exposure, use shielding (lead aprons) and follow medical‑imaging guidelines.
3. Why Digital Signalling? (Supplement 3.3)
Digital signals represent information as discrete levels – usually “0” and “1”. This simple change from a continuously varying analogue waveform gives three practical benefits, especially in the radio‑frequency and microwave parts of the spectrum.
3.1 Higher Data‑Transmission Rates
3.2 Longer Transmission Range – Accurate Signal Regeneration
- At the receiver a digital signal is interpreted as a clear “0” or “1”. This decision point allows a repeater or amplifier to regenerate the original clean levels, effectively resetting accumulated noise to (almost) zero.
- Analogue chain: Noise adds continuously; each amplification stage adds more noise → quality degrades with distance.
- Digital chain: Noise may cause occasional bit errors, but a correctly designed receiver can correct them and then output a perfect binary level. The regenerated signal can travel another long segment before the next regeneration.
3.3 Built‑in Error Detection & Correction
- Extra bits (parity, checksums, CRC, or more sophisticated forward error‑correction codes) are transmitted with the data.
- The receiver can detect most errors and, with FEC, often correct them automatically, further extending the reliable range.
4. Analogue vs Digital – Relevance to the EM Spectrum
| Aspect | Analogue signalling | Digital signalling (used mainly in RF & microwave bands) |
|---|
| Signal representation | Continuous variation of amplitude, frequency or phase | Discrete voltage or light levels (binary or M‑ary) |
| Noise sensitivity | Noise adds directly → gradual degradation | Noise can be filtered; bits are restored at repeaters |
| Data rate | Limited by bandwidth and S/N | Increased by using more levels per symbol (higher M) without widening bandwidth |
| Transmission range | Degrades with distance; repeaters add extra noise | Longer range; repeaters regenerate a clean digital signal |
| Error handling | Hard to implement | Parity, CRC, forward error correction built in |
5. Practical / Experimental Links (AO3)
- LED‑photodiode experiment: Transmit a binary message over a short optical fibre. Use a comparator circuit to restore the original square‑wave shape after a long fibre length, illustrating regeneration.
- Analogue vs digital radio test: Compare the signal‑to‑noise ratio of an analogue AM receiver with that of a digital DAB receiver at the same distance from the transmitter. Discuss why the digital receiver gives clearer audio.
- Data‑analysis task: Calculate the theoretical maximum data rate for a 5 MHz FM radio channel with S/N = 30 dB using the Shannon‑Hartley formula, then compare with the actual data rate of a digital radio broadcast (e.g., DAB).
6. Real‑World Examples
- Fibre‑optic internet: Near‑IR light pulses (digital) carry terabits per second over thousands of kilometres, with repeaters regenerating the signal.
- Digital TV & radio (DAB/DVB‑T): Use microwave‑frequency carriers (≈ 200 MHz–800 MHz) but encode programmes as digital data, giving higher picture quality, more stations and better resistance to noise.
- Mobile phones (3G/4G/5G): Employ QAM or PSK modulation in the 1–6 GHz band to achieve high data rates while keeping the network range acceptable.
- Low‑tech illustration – FM radio vs DAB: FM (analogue) varies continuously; DAB sends a stream of binary bits on the same frequency band, allowing more efficient use of spectrum and longer coverage.
Key Take‑aways
- The EM spectrum consists of seven regions; modern communication mainly uses the radio‑frequency and microwave parts.
- All EM waves travel at ≈ 3.0 × 10⁸ m s⁻¹ in vacuum.
- High‑energy radiation (UV, X‑ray, γ‑ray) can be harmful – simple safety measures minimise risk.
- Digital signalling provides:
- Higher data‑transmission rates by sending multiple bits per symbol (M‑ary modulation),
- Longer transmission range because repeaters regenerate a clean signal, removing accumulated noise,
- Built‑in error detection and correction for reliable communication.
- These advantages explain why today’s internet, mobile phones, digital TV and radio all rely on digital rather than analogue transmission.