Why must a signal be modulated onto a carrier to be transmitted, and how do AM, FM and digital modulation work?
Communications and wireless transmission: the need for a carrier, amplitude and frequency modulation, bandwidth, digital modulation (ASK, FSK), and the radio transmitter and receiver chain.
An Eduqas A-Level Electronics answer on communications and wireless transmission: why a high-frequency carrier is needed, amplitude modulation and frequency modulation, the bandwidth they occupy, digital modulation (amplitude-shift and frequency-shift keying), and the transmitter and receiver chain.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
Eduqas wants you to explain the need for a carrier, amplitude and frequency modulation, the bandwidth they occupy, digital modulation (ASK and FSK), and the radio transmitter and receiver chain. This is how information travels wirelessly.
The answer
The need for a carrier
Amplitude and frequency modulation
Bandwidth and digital modulation
The transmitter and receiver chain
Examples in context
Modulation underlies all wireless communication: AM and FM radio, the FSK tones of older modems and many low-power radio links, the phase-shift keying of WiFi and mobile data, and the on-off keying of simple remote controls. The carrier, modulator, amplifier and aerial of a transmitter, and the tuned circuit and demodulator of a receiver, are standard blocks. The reactance and tuned-circuit ideas come from the AC topic, and the digital data being sent is produced by the conversion and number-system topics.
Try this
Q1. State why an audio signal must be modulated onto a carrier for radio transmission. [2 marks]
- Cue. Audio frequencies radiate inefficiently (the aerial would be impractically large); a high-frequency carrier allows a sensible aerial to radiate it.
Q2. State which of AM and FM is more noise-immune. [1 mark]
- Cue. FM (the information is in the frequency, so amplitude noise is ignored).
Q3. State what is switched in frequency-shift keying (FSK). [1 mark]
- Cue. The carrier frequency (between two values for and ).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20206 marksExplain why an audio signal must be modulated onto a carrier before it is transmitted by radio, and compare amplitude modulation (AM) with frequency modulation (FM) in terms of noise immunity and bandwidth.Show worked answer →
Need for a carrier (up to 3 marks): an audio signal (roughly to ) has too low a frequency to radiate efficiently, because the aerial would need to be enormous (comparable to the wavelength). Modulating the audio onto a high-frequency carrier lets a practical-sized aerial radiate it, and different stations can use different carrier frequencies so they do not interfere.
AM versus FM (up to 3 marks): in AM the carrier's amplitude varies with the signal; AM is simple and uses less bandwidth, but it is susceptible to noise because noise adds to the amplitude. In FM the carrier's frequency varies with the signal; FM is far more noise-immune (noise mainly affects amplitude, which the receiver ignores) but uses more bandwidth.
Markers reward the aerial-size/efficiency reason for a carrier, AM varying amplitude (less bandwidth, more noise), and FM varying frequency (more bandwidth, better noise immunity).
Eduqas 20225 marksDescribe amplitude-shift keying (ASK) and frequency-shift keying (FSK) as methods of transmitting digital data, and state one advantage of FSK over ASK.Show worked answer →
ASK (up to 2 marks): in amplitude-shift keying the carrier is switched between two amplitudes (often on and off) to represent the binary s and s, so the digital data modulates the carrier's amplitude.
FSK (up to 2 marks): in frequency-shift keying the carrier is switched between two frequencies to represent and , so the data modulates the carrier's frequency.
Advantage of FSK (up to 1 mark): FSK is more noise-immune than ASK, because the information is in the frequency, not the amplitude, so amplitude noise and fading do not corrupt it as easily.
Markers reward ASK as two amplitudes (on/off) and FSK as two frequencies for the bits, and FSK's better noise immunity.
Related dot points
- Optical communication: the optical link (LED or laser source, fibre, photodiode receiver), total internal reflection, attenuation and bandwidth, and the advantages over copper.
An Eduqas A-Level Electronics answer on optical communication: the optical link of an LED or laser source, an optical fibre and a photodiode receiver, total internal reflection that guides the light, attenuation and bandwidth, and the advantages of fibre over copper cable.
- Digital communications: serial and parallel transmission, the data rate (bit rate and baud), multiplexing (time-division and frequency-division), and error detection with parity and checksums.
An Eduqas A-Level Electronics answer on digital communications: serial and parallel transmission and their trade-offs, the bit rate and baud, time-division and frequency-division multiplexing to share a channel, and error detection using parity bits and checksums.
- AC signals and reactance: amplitude, peak-to-peak, period and frequency of a sinusoid, root-mean-square values, and the frequency-dependent reactance of capacitors and inductors.
An Eduqas A-Level Electronics answer on alternating signals and reactance: amplitude, peak-to-peak, period and frequency of a sinusoid, the root-mean-square value and its relation to the peak, and the frequency-dependent reactance of capacitors and inductors that underlies all filtering.
- Analogue-to-digital conversion: sampling, quantisation and resolution, the sampling theorem and aliasing, quantisation error, and the trade-off between resolution and data rate.
An Eduqas A-Level Electronics answer on analogue-to-digital conversion: sampling a continuous signal, quantisation and resolution, the Nyquist sampling theorem and aliasing, quantisation error, and the trade-off between resolution, sampling rate and the data rate produced.
Sources & how we know this
- Eduqas GCE AS/A Level Electronics specification (A410QS) — WJEC Eduqas (2017)