How is information sent over a communication system, and why is modulation needed?
Communication principles and modulation: the structure of a communication system, the need for a carrier, amplitude and frequency modulation, bandwidth, data rate, and noise and distortion.
A focused answer to WJEC A-Level Electronics communication principles and modulation, covering the structure of a communication system, the need for a carrier, amplitude and frequency modulation, bandwidth and data rate, and the effects of noise and distortion.
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
Communication systems send information from a transmitter to a receiver, and modulation is the technique that makes long-range transmission possible. WJEC expects you to describe the structure of a communication system, explain why a carrier is needed, distinguish amplitude from frequency modulation, calculate bandwidth and data rate, and discuss noise and distortion. The AM sideband/bandwidth calculation and the AM-versus-FM noise comparison are dependable exam earners.
The answer
The structure of a communication system
Why a carrier is needed
Amplitude and frequency modulation
Bandwidth, data rate and noise
Bandwidth is the range of frequencies a transmission occupies. AM produces sidebands at the carrier plus and minus the modulating frequency, so its bandwidth is twice the highest modulating frequency. Data rate (bits per second) rises with bandwidth. Noise is unwanted random signal added in the channel, and distortion is unwanted change to the signal's shape; both degrade the received information.
Examples in context
- Example 1. AM versus FM radio
- AM radio fits more stations into a band because each needs less bandwidth, but suffers crackle from electrical noise (lightning, motors) because that noise changes the amplitude. FM radio sounds cleaner because the receiver clips amplitude noise before reading the frequency, which is why music stations use FM.
- Example 2. Why aerials are sized to the carrier
- An efficient aerial is comparable in length to the carrier's wavelength. A 1 MHz carrier has a wavelength of 300 m, so a tall mast works, whereas a 3 kHz audio signal would need a 100 km aerial. Modulating the audio onto the carrier is what makes a practical aerial possible.
- Example 3. Trading bandwidth for data rate
- A channel with more bandwidth can carry a higher data rate, which is why broadband uses a wide range of frequencies. The link between bandwidth and data rate is why faster connections need more spectrum, and why spectrum is a scarce, regulated resource.
Try this
Q1. An audio signal up to amplitude-modulates a carrier. Find the bandwidth of the transmission. [2 marks]
- Cue. Bandwidth .
Q2. State why frequency modulation is less affected by noise than amplitude modulation. [2 marks]
- Cue. The information is carried in the frequency, not the amplitude, so amplitude noise can be clipped off by the receiver before the signal is demodulated.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20205 marksAn AM radio station has a carrier of and transmits an audio signal with a maximum frequency of . State the frequencies of the sidebands produced, calculate the bandwidth of the transmission, and explain why a carrier is needed at all.Show worked answer →
Amplitude modulation produces sidebands at the carrier frequency plus and minus the modulating frequency.
Upper sideband: . Lower sideband: .
Bandwidth: twice the highest modulating frequency .
A carrier is needed because a low-frequency audio signal cannot be radiated efficiently from a practical aerial (the aerial would have to be many kilometres long). Modulating a high-frequency carrier lets the information be transmitted from a reasonable aerial, and different stations can use different carriers so they do not overlap.
Markers reward the two sideband frequencies, the bandwidth of , and the carrier justification (efficient radiation and channel separation).
WJEC Eduqas 20184 marksCompare amplitude modulation and frequency modulation in terms of how the information is carried and their susceptibility to noise.Show worked answer →
In amplitude modulation (AM) the amplitude of the carrier is varied in step with the information signal, while the carrier frequency stays constant.
In frequency modulation (FM) the frequency of the carrier is varied in step with the information signal, while the amplitude stays constant.
AM is more susceptible to noise because noise adds to the amplitude, and the receiver reads amplitude, so the noise is heard. FM is more resistant because the information is in the frequency, not the amplitude, so amplitude noise can be removed (clipped) by the receiver before demodulation. FM therefore gives better quality but needs a wider bandwidth.
Markers reward amplitude varied (AM) versus frequency varied (FM), and FM being less affected by noise because the information is not in the amplitude.
Related dot points
- Wireless and optical transmission: radio-wave transmission and the aerial, attenuation and noise, optical-fibre transmission, total internal reflection, and multimode versus monomode fibre.
A focused answer to WJEC A-Level Electronics wireless and optical transmission, covering radio-wave transmission and the aerial, attenuation and noise on a channel, optical-fibre transmission by total internal reflection, and the difference between multimode and monomode fibre.
- AC signals and reactance: peak, peak-to-peak and RMS values, frequency and period, capacitive and inductive reactance, and the phase relationship between voltage and current.
A focused answer to WJEC A-Level Electronics AC signals and reactance, covering peak, peak-to-peak and RMS values, frequency and period, capacitive and inductive reactance, how reactance varies with frequency, and the phase between voltage and current.
- Passive filters: the RC low-pass and high-pass filter, the cut-off (break) frequency, the frequency response and the half-power point, gain in decibels, and the Bode plot.
A focused answer to WJEC A-Level Electronics passive filters, covering the RC low-pass and high-pass filter, the cut-off (break) frequency, the frequency response and the minus 3 dB half-power point, gain expressed in decibels, and the Bode plot.
- Analogue-to-digital conversion: sampling, the sampling rate and the Nyquist criterion, quantisation, resolution and the number of bits, and quantisation error.
A focused answer to WJEC A-Level Electronics analogue-to-digital conversion, covering sampling and the sampling rate, the Nyquist criterion, quantisation into levels, resolution and the number of bits, and quantisation error.
- Audio systems: the audio signal chain, the mixer based on a summing amplifier, tone control with filters, voltage and power amplification, gain in decibels, and driving a loudspeaker.
A focused answer to WJEC A-Level Electronics audio systems, covering the audio signal chain, the mixer built from a summing amplifier, tone control with filters, voltage and power amplification, expressing gain in decibels, and driving a loudspeaker.
Sources & how we know this
- WJEC Eduqas GCE A-level Electronics specification — WJEC Eduqas (2017)