Amplitude (AM) and frequency modulation (FM) techniques (A-level only) Flashcards – AQA A-level Physics (7408)

What is modulation?

Varying a property of a carrier wave in accordance with an information signal so the information can be transmitted.

What are the roles of the information signal and the carrier in modulation?

The information signal contains the changes to be communicated; the carrier is generally a higher-frequency oscillation that conveys those changes.

How is information recovered from a modulated carrier at the receiving end?

From the variation imposed on the carrier.

What happens to the carrier wave in amplitude modulation (AM)?

Its amplitude varies with the instantaneous value of the information signal, while its frequency remains constant.

What does the envelope of an AM signal on a voltage–time graph follow?

The variation of the information signal.

How can the carrier frequency be found from an AM voltage–time graph?

Measure the period $T$ of the rapid carrier oscillations, then calculate $f = 1/T$.

How can the frequency of a periodic information signal be found from an AM voltage–time graph?

Measure the period $T$ over which the envelope repeats, then calculate $f = 1/T$.

What happens to the carrier wave in frequency modulation (FM)?

Its instantaneous frequency varies with the instantaneous value of the information signal, while its amplitude remains constant.

How does carrier oscillation spacing on a voltage–time graph indicate instantaneous frequency?

Closer oscillations indicate a higher frequency; farther-apart oscillations indicate a lower frequency.

How can the frequency of a periodic information signal be found from an FM voltage–time graph?

Measure the period $T$ over which the modulation pattern repeats, then calculate $f = 1/T$.

What is the carrier frequency in FM?

The unmodulated, or centre, frequency of the carrier.

How can carrier frequency be estimated from a graph spanning a complete modulation cycle?

Count the carrier cycles and divide by the elapsed time.

What is the bandwidth of a modulated signal?

The range of frequencies the signal occupies, expressed in hertz.

What is the required bandwidth for simple AM?

$2f_{\mathrm{M}}$, where $f_{\mathrm{M}}$ is the highest frequency in the information signal.

What is the bandwidth requirement for simple FM?

$2(\Delta f + f_{\mathrm{M}})$, where $\Delta f$ is the maximum deviation of the carrier frequency from its centre frequency, and $f_{\mathrm{M}}$ is the highest information frequency.

What is the data capacity of a communication channel?

The maximum rate at which information can be transmitted reliably, commonly expressed in bits per second.

Why does greater available bandwidth generally permit a greater data rate?

It allows the channel to support a wider range of signal frequencies.

Besides bandwidth, what constrains a communication channel’s data capacity?

Noise and the quality of the received signal constrain how reliably information can be distinguished.

How does the transmission medium affect a communication channel’s available bandwidth?

Bandwidth depends on the frequencies that can be used effectively through the medium.

How does the available bandwidth of optical fibre compare with that of copper conductors?

Optical fibre can provide very large bandwidth; copper has a more limited usable range that depends on the link.

What constrains the bandwidth available to an individual radio channel?

Its spectrum allocation and the need to share the spectrum.

Besides the transmission medium, what link-specific factors affect usable bandwidth?

The link’s equipment and operating conditions.