Communication

State what is meant by the amplitude modulation (AM) of a radio wave.

State two advantages of AM transmissions when compared with frequency modulation (FM) transmissions.

1. ______

2. ______

the wavelength of the carrier wave

$\text{wavelength}$ = ______ $\text{m}$

the maximum frequency of the transmitted audio signal.

$\text{frequency}$ = ______ $\text{kHz}$

Another audio signal with the same maximum frequency is transmitted using a different carrier wave frequency. The lowest frequency of this modulated wave is equal to the highest frequency of the modulated wave in (b).

Determine the frequency of this carrier wave.

$\text{frequency}$ = ______ $\text{kHz}$

State what is meant by the amplitude modulation (AM) of a radio wave.

the wavelength of the carrier wave

wavelength = ______ $\text{m}$

the bandwidth of the modulated wave

bandwidth = ______ $\text{kHz}$

the maximum frequency of the audio signal.

maximum frequency = ______ $\text{kHz}$

The power of a radio signal at a transmitter is $P_T$.
At a receiver, the received power $P_R$ is given by the expression

where $x$ is the distance, in metres, between the transmitter and the receiver.

For the transmission of this signal, the attenuation is $73\ \text{dB}$.

Determine the distance $x$.

$x$ = ______ $\text{m}$

State the name of this type of modulation of the carrier wave.

For this modulated carrier wave, determine the variation, if any, in:

1. its amplitude

2. its frequency.

the wavelength, in km, of the carrier wave

wavelength = ______ $\text{km}$

the bandwidth

bandwidth = ______ $\text{kHz}$

the frequency of the audio signal.

frequency = ______ $\text{kHz}$

State what is meant by the amplitude modulation (AM) of a radio wave.

the wavelength of the carrier wave

wavelength = ______ $\text{m}$

the bandwidth of the modulated wave

bandwidth = ______ $\text{kHz}$

the maximum frequency of the audio signal.

maximum frequency = ______ $\text{kHz}$

The power of a radio signal at a transmitter is $P_T$. At a receiver, the received power $P_R$ is given by the expression

where $x$ is the distance, in metres, between the transmitter and the receiver.

For the transmission of this signal, the attenuation is $73\ \text{dB}$.

Determine the distance $x$.

$x$ = ______ $\text{m}$

State two advantages of converting the signal into digital form for transmission.

State the sampled bits at time $4\text{ ms}$ and time $8\text{ ms}$.

$4\text{ ms}$: ______ $8\text{ ms}$: ______

Part of the signal received by the receiver, after the sampled signal has passed through the DAC, is shown in Fig. 5.3.

On Fig. 5.3, complete the line to show the received signal for time 0 to time $12\text{ ms}$.

The ADC in (b) is replaced with one that has a sampling frequency of $500\text{ Hz}$ and uses 3-bit sampling, with the least significant bit corresponding to $2\text{ mV}$.

On Fig. 5.4, sketch the signal that is now received, after passing through the DAC, from time 0 to time $12\text{ ms}$.

When audio signals are transmitted over long distances, modulation of radio waves is used.

Suggest a reason why modulation is used.

State a technical advantage and a technical disadvantage of using frequency modulation rather than amplitude modulation.

advantage: ______

disadvantage: ______

An audio signal of amplitude $2.0\ \mu\text{V}$ and frequency $4.2\text{ kHz}$ is to be transmitted using a carrier wave of amplitude $10.0\text{ mV}$ and frequency $100\text{ kHz}$.

Either amplitude modulation or frequency modulation may be used.

The amplitude modulation is at a rate of $1\text{ mV } \mu\text{V}^{-1}$.
The frequency modulation is at a rate of $5\text{ kHz } \mu\text{V}^{-1}$.

Complete Table 5.1 to show the maximum and minimum values of the amplitude and of the frequency of the modulated wave for each type of modulation.

Table 5.1

For the amplitude modulated wave in (b), determine the bandwidth.

bandwidth = ______ $\text{kHz}$

State two advantages of converting the signal into digital form for transmission.

State the sampled bits at time $4\ \text{ms}$ and time $8\ \text{ms}$.

$4\ \text{ms}$: ______ $8\ \text{ms}$: ______

Part of the signal received by the receiver, after the sampled signal has passed through the DAC, is shown in Fig. 5.3.

On Fig. 5.3, complete the line to show the received signal for time 0 to time $12\ \text{ms}$.

The ADC in (b) is replaced with one that has a sampling frequency of $500\ \text{Hz}$ and uses 3-bit sampling, with the least significant bit corresponding to $2\ \text{mV}$.

On Fig. 5.4, sketch the signal that is now received, after passing through the DAC, from time 0 to time $12\ \text{ms}.

State two advantages of the transmission of data in digital form, rather than analogue form.

1. ______

2. ______

A signal in an optic fibre is carried by an electromagnetic wave of frequency $1.36 \times 10^{14}\text{ Hz}$. The speed of the wave in the fibre is $2.07 \times 10^{8}\text{ m s}^{-1}$.

For this electromagnetic wave, determine the ratio:

ratio = ______

The attenuation per unit length of the signal in the fibre is $0.40\text{ dB km}^{-1}$. The input power is $1.5\text{ mW}$ and the output power is $0.060\text{ mW}$.

Calculate the length of the fibre.

length = ______ $\text{km}$

Suggest an advantage of greater bandwidth.

State what is meant by crosslinking.

In telecommunications, a signal power of $1.0\ \text{mW}$ is used as a reference power.
Signal powers relative to this reference power and expressed in dB are said to be measured in 'dBm'.

Show that a signal power of $13\ \text{dBm}$ is equivalent to $20\ \text{mW}$.

A signal of input power $20\ \text{mW}$ is transmitted along an optic fibre for an uninterrupted distance of $45\ \text{km}$.

The optic fibre has an attenuation per unit length of $0.18\ \text{dB km}^{-1}$.

Calculate the output power $P$ from the optic fibre.

$P$ = ______ $\text{mW}$

Telephone signals may be transmitted either by means of an optic fibre or by means of a wire pair.

State three advantages of the use of an optic fibre rather than a wire pair.

1. ______

2. ______

3. ______

the power of the background noise

power = ______ $\text{W}$

the maximum attenuation per unit length of the optic fibre that allows for uninterrupted transmission of the signal.

attenuation per unit length = ______ $\text{dB km}^{-1}$