AQA A Level Physics Paper 3 Section B: Electronics June 2024 question paper 7408/3BE

A toy manufacturer is designing a two-tone siren for use in small battery-operated cars. Figure 1 shows design Option 1.

Option 1 uses three separate signal generators feeding into a logic sub-system. The signal generators produce logic-compatible 9 V square waves of frequencies 1024 Hz, 1 Hz and 512 Hz.

The waveforms shown are not to scale.

Explain how the logic level applied at B in Figure 1 determines the output frequency at Q.

Write the Boolean algebra expression for output Q in terms of the inputs A, B and C. Use only the logic operations shown in Figure 1.

Option 1 is tested by replacing the 1 Hz signal generator with a manual input.

The manual input is provided by the combination of a push-to-make switch and a 10 kΩ resistor.

The combination produces the following voltages at its output:

• 0 V when the switch is not pressed

• 9 V when the switch is pressed.

Figure 2 shows the symbol for the push-to-make switch.

Figure 2

Complete Figure 3 to show how this switch and the 10 kΩ resistor are connected. Label the output V_out. You do not need to add details taken from Figure 1.

Figure 3

Figure 4 shows a generalised layout of an integrated circuit (IC) for an N-bit binary counter. Q₀ is the output that provides the least significant bit.

A signal generator feeds a square wave of frequency 1024 Hz into the clock of the IC. The N-bit binary counter generates the 512 Hz signal and the 1 Hz signal from separate outputs.

Deduce which of the outputs Q₀ to Q_N will provide the 1 Hz signal.

To make the two-tone siren, the manufacturer decides to use a new design, Option 2

Option 2 contains:

• one 1024 Hz signal generator

• one N-bit binary counter

• a new logic sub-system as shown in Figure 5.

Assume that:

• each type of logic gate has its own dedicated IC chip

• each separate signal generator is based upon its own IC chip.

Compare the number of ICs used in Option 1 with the number used in Option 2. Go on to explain one advantage of the manufacturer’s decision.

The short message service (SMS) on a mobile phone can send a maximum of 160 characters per message. Each character is represented by its own seven-bit binary code as it is converted to digital data.

The mobile phone transmits digital data at a rate of 8 kilobytes per second (kB s⁻¹) when using the SMS function.

Determine the minimum time required to send 160 characters.

Electrical noise can affect communication systems.

Describe one origin of electrical noise in a communication system.

Describe the effect that electrical noise can have:

• on the signal and

• on the communication system.

Figure 6 shows a noisy digital signal V_in which is applied to a circuit.

The circuit output V_out switches to:

• 5 V when the input voltage V_in falls below 1.8 V

• 0 V when the input voltage V_in rises above 3.2 V.

Draw, on Figure 7, the output signal V_out from the circuit.

Figure 8 shows an LC circuit that produces electrical oscillations when the switch is moved from position A to position B.

Which quantity in the LC circuit is analogous to the mass in a mass–spring system?

Tick () one box.

A radio receiver uses a parallel LC tuned circuit to select a radio station. Figure 9 shows the response of the tuned circuit.

Calculate the quality factor Q of the tuned circuit.

Another radio receiver is used to detect frequency-modulated (FM) radio waves.

Figure 10 shows the variation of amplitude with time for a carrier wave and an information signal.

Sketch, on Figure 10, the graph that represents the frequency-modulated (FM) signal.

An audio signal is transmitted on an FM music station. The transmission has a bandwidth of 186 kHz. The carrier wave has a maximum frequency deviation of 75 kHz.

Calculate the maximum frequency in the information signal.

Figure 11 shows the response of a photodiode for different values of reverse-bias

The photodiode is used as the input for a light-intensity meter.

The light intensity changes from 100 W m⁻² to 400 W m⁻².

Explain which value of V_R in Figure 11 should be used for this application. Go on to deduce the change in photocurrent for this change in light intensity.

Describe how a photodiode is used in a particle detector to detect sub-atomic particles.

The particle detector produces an analogue signal that is the input voltage V_in to an amplifier circuit.

Draw, on Figure 12, a circuit that uses a single operational amplifier to produce an amplification of +10

Use resistors with resistance values in the range 10 kΩ to 1 MΩ in the circuit.

On your diagram you should label:

• the value of the resistors

• the output of the circuit as V_out.

Do not show the power supplies for the operational amplifier.

Figure 12

Several telephone conversations need to be transmitted simultaneously between two locations.

Describe the process that allows these conversations to be carried by the same optical fibre.

In your answer you should explain:

• the importance of the sampling rate in the analogue-to-digital conversion

• the effect of resolution in this conversion

• the technique that is used to enable several conversations to be transmitted simultaneously.

You may use diagrams to help with your explanation.