AQA A Level Physics Paper 3 Section B: Electronics November 2020 question paper 7408/3BE

Figure 1 shows part of a circuit that includes a 4-bit binary counter. The main inputs and outputs of the counter are shown. The counter generates a sequence of binary codes representing the decimal numbers 0 to 7 Output Q₀ is the least significant bit of the binary codes.

The counter resets when the master reset pin MR receives a logic 1

The circuit requires the counter to reset when either one of two conditions is met.

Condition 1 Manual reset using the switch P to reset the counter to 0

Condition 2 Automatic reset when an appropriate binary code is produced at the counter outputs. This will cause the counter to continually cycle through the decimal numbers 0 to 7

Complete Figure 1 to show how both reset conditions can be met.

Do not show the power line connections to the integrated circuit.

A logic system is designed to identify prime numbers.

The binary codes from the counter are now applied to the inputs ABC of the logic system shown in Figure 2.

Input A takes the least significant bit of the binary code from the counter.

Output W becomes logic state 1 when a prime number 2, 3, 5 or 7 is detected. Otherwise output W is at logic 0

Write the Boolean algebra expression for output W in terms of the inputs A, B and C. The expression must contain only the four logic gate operations shown in Figure 2.

Complete Table 1, the truth table for the logic system in Figure 2.

The logic system in Question 01.2 is replaced with one that gives an output S using the same binary input codes CBA.

The Boolean algebra equation for output S is

Deduce which decimal numbers 0 to 7 will cause S to become logic 1

Complete Figure 3 by drawing the logic system for S.

You must use only the logic gate operations given in S = A B.(+ C)

Figure 3

Figure 4 shows the filter circuit that forms the first stage in an amplitude modulated (AM) radio receiver.

The circuit contains a 3.3 mH inductor and a variable capacitor.

The circuit is tuned to receive a radio station transmitting at a frequency of 1053 kHz.

Calculate the value of the capacitance needed to receive this station.

The circuit is retuned to receive a different radio station by setting the variable capacitor to a value of 9.3 pF.

Table 2 shows the capacitance range of four variable capacitors W, X, Y and Z.

Comment on the suitability of these capacitors for this application and state your preference.

Figure 5 shows part of the frequency response curve for a different filter circuit.

Determine the bandwidth of the filter circuit.

Calculate the Q factor of the filter circuit in Question 02.3.

The radio station is tuned using a different filter circuit with a very low Q factor.

State and explain one effect of this change on the sound heard by a listener.

Pulse code modulation (PCM) is used to encode live music as an uncompressed digital audio file. Sampling of the analogue signal is carried out at 44.1 kHz. A 16-bit system is used to encode each of the two channels that make up the stereo signal.

Explain why the sampling frequency used is suitable for this task.

Calculate the number of quantisation levels available on a 16-bit encoding system.

A recorded piece of stereo music lasts for 3.5 minutes.

Calculate the size, in megabytes, of the digital file needed to store this recording.

The music file is used by a call centre to play as background music while a phone call is on hold. However, the telephone network is designed to use a bandwidth of 0.3 kHz – 3.4 kHz.

Compare the quality of the music heard by the telephone caller with that of the original file heard when played directly from a compact disc.

Figure 6 shows a circuit containing a photodiode and an ideal operational amplifier. This circuit is used to monitor the intensity of monochromatic radiation.

What is the configuration of the operational amplifier circuit shown in Figure 6 ? Tick () one box.

Figure 7 shows the variation of photocurrent with intensity for the monochromatic radiation incident on the photodiode.

Radiation of intensity 3.0 W m⁻² is incident on the photodiode.

Show that the voltage at the non-inverting terminal (V +) of the operational amplifier is 1.9 V.

The intensity of radiation incident on the photodiode remains at 3.0 W m⁻².

Deduce whether the light-emitting diode (LED) in Figure 6 is on or off.

British embassies in Europe are to be connected to a new long-distance communication link. The link, in the form of a land-based cable, will support multiple simultaneous video conferencing as well as the transmission of sensitive government data.

The company installing the link has to consider the choice between using optic fibre or copper wire in the cables.

Compare the advantages and disadvantages of the two options for use in these cables. State which option you would advise the company to use.

For both types of cable refer to their:

• physical properties

• ability to reject external interference

• signal-carrying properties.