M Schemes 33

33 Marking scheme: Worksheet (A2) 1

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The audio signals have the same frequency. The amplitude of the signal/trace/carrier rises and falls with the same number of cycles (two in the time of the trace). The audio signals have different volumes/loudness/amplitudes. The amplitude of the trace rises and falls more in the bottom trace. b The time (or horizontal distance along each trace) for one rapid variation is the same in both traces. a

In amplitude modulation the amplitude of the carrier wave is altered to carry the signal (the frequency remains the same). In frequency modulation the frequency of the carrier wave is altered to carry the signal (the amplitude remains the same). b i 30 × 2 = 60 kHz ii 800 − 60 = 740 kHz iii Alters from 740 kHz to 860 kHz 6000 times a second. c More transmitters may be needed as the range of FM is less than that of AM. Equipment to transmit and receive FM is more expensive.

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i ii iii b i ii iii

Carrier wave Sidebands 5 kHz 2.5 × 10−5 s 2.0 × 10−4 s Correct amplitude-modulated shape 8 carrier wave oscillations per oscillation of the amplitude Correct times marked

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time for one wave of audio signal, b ii

time for one wave of carrier, b i

c

Correct shape Sidebands extending from 40 to 55 kHz and from 40 to 25 kHz

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AS and A Level Physics

40

55 f / kHz

Original material © Cambridge University Press 2010

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33 Marking scheme: Worksheet (A2)

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Analogue signal can have any value (within limits). Diagram to show analogue signal. Digital can have only a few values, e.g. two, and nothing in-between these values. Diagram to show digital signal.

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b The value of the signal is measured at regular intervals of time. The value obtained is converted into a binary number (with a certain number of bits). The binary numbers obtained are placed one after the other and form the digital signal.

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a 0101 and 1000 b Values shown as horizontal lines of 0.5 ms duration Values plotted: 5, 8, 8, 6, 2, 1, 2, 6, 8, 8 Graph correct with labels

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V / mV

8 6 4 2 0 0

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5 t / ms

Any variation in the signal that occurs between sampling is not detected Increasing sampling frequency decreases the time between samples Frequency at least 2 × signal, i.e. 600 Hz (frequency of signal ≈ 300 Hz) ii The variation in voltage can use more voltage levels (28 levels rather than 24 levels). The signal voltage at every sample is closer to the actual value. i

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⎛ 6.0 × 10 −3 ⎞ ⎟ = 164.7 dB ≈ 165 dB Signal-to-noise ratio (in dB) = 10 lg ⎜⎜ −19 ⎟ 2 . 0 10 × ⎝ ⎠

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a

1.0 × 10 −3 = 30 dB 0.001× 10 −3 b Signal becomes 0.001 mW or signal-to-noise ratio is 1 0 dB

Signal-to-noise ratio = 10 lg

AS and A Level Physics

Original material © Cambridge University Press 2010

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33 Marking scheme: Worksheet (A2)

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Any two reasons and explanation. Less attenuation so fewer repeater/regeneration amplifiers needed. More bandwidth so more data can be sent per second/more telephone calls made at once. Less interference/noise so fewer regeneration amplifiers needed. Lower diameter/weight so easier to handle/cheaper.

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i Any value less than 10 m (and more than 1 mm) ii Any value between 10 and 100 m b The sky wave uses reflection by the ionosphere for transmission. The ionosphere fluctuates in its ability to reflect.

Orbits around the Earth’s equator. Takes one day for a complete orbit. Stays over one point on the Earth OR height of orbit 36 000 km above Earth’s surface. b First satellites used wavelengths of about 5 cm; typically now between 1 mm and 1 cm. c Advantage: permanent link with ground station/dishes do not have to be moved. Disadvantage: greater time delay for signal OR further away so signal weaker.

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10 a

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11 The public switched telephone network connects every telephone through exchanges. Without exchanges too many telephones and interconnecting wires are needed. One cable can handle many telephone conversations at once. Sampling places a series of digital bits from many telephone conversations on one cable.

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AS and A Level Physics

Original material © Cambridge University Press 2010

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