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ELE2601 > #2: Modulation > Flashcards

#2: Modulation Flashcards

1
Q

What is ‘modulation’?

A

Superimposing a carrier and data signal.

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2
Q

What is a ‘carrier’ signal in the context of modulation?

A

A higher frequency signal that propagates (or transmits) well

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3
Q

What is a ‘modulating’ signal in the context of modulation?

A

A low frequency signal that you actually want to transmit (e.g. voice, music, even data)

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4
Q

What are the advantages of A.M., F.M. and D.M.?

A

A.M. = cheap electronics, simple F.M. = less susceptible to noise D.M. = A.M. + P.M. = faster bit rate

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5
Q

What does ‘superimposing’ mean from a mathematical perspective?

A

Multiplying two waves

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6
Q

Draw an example carrier, modulating, AM and FM signal

A
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7
Q

What is one way of de-modulating a modulated signal (also known as ‘Product De-modulation’)?

A

The product detector multiplies the incoming signal by the signal of a local oscillator with the same frequency and phase as the carrier of the incoming signal. After filtering, the original audio signal will result.

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8
Q

Draw an example of what a non-de-modulated AM signal might look like

A
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9
Q

What is the mathematical equation for [amplitude] modulating a signal?
Assume modulating signal = m(t)

A
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10
Q

In words, how does one [amplitude] modulate a signal?

A

A.M. = (Modulation x Carrier) + (Some Carrier)

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11
Q

What is the ‘Modulation Index’? Explain in words and mathematically.

A

a ratio of the amplitudes of the carrier and modulating signals. The extent of modulation done on a signal.

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12
Q

Draw the result of a spectrum analysis on a sample AM waveform and explain.

A

Tallest line = carrier signal

Two shorter lines = sideband signals from modulating signal

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13
Q

What is the amplitude + frequency of: a) the modulating signal, and b) the carrier signal?

A
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14
Q

What is the total power (i.e. carrier + modulating signal) for an A.M. signal?

A
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15
Q

Say you wanted to transmit a 10kHz signal via amplitude modulation. What would the bandwidth be?

A

20kHz

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16
Q

As the frequency of a modulated signal increases, the bandwidth required ____.

A

Increases

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17
Q

What are some drawbacks of A.M.?

A

Requires a wide bandwidth (sidebands)

Large power loss via carrier (carrier doesn’t have any useful information)

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18
Q

What is ‘DSB-SC AM’? Explain, and draw a spectrum plot. What is one advantage of this method?

A

Double-Sideband-Supressed-Carrier.

A mouthful that basically means there’s no actual amplitude of the carrier signal present. This means it’s much more electrically efficient.

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19
Q

What is ‘SSB’, and what are some advantages of this method? Draw a spectrum plot.

A

‘Single Side-band Modulation’.

There is only one side band present (no carrier, or upper/lower sideband). This means it takes up less bandwidth, and uses less power.

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20
Q

What is VSB, and why is it used?

A

‘Vestige Sideband’.

Keeps a small portion of the second sideband. This makes it easier to recover the carrier frequency for demodulation purposes.

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21
Q

Explain Frequency Modulation (FM).

A

The frequency of a modulating signal is modulated with a constant amplitude to form the FM signal.

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22
Q

What are the pros and cons of FM vs. AM?

A

FM = far less susceptible to noise;

AM = much smaller bandwidth.

23
Q

Frequency = rate of change of ____

A

phase

24
Q

How would you prove that frequency is the rate of change of phase (draw)? Express this mathematically.

A
25
Q

Illustrate the effect a following ‘input’ (i.e. modulating) waveform has on the FM output signal.

A
26
Q

What is the equation of a modulated FM signal? Explain these terms.

A

wct = carrier

kf = a constant

m(r).dr = sum of modulating voltages

27
Q

What does the constant ‘kf’ signify in this FM modulating equation?

A

A constant multipler that, when multiplied by the modulating input signal m(t) at some instant (t), determines the exacct frequency of modulation at that point.

28
Q

What is the modulation index (aka ‘Derivation Ratio’) of an FM signal with a basic sinusoidal input?

A
29
Q

What’s another simpler way of expressing this modulation index equation?

A
30
Q

How would one find the amplitude of the harmonics (i.e. frequency components) of an FM signal?

A

Look up the Bessel function table for that specific modulation index!

31
Q

What important factor does the maximum date rate (‘capacity’) depend uipon? Express this mathematically.

A

Noise.

C = B log (1 + S/N) (bps)

where:
C = Capacity

B = Bandwidtch

S/N = Signal/Noise ratio

32
Q

What is ‘QAM’ in terms of digital modulation?

A

Quadrature Amplitude Modulation

Gets a higher data rate (i.e. ‘throughput’) by encoding bits with amplitude and phase modulation.

33
Q

What is Differential Encoding? Sketch the D.E. equivalent of the following signal:

A

Instead of transmitting, say, a logic ‘1’ as +5V, differential encoding might transmit a +5V and a negative 5V. This prevents noise from affecting transmission.

34
Q

What is Manchester Encoding, and where is it used? Draw the M.E. equivalent encoding of the following signal.

A

Instead of looking for a certain voltage level for a logic ‘1’ (e.g. +5V), M.E. looks for a transition (e.g. high-low). It is used in Ethernet LANs.

35
Q

Why is there no ground wire in an ethernet cable?

A

A difference ground reference voltage at both ends would cause a current to flow and ruin the signal!

36
Q

What is an ethernet ‘pre-amble’?

A

A string of bytes the receiver looks for to signify that the transmitter is about to transmit actual data.

37
Q

Why doesn’t 100MBps ethernet use twisted pair?

A

Too much induced noise.

38
Q

What is multi-level encoding, and why is it used? Sketch an example of MLT-3.

A

As per Nquists’ sampling theory, the more voltage levels used, the greater the data throughput rate. MLT-3 uses +V, 0V, and -V (three voltage levels).

39
Q

What is a Tuned Radio Frequency receiver, and why is it not generally used in practise?

A

Simply a band-pass filter that can be adjusted to allow only a certain frequency through, and amplifies this frequency.

Difficult + expensive to build high-Q bandpass filters and such high frequencies.

40
Q

Draw a block diagram for a Tuned Radio Frequency receiver.

A
41
Q

Explain the concept of up/down conversion. Why is this necessary, and how does it relate to the Intermediate Frequency?

A

Expensive to build all electronics to deal with high frequencies all the way through. Solution; use an intermediate frequency as soon as possible once receiving signal. Must up/down convert to get to this Intermediate Frequency.

42
Q

What is ‘baseband’ and ‘passband’?

A

Baseband = low frequency modulating signal (e.g. 5kHz audio)

Passband = high frequency transmission carrier signal (e.g. 100kHz carrier)

43
Q

What is a heterodyne receiver, and why is it better than a Tuned Radio Frequency receiver?

A

It’s a method of demodulating a received signal.

Instead of needing a high-precision, high-frequency filter, a Heterodyne receiver just filters the spectrum very broadly (into the AM range), and multiplies this input signal with a local oscillator signal to demodulate the signal.

44
Q

Draw a block diagram of a Heterodyne receiver.

A
45
Q

What is ‘mixing’, and how does it relate to filtering?

A

Mixing = the multiplication of two sine waves. This creates a sum and difference frequency; the high one is filtered out, leaving just the low one (i.e. the demodulated signal) to come out.

46
Q

Why use a heterodyne receiver concept instead of the simpler T.R.F. filter approach?

A

In short: it’s easier to generate a frequency (via local oscillator) than it is to constantly re-calibrate a bandpass filter to be accurate.

47
Q

What does ‘heterodyne’ mean (i.e. what is its general concept?)

A

‘mixing’ (or multiplying) two signals (i.e. local oscillator + modulating signal)

48
Q

What is ‘Amplitude Shift Keying’ (ASK)? Draw a diagram.

A

a type of amplitude modulation that transmits digital data (e.g. 0 - 1) via an analog channel (expressed via A.M.)

49
Q

What is ‘Frequency Shift Keying’ (FSK)? Draw a diagram.

A

a method of transmitting digital data (1s and 0s) via frequency modulation

50
Q

What is ‘Quadrature Amplitude Modulation’ (QAM)? What does the ‘quadrature’ stand for? Draw a diagram.

A

a method of combining two AM signals and seperating them by 90 degrees (sort of like multiple electrical phases)

Quadrature = 90 degree phase shift

51
Q

What is ‘Quadrature Phase Shift Keying’ (QPSK)? Draw a diagram.

A

a method of transmitting two bits at once by using several different phases.

52
Q

Draw a block diagram of RF mixing.

A
53
Q

What’s a beat frequency?

A

Multiplying two signals (superimposing). Another name for mixing. Makes sort of like a fluttering sound (average and difference frequencies).

54
Q
A

ELE2601 (7 decks)

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