#2: Modulation Flashcards
What is ‘modulation’?
Superimposing a carrier and data signal.
What is a ‘carrier’ signal in the context of modulation?
A higher frequency signal that propagates (or transmits) well
What is a ‘modulating’ signal in the context of modulation?
A low frequency signal that you actually want to transmit (e.g. voice, music, even data)
What are the advantages of A.M., F.M. and D.M.?
A.M. = cheap electronics, simple F.M. = less susceptible to noise D.M. = A.M. + P.M. = faster bit rate
What does ‘superimposing’ mean from a mathematical perspective?
Multiplying two waves
Draw an example carrier, modulating, AM and FM signal
What is one way of de-modulating a modulated signal (also known as ‘Product De-modulation’)?
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.
Draw an example of what a non-de-modulated AM signal might look like
What is the mathematical equation for [amplitude] modulating a signal?
Assume modulating signal = m(t)
In words, how does one [amplitude] modulate a signal?
A.M. = (Modulation x Carrier) + (Some Carrier)
What is the ‘Modulation Index’? Explain in words and mathematically.
a ratio of the amplitudes of the carrier and modulating signals. The extent of modulation done on a signal.
Draw the result of a spectrum analysis on a sample AM waveform and explain.
Tallest line = carrier signal
Two shorter lines = sideband signals from modulating signal
What is the amplitude + frequency of: a) the modulating signal, and b) the carrier signal?
What is the total power (i.e. carrier + modulating signal) for an A.M. signal?
Say you wanted to transmit a 10kHz signal via amplitude modulation. What would the bandwidth be?
20kHz
As the frequency of a modulated signal increases, the bandwidth required ____.
Increases
What are some drawbacks of A.M.?
Requires a wide bandwidth (sidebands)
Large power loss via carrier (carrier doesn’t have any useful information)
What is ‘DSB-SC AM’? Explain, and draw a spectrum plot. What is one advantage of this method?
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.
What is ‘SSB’, and what are some advantages of this method? Draw a spectrum plot.
‘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.
What is VSB, and why is it used?
‘Vestige Sideband’.
Keeps a small portion of the second sideband. This makes it easier to recover the carrier frequency for demodulation purposes.
Explain Frequency Modulation (FM).
The frequency of a modulating signal is modulated with a constant amplitude to form the FM signal.
What are the pros and cons of FM vs. AM?
FM = far less susceptible to noise;
AM = much smaller bandwidth.
Frequency = rate of change of ____
phase
How would you prove that frequency is the rate of change of phase (draw)? Express this mathematically.
Illustrate the effect a following ‘input’ (i.e. modulating) waveform has on the FM output signal.
What is the equation of a modulated FM signal? Explain these terms.
wct = carrier
kf = a constant
m(r).dr = sum of modulating voltages
What does the constant ‘kf’ signify in this FM modulating equation?
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.
What is the modulation index (aka ‘Derivation Ratio’) of an FM signal with a basic sinusoidal input?
What’s another simpler way of expressing this modulation index equation?
How would one find the amplitude of the harmonics (i.e. frequency components) of an FM signal?
Look up the Bessel function table for that specific modulation index!
What important factor does the maximum date rate (‘capacity’) depend uipon? Express this mathematically.
Noise.
C = B log (1 + S/N) (bps)
where:
C = Capacity
B = Bandwidtch
S/N = Signal/Noise ratio
What is ‘QAM’ in terms of digital modulation?
Quadrature Amplitude Modulation
Gets a higher data rate (i.e. ‘throughput’) by encoding bits with amplitude and phase modulation.
What is Differential Encoding? Sketch the D.E. equivalent of the following signal:
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.
What is Manchester Encoding, and where is it used? Draw the M.E. equivalent encoding of the following signal.
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.
Why is there no ground wire in an ethernet cable?
A difference ground reference voltage at both ends would cause a current to flow and ruin the signal!
What is an ethernet ‘pre-amble’?
A string of bytes the receiver looks for to signify that the transmitter is about to transmit actual data.
Why doesn’t 100MBps ethernet use twisted pair?
Too much induced noise.
What is multi-level encoding, and why is it used? Sketch an example of MLT-3.
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).
What is a Tuned Radio Frequency receiver, and why is it not generally used in practise?
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.
Draw a block diagram for a Tuned Radio Frequency receiver.
Explain the concept of up/down conversion. Why is this necessary, and how does it relate to the Intermediate Frequency?
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.
What is ‘baseband’ and ‘passband’?
Baseband = low frequency modulating signal (e.g. 5kHz audio)
Passband = high frequency transmission carrier signal (e.g. 100kHz carrier)
What is a heterodyne receiver, and why is it better than a Tuned Radio Frequency receiver?
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.
Draw a block diagram of a Heterodyne receiver.
What is ‘mixing’, and how does it relate to filtering?
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.
Why use a heterodyne receiver concept instead of the simpler T.R.F. filter approach?
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.
What does ‘heterodyne’ mean (i.e. what is its general concept?)
‘mixing’ (or multiplying) two signals (i.e. local oscillator + modulating signal)
What is ‘Amplitude Shift Keying’ (ASK)? Draw a diagram.
a type of amplitude modulation that transmits digital data (e.g. 0 - 1) via an analog channel (expressed via A.M.)
What is ‘Frequency Shift Keying’ (FSK)? Draw a diagram.
a method of transmitting digital data (1s and 0s) via frequency modulation
What is ‘Quadrature Amplitude Modulation’ (QAM)? What does the ‘quadrature’ stand for? Draw a diagram.
a method of combining two AM signals and seperating them by 90 degrees (sort of like multiple electrical phases)
Quadrature = 90 degree phase shift
What is ‘Quadrature Phase Shift Keying’ (QPSK)? Draw a diagram.
a method of transmitting two bits at once by using several different phases.
Draw a block diagram of RF mixing.
What’s a beat frequency?
Multiplying two signals (superimposing). Another name for mixing. Makes sort of like a fluttering sound (average and difference frequencies).
