Modulation Flashcards
What is channelisation?
This is placing intended signals in different bands to prevent interference
What are some important considerations when deciding a frequency to transmit data over?
> Data transmission rate is limited by signal frequency. Too low of a frequency will not allow a high enough data rate. Too high of a frequency will not travel far enough
> Antenna size is dependent on the wavelength of the signal
What is the purpose of modulation?
To transmit more data at lower frequencies e.g. voice signal with a 4kHz bandwidth
What are the two types of analogue modulation?
> Amplitude modulation
> Frequency modulation
How does amplitude modulation work?
A carrier signals amplitude varies according to the amplitude of the input signal. Because the carrier wave is at a higher frequency it can transmit the lower frequency input signal and retain the clarity
How does frequency modulation?
The frequency of the carrier signal is changed depending on the frequency of the input signal. The width of frequencies used is the same when translated to a higher frequency
How does the receiver decode the modulated signal?
The receiver is fed in the carrier frequency and then this can be removed from received signal
What is the mathematical equation for analogue modulation of a signal for transmission?
Amplitude = cos(2πt) × cos(2πqt)
To increase the frequency q is a multiplier of the input signal.
To vary the amplitude with the input signal the whole thing is multiplied by the original signal ( cos(2πt) ). [IMAGE 15]
What happens to the signal when you modulate it with the the equation for analogue modulation?
The signal gets split due to the double angle identities.
cos(a)cos(b) = 0.5( cos(a+b) + cos(a-b) )
[IMAGE 16]
How does demodulation work mathematically?
The incoming signal is multiplied by the carrier signal. This results in the original signal and higher frequency sinusoids. [IMAGE 17]
How are the higher frequency sinusoids removed?
Using filter circuits
What does a lowpass filter do?
Filters out higher frequencies and allows lower frequencies to pass through
What does a highpass filter do?
filters out lower frequencies and allows higher frequencies to pass through
What is a bandpass filter?
This only allows a specific range of frequencies to pass through. This can isolate a certain range of frequencies
What are the 4 different methods of digital modulation?
ASK = Amplitude shift keying PSK = Phase shift keying FSK = Frequency shift keying PAM = Pulse amplitude modulation [IMAGE 18]
What is the equation for digital modulation that combines PSK,FSK,PAM?
x(t) = Asin(2πft + ϕ) + B
Which part of the equation for digital modulation displays PSK?
x(t) = Asin(2πft + ϕ) + B When a binary value a 1 then the phase is shifted by ϕ. ϕ = binary value × phase shift
Which part of the equation for digital modulation displays FSK?
x(t) = Asin(2πft + ϕ) + B When a binary value is a 1 then the frequency (f) part of the equation changes. f = carrier frequency + binary value × frequency change
Which part of the equation for digital modulation displays PAM?
x(t) = Asin(2πft + ϕ) + B When a binary value is a 1 then the amplitude (A) part of the equation changes. A = A0 + binary value × Amplitude change
What are the advantages of using analogue modulation (2)?
> Less bandwidth > More accurate representation of wave
What are the disadvantages of using analogue modulation (6)?
> Low noise immunity > No signal conditioning and processing is used > Low quality of service > You can only use frequency division multiplexing rather than time devision or other > You can only transmit analogue signals > More difficult to design than digital signals
What are the advantages of using digital modulation (7)?
> High noise immunity > High levels of security > Support complex signal conditioning and processing techniques > High QOS > Lots of different modulation techniques can be used such at frequency division, time division and other > Supports digital signal transmission > Easily designed using software
What are the disadvantages of digital modulation?
> Large bandwidth > Less accurate signal due to quantization
What is the block diagram of digital voice communication on the transmitter side?
Input signal 1. Low pass filter 2. Analogue to digital converter 3. Source coding 4. Channel coding 5. Symbol mapping 6. Pulse shaping 7. Frequency up-conversion Output signal
