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Modulación Analógica (AM-FM)
Cx Eléctricas 09 – E.Tapia
Modulación de Onda CC (CW)
Representación en dominios t-f Efectos del ruido en los receptores
correspondientes
Modulation -Demodulation
Ix transmission in presence of noise Ix bearing signals or baseband signals Transmitter-Channel-Receiver Frequency shifting on Tx – Modulation using a carrier Frequency shift back on Rx –Demodulation
Modulation
Carrier is sinusoidal wave
Amplitude, frequency, or
phase are varied with a
modulating wave - signal
Amplitude Modulation
Message signal m(t) and
carrier c(t) are independent
Carrier amplitude is varied
about a mean value (Ac),
linearly with m(t)
Ka is the modulation sensiviy
measured in 1/volt
Some issues on AM
Overmodulation
Leads to envelope distortion. The demodulator will
track a false envelope and information will be lost.
fc >>>> W – the message bandwidth
Easy envelope visualization and tracking
Frequency Domain
Note that
Mod-Demod are implemented using non-linear devices
Demod are often envelope detectors
AM Power and AM Bandwith
Not efficient at power use (tx of c(t))
Sidebands are related each other >>>> just one is needed
Hence >>>> avoid c(t) transmission and duplicate sidebands
Linear Modulation
DSB-SC- (Double SideBand-Supressed Carrier)
Coherent Detection
Note that
Non coherent detection may
lead to null quadrature effect
Need coherent local oscillator
at demodulation >>
complexity >> the price
SSB MOdulation
DSB-SC + Filtering for Sideband Removal
Highly selective filters from cristal oscillators
Coherent detector >> low power pilot carrier addition is added at
transmission
VSB – Vestigial Sideband Modulation
More on VSB
Frequency Modulation (FM)
f is the frequency deviation
is the modulation index defined as f /fm
Which is the FM angle?
<< 1 radian is known as narrowband FM
>> 1 radian is known as wideband FM
Noise in CW Modulation
Chanel Model is AWGN
Power spectral density is No/2
Receiver model defined by a
bandpass filter and a
demodulator model
SNRs
SNR I (Input)
Ratio of the average power of the modulated
signal s(t) to the average power of the filtered
noise
SNR o (Output)
Ratio of the averaged power of the
demodulated signal to the power of noise
measured at the receiver output
SNR c (Channel)
Ratio of the averaged power of the modulated
signal to the average power of noise in the
message bandwith both at the receiver input
Noise in DSB Coherent Detection
s(t) is the DSB component of
x(t)
C is system dependent scaling
factor
m(t) sample from stationary
process of zero mean and S(f)
Hence compute SNRC, DSB
Figure of Merit in Coherent Detection
The quadrature component of noise is rejected in coherent detection
The average power of filtered noise n(t) is
Same for nI(t)
Figure ….
The same holds for SSB
NO way to improve SNR
by increasig bandwith use
in DSB w.r.t SSB
The effect of modulation
is just frequenxy shifting
Noise in AM
From the SNR at the channel (C, AM) we desire the SNR at the output , demodulator – envelope
Phasorial Analysis
Figures of Merit
Always << 1for AM envelope
receivers
Equal to 1 for DSB, SSB
Caused by waste of power on
carrir transmission
Existence of threshold effect
Threshold effect in AM Detectors
Noise Effects in FM
Limiter: clipp and round so that amplitude is independent of the carrier amplitude at the receiver input.
Noise Model for FM
R(t) is Rayleigh Phase is uniform
Signal Model for FM
Signal and Noise in FM
Discriminator Output
Provided the carrier to noise is high
FM Discriminator: S2N
Cont’
The carrier power has noise quoting effect in FM Recall that
The average signal transmitted power is kf2P
How can we improve S2N in FM?
The conclusion
FM provides a mechanism for the exchange of improved noise performance
by increased transmission bandwidth
FM can also reject other FM signals closed to the carrier frequency provided
interferent signal are weaker w.r.t. the target FM input
Threshold Effect in FM
Assumption Carrier to Noise ratio at the discriminator input >> 1
Violation to this assumption FM receiver breaks. From breaks to sputtering sounds. The
formula does not hold.
No signal but Noise
Ac >> nI , nQ
Ac << nI , nQ
P1 noves to the origin and random phase is observed
is around
Alternatevely
Clicks are heard after the low pass filter
Threshold Effect
As is decreased the
rate of clicks grows
Rate of clicks is high
threshold occurs
Designing an FM System
Given D ()
Compute BT
Given BT and N0 (Noise
power per unit bandwidth)
Determine AC to keep
above the threshold
FM Threshold Reduction
FM demodulator with negative feeback (FMFB) or PLL
FM Threshold Reduction
The VCO output
The phase comparator output
FM Threshold Reduction (cont)
FM Threshold Reduction (cont)
FM Threshold Reduction (cont)
Linear Model of the PLL-FM Demodulator
PreEmphasis - Deemphasis
Pre at transmitter
De- at the receiver
Pre-emphasis & De-emphasis
Pre at transmitter
De- at the receiver
Conclusions