modulación analógica (am-fm) cx eléctricas 09 – e.tapia

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