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) K a 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. f c >>>> 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 /f m

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 N o /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 SNR C, 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 n I (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 k f 2 P

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 >> n I, n Q Ac << n I, n Q  P 1 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 B T Given B T and N 0 (Noise power per unit bandwidth)  Determine A C 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)

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