SNS COLLEGE OF TECHNOLOGY An autonomous institution Coimbatore – 35 Department of Electronics and Communication Engineering UNIT III DOUBLE TUNED DEMODULATOR 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

FM Modulator and Demodulator Direct FM Indirect FM FM demodulator Direct: use frequency discriminator (frequency-voltage converter) Ratio detector Zero crossing detector Indirect: using PLL Super heterodyne receiver FM broadcasting and Satellite radio 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R Review 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R FM Direct Modulator Direct FM Carrier frequency is directly varied by the message through voltage-controlled oscillator (VCO) VCO: output frequency changes linearly with input voltage A simple VCO: implemented by variable capacitor Capacitor Microphone FM generator 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R FM Direct Modulator Direct method is simple, low cost, but lack of high stability & accuracy, low power application, unstable at the carrier frequency Modern VCOs are usually implemented as PLL IC Why VCO generates FM signal? 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R Indirect FM Generate NBFM first, then NBFM is frequency multiplied for targeted Δf. Good for the requirement of stable carrier frequency Commercial-level FM broadcasting equipment all use indirect FM A typical indirect FM implementation: Armstrong FM 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R Indirect FM Generate NBFM signal with a very small β1 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R Indirect FM Then, apply frequency multiplier to magnify β Instantaneous frequency is multiplied by n So do carrier frequency, Δf, and β What about bandwidth? 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

Analysis of Indirect FM 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R FM Demodulator Four primary methods Differentiator with envelope detector/Slope detector FM to AM conversion Phase-shift discriminator/Ratio detector Approximates the differentiator Zero-crossing detector Frequency feedback Phase lock loops (PLL) 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R FM Slope Demodulator Principle: use slope detector (slope circuit) as frequency discriminator, which implements frequency to voltage conversion (FVC) Slope circuit: output voltage is proportional to the input frequency. Example: filters, differentiator 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R FM Slope Demodulator Block diagram of direct method (slope detector = slope circuit + envelope detector) 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R Bandpass Limiter A device that imposes hard limiting on a signal and contains a filter that suppresses the unwanted products (harmonics) of the limiting process. Input Signal Output of bandpass limiter Bandpass filter Remove the amplitude variations 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R Phase Discriminator Foster-Seeley/phase shift discriminator uses a double-tuned transformer to convert the instantaneous frequency variations of the FM input signal to instantaneous amplitude variations. These amplitude variations are rectified to provide a DC output voltage which varies in amplitude and polarity with the input signal frequency. Ratio detector Modified Foster-Seeley discriminator, not response to AM, but 50% 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R FM Demodulator PLL Phase-locked loop (PLL) A closed-loop feedback control circuit, make a signal in fixed phase (and frequency) relation to a reference signal Track frequency (or phase) variation of inputs Or, change frequency (or phase) according to inputs PLL can be used for both FM modulator and demodulator Just as Balanced Modulator IC can be used for most amplitude modulations and demodulations 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R FM Broadcasting The frequency of an FM broadcast station is usually an exact multiple of 100 kHz from 87.5 to 108.5 MHz . In most of the Americas and Caribbean only odd multiples are used. fm=15KHz, f=75KHz, =5, B=2(fm+f)=180kHz Pre-emphasis and de-emphasis Random noise has a 'triangular' spectral distribution in an FM system, with the effect that noise occurs predominantly at the highest frequencies within the baseband. This can be offset, to a limited extent, by boosting the high frequencies before transmission and reducing them by a corresponding amount in the receiver. Block diagram and spectrum Relation of stereo transmission and monophonic transmission 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

FM Stereo Multiplexing (a) Multiplexer in transmitter of FM stereo. (b) Demultiplexer in receiver of FM stereo. 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R TV FM broadcasting fm=15KHz, f=25KHz, =5/3, B=2(fm+f)=80kHz Center fc+4.5MHz Eye cells structure 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R Frequency Allocation 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R THANK YOU 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R