1. 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

2. 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

3. 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R
Review
16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

4. 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

5. 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

6. 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

7. 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

8. 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

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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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 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

17. 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

18. 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

19. 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R
Frequency Allocation
16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R

20. 16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R
THANK YOU
16EC304 - Communication Theory/UNIT III/ SATHISH KUMAR R