FM

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FM DEMODULATORS
Prof. Dr. G.Aarthi,
Associate Professor, SENSE

2. What is a Demodulator?
• Demodulation is the act of extracting the original
information-bearing signal (modulating signal) from a
modulated carrier wave.
• A demodulator is an electronic circuit used to
recover the information content from the modulated
carrier wave.
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3. What is FM Demodulator
• An electronic circuit in which frequency variations of
modulated signals are converted to amplitude
variations first, with the help of tuned circuit
• And then original information is extracted with the
AM demodulation techniques say diode detector.
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4. Types of FM Demodulators
FM
Demodulation
Indirect
Direct
• Slope Detector
• Balanced Slope Detector
• Foster-Seeley Phase Discriminator
• Ratio Detector
Phase Lock Loop(PLL)
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5. Basic FM Demodulator
TUNED
CIRUIT
Frequency
Variations
Amplitude
Variations
NOTE: Amplitude Variations are added to wave
according to frequency variations, and frequency
variations remain present in wave.
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6. FM
Wave
Output
of Tuned
Circuit
Basic FM Demodulator
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7. Basic FM Demodulator
• The most basic circuit employed as FM demodulator
is parallel tuned LC circuit, often known as slope
detector.
• The carrier frequency should fall on one side of
resonant frequency and
• The entire frequencies should fall on linear region of
transfer curve of tuned circuit.
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8. Slope Detector
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FM
Source
Tank Circuit Envelope Detector Circuit
Voltage versus Frequency Curve
Detector
Output

9. Transfer
Curve
Output
Slope Detector
Transfer
Characteristics
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10. Transfer
Curve
Output
Slope Detector Transfer
Characteristics
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11. SLOPE Detector
• The output is then applied to a diode detector with
RC load of suitable time constant.
• The circuit is, in fact, identical to that of AM detector.
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12. Advantages of Slope Detector
• Simple - can be used to provide FM demodulation
when only an AM detector is present.
• Enables FM to be detected without any additional
circuitry.
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13. Limitations of Slope Detector
• It is inefficient, as it is linear in very limited frequency
range.
• It reacts to all amplitude changes.
• It is relatively difficult to tune, as tuned circuit must
be tuned to different frequency than carrier
frequency.
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14. Balanced Slope Detector
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15. Balanced Slope Detector
• This circuit uses two slope detectors, connected in
back to back fashion, to opposite ends of center-
tapped transformer.
• And hence fed 1800 out of phase.
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16. • The top secondary circuit is tuned above the IF by an amount
f, and bottom circuit is tuned below IF by f.
• Each circuit is connected to diode detectors with suitable RC
loads.
• The output is taken across series combination of loads, so that
it is sum of the individual outputs. 16

17. Balanced Slope Detector
Output of T’ at fc+f
(+10 to +15V)
Output of T’ at fc-f
(+5 to +10V)
Output of T’’ at fc-f
(-10 to -15V)
Output of T’’ at fc+f
(-5 to -10V)
5V
10V
15V
-5V
-10V
-15V
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18. Balanced Slope Detector
• When input frequency = fc
– Then output of T’(+Ve)= output of T’’ (-Ve)
– So sum of outputs of T’ and T’’ = Zero
• When input frequency = fc+f
– Then output of T’(+Ve) > output of T’’ (-Ve)
– So sum of outputs of T’ and T’’ = +Ve
• When input frequency = fc-f
– Then output of T’(+Ve) < output of T’’ (-Ve)
– So sum of outputs of T’ and T’’ = -Ve
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19. Combined Transfer Curve
Useful Range
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20. Balance Slope Detector- Advantages
• This circuit is more efficient than simple slope
detector.
• It has better linearity than the simple slope detector
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21. Balance Slope Detector- Drawbacks
• Even more difficult to tune, as there are three
different frequencies to be tuned.
• Amplitude limiting still not provided.
• Linearity, although better than single slope detector,
is still not good enough.
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