Lecture 22 Threshold effects in FM.pptx

1. AM Signal Generation – Type 1
Lecture-5

2. AM Signal Generations

3. DSB-FC

4. Square law diode modulation (DSBFC)
• Used for generating DSBFC signal : Circuit Diagram:

5. Square law diode modulation (DSBFC)
• A Square-law modulator requires
three features: a means of summing
the carrier and modulating waves, a
nonlinear element, and a band pass
filter for extracting the desired
modulation products.
• Semi-conductor diodes and
transistors are the most common
nonlinear devices used for
implementing square law
modulators.

6. Square law diode modulation (DSBFC)
Derivation:
• Diode is a non-linear device which satisfies power law.
Power law :
• But power law is practically non-feasible due to the infinite
series and hence limited to the first 3 terms which is called as
square law:
• Using this law, AM signal can be generated
• Let
• Substitute in square law equation and find the output ‘i’
keeping a0, a1 and a2 as constant.
..
..........
3
1
3
2
1
2
1
1
0
e
a
e
a
e
a
a
i 



2
1
2
1
1
0
e
a
e
a
a
i 


t
E
t
E
e m
m
c
c 
 sin
sin
1 


7. Square law diode modulation (DSBFC)
Derivation:
• After expansion, the equation becomes:
• Now, using the tuned circuit, select only the frequencies
which are available in AM signal. Thus the output becomes,
   
 
t
t
E
E
a
t
E
a
i m
c
m
c
m
c
c
c




 



 cos
cos
sin 2
1

8. SQUARE LAW MODULATOR cont…
• SPECTRAL RELATIONSHIP IN A SQUARELAW MODULATOR
• SPECTRUM OF BASEBAND SIGNAL
• SPECTRUM OF V2(t)

9. Need for suppression of carriers
• Carriers are suppressed to save the power in AM signal
Power saving formula:
• If the carrier alone is suppressed (DSBSC), the power
saving is 66.66%
• If the carrier and one of the sidebands (SSBSC) is
suppressed, then the power saving is 83.33%
Types of Suppressed carrier systems:
1) DSBSC
2) SSBSC

10. Qs