2. Types of AM modulator
โข Amplitude modulators - AM wave generating circuits
Amplitude
Modulators
Nonlinear
Square law
Balanced
Linear
Low level
Product
Transistor
โข Based on power level
3. Low level modulators
โข Generation takes place at low power level
โข Modulation takes place prior to the output element
โข Linear amplifiers are preferred
โข Advantages:
โข Less modulating power is required
โข Disadvantages:
โข Extremely inefficient due to the usage of linear amplifiers
4. High level modulators
โข Generation takes place at higher power level
โข Modulation takes place at the output element
โข Class C amplifiers are preferred
โข Advantages:
โข Extremely efficient due to the usage of Class C amplifiers
โข Disadvantages:
โข High modulating power is required
5. Linear modulator
โข The Devices are operated in the linear region of its
transfer characteristics.
โข The relation between the amplitude of the modulating
signal and the resulting depth of modulation in linear.
โข Switching modulator
โข Transistor modulator
6. Nonlinear modulator
โข Devices used in this modulation are operated in the
non linear region of its characteristics.
โข Uses non linear property of diode, BJT and FET
โข Simple diode can be used as a nonlinear modulator
โข Undesired frequency terms are filtered using BPF
โข Square law modulator
โข Balanced modulator
โข Product modulator
7. Square law modulator
Let the modulating and carrier signals be denoted as Vm (t) and Vc (t) respectively.
These two signals are applied as inputs to the summer (adder) block. This
summer block produces an output, which is the addition of the modulating and
the carrier signal.
8. Square law modulator
โข Semiconductor diodes and transistors are commonly used
โข Filtering done using single or double tuned filter (BPF)
9. Square law modulator
โข The input to the nonlinear device is given as
V1(t) ๏ฝVm (t) ๏ซVc (t) ๏ญ ๏ญ ๏ญ ๏ญ ๏ญ (1)
V1(t) ๏ฝVm cos๏ทmt ๏ซVc cos๏ทct ๏ญ ๏ญ ๏ญ ๏ญ ๏ญ (2)
โข The input-output relation for a nonlinear device is given as
10. Square law modulator
โข The LC circuit acts as a bandpass filter and is tuned to the
frequency of fc and its bandwidth is equal to 2fm
โข Therefore it allows ฯc, ฯc+ฯm, ฯc-ฯm
11. Square law modulator
โข The filter circuit is tuned to the frequency of fc and its bandwidth is
equal to 2fm
โข Therefore it allows ฯc, ฯc+ฯm, ฯc-ฯm
โข The output of the BPF is given as
V0 (t) ๏ฝ aVc cos๏ทct ๏ซ bVmVc[cos(๏ทc๏ซ๏ทm)t ๏ซ cos(๏ทc๏ญ๏ทm)t]๏ญ ๏ญ ๏ญ ๏ญ ๏ญ (6)
12. Non linear Modulators-Drawbacks
โข Heavy filtering is required to remove the unwanted terms present
in the output of the modulators.
โข The output power level is also very low.
โข Hence a substantial linear amplification is necessary to bring the
power upto the desired level.
14. Switching modulator
โข Simple diode can be used as an AM switching modulator
โข Consider the diode is ideal and carrier is stronger
message signal
than the
โข Diode is forward biased โ Positive half cycle of the carrier [c(t)>0]
โข Diode is reverse biased โ Negative half cycle of the carrier [c(t)<0]
15. Switching modulator
โข The transfer characteristic of the diode is approximated by a
piecewise linear time varying relationship
โข The input voltage is given as
V1(t) ๏ฝVc cos๏ทct ๏ซ m(t) ๏ญ ๏ญ ๏ญ ๏ญ ๏ญ (1)
16. Switching modulator
0;c(t) ๏ผ 0
2
๏ท ๏ญ ๏ญ ๏ญ ๏ญ ๏ญ (2)
๏ธ
๏จ
โข The transfer characteristic of the diode is approximated by a
piecewise linear time varying relationship
โข The input voltage is given as
V1(t) ๏ฝVc cos๏ทct ๏ซ m(t) ๏ญ ๏ญ ๏ญ ๏ญ ๏ญ (1)
โข The resulting voltage is given as
๏ฆV1(t);c(t) ๏พ 0๏ถ
V (t) ๏ฝ๏ง
17. Switching modulator
0;c(t) ๏ผ 0
2
๏ท ๏ญ ๏ญ ๏ญ ๏ญ ๏ญ (2)
๏ธ
๏จ
โข The transfer characteristic of the diode is approximated by a
piecewise linear time varying relationship
โข The input voltage is given as
V1(t) ๏ฝVc cos๏ทct ๏ซ m(t) ๏ญ ๏ญ ๏ญ ๏ญ ๏ญ (1)
โข The resulting voltage is given as
๏ฆV1(t);c(t) ๏พ 0๏ถ
V (t) ๏ฝ๏ง
โข Mathematically, Equation (2) is expressed as
V2 (t) ๏ฝ [Vc cos๏ทct ๏ซ m(t)]gp (t) ๏ญ ๏ญ ๏ญ ๏ญ ๏ญ (3)
โข Where gp(t) is a periodic pulse train
18. Switching modulator
โข Representing gp(t) by its Fourier series we have
โข Sub Eq.(5) in Eq.(3), the filter output equation is given as
โข The unwanted components are removed by the BPF with the mid-
band frequency fc and bandwidth 2W
cos[2
๏ซ ๏ฅ
๏ฅ
n๏ฝ1 2n๏ญ1
1 2 (๏ญ1)n๏ญ1
2 ๏ฐ c
๏ฐf t(2n ๏ญ1)]๏ญ ๏ญ ๏ญ ๏ญ ๏ญ (4)
p
g (t) ๏ฝ
2
V 4
0
V (t) ๏ฝ
c
c
c
๏ฐV
cos๏ท t[1๏ซ m(t)]๏ญ ๏ญ ๏ญ ๏ญ ๏ญ (6)
c
p c
g (t) ๏ฝ
1
๏ซ
2
cos๏ท t ๏ญ
2
cos3๏ท t ๏ซ....๏ญ ๏ญ ๏ญ ๏ญ(5)
2 ๏ฐ 3๏ฐ
๏ฐVc
Amplitude Sensitivity k ๏ฝ
4