This document discusses two methods of generating frequency modulated (FM) signals: direct and indirect. The direct method modulates the carrier frequency directly using reactance modulators or varactor diode modulators. This allows for simplicity but lacks stability. The indirect or Armstrong method first phase modulates the signal, then uses a nonlinear circuit and mixer to change the carrier frequency. This provides greater stability and flexibility with higher frequency deviations compared to the direct method.

1. Generation of FM Signals
By:-
• Dharit Unadkat - 130870111039
• Jenish Thumar -130870111038
• Shivam Rai -130870111029
Guided By:-
• Prof.Bharat Tank
• Prof.Ketan Goswami
Class:-
EC-5th Sem,
Parul Institute Of Technology,
Limda.

2. There are two basic methods of
generating frequency Modulated signals
•Direct Method
•Indirect Method
Methods of FM generation

3. Classification of FM generation
method
DIRECT
METHOD
REACTANCE
MODULATOR
INDIRECT
METHOD
ARMSTRONG
METHODVARACTOR
MODULATOR
Methods of
FM
generation

4. Direct method
• In direct FM generation method, the instantaneous
frequency of the carrier is changed directly in proportion
with the message signal.
• For direct generation of FM signal Reactance modulator
and Varactor diode Modulator used.

5. FM
wave
Reactance modulator
• In the reactance modulator, a transistor or FET is
operated as a variable reactance(inductive or capacitive)
• If L or C of a tuned circuit of an oscillator is changed in
proportion with the amplitude of modulating signal then
FM can be obtained across the tuned circuit as shown
below.

6. Varactor diode modulator
• A varactor diode is a semiconductor diode whose
junction capacitance varies linearly with the voltage
applied across it. The varactor diode must be reverse
biased.

7. Advantages of direct method
• Simplicity of the modulator circuit
• Low cost
Disadvantages of direct method
• This type of modulator is not stable enough. It is a
function of various parameters such as temperature,
device ageing etc.

8. Indirect method
• This method of FM generation is also known as
Armstrong method of FM generation.
• In this method the FM is obtained through phase
modulation.
• In this method crystal oscillator is used for frequency
stability.

9. Armstrong modulator

10. Summary
•Direct FM generation:
• The carrier freq is directly varied by the input signal
• Frequency drifting is a problem
• freq deviation < 5KHz
•Indirect FM generation:
• Use nonlinear circuit to get multiplier
• Can use mixer to change the carrier frequency
• Combination of mixer and multiplier provides flexibilities.

11. THANK YOU