Direct Method: when the frequency of carrier is

modulated by information signal

Indirect Method: when the phase of carrier is

modulated by information signal

FM GENERATION

1

Generation of FM waves

1.Varactor diode modulator( Direct method)

2.Reactance modulator( Direct method)

3.Crosby FM direct transmitter

4.Armstrong method (Indirect method)

2

- more capacitance.

- less capacitance.

dc /

3

Varactor Diode FM modulatordc /

4

CIRCUIT DIAGRAM OF VARACTOR DIODE MODULATOR

5

. 

                                                                           

FET REACTANCE MODULATOR

6

7

Modulating signal

Career

Frequency modulation

Phase modulation

8

INDIRECT FM GENERATION

Integrator

9

10

+ =

11

FREQUENCY UP-CONVERSION

With FM and PM modulators, the carrier at the output is

generally somewhat lower than the desired frequency of

transmission.

There are two basic methods of performing frequency up

conversion.

1. Heterodyning process

2. Frequency multiplication12

HETERODYNE METHOD OF FREQUENCY UP-CONVERSION

13

MULTIPLICATION METHOD OF UP-CONVERSION

14

BLOCK DIGRAM OF CROSBY DIRECT FM TRANSMITTER

15

Block Diagram of FM Receiver

16