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x FM t AC t AC fct f x d 6-
If x ( ) Am cos[ 2 fm , then
( ) cos[ 2 sin( 2 f t )] f
f A x t A ft m m
m FM ^ c c
Ac cos[ 2 fct sin( 2 fmt )]
Where
( t )= instantaneous modulating frequency
f (^) c carrier frequency f (^) m modulating frequency
= modulation index = ( ) m
m f
f A
The frequency of FM signal can be expressed as
[ 2 sin( 2 )] 2
ft f t dt
d f c m
f c fm sin( 2 fmt ) 6-
From Eq. 6-3 we can find that the frequency of frequency modulated signal occurs frequency deviation from the centre frequency of the carrier when the intelligence amplitude is variation.
2.2 Frequency Modulation Based of LM 566 VCO
The circuit of Fig 2.1 is a frequency modulator based on voltage controlled oscillator (VCO) IC, LM 565. If the switch SW 1 is open, this circuit is a typical VCO whose output frequency is determined by the values of C 3 and VR 1 , and the audio input voltage. If the values of C 3 and VR 1 are fixed, the output frequency is directly proportional to the voltage difference between pins 8 and 7. (V 8 -V 7 ).
TRES
Fig 2.
If the SW 1 is closed, the voltage divider constructed by R 1 and R 2 provides a DC level to the audio input (pin 7). By adjusting the R 1 , we can easily tune the VCO centre frequency fo. when an audio signal is applied to the audio input, the output frequency will generate frequency deviations around fo in the variation of audio amplitude. Thus, a frequency modulated signal is obtained.
3. Experimental Work
3.1 Material Required
a) FM Modulation-Demodulation circuit trainer --------- 1 b) DC Power Supply --------- 1 c) Oscilloscope ---------------- 1 d) Function Generators ------ e) DMM -----------------------
Part-I: LM 566 Characteristic measurement
F, f o =50 KHz)
Input Voltage (V)
Output Frequency (KHZ)
Table 3.
Fig 3.
(C 3 = 0.001 F, f o =100 KHz)
Input Voltage (V) 10.3^ 10.5^ 10.7^11 11.2^ 11.4^ 11.
Output Frequency (KHZ)
Table 3.
Fig 3.
Part-II: LM 565 Frequency Modulator
3.4 Procedure
- Set up the FM Modulation Trainer circuit unit shown in Fig 3.1.
- Connect +12V power supply. (When connecting the power supply, take care of the polarity).
3. Close the Switch (SW) at pin 9 to set the capacitor to 0.01 F. Adjust the 5K
potentiometer at (pin 8) to get the output frequency of 50 KHz.
- Close the switch at (pin 7) and connect a 250 mV, 1KHz sine wave to the audio input (AF IN). Using the oscilloscope , observe the output waveform (O/P) and record the result in Table 3.3.
- Change the audio frequency to 3KHz and 5 KHz sequentially. Observe the output waveforms corresponding to the audio inputs and record the results in Table 3.3.
- Change the audo input to a 500 mV, 1KHz sine wave. Observe the output waveform and record the result in Table 3.4.
- Change the audio frequency to 3KHz and 5 KHz sequentially. Observe the output waveforms corresponding to the audio inputs and record the results in Table 3.4.
F, f o =50 KHz)
Input Frequency Input Waveform Output Waveform
1 KHz
3 KHz
5 KHz
Table 3.
4. Questions 1. Reviewing the circuit of Fig 3.1, what is the function of R 1 and R 2 when the SW 1 is closed?
