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EXPERIMENT #4
FREQUENCY MODULATION
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Purpose:
The objectives of this laboratory are:
1. To investigate frequency modulation characteristics in the frequency domain.
2. To implement a classical double-tuned FM demodulator and measure its characteristics.
3. To implement a modern PLL FM demodulator and measure its characteristics.
4. To investigate the effect of FM signal bandwidth on the detected signal-to-noise ratio.
Equipment List
1. PC with Matlab and Simulink
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Frequency Modulation
FM results when the time derivative of the phase of the carrier is varied linearly with the
message signal m(t). The frequency deviation is proportional to the derivative of the phase
deviation. Thus, the instantaneous frequency of the output of the FM modulator is maximum
when the message signal m(t) is maximum and minimum when m(t) is minimum.
Carson’s Rule:
Carson’s Rule is used to determine the bandwidth of the FM wave. According to Carson’s
Rule, the bandwidth is given by:
BW = 2(�+1)fm Hertz.
Laboratory Procedure
Determining Constants:
Before proceeding to perform the experiment, the following steps were performed:
1. Calibrate the multiplier and determine the multiplier constant.
2. Determine the VCO conversion constant K
o
3. Set the VCO’s frequency for 5 kHz.
4. Verify the outputs of the 1
st
order Low Pass Filter.
1. Multiplier Constant, Km
With a 1V p-p sinusoidal voltage at both inputs of the multiplier, the output was observed and
the multiplier constant was calculated to be 0.206.
2. VCO Conversion Constant, Ko
An external voltage may control the output frequency of the VCO. The change in the output
frequency per change in the dc input voltage was measured.
K
o
= �f / �v
K
o
= 1 / 0.5 = 2 kHz/sec/volt
�K
o
= 4�10
3
= 12566 rad/sec/volt
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FM Transmission:
The following schematic was implemented.
Figure 4 A (a) FM detector
Figure 4 A (b) FM Input signal
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Figure 4 A (c) PSD of message signal
Figure 4 A (d) Limter - Unmasked
