EXPERIMENT NO.8
FREQUENCY MODULATION COMPUTER SIMULATIONS
Objective A
Concepts of Frequency Modulation
Practical to be used: Concept of Frequency Modulation and Frequency Modulator
1. Choose two voltages levels at the control input to the oscillator and measure the corresponding
output frequencies. Then calculate the 'frequency slope' of the oscillator in kilohertz per volt.
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2. Can you see any amplitude variation over the frequency range? Should there be any? Explain.
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3. Is it easier to measure the frequency range on the oscilloscope or on the spectrum analyser?
Explain.
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4. What relationship can you derived between voltage and frequency?
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In this practical the variable voltage used to control the VCO frequency has been replaced by a
sine wave oscillator. This sine wave now frequency-modulates the carrier. Set Carrier level to
about half scale. Look at the signal at 4 with the oscilloscope. Turn the modulation level up and
down and observe the effect. Notice that the frequency is changing.
Use the spectrum analyzer to examine the sidebands of the signal.
5. By looking at the spectrum of the modulated signal, can you estimate the frequency of the
modulating signal? Explain.
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6. Would it be equally easy to estimate the bandwidth of the modulating signal from the spectrum if
the modulating signal were complex, having many frequencies?
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7. As the modulation level varies, how constant is the carrier-frequency component of the
modulated signal?
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8. As the modulation level varies, how constant is the amplitude of the modulated signal?
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Objective B
FM Spectrum
Practical to be used: FM Spectrum with 5 kHz Source
1. If the modulating frequency is 5 kHz and the deviation is 50 kHz, calculate the modulation index.
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2. Calculate the signal bandwidth using Carson's rule.
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Note: BW (Carson’s Rule) = 2 (fm + )
Where fm = modulating frequency
 = frequency deviation
3. What relationship can you get between bandwidth and the modulation level?
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4. If a bandpass filter were to be added at the input of an FM detector what factors determine the
bandwidth required?
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5. If you would increase the frequency of the modulating signal, what would happen to the
deviation? to the bandwidth?
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Objective C
Demodulation of Frequency Modulated Signals
Practical to be used: Quadrature Detector, Phase Locked Loop Detector
1. What can you say about the output (amplitude) of the phase comparator relative to the phase
difference?
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2. What should be the passband of the output filter?
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1. What is the output of the phase detector when there is no modulation?
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2. Compare the output of the filter to the output sent back by the VCO?
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Objective D
Limiters and Effect of Noise on FM Demodulation
Practical to be used: Quadrature Detector with Limiter, PLL with Limiter
1. In the absence of a limiter, does the demodulated output depend on the carrier amplitude?
Explain.
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2. With the limiter in use, how does the demodulated output vary with carrier amplitude?
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3. How does the waveform of the input to the phase comparator (point 9) differ with and without
the limiter?
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4. Above what amplitude is the limiter designed to clip off?
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1. In the absence of a limiter, with maximum noise, describe the demodulated output.
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2. With the limiter in the circuit, describe the effect of adjusting the noise level, minimum to
maximum, on the demodulated output.
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3. Tapping at monitor point 4, in the spectrum of the frequency modulated signal, describe the
effect of adjusting the noise level. What signal frequencies are affected by noise?
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4. What does the term SNR stand for and what does it mean?
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5. Why do you think it is more convenient to express it in decibels?
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6. Why is it that the use of the limiter reduces the noise on the output?
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7. With the limiter in circuit, observe that reducing the carrier level causes an increase in the noise
on the output. Why is this?
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1. Compare the effect of limiter in this practical and in the previous?
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1. How does limiting the signal improve the signal to noise ratio?
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2. How does the performance of the PLL demodulator compare with the phase comparator
demodulator?
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