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DAR ES SALAAM INSTITUTE OF TECHNOLOGY (DIT) DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION ETU 07209: ANALOGUE COMMUNICATION LABORATORY LABORATORY TASK THREE (a) FREQUENCY MODULATION & DEMODULATION Frequency Modulation and Detection Objectives This Laboratory has 3 tasks. Task one  Generate FM...

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DAR ES SALAAM INSTITUTE OF TECHNOLOGY (DIT)

DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION ETU 07209: ANALOGUE COMMUNICATION LABORATORY LABORATORY TASK THREE (a) FREQUENCY MODULATION & DEMODULATION

Frequency Modulation and Detection Objectives This Laboratory has 3 tasks. Task one 

Generate FM bandpass signals using Generator/Voltage Controlled Oscillator..

the

ICL8038

Precision

Waveform

Task two 

Use the LM565 Phase-locked-loop for demodulating FM signals.

Task three 

Investigating the performance of the slope detector.

Task One The objective is to generate FM signals using a voltage controlled oscillator and observe the waveforms in the time and spectral domains. Circuit Diagram

Figure 1: Frequency modulation circuit from ICL8038.

Procedure 

 

Implement the Frequency Modulator circuit shown in Figure 1 below. The output of the VCO when no modulation is applied (i.e. the carrier frequency) is given by f = 0.33/RC, where R = RA = RB = RL. Design the circuit for a carrier frequency of 30 kHz. Test the system response by feeding a 1-Hz audio-frequency tone to the FM sweep input pin. Observe the waveforms on the oscilloscope. Experiment with varying the modulating signal frequency and amplitude. Observe the input and output waveforms. Determine the range of modulation indices and modulating frequencies at which this circuit will operate. Digitally capture a few input/output waveforms in this range. Confirm your observations with theoretical predictions.

Task Two The objective is to use the LM565 Phase-locked-loop for demodulating FM signals and observe the waveform. Circuit diagram

Figure 2: FM demodulator circuit.

Procedures   

Design the test circuit shown in Figure 2. Choose the free-running frequency of the PLL equal to the carrier frequency of the FM signal. The design equation is f0 = 0.3/RC, where R = R1 + R2. Link the modulator-demodulator circuits and observe signal progression from Input to the output. Note you will need to use an op-amp buffer circuit between the two circuits so that the demodulator does not load the VCO.

Task Three Investigating the performance of the slope detector Circuit diagram

Figure 3: Slope detector circuit.

Procedures 



Design the slope detector circuit shown in Figure 3. The slope detector is nothing but a frequency-to-voltage converter followed by an AM envelope detector. The design equation for the frequency-to-amplitude converter is fco1 = 1/(2 1C1) where fco1 is the cut-off frequency of the low-pass filter R1C1 . fco1 is chosen to be at the carrier frequency of the FM signal. The design equation for the envelope detector circuit is similar and was provided in Lab 1. As before, C3 is a coupling capacitor, chosen such that C3 >> C2 . As part of your initial design parameters, you can choose an FM carrier frequency of 30kHz and a maximum modulating frequency of 15 KHz to design your circuit. Use the slope detector circuit designed to recover the baseband signal. Observe the output. o o o

Vary the modulating frequency and observe the input and output waveforms. Determine the range of performance of the slope detector, in terms of modulation frequency and modulation index. Digitally capture samples of the input and output waveforms in this range

NB: 

At each stage, note your observations and carefully conclude on the results obtained....