Lab 6: Frequency Modulation and Demodulation
FM Modulation is widely used in communication systems. The most well known use is in
FM broadcasting. Digital FM is used in modems and multitone selective signaling
Double-check your circuit connections before powering up the circuit. The IC chips can
be easily damaged if incorrectly connected.
This experiment demonstrates some of the principles of VCO operation using the NE566
integrated circuit. Mathematical theory is demonstrated in both the frequency and time
domains. A simple wideband demodulator demonstrates one method of recovering a
frequency modulated signal.
1. Spectrum Analyzer
3. Krohn-Hite Filter
4. Digital Multimeter
5. Function Generator
6. Power Supplies (+5 vdc, -5 vdc)
In this lab we will learn to use VCO chip NE566 to produce FM signal. Figure 1 is a
simple block diagram of a complete FM system. The information source is often audio,
e.g. speech or music or tones. The voltage output of the source is called the modulation
signal and can be produced by a microphone, CD player, or audio signal generator. The
FM generator (NE566) is a square-wave signal source with a voltage controlled
frequency. This is usually referred to as a voltage controlled oscillator (VCO). The FM
generator can also be thought of as a voltage-to-frequency converter. The modulation
signal affects both the magnitude of the frequency change and the rate at which it
changes. Figure 1 shows the FM circuit which will be used for this experiment. This
board has two inputs:
1. Power supply (+Vcc = 5v, ground, and –Vcc = -5v).
2. Modulation signal. This voltage should not exceed 2 volts peak-to-peak for
linear operation of the VCO.
The output frequency f out is proportional to the voltage across pins 5 and 8 of the
NE566. More specifically:
f out D f V85 C
where C is a constant.
By applying a modulation signal to V85, an FM signal results.
HINT: Make sure that all of your waveforms are correct and make sense. Ask the lab
instructor to explain them if necessary.
1. MODULATION: Use the same parameters of lab#5 to generate FM signal. Make
sure it is correct before connect it to the demodulation parts.
2. VOLTAGE COMPARATOR: Observe the waveforms at TP-3. What are the
differences between TP-3 and TP-2. Why do we need this step before
3. DEMODULATION: Sketch the waveforms of pin5 and pin6 of 74121. What are
the differences? Design the RC filter yourself. What are the values of R and C you
use? Sketch the waveform before and after the Krohn-Hite filter. What are the
high/low cutoff frequencies you use for Krohn-Hite filter? Compare the
demodulated signal m(t ) and the source message m(t ) . Is there any distortion?
Comment on your results.
Include all measurements, waveform sketches, and calculations for the steps given above.
Fig. 1 Reference diagram, only NE566 is used in this lab