Electrical Communications Systems
Course No. 0909-331-01
Laboratory Project 3
Frequency Modulation and
This project has 3 parts. In Part 1,
you will investigate the performance of the slope detector. In Part 2,
you will generate FM bandpass signals using the ICL8038 Precision
Generator/Voltage Controlled Oscillator. In Part 3 you will use the
Phase-locked-loop for demodulating FM signals.In all parts, you will
the system with single-tone FM ( w and w/o added Gaussian noise) and
FM signals (w/o added noise).
Equipment and Software
33120A Function Generator/Arbitrary Waveform Generator
- HP8591EM EMC Analyzer/ HP Infinium
54657A FFT Module.
- 1N4148 Fast Switching Diode
- ICL8038 Precision Waveform
Oscillator. Download data-sheet.
- LM565 Phase-locked-loop. Download data-sheet.
- Assorted Resistors and Capacitors (based
- PC speakers
- HP IntuiLink Suite, HPVEE or Matlab
- Audio Player Software on PC
- CD with your favorite music!
- Design the slope detector circuit shown
1. The slope detector is nothing but a frequency-to-voltage converter
by an AM envelope detector.The design equation for the
converter is fco1 = 1/(2pR1C1)
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
The design equation for the envelope detector circuit is similar and
provided in Lab Project 2. As before, C3
is a coupling capacitor, chosen such that C3 >> C2
. As part of your initial design parameters, you can choose an FM
frequency of 30kHz and a maximum modulating frequency of 15 KHz to
Figure 1: Slope detector circuit.
- Using the HP Arb. Function Generator,
FM wave with carrier frequency of 30 kHz and modulation frequency
1 kHz (single tone).
- Vary the frequency deviation and
and output waveforms.
- Vary the modulating frequency and
and output waveforms.
- Determine the range of performance of
in terms of modulation frequency and modulation index.
- Digitally capture samples of the input
waveforms in this range
- Perform a spectral analysis of these
- Listen to the output signal using the
Compare with the pure tone of the same frequency.
- Repeat the experiment by digitally
signals with varying SNR. Figure 2 shows the overall block diagram for
this part of the lab project.
- At each stage, note your obervations and
The obective is to generate single- and
FM signals using a voltage controlled oscillator and observe the
in the time and spectral domains.
- Implement the Frequency Modulator circuit
the data sheet (reproduced in Figure 2 below). The output of the VCO
no modulation is applied (i.e. the carrier frequency) is given by f =
where R = RA = RB = RL.Design the
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
on the oscilloscope.
Figure 2: Frequency modulation application circuit from
- Experiment with varying the modulating
and amplitude. Oberve the input and output waveforms.
- Determine the range of modulation indices
frequencies at which this circuit will operate.Digitally capture a few
input/output waveforms in this range and perform spectral analysis.
your obervations with theoretical predictions.
- Experiment with feeding in multi-tone
signals at the modulating signal input of this circuit.
- Use the slope detector circuit you have
in Part A to recover the baseband signal (for both single- and
Observe and listen.
The obective is to generate an FM signal using
a voltage controlled oscillator and observe the waveforms in the time
- Design the test circuit shown in Figure
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.
Figure 3: FM demodulator circuit using the LM565 PLL.
Part 4: XTRA CREDIT
- Test the circuit by feeding in an
from the arbitrary function generator, observe (waveform and spectrum)
and listen to the audio output.Vary the input signal amplitude, the
signal frequency and the modulation index.Experiment with the capture
and lock range of the PLL.
- Test the product detector circuit by
synthesized FM signals with varying SNR. When does the demodulator fail
to detect the message signal?
- Link the modulator-demodulator circuits
signal progression from input to output. You will need to use an op-amp
buffer circuit between the two circuits so that the demodulator does
load the VCO.
- Sections 4.13, 4.14 and 5.6 of textbook.
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