GW Instek GRF-1300A - Manual (Page 72)

GRF-1300A User Manual and Teaching Materials
Experiment 6: FM signal measurement
Relevant
information Since frequency modulation is a common type of modulation,
it is important to learn the principles and characteristics of FM
waves. Compared to AM waves, the amplitude of an FM wave
doesn’t carry the modulating signal information. This allows an
amplitude limiter to be used to eliminate the magnitude
interference before demodulation. The noise power spectral density
in an FM wave band is evenly distributed at the input terminal. But
due to frequency modulation, it is affected by frequency at the
output terminal. Because the bandwidth of a modulated signal is
far less than the FM wave bandwidth, it can pass through a low-
pass filter to attenuate noise and increase the output signal to noise
ratio during demodulation. FM waveforms are advantageous as
they utilize power efficiently and have a high degree of fidelity as
they rely on the phase of the modulated signal and not the
amplitude to carry the baseband signal. The FM circuit in this
experiment uses a phase-locked loop. The phase-locked loop circuit
principles described earlier can be used to study the application of
a phase-locked loop circuit for this section.
Item Equipment Quantity Note
1 Spectrum analyzer 1 GSP-730
2 RF & Communication Trainer 1 GRF-1300A
3 RF wire 2 100mm
4 RF wire 1 800mm
Experiment
equipment
5 Adapter 1 N-SMA
Experiment
goals 1. Understand the working principals of frequency modulation.
2. Use a spectrum analyzer to measure the FM characteristics of an
FM wave.
3. Master phase-locked loop principals that are used in FM.
Experiment
principles 1. Time domain analysis.
Frequency modulation is a type of modulation in which the
instantaneous frequency deviation of the modulated signal with
respect to the frequency of the carrier signal is directly
proportional to the instantaneous amplitude of the modulating
signal.
Assume that the modulating signal is
u
Ω
(t) = U
Ωm
cosΩt
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