Instruction manual

BLOCK 2 - RECOVERING THE DATA
Theory of Operation
In dealing with FM receivers, there are some terms
that must be defined. First, let’s determine the term
deviation as the frequency swing of the incoming
FM signal. When no modulation is present, the
incoming signal is a fixed frequency
carrier wave
(Fc) . Positive deviation (Fp) is the increase in Fc
due to modulation, and negative deviation (Fn) is
the decrease in Fc due to the modulation. The
detector must be linear over the maximum total
deviation (Fp max. - Fn max.) produced by the
transmitter or distortion will occur.
Before the second i-f signal reaches the detector, it
is applied to a
limiting amplifier inside the
integrated circuit. A limiting amplifier is designed to
remove any amplitude variations in the signal by
amplifying the signal well beyond the limit of the
amplifier. The frequency variations (FM) are not
affected by the limiter. The limiter removes the
affects of fading (driving through a tunnel) and
impulse noise (lightning and ignition). These affects
produce significant unwanted amplitude variations
in the received signal, but minor frequency
variations. Noise immunity is one of the great
advantages of frequency modulation over amplitude
modulation. The remaining signal contains only the
frequency modulation as shown in Figure 15.
Figure 15
Original Transmitted
Signal
Received Signal with
Noise and Fading
Received Signal After
Limiting Amplifier
The AR-2N6 Radio Kit uses a quadrature detector
to demodulate the FM signal. After the noise is
removed by the limiter, the signal is internally
connected to the quadrature detector. A parallel
tuned circuit must be connected from pin 12 to the
power supply. This circuit produces the 90
O
phase
needed by the quadrature detector. A resistor
shunting this coil sets the peak separation of the
detector. If the value of the resistor is lowered, it will
increase the linearity, but decrease the amplitude of
the recovered audio as shown in Figure 16.
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