Instruction manual

Figure 16
Notice how the linearity improves for a given amount
of frequency deviation when the positive and
negative peaks are further apart. Figure 16 also
shows how the amplitude of the output would drop
for the same frequency deviation on a more linear
detector.
The quadrature detector combines two phases of
the i-f signal that are 90
O
apart, or in quadrature, to
recover the modulation (see Figure 17a). The
shifted signal is used to gate the non-shifted signal
in a synchronous detector. When the frequency
increases above the carrier frequency (positive
deviation), the phase shift increases causing a
decrease in the width of the gated impulse output
(see Figure 17b). In a similar manner, when
negative deviation occurs, the gated impulse output
will widen (see Figure 17c). The gated output is
then filtered to remove the pulses and recover the
modulating signal.
-13-
Figure 17
(a)
(b)
(c)
Signal from Limiter
No Modulation
90
O
Phase Shift
Gated Output
Signal from Limiter
Positive Deviation
Greater than 90
O
Phase Shift
Gated Output
Signal from Limiter
Negative Deviation
Less than 90
O
Phase Shift
Gated Output
Linear Area
Narrow Band
Amplitude
Frequency Frequency
Amplitude
Same Deviation
Lower R