
315
Fig. 18.15 (a) Circuit of phase discriminator for f.m. detection, arranged to bring out
the
principle: L
p
and L
s
are the primary and secondary windings of the input r.f.
transformer;
(b) phasors of voltages marked, for carrier wave frequency. Broken lines
indicate
the effect of f.m. deviation
But as they are connected in opposite polarity, the net output is nil.
When the carrier wave is frequency-modulated it swings off the
resonance peak and, as explained in Sec. 8.11, the impedance of a
tuned circuit changes from resistance to inductance on one side and
capacitance on the other. The phase swings correspondingly, as
indicted by the broke ...