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The circuit of the P.E.C. includes a de-empha-
sis network to compensate for high frequency
pre-emphasis that was added to the transmitted
audio signal; it thus provides a flat audio fre-
quency response. This network also attenuates
frequencies above the audio range.
The audio output signal of the ratio detector is
applied both to Q7, the emitter follower and, as
an AFC voltage, to Q2 oftheFMtuning unit. Any
shift of the IF from the 10.7 megacycle center
frequency appears as a DC voltage which is ap-
plied to diode D1; this is commonly called AFC
voltage. The DC voltage shift on D1 in turn
changes the oscillator capacity, thereby ef-
fectively retuning it and causing it to track the
particular station that is being received.
AUDIO SECTION
The audio signal from the ratio detector is
applied to Q7, the emitter follower, which acts
as a matching networkbetween the ratio detector
and the Volume control. After the Volume control,
the audio signal is amplified bydriver amplifier
Q8 and then to the Tone control and to driver
transformer T6. Transformer T6 couples the
audio signal to Q9 ahd Q10, ttte push-pull audio
amplifier stages. From Qg and Q10 the signal,
is coupled to the speaker and earphone jack
through T?, the audio output transformer. When
an earphone is plugged in, the built-in speaker
is automatically disconnected from the circuit
and the audio signal goes instead to the ear-
phone.
Operating power is supplied by six 1-1/2 volts
size C flashlight batteries.
IF-LIMITER SECTION
The 10.? megacycle signal from the FM tuning
unit is appliedto Q4, the first IF amplifier. From
Q4, the IF signal passes through transformer T3'
which passes only the difference frequency of 10.?
megacycles, to Q5, the second IF amplifier. After
additional amplification in Q5, the signal is
applied through T4, the second IF transformer,
to limiter stage Q6, which limits on all but the
weakest signals.
Stages Q4 and Qb act as IF amplifiers for weak
signals, and as limiters for strong signals.
Limiter stages are used to remove any AM
(amplitude modulation) of the IF signal, and to
limit the signal to the proper level for the ratio
detector. The IF signal from limiter Q6 is next
applied to ratio detector transformer, T5.
RATIO DETECTOR
Ideally, the IF signal applied to the ratio de-
tector should be of constant amplitude, but con-
tain variations in frequency which correspond to
tfie audio signal being transmitted by the FM
station. The amount of frequency deviationfrom
the center frequency of 10.7 megacycles de-
termines the amplitude of the resulting audio
signal. The number of times per second thatthe
IF signal deviates from 10.? megacycles de-
termines the resulting audio frequency. The
combinatlon of diodes D3, D4, and transformer
Tb detect frequency variations in the IF signal,
and change these variations into anaudiosignal.
Most of the resistors and capacitors used inthe
ratio detector are encapsulated in a P.E.C. (pack-
aged electronic circuit). The use of this P.E.C.
eliminates any variations in performance that
could result from different wiring techniques.
