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642 RICHARDSON'S BLUEBOOK OF PROJECTION
tuned by adjustment of the same control. This amplifier filter circuit will reject all the multitude of signals received by the antenna except those of the radio frequency to which the circuit has been tuned.
ANTENNA
TO DEMODULATOR OS DETECTOIZ,
Figure 226
Demodulation
(4) The output of Figure 226 is a modulated radio frequency current of the type shown in the right of Figure 225. This current must be separated into its radio frequency and audio frequency components. That is done in what is called a demodulator or detector circuit, which includes a tube or a part of a tube. The detector is essentially a rectifier converting the output of Figure 226 into pulsating direct current. That current is then applied to a circuit that can accept the audio frequency component but cannot accept the radio frequency component. _ The final output of the detector or demodulator circuit is therefore simply audio frequency — the same audio frequency that was used to modulate the radio frequency in the broadcasting station. This audio frequency output of the detector or demodulator stage of the radio receiver is then wired to a common sound amplifier from which it is passed to a loud speaker. The modulator's rectifying elements and the first elements of the sound amplifier often are combined in one tube.
The Superheterodyne Receiver
(5) The superheterodyne receiver is shown in block schematic in Figure 227. The output of the antenna is applied to an r.f . amplifier as in the case in Figure 226.