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RADIO RECEIVERS
643
In a small, inexpensive superheterodyne receiver the r.f amplifier may be omitted and the antenna signal may be applied directly to the first detector.
(6) The phenomenon known as heterodyning produces a "beat" frequency or component when two different frequencies are present simultaneously. If the two frequencies are, say, 1,000,000 cycles and 500,000 cycles, their presence in the same wire will produce a "sum frequency of 1,500,000 cycles and also a "difference" frequency of 500,000 cycles. The oscillator in Figure 227 incorporates in its circuit a variable condenser which is
OSCILLATOG
II
B.F.
amplifies
MIXEZ
converter
FIRST
detector.
IF.
AMPLIFIER
SECOND DETLXT02
A.F. AMPLIFIER.
J LOUDSPEAKER
Figure 227
ganged with the other variable condenser of the superheterodyne receiver in such a way that the oscillator always generates a frequency which will heterodyne that frequency being received at the moment to produce a "beat frequency" of a predetermined number of cycles. If for example, the i.f. amplifier portion of Figure 227 is designed to work at 500,000 cycles, the oscillator will generate a 300,000 cycle current when the tuning control is set to receive an 800,000 signal, thus producing a 500,000 cycle intermediate frequency. But if the tuning control is shifted to receive a 1200 kilocycle signal the corresponding change in the setting of the tuning condenser of the oscillator will make the oscillator deliver a 700,000 cycle output, again producing a 500 kilocycle intermediate frequency.
(7) Sometimes the oscillator and the mixer of the first detector are two separate tubes but very often they are a single tube known as a pentagrid converter, a tube with one cathode and one plate, and five grids interposed in succession between cathode and plate. Some of these