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Signal should be heard with about
the same strength as at E.
  
The function of the i.f. transformer
is not to produce voltage gain but to
provide (1) a tuned plate load for the
preceding tube, (2) a tuned grid
circuit for the following tube, and (3)
a tuned coupling impedance between the
tubes.
  
The primary side is a parallel resonant
circuit tuned to the desired fixed
intermediate frequency. Taking the
plate circuit alone we have Fig. 5.
Fig. 5. Primary portion of a tuned intermediate-frequency transformer.
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Considering the tube as an a.c. generator
of internal resistance rp we may
redraw the diagram as shown in Fig. 6.
Fig. 6. Equivalent circuit of Fig. 5.
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Plainly, the a.c. voltage generated
by the tube is applied to the resonant
elements in parallel. For this
connection the impedance of the resonant
circuit is a maximum at the resonant
frequency. Since the total voltage is
divided between rp and the load impedance,
the voltage across the load is a maximum when the
load impedance is a maximum. For this reason the voltage
across the primary is greatest at resonance.
  
The tuned secondary may be represented as shown in Fig. 8.
Fig. 8. Secondary portion of a tuned intermediate-frequency transformer.
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Since the e.m.f. "source" is the induced voltage in the
secondary coil, the equivalent circuit of Fig 9 may be used.
Fig. 9. Equivalent circuit of Fig. 8.
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Plainly, the resonant elements are in series with the
source. Maximum voltage occurs across the condenser
at (or very near) the resonant frequency.
  
In the i.f. transformer, the primary and secondary are
coupled together through mutual inductance, so a voltage
applied at the primary terminals reappears at the
secondary terminals. In the usual circuit the voltage ratio
between input and output is seldom greater than one.
  
The main function of this transformer is to pass a narrow
band of desired frequencies and to reject all others.
Rejection occurs because the combination of tubes and
tuned transformer will not amplify the undesired frequencies.
  
If the i.f. channel is tuned to 455 kc. and has a bandwidth
of 10 kc., the lowest frequency is 450 kc. and the highest,
460 kc. These extremes correspond to a ratio of 46/45
or 1.02. In music a ratio of 2 to 1 is an octave. The
ratio of a semitone is equal to the twelfth root of 2, or 1.06.
The width of the i.t response is thus seen to be much less
than a half tone.
  
When it is remembered that some audio transformers
will pass frequencies from 20 to 20,000 cycles, or about
ten. octaves, it is evident that the response of the i.f.
channel is confined to a very narrow band of frequencies.
  
Broadcast frequencies extend from about 550 kc. to 1600
kc. The ratio between these extremes is roughly 1 to 3.
This is an octave plus a fifth, a total of 19 semitones.
  
When this frequency range is divided into 10 kc. bands
it yields a total of 105 channels.
  
The piano tuner has trouble enough to divide the interval
of a twelfth into 19 equal parts. Yet the results are
coarse compared with the tuning of a superheterodyne,
which can slice the same interval into 105 pieces, adjoining
but not overlapping!
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