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Point H--Grid of I.F. Tube

Back    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.
   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.
   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.
   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.
   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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