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Signal should be heard with increased strength. Normal "conversion gain" is about 20 to 60 times, with a.v.c. at minimum.
Within the second tube in Fig. 4, a so-called "converter," is the seed of the superheterodyne principle. Only the prophetic insight of genius could have foreseen that the simple "mixing" of two frequencies to produce a third would become the master key for unlocking any one door of a thousand. Now it is precisely this apparently simple process of "mixing" that is most difficult to understand. The commingling of two frequencies does not, of necessity, produce a third. It can be demonstrated that when two audio frequencies are sounded together so that the ear perceives a thrid tone, there are, in fact, still only two frequencies present. The third frequency is considered to be "subjectrive;" its perception is attributed to the nonlinear response of the ear. In a superheterodyne the incoming signal voltage is "mixed" with a highfrequency voltage generated within the receiver. The mixing is done electronically in the "first detector" (mixer). If this tube were a straight voltage amplifier the output would be an amplified reproduction of the input; in this case, two frequencies and nothing more. But the mixer is so biased as to have a nonlinear response. It distorts the input waves and produces waves of other frequencies, among which are the sum and difference frequencies of the applied voltages. In general at least four frequencies will be found upon the plate of the mixer tube, namely:
The plate impedance of the mixer tube is a parallel resonant circuit tuned to the difference frequency. Since amplification depends upon the magnitude of the plate impedance (see discussion under B), a voltage at resonant frequency will be amplified to a far greater degree than will the other voltages. This difference frequency is known as the intermediate frequency (i.f.). Further amplification of the modulated wave is carried on through circuits designed and adjusted to resonate at this frequency. It is the function of,the mixer tube to generate the i.f. signal when an r.f. signal is applied at the grid. It is the function of the tuned circuits to select and amplify this i.f. signal to the exclusion of other frequencies. Trouble With an audible signal only, it is not possible to identify the various frequencies that may appear on the plate of the mixer tube. If the signal is heard clearly and with noticeable gain, the performance may be considered satisfactory. When a more elaborate signal-tracing instrument is used, the various frequencies may be identified, if necessary. It is particularly important that the intermediate frequency be correct. If the signal can not be heard at E with proper gain, the following troubles may be looked for:
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