Tab Electronics Guide to Understanding Electricity ... - Sciences Club

Radio and Television
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tude variations of the carrier wave. In actuality, it is carried on the top
and the bottom of the carrier wave, where the bottom is a “mirror image”
of the top. However, at any given instant, the audio component voltages
(but not the RF voltages) of the upper and lower halves tend to cancel
each other out; they are 180 degrees out-of-phase with each other. Carefully
examine where the positive and negative peaks of the two signals exist
at several instants. The RF signal alternates its peaks. The audio does not.
If a diode were used to “half-wave-rectify” this signal, the result would
be the full program signal (either the top, or the bottom) and half of the
carrier wave, with no phase cancellation. In other words, it would be just
like cutting the waveform horizontally, through the middle, and removing
half of this energy to pass along to the audio stages. Referring back
to Fig. 16-4, the half-wave rectified IF signal is applied across the RC network
of C1 and P1.
The value of C1 is chosen to filter out the high-frequency IF carrier.
This capacitance value has a low reactance to frequencies in the IF range,
but it has a very high reactance to the lower audio frequencies. In this
manner, the IF carrier is shunted to ground. The audio component, however,
seeks the lower-impedance path through the pot (P1), to the wiper,
and on to the audio amplifier stages. P1 is the audio volume control.
FM detection is a little more complicated. FM detection circuits are
called discriminators or autodetectors. Figure 16-5 is technically an autodetector.
The primary and secondary of the input transformer, together
with C1 and C2, are both designed to be resonant at the FM IF of 10.7
MHz. The frequency variations making up the program signal cause the
resonant circuit to look more inductive (or more capacitive, depending on
the direction of the frequency variation). This, in turn, creates unequal
voltage levels across D1 and D2, in proportion to the original program
signal. C3 and C4 filter out the unwanted IF (in a fashion similar to
blocking capacitor in the AM detector), and the remaining program signal
is applied to an audio amplifier for reproduction. The discriminator
in a quality FM receiver is much more complicated than the simplified
circuit of Fig. 16-5, but the operational principle is the same.
Television
The majority of the principles that apply to radio also apply to television.
The television RF signal (called the composite video signal) is transmitted
from a broadcast station. It is received by a TV antenna, and the