Чтение общенаучной литературы. Кытманова О.А. - 14 стр.

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15
former being used in point-to-point radio telephone and telegraph service while the
latter are designed for the reception of sound and visual programs.
Radio Receivers
The simplest possible receiver is exactly the same as used for spark
telegraphy. The manipulations necessary for the operation of this receiver are
the same as for any spark receiver; the antenna circuit and the closed circuit must
be tuned to the incoming high frequency, and the coupling between the antenna
circuit and the closed circuit should ordinarily be made loose. The e. m. f.
impressed upon the receiving antenna, due to the electromagnetic waves
emanating from the transmitter is known to produce a current in receiving antennas
which will be a reproduction of the current in the transmitting antenna. Assume,
for the sake of simplicity, the rectifier used in the receiving circuit to have a
characteristic such that a negative e. m. f. impressed upon the circuit of the rectifier
produces no current whatsoever, a positive e. m. f. producing a current varying
directly with the e. m. f. As a matter of fact, the e. m. f. impressed upon the
receiving antenna and transferred to the rectifier circuit by suitable coupling coils
will produce a current in the rectifier circuit. The current, though unidirectional, is
yet one changing at high frequency, and as such it cannot flow through the high
impedance winding of the telephone receiver, the current in the receiver being the
average current. It will be noted that the current in the telephone receiver which
corresponds to a period of activity of the microphone at the distant transmitting
station, is one which changes periodically, between a maximum and a minimum, at
the "modulating frequency", on the other hand, the current corresponding to a
period during which the microphone transmitter is idle, is constant.
Characteristics of Broadcast Receivers. The most important characteristics of a
receiver for radio-telephone signals are the sensitivity, the selectivity, and the
fidelity. The sensitivity represents the ability of the receiver to respond to small
radio-signal voltages, and is measured quantitatively in terms of the voltage that
must be induced in the antenna by the radio signal to develop a standard output
from the power amplifier. This standard output has been arbitrarily chosen as 0.05
watt in a non-inductive load resistance having a value corresponding to the load
resistance into which the power amplifier is designed to operate. The sensitivity is
arbitrarily defined as the effective value of the carrier voltage that must be induced
in the antenna to develop this standard output when the carrier is modulated 30 per
cent at a frequency of 400 cycles. The sensitivity is measured with the radio
receiver tuned to give maximum response at the carrier frequency involved and
with the volume controls adjusted for maximum volume. j Selectivity is the
property that enables a radio receiver to discriminate between radio signals of
different carrier frequencies. Selectivity cannot be defined in a single term as can
sensitivity but must be expressed in the form of curves, which shows the amount
by which the signal input must be increased in order to maintain the standard
output as the carrier frequency is varied from the frequency to which the receiver is
tuned. These curves therefore indicate the extent to which interfering signals are

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