(2) In the circuit shown in B of figure 4-1, resistors R1 and R4 form a
voltage divider and establish the no-signal negative (forward) bias on the
base. The AGC voltage from the second detector is negative with respect
to ground and is fed to the base through dropping resistor R2. When the
dc output of the second detector increases (because of a high carrier
signal input to the detector), the negative dc voltage fed to the base of
transistor Q1 through dropping resistor R2 increases the net negative
(forward) bias on the base and increases the emitter current and,
therefore, the collector current. The flow of increased collector current
through resistor R5 reduces the collector voltage.
The gain of the
amplifier is also reduced.
When the dc output of the second detector
decreases, the net forward bias of transistor Q1 decreases.
Emitter and
collector current decreases; collector voltage increases, and the gain of
the amplifier increases.
(3) Capacitor C3 ac-bypasses resistor R5 to ground.
Correspondingly
referenced circuit elements in both A and B of figure 4-1 perform the same
circuit function.
4-2.
SQUELCH
The limiter and discriminator-derived squelch voltages discussed in TM 11-668
are developed through the use of either the noise or carrier signals. When either
the noise or the carrier signals are used to develop the squelch voltage, the type
of squelch is referred to as old.
A third method is also used in some FM
transmitters and receivers, which is referred to as either tone or new. The tone
squelch is derived from a 150-Hz tone. This tone modulates the carrier signal in
the transmitter and is transmitted along with the intelligence.
The receiver
squelch circuit recovers the 150-Hz tone and develops the squelch voltage for use
in the receiver.
4-3.
THE VOLTMETER IN RECEIVER ALIGNMENT
a. The proper alignment of a radio receiver requires a means of measuring the
output when a known signal voltage is applied at the input. The signal is usually
supplied by a signal generator while the output is measured by an oscilloscope,
output meter, or voltmeter. If a voltmeter is used, its input resistance must be
greater than the resistance of the circuit to be measured, or the readings will be
inaccurate.
b. The input resistance of a voltmeter is determined by a number of resistors,
or multipliers, connected in series with the meter movement (galvanometer). Adding
multipliers makes it possible to extend the range of the voltmeter.
The
sensitivity of a meter is determined by dividing the total series resistance by the
full-scale reading in volts.
Thus, if the total series resistance of a meter is
50,000 ohms for the 50-volt scale, the meter sensitivity is
= 1,000 ohms
per volt.
In general, 50 volts a 1,000-ohm-per-volt meter is satisfactory, but
more accurate measurements in high-resistance circuits require the use of a meter
with greater sensitivity, such as the 20,000-ohm-per-volt meter or the vacuum-tube
voltmeter (VTVM).
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