oscillator frequency, which is above the incoming signal frequency by the amount of
the IF.
Capacitor C5 is normally ganged with the tuning capacitors of the RF
b. Biasing Arrangements. Self-biasing resistor R1 has a degenerative effect on
the oscillator, since the voltage drop across it causes the emitter to go positive
during the conduction part of the cycle.
The result is that excessive collector
current and tank circuit loading is prevented, which aids in maintaining oscillator
stability.
Forward base-emitter biasing is obtained by the R2-R3 voltage divider
connected in series with the +6.5 voltage source.
AGC is not used, because the
oscillator circuit must be self-regulating to maintain a stable output level.
c. Output Voltage.
The output voltage is coupled to the mixer from the low-
The low-impedance
tap is necessary to match the input impedance of the mixer base-emitter circuit.
d. Ganged Tuning.
In the foregoing circuit diagrams, a ganged capacitor
arrangement is used for tuning.
Many transistor radio receivers use ganged
variable slug tuning rather than ganged capacitor tuning.
In slug tuning, the
tuning coils are equipped with movable ferrous cores. Sliding ganged cores in and
out of the inductors simultaneously varies the impedance of all the tuning coils.
Such arrangements require the use of fixed capacitors. The slug tuning method is
popular with radio receiver designers because of its low cost and small size.
LESSON EXERCISES
In each of the following exercises, select the ONE answer that BEST completes
the statement or answers the question.
Indicate your solution by circling the
letter opposite the correct answer in the subcourse booklet.
1. Four typical antenna-coupling circuits are shown in figure 159 of TM 11-665.
A brief comparison of these circuits shows that the greatest sensitivity and
selectivity will be produced by the circuit shown in sketch
a. A.
c. C.
b. B.
d. D.
2. Bandswitching
is
used
in
communication-type
receivers
when
design
difficulties limit the use of some circuit components. Bandswitching is necessary
in these receivers because
a. tracking and alignment are simplified.
b. different tuning capacitors are required for each tuning range.
322 L2
11
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