DESCRIPTION AND OPERATION
K.
During the next half-cycle of resultant voltage (shaded areas), the bases of transistors Q5 and Q6
become negative enough with respect to their emitters so as to cause astate of forward bias in both
transistors,
voltage.
In other words, both transistors are conducting during this second half-cycle of resultant
L.
It will be noted at this time that the base of transistor Q5 is more forward-biased than that of the base
of Q6. With our representative values taken into consideration, this means there is a-15 vac signal
at the base of Q5 and only a-5vac signal at the base of Q6. Transistor Q5. being more forward-
biased than that of Q6 results in heavier collector current flowing through the clockwise rotation control
winding of the motor than that of the counterclockwise control winding applied from the collector output
of transistor Q6.
NOTE
In some cases, the 47 cps oscillator reference voltage will
be at ahigher amplitude than the ADF signal. This is
dependent upon the relative position of the loop antenna
"pickup” with respect to the angle and distance of the trans¬
mitting station. Whether the ADF signal is at ahigher or
lower amplitude than the reference voltage, the motor
control amplifier essentially operates the same. The only
difference being that when the ADF signal is lower in ampli¬
tude than the reference voltage, transistors Q5 and Q6
alternately conduct during each half cycle of resultant
voltage. The servo motor responds only to the output
developed from the heavier conducting transistor.
Hence, the motor "sees” only the difference between both collector output currents. Since more current
Is flowing from the collector output of Q5, the motor responds to this output only and momentarily
r o t a t e s i n a c l o c k w i s e d i r e c t i o n .
M.
N.
As explained previously, the servo motor, mechanically linked to the r-f resolver rotor, causes the
resolver rotor coil to also rotate in aclockwise manner. This in turn, decreases the mutual inductance
between the rotor coil magnetic field and the field surrounding the stator coils, until apoint of zero
voltage at the resolver output is attained, at which time the system is at "null".
Consequently, the variable 47 cps ADF signal is absent at the bases of transistors Q5 and Q6. Hence,
the motor stops rotating and the resolver rotor coll stops rotating at aposition that is 45 degrees
relative to the stator coils. The ADF pointer, mechanically coupled to the r-f resolver rotor shaft
also stops rotating at the 45 degree indication on the calibrated dial.
In effect, the ADF pointer is representative of the r-f resolver rotor coil and the calibrated dial is
representative of the stator coils. The effected result as observed on the indicator is the angular
relationship of the resolver rotor coil with respect to the stator coils. This in turn, is representative
of the aircraft's relative bearing from the transmitting station.
Figure 9illustrates the aircraft in aposition of 225 degrees bearing relative to the transmitting station.
In this case, it is required that the servo motor armature must rotate counterclockwise enabling the r-f
resolver rotor coil to stop at the 225 degree angle ("true" null).
The circuit operates identically to that shown in Figure 8. except that the ADF variable signal is
reversed in phase. Consequently, transistor Q6 conducts heavier during the negative half-cycles of
resul tant volt age. Henc e , the servo moto r ar matur e ro t ates in acou n tercl ockwi se direct ion.
Figure 10 illustrates the aircraft pointing to the station or with arelative bearing of zero degrees.
When the condition exists, only the 47 cps oscillator reference voltage appears at the bases of
transistors Q5 and Q6. This is due to the absence of the loop r-f "error" signal to the input of the
receiver. In other words, the ADF System is "nulled" out.
O,
P.
Q.
R.
S.
APRIL 1965
Page 19
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