BENDIX ADF-T12B Maintenance Manual page 13

D E S C R I P T I O N A N D O P E R A T I O N
O . During the negative "swing" of the resultant signals, both transistors are "turned on" or conducting.
D u e t o t h e h i g h e r a m p l i t u d e o f t h e s i g n a l a t Q 5 , h e a v i e r c o l l e c t o r c u r r e n t fl o w s a t t h e o u t p u r o f Q 5 t h a n
that of Q6. The voltages developed at the respective transistor outputs are filtered and applied to both
windings of the servo motor as d-c voltages of different amplitudes and polarity.
If the positive voltage developed across the motor from the output of Q5 causes clockwise rotation of
the servo motor, the positive voltage developed from the output of Q6 will cause counterclockwise
r o t a t i o n o f t h e m o t o r .
P .
The motor responds only to the differential between the two transistor outputs. In this case, the motor
will rotate clockwise due to the higher amplitude signal current derived from the output of Q5,
Q .
R .
The r-f resolver rotor coil, mechanically coupled to the armature of the servo motor begins to rotate
c l o c k w i s e a s d o e s t h e A D F i n d i c a t o r p o i n t e r, w h i c h i s l i n k e d t o t h e r - f r e s o l v e r r o t o r c o i l .
The motor continues to turn until the developed magnetic field surrounding the rotor coil and the mag¬
netic field surrounding the stators result in zero voltage at the output of the resolver.
S.
At this point, the loop r-f signal is absent in the receiver. Since phase comparison cannot be made
with both the sense r-f and oscillator switching signals, the motor stops rotating and the indicated
bearing as observed on the ADF will read 90 degrees.
T .
U . I f c r o s s - w o u n d c o i l B r e c e i v e s t h e m a x i m u m s i g n a l a n d A t h e m i n i m u m , t h e o p p o s i t e e f f e c t o c c u r s .
That is, the incoming loop r-f signal will lag the sense r-f signal by 90 degrees.
The additional 90 degree phase shift, results in the loop r-f signal becoming 180 degrees out-of-phase
w i t h t h e s e n s e r - f s i g n a l .
V .
Consequently, due to the switching action of the balanced modulator, the demodulated ADF signal as
compared to the power oscillator switching signal across transformer T1 in the servo amplifier unit is
opposite to that explained previously. As aresult, the motor rotatates in acounterclockwise manner
and the indicator pointer will stop at 270 degrees on the dial.
W .
t
\
Due to the inertia of the motor together with noise modulation, in some cases, the ADF pointer will
"overshoot" the "true" null position by approximately 5degrees. The pointer will then reverse its
r o t a t i o n a n d s t o p a t t h e t r u e n u l l p o s i t i o n .
X .
Once the rotor coil passes through the "true" null position (overshoots), areversal in phase of the
l o o p r - f s i g n a l a p p e a r s a t t h e r e s o l v e r o u t p u t . T h i s i s d u e t o t h e f a c t t h a t t h e r o t o r c o i l h a s p a s s e d
through the magnetic field created in the stator coils by afactor of 180 degrees with relation to the
position of the transmitting station. As aresult, the rotor begins cutting the magnetic field of the
stator coils with reversed polarities to that previously encountered. Hence, the variable ADF signal
in the servo amplifier is reversed by 180 degrees. When compared with the reference 47 cps voltage,
the resultant signal causes the servo motor to reverse its rotation, which in turn, brings the pointer
back to the "true" null position.
The 47 cps modulated loop r-f output of the balanced modulator is amplified by modulator isolation
amplifier Q2 and applied to the input of sense r-f amplifier Q3 where it is further amplified and
alternately added to the sense r-f signal. The resultant output of the sense r-f amplifier is applied
to the input of the mixer stage, where, together with the output of the local oscillator, it is converted
to an I-F frequency of 142. 5kc. The signal is further amplified through the three stages of I-F and the
output applied to the 2nd. detector where the audio, together with the 47 cps modulation component is
r e c o v e r e d f r o m t h e r - f s i g n a l .
The output of the 3rd I-F amplifier is also applied to an automatic gain control (AGO) detector. The
d-c component of the demodulated output of the AGC detector is amplified by the AGC amplifier and
applied to the r-f amplifier stage, and both the 1st and 2nd I-F amplifier stages. The AGC voltage
lowers the gain of the system upon reception of an r-f signal above apre-determined amplitude. A
1000 cps CW oscillator is included in the Model 201B, B-1, DReceiver to enable the tuning in of weak o:
distant stations. The output of the oscillator modulates the output of the 2nd I-F amplifier.
The 47 cps modulated audio signal is applied to the 1st audio amplifier (in the receiver) where it is
a m p l i fi e d a n d a p p l i e d t o t h e i n p u t o f 1 s t a u d i o a m p l i fi e r Q 1 i n t h e s e r v o a m p l i fi e r - i n d i c a t o r. T h e a u d i o
signal is also applied to two more stages of amplification in the ADF receiver where it is finally re¬
p r o d u c e d i n t h e h e a d s e t o u t p u t .
Y .
Z .
A A .
B B .
(
A p r i l 1 9 6 5
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