DESCRIPTION AND OPERATION
f
D. TUNING CONTROL
The tuning control tunes the receiver to aspecific frequency within the selected frequency band.
Tuning is accomplished by rotating the tuning knob until the desired frequency designation on the
tuning dial aligns with the lubber line on the dial window or until the maximum indication on the
tu n i n g m e t e r i s o bt a in e d .
The low and medium frequency bands are calibrated in kilocycles. The 850 -1750 kc band is
calibrated in hundreds of kilocycles.
(1)
(2)
E . V O L ( V o l u m e ) C o n t r o l
This knob is mechanically linked to the wiper-arm of apotentiometer. The position of the wiper-
a r m d e t e r m i n e s th e au d i o g a i n o f t h e r e ce i v e r.
(1)
F. TE S T BU T TO N
The spring-loaded TEST button provides aquick operational check of the ADF-T-12B, C^stem.
When the receiver (in the ADF mode) is tuned to astation, pressing the TEST button will cause
the indicator pointer to rotate away from the indicated bearing. If the system is functioning
properly, the indicator pointer will return to the station bearing upon release of the TEST button.
7 . P R I N C I P L E S O F O P E R AT I O N ( S e e F i g u r e 2 a n d 3 )
As indicated in figure 2the cross-wound coils of the fixed loop antenna are connected to the cross¬
wound coils of the r-f resolver located in the Servo Amplifier-Indicator. The voltages induced across
the coils of the fixed loop antenna by the received signal cause proportional currents to flow through
the stator coils of the r-f resolver. The currents, in turn, produce proportional magnetic fields that
combine algebraically to produce aresultant magnetic field. The resultant magnetic field assumes the
same conditions as the induced signal voltage at the fixed loop antenna.
The magnetic field surrounding the stator coils of the r-f resolver induces avoltage in the resolver
rotor coil. The amplitude and phase of this induced voltage is determined by the axis of the rotor coil
with respect to the axis of the magnetic field created by the stator coils. When the two axes are dis¬
placed by zero or 180 degrees, that is, parallel to each other, the induced rotor voltage is at amaxi- -
mum. Similarly, there are two positions of the rotor coil that produce zero voltage. This occurs
when the two axes are displaced at right angles to each other, that is, 90 and 270 degrees.
The induced voltage developed across the r-f resolver rotor coil is the "error" input signal to the
servo loop" formed by the resolver rotor, the receiver, servo amplifier and the servo motor,
receiver converts the r-f rotor "error" voltage into alow-frequency motor control voltage, which is
amplified and phase compared in the servo amplifier-indicator. The resultant signal is applied to the
control windings of the d-c servo motor. The servo motor rotates and causes the resolver rotor coil
to rotate to aposition corresponding to zero output voltage. At this point, there is no input voltage
applied to the receiver from the resolver rotor coil and therefore there is no low-frequency signal
applied to the servo system. As aresult, the servo motor stops rotating. Apointer, attached to the
resolver rotor coil indicates the relative bearing of the aircraft from the "tuned-in" transmitting
station as read against the dial.
The direction from which the transmitted radio wave is received, that is, from the left or right of the
transmitting station, is therefore determined by the zero voltage or "null" position of the resolver
rotor coil with respect to the induced magnetic field surrounding the stator coils. There are two
positions of "null" (refer to paragraph B) and they occur 180° apart. The "null" position that causes
the indicator pointer to point to the true direction of the transmitting station is called the "true" null.
The other "null", displaced 180 degrees from the "true" null is called the "false" null. Ameans of
discerning between the "true" and "false" null indications is incorporated in the system such, as to
cause the pointer to indicate "true" null at all times. The manner in which this is accomplished is as
follows:
Due to the design characteristics of the cross-wound coils of the fixed loop antenna, the incoming loop
r-f signal will either lead or lag the incoming sense r-f signal by 90 degrees. Whether the loop r-f
signal leads or lags the sense r-f signal, is dependent upon the position of the loop antenna and resolver
r o t o r c o i l w i t h r e s p e c t t o t h e t r a n s m i t t i n g s t a t i o n .
A.
B.
{
C.
The
D.
E.
1
April 1965
(Revised April 1967)
Page 6
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General