BENDIX ADF-T12B Maintenance Manual page 12

D E S C R I P T I O N A N D O P E R A T I O N
In order for the ADF indicator pointer to rotate in the proper direction and stop rotating at the correct
aircraft to station relative bearing, aphase comparison or sampling between the loop r-f and sense r-f
signals must be performed. The result, or phase of this "sampling" will determine the direction of
rotation of the servo motor. This, in turn will cause the ADF indicator pointer to rotate in the proper
direction. This "sampling" method is accomplished as follows;
F .
The loop r-f signal is phase-shifted an additional 90 degrees through means of capacitor CIO, Cll or
C12 (depending upon the position of the band selector switch). Depending upon whether the loop r-f
signal originally leads or lags the sense r-f signal by 90 degrees, the additional phase shift will cause
the loop r-f signal to be either in-phase or 180 degrees out-of-phase with the sense r-f signal.
G .
The 47 cps low frequency switching action of the balanced modulator circuit, moduiates or switches the
incoming r-f signal at the loop antenna in such amanner as to alternately switch the loop r-f signal in-
phase and 180 degrees out-of-phase with the constant-phase sense r-f signal during each complete cycle
of 47 cps switching voltage. The modulated loop r-f signal is amplified by isolation amplifier Q2 and
the output combined with the incoming sense r-f signal. The low frequency modulated loop r-f signal
alternately adds to and subtracts from the sense r-f signal and as aresult, during one half-cycle o*'
switching voltage (47 cps) either an addition or subtraction takes place with the sense r-f signal.
Whether the loop signal, during 1st half-cycle of switching voltage, adds to or subtracts from the sense,
is dependent upon the relative position of the r-f resolver rotor coil with respect to the field induced by
the resolver stator windings. This in turn, is dependent upon the position of the loop antenna with
respect to the transmitting station. The following example will facilitate the explanation given above:
H .
With the tuned-in transmitting station at arelative bearing to the aircraft of 90 degrees to the right,
the loop antenna will receive the incoming r-f signal at amaximum level in one of the internally cross¬
wound coils and at aminimum level in the other. Assume the two loop coils as Aand B. Coil Abeing
the coil that receives the signal at amaximum level. It will further be assumed that reception at coil
Acauses a90 degree lead with respect to the constant-phase signal received at the sense antenna.
Cross-wound coil A, directly connected to one pair of stator coils of the r-f resolver creates amaxi¬
mum magnetic field in that pair of stator coils. Cross-wound coil Bconnected to the other pair of
stator coils creates aminimum or virtually zero magnetic field. Depending upon the position of
resolver rotor coil with respect to the induced magnetic field, the loop r-f signal will either lead or
lag the incoming constant-phase sense r-f signal at the sense antenna by 90 degrees. In the example
cited, the rotor coil is in aposition such, that causes the loop r-f signal to lead the sense r-f signal
by 90 degrees. Afurther 90 degree phase shift causes the loop r-f signal to become in-phase with the
sense r-f signal.
The first half-cycle of balanced modulator switching voltage, reverses the loop signal 180 degrees or
causes it to become 180 degrees out-of-phase with the sense r-f signal. Upon mixing, the two signals
cancel each other and as aresult, zero r-f voltage exists at the output of the r-f amplifier. (Assuming
equal sense and loop signals). During the next half-cycle of balanced modulator switching voltage, the
loop r-f signal is reversed or "switched" back to its previous state, that is, to an in-phase condition
with the sense r-f signal. Upon mixing, the two signals aid each other and as aresult, maximum r-f
voltage exists at the output of the r-f amplifier.
Therefore, for acomplete cycle of balanced modulator switching voltage, the loop r-f signal alternately
subtracts from and adds to the constant-phase sense r-f signal. During amplification and demodulation,
the resultant ADF signal maintains the same phase with respect to the power oscillator switching voltage
in the servo amplifier unit.
The ADF signal, in this case, is at ahigher amplitude than the oscillator switching voltage due to the
90 degree relationship in the position of the resolver rotor coil with respect to the stators.
The power oscillator signal, as applied to windings 7and 9of phase comparison transformer T1 (motor
control amplifier) results in the ADF signal either being in-phase with the switching signal at winding
7and out-of-phase with the switching signal at winding 9or vice versa. From our example, we will
assume that the former condition exists. Consequently, at winding 7of transformer Tl, both the ADF
and switching signals aid each other and therefore add in amplitude while at winding 9both signals cancel
e a c h o t h e r a n d t h e r e f o r e s u b t r a c t i n a m p l i t u d e .
I .
J .
K .
L .
M .
The resultant signals as observed at the bases of transistors Q5 and Q6 are such, that during the
positive "swing" of the resultant signals, both transistors are "shut-off or non-conducting. Hence,
the servo motor "sees" zero voltage. Consequently, the motor does not rotate.
N .
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