DESCRIPTION AND OPERATION
(11)
The modulated r-f output of the balanced modulator is amplified and demodulated by the conven¬
tional superheterodyne circuits of the ADF receiver. The output of the receiver consists of a
47 cps voltage that is either in-phase or 180 degrees out-of-phase with the output of the 47 cps
low-frequency power oscillator located in the servo amplifier-indicator.
The power oscillator output voltage is applied to both the balanced modulator and the phase com¬
parison circuits.
(12)
9. RECEIVER (REC) OPERATION
A.
When the ADF Receiver function switch is in the REC position, the ADF-T-12B, CSystem operates as
aconventional superheterodyne receiver, with the input signal supplied from the sense antenna only.
The output consists of an audio signal that produces aural reproduction in a500 ohm headset and into a
3 . 2 oh m s p e a k e r w h e n t h e o p t i o n a l M o d e l 1 0 2 A , B A u d i o A m p l ifi e r i s u s e d .
B.
When operating the system in the REC position the following ADF circuits and components become in¬
operative:
1. Model 2321E Fixed Loop Antenna
2. L o op r - f am p l i fi er c i r c u it
3 . B a l a n c e d m o d u l a t o r c i r c u i t
4. I s o la t i o n a m pl i fi er c i r cu i t
5. M o d el 5 5 1 ( )S e r v o A mp l i fie r - In d i ca t o r
6. CW Oscillator (Models 201B, B-1, Donly)
C.
Operating in the CW position is the same as operating in the REC position except the 1000 cps CW
oscillator is available for assistance in tuning in stations.
D,
Figure 5illustrates the A+ power distribution as witnessed through the action of the function selector
switch (S7) on the front panel of the receiver. For ease of understanding, the wafer switch is shown
as afour-pole four-position switch. The switclj terminal numbers correspond to the wafer terminal
numbers located in the schematic diagram (Figure 401).
10. DETAILED aRCUTT DESCRIPTION: BALANCED MODULATOR
A.
The balanced modulator stage consists of r-f transformers T-1 and T-2 together with aring modulator
bridge comprised of diodes CRl through CR4. The circuit is re-drawn for ease of explanation in
F i g u r e s 6 a n d 7 .
The purpose of the balanced modulator stage is to modulate or switch the incoming loop r-f signal 180
degrees at arate of 47 cps and mix this signal with the incoming sense r-f carrier. The phase of the
resultant signal is such, as to drive the servo motor in the proper direction. The degree of rotation
being dependent upon the amplitude of the received r-f signals at the loop and sense antennas.
The output of the 47 cps power oscillator located in the servo amplifier-indicator is applied to the in¬
put of the balanced modulator through the center-tapped windings or r-f transformers T-1 and T-2.
It will be assumed the first half-cycle of the applied 47 cps voltage causes the center-tap of trans¬
former T1 to become negative and the center-tap of T2, positive.
Therefore, during the first half-cycle of the incoming switching voltage, diodes CR2 and CR3 are con¬
ducting in the direction shown in Figure 6, and diodes CRl and CR4 are considered "open" or non¬
conducting. This action is realized due to the existing distributed polarities across the individual
di o d es . I n t h is c a se , d io d e s C R 2 a n d CR 3 a re f o rw a r d b i a s e d to c o nd u c ti o n be c a us e of t h e a s s um e d
phase of the incoming half-cycle. That is, the cathodes of both CR2 and CR3 are more negative with
respect to their anodes which is the primary requirement of conduction through the two diodes. On the
other hand, diodes CRl and CR4 develop polarities of the opposite direction and therefore cannot con¬
duct during the first half-cycle of applied switching voltage.
During the first-half cycle of applied switching voltage, the incoming loop r-f signal is switch 180
degrees by the half-cycle of switching voltage conducted through diodes CR2 and CR3.
B.
C.
D.
E.
F.
April 1965
(Revised April 1967)
Page 13
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General