Section III
Paragraphs 3-6 to 3-9
voltage from the AFC amplifier signal whenever the
applied to cathode-coupled limiters V-1405 and V-1404.
center frequency of the input signal varies from the
The output level of the limiters is controlled by a
9960 cps center frequency. This d-c voltage, positive
potentiometer in each cathode circuit. Further filter-
or negative depending on the direction of error is
ing is provided by filters Z-1402 and 2-1404. The 90
filtered to remove the 30 cps component and applied
and 150 cps voltages of constant amplitude appearing
across R-1436 and R-1435 are connected to the LOC-
to the grid of modulator tube V-1407B through R-1474
and R-1475.
GS attenuator. When the LOC-GS control is set in the
middle of the white center sector on the front panel,
equal amounts of 90 and 150 cps signal appear at the
grid of V-1408B. The amplitude of 90 and 150 cps
modulates the 9960 cps FM signal can be shifted in
phase one degree. This allows a one degree incre-
signals here is such that each signal will modulate the
ment to be placed on any azimuth selected on the VOR
r-f carrier 40%. When the control is rotated to the
switch. The 30 cps reference voltage is obtained from
top line on the left segment, the 90 cps level will fall
terminal 8 of T-1401 and applied to an RC network
and the 150cps level will rise until the 150cps voltage
whose constants can be changed by operating the 1
is 3.0 db higher than the 90 cps voltage. If the control
DEGREE switch. The shifted reference voltage
is set on the bottom line there is a 3.75 db difference.
On the right segment the situation is reversed and the
appears across the DEVIATION potentiometer R-1421.
90 cps level becomes greater than the 150 cps level.
3-7. The 1000 cps signal for checking the aural out-
put of a receiver is obtained from the 9960 cps oscil-
is practically the same on TONE LOC as it is on
lator by switching in a 1000 cps feedback network in
place of the 9960 cps network. The frequency modu-
GLIDE SLOPE . Some resistor values in the LOC-GS
lator is disconnected and the AFC is inoperative when
attenuator are changed by switching. See figure 3-4.
the oscillator is operating at 1000 cps.
The 15, 000 ohm resistors on each side of R-1418A in
the glide slope circuit are replaced with 2200 ohm re-
sistors in the tone localizer circuit. The 90 and 150
90 and 150 cps signals for glide slope and localizer
cps levels at the center position of the LOC-GS control
are still equal, but when the control is set to the top
operation are generated by distorting the 30 cps signal
line of the right or left segment the difference in level
and selecting the third and fifth harmonics by means
between the 90 and 150 cps signals is now 6.0 db. At
of filters. Considerable 30 cps voltage is applied
the bottom line of either segment the level difference
across L-1401, a small audio reactor whose core is
insufficiently large for the level involved and cons e-
is now 7.5 db. Since only 20% modulation is required
quently saturates. This saturation causes the voltage
for each tone for an on-course signal in the TONE LOC
drop across R-1430 and C-1422C to be distorted and
position, the level applied to the grid of V-1408B is
very rich in harmonics. The 3rd and 5th harmonics
reduced with the voltage divider consisting of R-1586,
are separated with filters Z-1401 and Z-1403 and
R-1442 and R-1443.
13
