LOC/GS MODULATION

TM 11-6625-2976-40

1-16. MODULATING CIRCUITS. (CONT)

LOC/GS MODULATION

Refer to FO-5 while reading the following text.

Operating voltage is applied to LOC modulating circuits in circuit card assembly and LOC rf module A4

by closing front panel switches S2, S16, S12, and S15. Operating voltage is applied to GS modulating

circuits in circuit card assembly and GS rf module A3 by closing switches S2, S10, and S12.

Crystal oscillator Y1 generates a 3.932 MHz signal and applies it to a frequency divider whose output is

30 Hz. The 30 Hz signal is applied through a limiter to limit the gain of the signal. From the limiter,

30 Hz is applied to both 90 Hz and 150 Hz band pass filter circuits. The 90 Hz band pass filter circuitry

consists of amplifiers which accept and amplify third order harmonics to produce the 90 Hz signal.

Amplifiers in the 150 Hz band pass filter circuitry accept and amplify fifth order harmonics to produce

the 150 Hz signal. Test points J4 and J3 are provided to check for 90 Hz and 150 Hz LOC and GS modu-

lating signals. These signals are applied from the band pass filter circuitry through the normally

closed front panel 90 Hz and 150 Hz DELETE switches to LOC or GS mode selection circuits in the

circuit card assembly. Depressing either the 90 Hz or 150 Hz DELETE switches opens the respective

signal path and removes the signal from the circuitry.

Mode Selection

The LOC and GS mode selection circuits are controlled by their corresponding front panel LOC and GS

controls.

LOC Mode Selection. In addition to LOC 90 Hz and 150 HZ DELETE, test set provides the following LOC

operational modes:

1. On-course (LOC OC)

Variable (LOC VARIABLE)

Left/right (LOC LEFT/RIGHT)

1020 Hz modulation

When the on-course mode is selected by the front panel LOC LEFT/OC/RIGHT switch, the circuit card

assembly will apply the signal through corresponding circuitry while inhibiting the other modes. The

on-course mix of 90 Hz and 150 Hz is applied through the LOC OC amplifier and LOC OC/VAR select

switch to the LOC output amplifier where the signal is amplified and applied to LOC rf module A4. Test

point J2 is provided to check the output modulating signal.

Setting the LOC on-course switch to LEFT or RIGHT allows either 90 Hz or 150 Hz signal to be amplified

by the LOC output amp. In this manner, the appropriate LOC LEFT or RIGHT conditions are

transmitted.

Selection of variable mode is accomplished by pressing the LOC VAR Iamp/switch once causing the

lamp to blink. The LOC VAR switch then overrides the LEFT/OC/RIGHT switch selection and controls

the signal path. The amplitudes of the 90 Hz and 150 Hz modulating signals are adjusted by front panel

variable control R2 before passing through the LOC VAR circuits and LOC output amp to rf module A4.

Pressing the VAR lamp/switch a second time disables the VAR mode and returns operational control to

the LEFT/OC/RIGHT switch.

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