Marker Beacon System Component Location
Figure 1
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(1)
The marker beacon receiver is a single-conversion superheterodyne unit, fixed-tuned to receive 75-MHz amplitude modulated signals. The 75-MHz incoming signal from the antenna is applied through a selective LC filter to a crystal controlled converter where it is mixed with a 70.4-MHz local oscillator signal to generate a 4.6-MHz intermediate frequency (IF) signal. This difference or IF frequency is selected by a 4.6-MHz IF filter and applied through three stages of IF amplification to increase overall receiver selectivity.
(2)
The audio component (400, 1300, or 3000 Hz) of the 4.6-MHz IF signal is detected in the audio and AGC detector and amplified by the first audio amplifier.
(3)
Two audio outputs are provided by the audio and AGC detector. The AGC audio output is rectified through the AGC gate and applied to all three IF amplifier stages where it provides automatic gain control of the incoming signal. The second audio output drives the first and second audio amplifiers.
(4)
The second audio amplifier output divides and drives the audio power amplifier and the audio lamp driver. The output signal from the audio power amplifier is fed through an output transformer to the flight interphone system for aural monitoring of the marker beacon system. Audio output from the audio lamp driver is fed through a three-stage frequency selective filter which separates the audio into its 400, 1300 or 3000 Hz component and drives the respective 400, 1300, or 3000 Hz lamp switches which turn on the corresponding marker beacon lights.
(5)
AF 172-182, 185-199; AT ALL; AZ ALL; LH 051-099; GN ALL; MD ALL; SN ALL; VM ALL; The self-test circuit supplies a 75-MHz amplitude modulated test signal to the input of the crystal controlled converter stage when the push-to-test (PTT) button on the front of the receiver is pressed. The test signal is generated by a switched three-frequency (400, 1300, and 3000 Hz) multivibrator which sequentially modulates a 75-MHz test signal oscillator.
B. Functional Description (IB ALL) (Fig. 2, Sheet 2)
(1) The TRF marker beacon receiver is fixed-tuned to receive 75-MHz amplitude modulated signals. A crystal filter provides receiver selectivity. From the crystal filter, the signal is applied to an rf amplifier and detector where it is amplified and the 75-MHz rf component is removed. Output from the rf amplifier and detector is fed to a parallel combination of three selective audio filter-amplifier stages which reject undesirable audio signal components and provide separate 400-, 1300-, and 3000-Hz audio output channels to the respective indicator lamp driving circuits. A portion of the output from each filter-amplifier stage is recombined into a common channel and then amplified to the flight interphone system which provides aural monitoring of the marker beacon signal through the audio selector panels.
EFFECTIVITY
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(2) The self-test circuit supplies a 75-MHz amplitude modulated test signal to the input of the crystal filter stage when the push-to-test (PTT) button on the front of the receiver is pressed. The test signal is generated by a switched three-frequency (400-, 1300-, and 3000-Hz) multivibrator which sequentially modulates a crystal controlled 75-MHz test signal oscillator.
C. Functional Description (AF 101-120, 123-156, 158-159) (Fig. 2, Sheet 3)
(1)
The marker beacon receiver is a single-conversion superheterodyne unit, fixed-tuned to receive 75-MHz amplitude modulated signals. The incoming 75-MHz signal passes through a 6 db pad (to improve VSWR) to a four-stage passive rf filter network providing image rejection and narrow bandpass. The filtered input signal is applied to a mixer where it is combined with a 70.5-MHz crystal controlled local oscillator signal to generate a 4.5-MHz intermediate frequency (IF) signal. This difference or IF frequency is applied through three stages of IF amplification to increase overall receiver selectivity. The audio component (400, 1300, or 3000 Hz) of the 4.5-MHz IF signal is detected by the audio detector.
(2)
An AGC circuit output is applied to the first two IF amplifier stages and the mixer to provide automatic gain control of the incoming signal.
(3)
The detected audio output is applied, through an audio amplifier to an output audio amplifier and to three series, filter/transformer circuits, an audio transformer, and a feedback transformer. The filter/transformer circuits separate the audio into its 400-, 1300-or 3000- Hz components and provide the power to illuminate the corresponding marker lights. The audio/transformer provides an audio output signal which is fed to the flight interphone system for aural monitoring of the marker beacon system. The feedback circuit provides a negative feedback to the first audio amplifier causing the lamps to light and go off sharply with no flicker.
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