TM 11-6625-3059-10
PRINCIPLES OF OPERATION
c.
Amplitude Modulation (AM) Circuits. The 1.0 MHz intermediate frequency signal from the sampling mixer
is coupled to amplifier circuits through a flat-amplitude, low-pass filter. An electronic variable attenuator ensures that a
constant intermediate frequency signal level is maintained at the input of a linear active detector circuit, which recovers
the amplitude modulation component of the intermediate frequency signal. Additional filtering removes unwanted signals
from the recovered audio. Constant AM channel gain is maintained through an agc amplifier, driven by a dc output
signal from the detector circuit. The agc amplifier controls the attenuators in the amplitude modulation circuits and the
radio frequency circuits. The recovered audio is supplied to the audio circuits. A leveled intermediate frequency signal is
also supplied by the amplitude modulation circuits to the front panel IF OUT connector when the % AM or LEVEL
FUNCTION is selected.
NOTE
The preceding description of attenuator control assumes that the front panel FUNCTION
LEVEL SET TO 10.00-AUTO control is set to the AUTO position. If the LEVEL control
is switched from the AUTO position, manual adjustment of attenuator control signals,
using the LEVEL control, is possible.
d. Frequency Modulation (FM) Circuits. The 1.0 MHz intermediate frequency signal from the sampling mixer in the
radio frequency RF circuits is coupled to a limiting circuit through a flat-delay, low-pass filter. The limiting circuit
prevents amplitude modulation of the intermediate frequency (IF) signal from affecting the frequency modulation (FM)
detector circuits. The FM detector circuit is a pulse-counting frequency discriminator. The linearity of this circuit insures
accurate, low distortion detection of FM signals. The recovered audio signal is coupled to the audio circuits through
additional filters that remove unwanted signals from the recovered audio. The frequency modulation (FM) circuits also
contain search circuitry which supplies the local oscillator control signal to the radio frequency RF circuits for automatic
tuning of the local oscillator. This control signal tunes the local oscillator automatically, as required, to attain the 1.0 MHz
intermediate frequency. Once frequency lock has been attained, the frequency of the local oscillator is held constant by
a dc output from the FM detector circuit in the frequency modulation (FM) circuits. The limited intermediate frequency
(IF) signal from the frequency modulation circuits is available at the front panel IF OUT connector when the FUNCTION
kHz DEV has been selected.
e. Audio Circuits. The recovered audio signal from the amplitude modulation (AM) circuits or the frequency
modulation (FM) circuits, as determined by the selected function, is supplied to the audio circuits. Baseband processing
circuitry in the form of cascadable high-pass and low-pass or de-emphasis filters allows precise control of post-detection
bandwidth. The baseband circuitry also contains additional amplification and precision peak detectors. The precision
peak detectors convert the recovered audio into a proportional dc signal, which is supplied to the digital panel meter
(DPM) circuits.
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