TM 11-6625-3059-10
Section III. Technical Principles of Operation
PRINCIPLES OF OPERATION
Introduction
a. AM/FM Modulation Meter ME-525A/USM is a compact, solid-state amplitude modulation (AM) and frequency
modulation (FM) modulation meter that covers the carrier frequency range of the signal under test from 0.01 to 1.2 GHz.
Amplitude or frequency modulation is displayed on a four-digit light-emitting diode (LED) display, which provides a
maximum resolution of + 10 Hz deviation or 0.01% AM.
b. Operation of the ME-525A/USM is fully automatic. The largest signal present at the RF IN connector is
converted to a 1.0 MHz intermediate frequency signal, which is automatically leveled. With a fixed carrier level, the
modulation depth of AM signals is directly proportional to the peak amplitude of the recovered audio. The deviation of an
FM signal is also directly proportional to the peak amplitude of the recovered audio. The modulation component of the
input signal is recovered and converted to a proportional dc level, which drives the four-digit LED display to provide
calibrated indications of amplitude modulation or frequency modulation. IF level may also be displayed. Pushbutton
selection of baseband filtering assures accurate modulation readout with minimum residual modulation error.
2. Simplified Block Diagram
a. General. For purposes of this discussion, the circuits of the ME-525A/USM can be grouped by function as
follows:
radio frequency circuits
frequency modulation circuits
amplitude modulation circuits
audio circuits
digital panel meter (DPM) circuits
logic circuits
power supply circuits
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