Section III. Technical Principles of Operation

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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