Belar AMM-3A AM Modulation Monitor




The Belar AMM-3A AM Modulation Monitor is an all-solid-state precision AM demodulator which exceeds the US Federal Communications Commission standards for measuring the total modulation characteristics of medium-frequency AM broadcast transmitters.


The detector circuitry of the AMM-3A is non-frequency discriminating, so the unit is suitable for use with shortwave transmitters as well. The AMM-3A employs a unique analog divider circuit which maintains the accuracy of all indications despite changes in carrier level. Simultaneous indications of negative and positive modulation appear on two semi-peak reading meters. Negative and positive modulation peaks are simultaneously indicated on two thumbwheel-programmable peak indicators, as well as individual fixed negative 100% and positive 125% modulation peak indicators. Front-panel pushbuttons select meter indications of relative carrier level and AM noise. The AMM-3A incorporates a carrier-level alarm which indicates when the carrier level falls outside the range required for accurate indications. Also included are an adjustable modulation-level alarm, and a calibration reference to check the accuracy of the readings at any time. An internal jumper can be set to apply the USA NRSC de-emphasis characteristic to the audio outputs.

  • Two modulation meters for simultaneous measurement of positive and negative modulation
  • Two adjustable peak modulation indicators, thumbwheel programmable from 1-99% negative and 1-199% positive modulation.
  • Separate, dedicated 100% negative and 125% positive peak indicators
  • All modulation indications independent of carrier level
  • Semi-peak reading modulation meters respond accurately to short duration program peaks
  • Built-in Carrier Level Limit (Loss) and Modulation Level Alarms with dry relay contact outputs
  • Built-in modulation calibration check
  • Linear-phase detector filter provides accurate modulation indications without overshoot.
  • Switchable NRSC de-emphasis

In addition to the AMM-3A, Belar has a number of analog remote meter panels for use with Belar AM monitors. The MP-7A meter panel is designed for use with the AMM-3A or AMM-3 AM Modulation Monitor. It features the same large backlit meters and all the LED’s as the AMM-3A or AMM-3 for easy remote monitoring.

Technical Specification

Technical Specification

RF Frequency Range:    
260 kHz to 50 MHz

RF Sensitivity:    
5 Vrms to 10 Vrms

RF Input Impedance:  
1000 ohm standard, 50 ohm optional

RF Input:     
Rear-panel BNC connector

Positive Modulation Meter Range:  
0% to 133%

Negative Modulation Meter Range:  
0% to 100%


Peak Modulation Indicators:
Negative peak range 1% to 99% in 1% increments
Positive peak range 1% to 199% in 1% increments

Fixed Indicators:
-100%, +125%

Frequency Response:  
+0.0 dB, -0.5 dB, 20 Hz to 15 kHz

Envelope Pulse Response:   
Overshoot <1%

Modulation Level Alarm:    
Adjustable threshold, fixed timeout (-15 seconds)

Carrier Level Meter Range:    
0% to 133%

Carrier Limit (Loss) Alarm:   
50% & 133% relative level

Audio Monitoring Output:    
+20 dBu (4.9 Vrms), 600 ohm active-balanced

Audio Test Output (proof of performance): 
2.5 Vrms, rear-panel BNC connector

Auxiliary Detector Output:   
5 Vrms

Audio Response:  
+0.0 dB, =0.5 dB, 20 Hz to 15 kHz
(detector response -3 dB at 50 kHz)

0.25% maximum at 99% modulation

Signal-to-Noise Ratio:
>_80 dB (de-emphasis out)

AM Noise Level Metering Range:   
-40 dB to -70 dB

Remote Outputs:
Adjustable negative and positive peak indicators, -100% and +125% peak modulation indicators; positive and negative remote modulation meters, carrier level reference, modulation level alarm, carrier limit (loss) alarm

Operating Temperature:    
0°C to +50°C

Power Consumption:  
8 watts, 100-240 V, 50-60 Hz

5¼”H ´ 19″W ´ 8½” D (EIA rack mount)

Shipping Weight:     
11 lbs. (5 kg)

© Copyright 2000 Belar Electronics Laboratory, Inc. All rights reserved.

New in 2015 – 3 Year Factory Warrant to all Belar Products 

Product Resources

Product Resources

Belar AMM-3A Manual

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