Belar AMM-4 AM Digital Frequency Monitor



The Belar AMM-4 is a digital AM frequency monitor designed especially for automatic broadcast transmitter monitoring. The counter will accurately monitor any frequency from 10 kHz to 50 MHz. A large 3½ digit LED display provides a range of ±1999 Hz deviation from the assigned channel. A front panel LED indicator warns of low RF level or loss of carrier; an optional relay contact can be provided. The monitor also provides two off-frequency alarms which are inhibited so that three successive errors are required to signal an alarm. This prevents false off-frequency conditions. For example, of the carrier frequency exceeds ±10 Hz, a front panel LED indicator is immediately actibated into a flashing state. This warns of an impending off-frequency condition. After three successive counts in this condition, the monitor will actibate an optional relay contact for remote alarms. It also changes the front panel indicator to a continuous ON state. If the frequency exceeds ±20 Hz for three sucessive counts, then the second front panel indicator is activated as well as an optional relay contact.

Another feature of the AMM-4 is the invalid count alarm. If this condition arises due to low RF level or a malfunction in the counter, the frequenct alarms are held in their OFF condition; however, a front panel LED indicator and optional relay contact are provided to warn of an invalid count.

If the counter is driven by a modulated source, a count inhibit input is provided. This input is controlled by the 100% negative output of a modulation monitor; thus if the modulation level exceeds 99% negative, then the display and alarms would be held at the state they were in prior to the overmodulation fault.

Relay contact outputs (described above) are available as options. A switchable 1 MHz input/output connector is provided for frequency comparison.



  • Inhibited off-frequency alarms
  • ±10 Hz Off-Frequency Alarm
  • ±20 Hz Off-Frequency Alarm
  • RF Level Alarm
  • Invalid Count Alarm
  • Count Inhibit AlarmPILOT
  • External timebase input
  • 1 MHz output
  • Large 3½ digit LED display
  • Low profile design
  • Designed for ATS Operation

Option 01 – Three Alarm Relay Card AMM-4 available 

Technical Specification

Technical Specification

Frequency Range:  
10 kHz to 50 MHz

Large 3½ digit LED

Display Range:
±1999 Hz

100 mV, Unmodulated
2 Vrms, 99% modulation

Gate Time:  
2 seconds

0.5 Hz

Time Base (internal):    
6 MHz

±1 x 10^-6 per year

Time Base (External):
1 MHz TTL compatible level

10 Hz alarm:    
Front panel LED, inhibited for three successive errors

20 Hz alarm:     
Front panel LED, inhibited for three successive errors

RF alarm:    
Front panel LED, warns of low RF level or carrier fail condition

Invalid Count Alarm:    
prevents false counts due to low RF level or problem in counter

Count Inhibit Input:    
External contact required capable of sinking 2mA

Level Alarms:
3 LED Indicators

1.75″H x 10.5″D x 19″W (EIA Rack Mount)

Power Requirements:   
115/230 VAC, 50/60 Hz

© 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-4 Manual

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