The Belar AMM-4 is a digital AM frequency counter specially designed for automatic monitoring of broadcast transmitters. The counter accurately monitors 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 that are inhibited so that three successive errors are required to signal an alarm. This prevents false off-frequency conditions from occurring. For example, when the carrier frequency exceeds ±10 Hz, a front panel LED indicator immediately goes to a flashing state. This warns of an impending off-frequency condition. After three successive counts in this condition, the monitor will activate 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 successive counts, 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 occurs due to a low RF level or counter malfunction, the frequency alarms remain in their OFF condition; however, a front panel LED indicator and an optional relay contact are provided to warn of an invalid count.
If the counter is driven by a modulating source, a count inhibit input is provided. This input is controlled by the 100% negative output of a modulation monitor; therefore, if the modulation level exceeds negative 99%, the display and alarms will remain in 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.