Off-Air AM Modulation Monitor

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The Model 520 is a compact and accurate AM Modulation Monitor with a bright, easy-to-read display. In addition to one pair of peak flashers preset at the –100% and +125% absolute-modulation limits, a second pair of flashers may be user-set at alternative trip points.

Alarm tallies are available on the rear panel, and an RS-232 port allows data logging by computer or modem. An outdoor active antenna is optionally available for difficult reception situations.


  • Mono-AM Modulation Monitor with built-in preselector

  • Accurate off-air modulation measurements

  • Remote alarms for overmodulation, carrier-loss and program-loss



Tuning Range

530kHz to 1710kHz in 1kHz increments; programmed with a DIP-switch under the top cover.

Total Modulation Measurement

The front-panel bargraph readout is peak-responding and incorporates a “floating” peak-hold display. It may be switched between positive or negative carrier modulation and indicates modulation peaks from 22% to 130% in 2% increments. The display also indicates incoming RF carrier level.

Peak Flashers

Preset “absolute limit” flashers are factory calibrated at –100% and +125% carrier modulation. Separate POS and NEG flashers may be set by the user at values ranging from 80% to 130%.

Accuracy and Calibration

Modulation readings are referenced to the incoming unmodulated carrier level, obviating the need for an internal calibration source. RF gain for off-air measurements is servo-stabilized, and a relative measurement of the RF input level may be monitored on the front-panel bargraph readout.

Optional Antenna

The optional outdoor antenna is a compact, broadband ferrite-rod design with an integral preamplifier powered by the Model 520. The antenna is completely weatherproof and attaches easily to an existing mast or roof-vent pipe. The antenna may be located up to 100 feet from the Monitor. Cabling is not included. (The antenna uses common 75-ohm coax cable fitted with “F” connectors on both ends.)

RF Inputs

  • The 75-ohm antenna input (F connector) is phantom powered for the optional active antenna.  A random “longwire” antenna may prove adequate in high-signal areas with low levels of interference.

  • A high-level input (BNC connector) accepts a direct RS sample in the 1V–10V r.m.s. level range.

Measurement Bandwidth

Carrier amplitude demodulation extends to 10kHz, ±0.2dB

Audio Response

±0.5dB, 20Hz–10kHz; follows NRSC de-emphasis

Audio Distortion

Less than 0.5% THD at 100% modulation

Audio Noise

Better than 50dB below 100% modulation

Audio Inputs

  • Active-balanced program audio output (XLR connector) on rear panel; 200-ohm resistive source
  • Front-panel headphone jack

Alarm Outputs

Open-collector transistor saturations to ground for –100% modulation, +125% modulation, loss of carrier and loss of program audio.

Data Output

An RS-232 serial data port (DB9 connector) supplies raw modulation information for remote analysis or archiving.

Power Requirement

105–130VAC or 210–255VAC, 50/60Hz; 10W

Size and Weight

13/4”H x 19”W x 7”D (1U); 7 lbs.