FM Modulation Monitor

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SKU: INO-531N Category:


FM Modulation Monitor


Incorporating all the necessary features for station setup, regulatory compliance, and remote monitoring, Inovonics’ 531N is the ultimate choice for essential FM signal monitoring. Dependable, direct input and off-air reception lets you keep a sharp eye on your total RF signal performance whether you’re right at the transmitter site or half way around the world.

The high-resolution LED bargraph displays are easy to read, and a “floating dot” program peak marker eliminates any ambiguity in the total-mod measurement. Off-air readings are qualified by Inovonics’ exclusive multipath indicator, which also aids in antenna alignment during initial station build-outs. In addition, readouts of signal strength and synchronous AM noise qualify the incoming signal and validate measurements.

Synchronous AM Noise Monitoring – an exclusive with Inovonics.

Rear-panel alarms constantly check for Audio Loss, RF Loss, Multipath and Overmodulation, while advanced notifications alert personnel with e-mail or SMS messaging when any or all of the alarms occur.

WEB INTERFACE: To view the 531N web interface in action, simply follow this link in your web browser:



  • Complete set of metering and measurement tools to ensure FM broadcast regulatory compliance worldwide.
  • Separate inputs for Antenna, High Level ʻdirectʼ RF, and composite signal measurements.
  • Full-time display of Signal Strength and Multipath effects, plus Synchronous AM noise output (BNC) to validate readings and aid in antenna alignment.
  • Synchronous AM Noise Monitoring – an exclusive with Inovonics
  • Composite and Balanced analog Left and Right line outputs.
  • Front panel metering for subcarrier injection levels: 38kHz, 57kHz (RDS), 67kHz, & 92kHz.
  • Advanced Metering includes an FFT Spectrum Analyzer for the MPX, Left/Right XY plots and audio levels over time
  • Alarms for Peak Overmodulation, Signal Loss, Program Audio Loss and Multipath.
  • Rear-panel closures to ground enable remote alarm indications and preset station switching.
  • “Responsive” Remote Control Web interface with live audio streaming.
  • SNMP control and support.



TypeDouble-conversion superhet circuit with proprietary ultra-linear, noise-rejecting pulse-counting FM detector
Tuning Range76MHz – 108MHz  in 100kHz increments
Station Presets6 user-programmable pushbutton presets; local or remote selection
RF InputsF connector for 75-ohm antenna
BNC connector for high-level 50-ohm RF sample; 1V - 7V r.m.s.
Receiver Sensitivity10μV (10dBf) for 50dB mono quieting; 250μV (60dBf) required for valid Total Modulation reading
Receiver Selectivity±0.25dB, 20Hz - 50kHz; –1.5dB or less at 100kHz (see Figure 1); –35dB (typ) at 200kHz (see Figure 2)
Baseband OutputBNC composite/MPX demod output; 3Vp-p at 100%-modulation.
RSSI (Incoming Signal Level)Independent front-panel LED bargraph metering of the relative strength of the incoming carrier
Multipath Distortion MeasurementIndependent front-panel LED bargraph metering of the relative degree of multipath reception-distortion effects
AM Noise MeasurementFront-panel metering may be switched to show the relative level of the incidental, synchronous AM noise component of the FM carrier. AM noise is routed to a separate BNC output and also monitored by headphones when selected.
Baseband InputAdjustable BNC composite/MPX input to stereo decoder and subcarrier measurement circuitry. Accepts 100%- modulation levels (typical 3V p-p) equating to ±75kHz carrier deviation.
Composite/MPX Metering58-segment LED bargraph display has quasi-peak response with floating peak hold. Switchable between positive deviation, negative deviation and highest of either
Meter Resolution1% accuracy between 80% and 100% carrier modulation; 2% resolution between 46% and 80% (100% = ±75kHz carrier deviation)
Metering IntegrationUser-selectable at 0.1ms, 0.2ms, 0.5ms and 1.0ms
Meter Frequency Response+0/–1%, 10Hz - 100kHz
Peak FlasherProgrammable in 1% increments between 95% and 120% of total carrier modulation.
Switch-Selected Composite/MPX InputDirect connection to the demodulated output of the offair receiver section
BNC connector accepts an external baseband input 3V p-p (adjustable) equivalent to full (±75kHz) carrier deviation
Program Audio OutputsBalanced XLR Left & Right channel stereo program outputs deliver +4dBm at 100% monaural modulation (±75kHz carrier deviation)
Front-panel headphone jack
Demod Metering DisplayDual LED bargraph displays show Left and Right or L+R and L–R demodulated program audio. The display is peak responding between +10dB and –30dB, and average responding between –30dB and – 64dB.
Stereo Audio Frequency Response±0.5dB, 10Hz - 15kHz
Stereo Signal-to-Noise RatioUnmodulated (stereo) carrier noise is better than 65dB below 100% modulation with de-emphasis applied.
Distortion<0.075%THD at 400Hz 100% Modulation Mono. No De-Emphasis, unweighted
Stereo Separation55dB, 50Hz - 15kHz with an external composite/MPX input. In the off-air mode, receiver selectivity limits separation to >45dB, 50Hz - 15kHz (see Figure 3).
Stereo CrosstalkBy the M/S or S/M (stereo-difference) method, >60dB, 50Hz - 5kHz, derated to >45dB at 15kHz with external composite/MPX input. In the off-air mode receiver selectivity limits crosstalk to >50dB, 50Hz - 5kHz, derated to >35dB at 15kHz (See Figure 4).
Stereo/Mono SwitchingMode switching is automatic with a front-panel indicator; monaural reception may be forced with a front-panel button or from the remote Web Interface.
Program De-EmphasisMay be turned on and off from front panel or Web Interface (indicator provided); internal jumper selects 50μs or 75μs characteristic.
Meter Scaling and Resolution/Accuracy58-segment LED bargraph display measures stereo pilot and subcarrier levels between 2.6% and 14% with 0.2% resolution and accuracy. Injection percentages refer to 100% = ±75kHz carrier deviation.
Metering CharacteristicPeak-responding in all modes
Measurement Filters19kHz Stereo Pilot Tone
38kHz Residual Stereo Subcarrier
57kHz RDS/RBDS Data Subcarrier
67kHz SCA Audio Subcarrier
92kHz SCA Audio Subcarrier
Peak FlasherGround-closure output gives remote indication coincidental with the front-panel Peak Flasher
Program Audio LossFront-panel LED and ground-closure (remote) alarm is programmable from the front panel to indicate when either stereo channel remains 10dB or more below 0dB for a period that may be set between 10 seconds and 2 minutes.
Low SignalFront-panel LED and remote ground-closure alarm is fixed at a level of incoming RF signal below which modulation measurements are not valid.
MultipathFront-panel LED and remote ground-closure alarm is fixed at a degree of multipath distortion above which modulation measurements are not valid.
Rear Panel ConnectorAn RJ45 jack accepts TCP/IP Network Connections for remote setup and operation.
SettingsDHCP or Static IP address can be set from the front panel.
SNMPSupports SNMP remote monitoring and control (MIB file can be downloaded directly from the 531N)
SMTPSupports email services with or without SSL.
Dynamic DNSSupports,, and
Power Requirements105 - 130VAC or 210 - 260VAC, 50/60Hz; 20 watts
Size and Weight3½”H x 19”W x 12”D (2U); 14 lbs. (shipping)
Environmental0°C - 50°C operating temperature range; 95% noncondensing relative humidity; up to 3000 meters AMSL