Inovonics 531N FM Modulation Monitor

531N FM Modulation Monitor

• FM Modulation Monitor
• Tunable Off-Air
• 76MHz – 108MHz
• Alarms
• SCA Injection
• RDS Decoding
• AM Noise
• Full Network Control and Monitoring


  •  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 531N adds new monitoring and control features which are accessible remotely via the Web interface.

  • 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.

  • 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.



  • 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.
  • 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.



Type: Double-conversion superhet circuit with proprietary ultra-linear, noise-rejecting pulse-counting FM detector.
Tuning Range: 76MHz to 108.1MHz in 100kHz increments. 
Station Presets: 6 user-programmable pushbutton presets; local or remote selection. 

RF Inputs:
1) F connector for 75-ohm antenna.

2) BNC connector for high-level 50-ohm RF sample; 1V - 7V r.m.s. 

Receiver Sensitivity: 10µV (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 Output: BNC 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 Measurement: Independent front-panel LED bargraph metering of the relative degree of multipath reception-distortion effects. 
AM Noise Measurement: Front-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 Input: Adjustable BNC composite/MPX input to stereo decoder and subcarrier measurement circuitry. Accepts 100%- modulation levels of typical 3V p-p equating to ±75kHz carrier deviation.
Composite/MPX Metering: 58-segment LED bargraph display has quasi-peak response with floating peakhold. Switchable between positive deviation, negative deviation and highest of either.
Meter Resolution: 1% accuracy between 80% and 100% carrier modulation; 2% resolution between 46% and 80%. (100% = ±75kHz carrier deviation.)
Metering Integration: User-selectable at 0.1ms, 0.2ms, 0.5ms and 1.0ms.
Meter Frequency Response: +0/–1%, 10Hz - 100kHz.
Peak Flasher: Programmable in 1% increments between 95% and 120% of total carrier modulation.

Switch-Selected Composite/MPX Input:
1) Direct connection to the demodulated output of the off-air receiver section.
2) BNC connector accepts an external baseband input 3V p-p (adjustable) equivalent to full (±75kHz) carrier deviation. 

Program Audio Outputs:
1) Balanced XLR Left & Right channel stereo program outputs deliver +4dBm at 100% monaural modulation (±75kHz carrier deviation).
2) Front-panel headphone jack. 

Demod Metering Display: Dual 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 Ratio: Unmodulated (stereo) carrier noise is better than 65dB below 100% modulation with deemphasis applied.
Distortion: <0.075%THD at 400Hz 100% Modulation Mono. No De-Emphasis, unweighted
Stereo Separation: >55dB, 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 Crosstalk: By 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 Switching: Mode switching is automatic with a front panel indicator; monaural reception may be forced with front-panel button or from the remote web interface.
Program De-Emphasis: May be turned on and off from front panel or web interface (indicator provided); internal jumper selects 50μs or 75μs characteristic.

Meter Scaling and Resolution/Accuracy: 58-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 Characteristic: Peak-responding in all modes.

Measurement Filters:
1) 19kHz Stereo Pilot Tone
2) 38kHz Residual Stereo Subcarrier 
3) 57kHz RDS/RBDS Data Subcarrier 
4) 67kHz SCA Audio Subcarrier 
5) 92kHz SCA Audio Subcarrier 

Peak Flasher: Ground-closure output gives remote indication coincidental with the front-panel Peak Flasher.
Program Audio Loss: Front-panel LED and ground-closure (remote) alarm. 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 Signal: Front-panel LED and remote ground-closure alarm; fixed at a level of incoming RF signal below which modulation measurements are not valid.

Multipath: Front-panel LED and remote ground-closure alarm; fixed at a degree of multipath distortion above which modulation measurements are not valid.

Rear Panel Connector: An RJ45 jack accepts TCP/IP Network Connections for remote setup and operation. 
Settings: DHCP or Static IP address can be set from the front panel.
SNMP: Supports SNMP remote monitoring and control (MIB file can be downloaded directly from the 531N).
SMTP: Supports email services with or without SSL.
Dynamic DNS: Supports, and
Power Requirements: 105–130VAC or 210–260VAC, 50/60Hz; 20 watts. 
Size and Weight: 3½”H x 19”W x 12”D (2U); 14 lbs. (shipping).
Environmental: 0°C - 50°C operating temperature range; 95% non-condensing relative humidity; up to 3000 meters AMSL.