Nautel NX25 – 25kW AM Transmitter

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The NX25 stands apart compared to other 25kW AM transmitters on the market today with efficiency of 88%, compact enclosures that are 1/3 to 1/4 smaller, and digital transmission capability.

NX Advanced User Interface (AUI)

The NX25 features a 17 inch color LCD screen with a wide range of configurable displays. The AUI can be controlled by touch screen, or via a mouse and keyboard. Log in to the AUI on the NX25 using a web browser on your PC or mobile device to gain access to unprecedented instrumentation and control whether you are at the transmitter site, the office or home.



Some of the features of the AUI include:

  • Real-time Impedance locus measurement
  • Instrument grade spectrum analyzer
  • IBOC modulation analyzer
  • Module level monitoring and control
  • Logging of all functions

Screens are easy to set up and read, and clearly display the parameters you need to see.

100% Remote Access

No matter where you are, you’re only moments away from ensuring your NX25 transmitter is operating optimally. Pull up a web browser, enter your transmitter’s IP address, and you’re connected. The complete local NX25 display functionality is available via the internet. Users can access status, controls, alarms, logs and reports.

Control Redundancy

The touch screen interface is implemented as a non-critical functional unit and may be completely removed from the system without affecting transmitter operation.The NX25 also supports SNMP and traditional direct wired contact closure capability for local or remote control.

Real-Time Impedance Measurement

The NX25 uses Nautel’s patent-pending real-time impedance measurement technology which compares the transmitted voltage and current waveforms to measure the load impedance over the operational frequency range. Because this system can measure impedance without requiring artificial swept tones, the antenna system impedance locus can be measured without taking the station off-air. Impedance is measured at the combiner so no correction for harmonic filter phase is required. This capability allows the antenna system load impedance can be optimized and monitored while the transmitter is operating normally.The NX25 also includes a built in directional coupler for spectrum measurement.Traditionally transmitters have utilized a voltage or current sensor. The directional coupler increases the accuracy of spectrum measurement into a real antenna load which is quite important when measuring adjacent channel emissions.

Class Leading Space Efficiency:

The NX25 is smaller than any other transmitter in its class. Yet even with such a compact footprint the NX25 offers easy and spacious access to all major serviceable components and modules.

On-Air Serviceability

NX25 transmitters are ruggedly engineered to provide easy on-air service and maintenance. In the NX25, 10 amplifiers combine to deliver up to 37.5 kW of average power (carrier plus modulation). At all power and modulation levels, all modules contribute equally to the final output. If an amplifier fails, no stress is imposed on the remaining modules and spectral integrity is never compromised. Maintenance can be performed whenever it is convenient. Generous ventilation is provided by redundant brushless DC ball bearing fans mounted in removable trays below the power modules. Airflow is unaffected by AC supply variations, further ensuring cool operation and long term reliability.

Automatic Standby

The most critical part of a transmitter is the exciter section, which provides the RF carrier and modulation control signals to the power modules. A unique feature of Nautel transmitters is the complete duplication of these circuits. Should a failure occur in the RF drive or modulator drive, the transmitter automatically switches over to the built-in standby exciter. This dramatically enhances the already high operational reliability inherent in the modular solid-state design.

Unattended Operation

The NX25 transmitter is built to stay on the air without human supervision. The NX25 was designed with a high VSWR threshold of 1.5:1 at rated power with 100% modulation. With more extreme VSWR values, power is automatically reduced to a safe level. A unique circuit also dynamically stabilizes power against AC line voltage variations. After an AC power loss, over voltage or RF overload, prior operating status is automatically restored.

Powerful Building Blocks

The building block of the NX25 is an integrated RF amplifier/modulator with a carrier power capability of 2500W. These power modules plug into the front of the transmitter, making service easy. The Class-D RF amplifier uses four transistors that can be replaced without soldering using only a screw driver. Due to advances in amplifier technology, this amplifier is so efficient (96%) that it is capable of operating at over 10kW continuously. This power capability results in very low transistor junction temperatures assuring reliable operation even in the highest ambient temperatures.

Redundant Architecture

Redundancy features and protection systems help you maintain operation without a site engineer, reducing your operating costs even further.

The NX25 offers:

  • Redundant Exciters
  • Redundant Modulators
  • Parallel/redundant fans
  • Redundant low voltage power supplies
  • Redundant amplifier control power supplies
  • Failsafe manual and remote control


Technical Specification

Technical Specification

Transmitter Type

  • Medium wave, AM, 100% solid state


  • Ten RF power modules, each including a single integrated RF amplifier/modulator
  • No frequency dependant parts in RF module
  • Each module is hot pluggable and has internal microcontroller for protection and monitoring over a serial bus
  • Short circuit protection at the module level offers an added layer of protection
  • Dual redundant digital single board exciters utilizing FPGA/DSP technology with automatic
  • changeover
  • Pre-correction utilized specifically to improve digital performance
  • Redundant low voltage power supplies

RF Output Power

  • Maximum: 37.5 kW RMS
  • Carrier Power Range: 0 to 27.5 kW
  • Up to 62 preset levels, presets may include other operational parameters such as DRM vs Analog and different program inputs
  • Output level stabilized against AC supply voltage variations
  • Built in Dynamic Carrier Control
  • Built in AM stereo

RF Output Connection

  • 3 1/8 EIA female standard, other outputs on request

RF Output Impedance

  • 50 ohms, unbalanced


  • 88% typical at 25 kW


  • 4000 peak reflected watts (1.5:1 VSWR @ 25 kW, 100% modulation) results in instantaneous power shutback
  • 1000 reflected watts RMS (1.5:1 VSWR @ 25 kW, 0% modulation) results in a graceful power reduction

Frequency Range

  • 531 kHz to 1,620 kHz.
  • “Quick frequency change capability”

Frequency Stability

  • ±2 ppm over temperature range
  • Optional 300 ppb ovenized source with GPS option

Modulation Type

  • 6 phase direct digital modulation
  • 1.8 MHz modulation sample rate

Modulation Capability

  • 140% positive peak modulation to 25 kW
  • 130% positive peak modulation to 27.5 kW

Spurious and Harmonic

  • 80 dB or more below carrier at 25 kW
  • Meets ITU-R SM.328-10
  • Meets ITU-R SM.329-9




  • 340 V to 510 V, 3 phase or to customer specifications

Power Supply Variation

  • ±10% voltage, 47 Hz to 63 Hz*

Power Consumption

  • 28.4 kW typical at 25 kW, 0% modulation
  • 42.5 kW typical at 25 kW, 100% modulation
  • Cos (theta)
  • 0.95 typical




Temperature Range

  • 0°C to + 50°C
  • Derate 3°C per 500 m above sea level (2°C per 1,000 ft)

Humidity Range

  • 0% to 95% non-condensing


  • 0 m to 4,000 m (0 ft to 13,000 ft)

Cooling Air Requirements

  • 2,550 m3/hr (1,500 CFM)


  • Meets EN60215: 1996 Safety Requirements for Radio Transmitting Equipment





  • 184.2 cm H x 95.7 cm W x 120 cm D
  • (72.4″ H x 37.7″ W x 47.2″ D)


  • Main transmitter cabinet with modules installed: 567 kg (1250 lbs)
  • Power transformer: 262 kg (578 lbs)



Analog Broadcast Inputs

  • Dual AES-EBU Digital Audio inputs* adjustable from -30dBFS to 0dBFS for 100% modulation
  • 600 ohms balanced analog audio input +10 dBm nominal for 100% modulation, adjustable from -10 to +12 dBm

Digital Broadcasting Inputs

  • I,Q over AES-EBU, SRC available* with sample rate converter
  • I,Q over LVDS, 3 pairs, Clock, Data, frame sync
  • *Two AES-EBU inputs provided and may be used for either analog audio or digital I,Q inputs
  • Optional Embedded DRM Generator/Content Server
  • Optional HD Radio Generator (Exgine™)

Frequency Response

  • +0.2 dB/-0.8 dB, 30 Hz to 10,000 Hz.
  • Optional audio input filters available to meet regional bandwidth restrictions

Total Harmonic Distortion

  • Better than 0.8% (THD), 30 Hz to 10,000 Hz at 95% modulation (typical)

Intermodulation Distortion

  • SMPTE 1:1 Ratio, 60Hz/7kHz, 95% Mod Peak – 0.5% @ 25 kW (typical)
  • DIM-B, 2.96kHz/9kHz, 80% Mod Peak – 0.5% @ 25 kW

Carrier Shift

  • 0.5% or less

Hum and Noise

  • -65 dB or better below 25 kW, 100% modulation


  • Extensive Control/Monitoring/Troubleshooting system through 17″ front panel LCD touchscreen.
  • Touch panel control system is non-critical and may be removed from the system without affecting transmitter operation or remote control/monitor via direct wired connections. Redundant back-up control interface provides control in case of front
  • panel computer system failure. Built in instrumentation providing detailed spectrum/impedance and modulation analysis.


  • Cube
  • DC Voltages (B+, PA and 15V)
  • DC Current
  • Sample Levels (PDM and RF Drive)
  • Fan Speeds
  • Heat Sink Temperature
  • Rack
  • DC Voltage levels (15V, 5V, 30V, 48V and B+)
  • Rectifier Fan Speeds
  • DC Current
  • Rectifier Heat Sink Temperature
  • AC Voltage
  • Exciter
  • Output Current (RMS, Peak, Carrier)
  • Output Voltage (RMS, Peak, Carrier)
  • Forward Power (RMS, Peak, Carrier)
  • Reflected Power (RMS, Peak, Carrier)
  • Audio/Modulation Levels (RMS and
  • Peak – Positive/Negative)
  • Load Impedance
  • Controller
  • PDM and RF Drive Levels
  • Ambient Temp

RF Monitor

  • RF monitor is a power sample (using a directional coupler) that will allow for accurate audio performance measurements


  • Easy access to current transmitter operating state, past and present alarm conditions and historical trends of both digital and analog channels


  • Intuitive easy to read built in scheduler
  • Up to 144 yearly rules can be defined by user
  • Remote Control/ Monitoring

Three Remote interfaces:

  • Direct wired optically isolated inputs and open collector outputs
  • Web interface – All locally available control is available over TCP/IP web interface
  • SNMPv1



Specifications defined in a laboratory environment with high grade source and demodulation equipment. Standard factory measurements does not include all items.



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