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NX Series 25 – 2000 kW Tim Hardy Head of Engineering
© Nautel Limited 2008 This presentation has been produced for Nautel customers and agents and is not for distribution without the expressed written consent of Nautel.
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Product Overview – NX 50 20 hot pluggable RF power modules combine to deliver up to 75kW of average power (carrier plus modulation) RF Power Capability 55 kW RF maximum output power 140% positive peak modulation at 50kW 125% positive peak modulation at 55kW 100% positive peak modulation at 50kW with 4 modules removed 1.5:1 VSWR threshold at 50kW 88% AC to RF Efficiency (90% NX100) 72.5” (1.84m) Read information on slide. 44” (1.12m) 38” (0.96m)
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Advanced User Interface
NX50 Block Diagram ExGine Embedded IBOC Generator Mod Drive 1-3 2.5kW EtoX IP Stream Program Inputs, AES/EBU, Analog L&R, I&Q Mod Drive 4-6 2.5kW Control/Interface Card Mod Drive 7-9 2.5kW Dual T Output Matching Network DSP Exciter Combiner TCP/IP Webserver / Remote Control RF Drive 2.5kW DSP Exciter Manual Control 2.5kW RF Driver 15VDC PS RF Driver 15VDC PS Fan 48VDC PS Fan 48VDC PS 2.5kW Advanced User Interface 17” LCD GUI Controller LVPS Controller LVPS 20 Modulator / RF Amps Mains Transformer Phase Control Rectifier Choke Input Filter AC INPUT 400V DC
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RF Amplifier/Modulator Module
Modulator Stage RF Amplifier Stage SiC Rectifiers (x3) B+ Supply Modulator Filter Digitally Controlled Modulator Drivers (x3) Digitally Controlled RF Drivers (x4) RF FET (x4) Modulator FET (x3)
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RF Amplifier / Modulator Module
Hall Effect Current Sensor Replaceable Fuse RF FET’s Replaceable with a screwdriver RF Driver Modulator Input RJ45 Drive Transformers Modulator Stage RF Amplifier Stage Micro Controller >96% PA Efficiency Amplifier runs very cool for maximum MTBF even at 1710 kHz
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RF Amplifier / Modulator Module
Broadband: No adjustments across the AM band 525 – 1710 kHz Hot pluggable. Heavy gold connectors both sides 3 Phase Modulator with High Efficiency Silicon Carbide Modulator Diodes Balanced RS422 digital inputs for modulation and RF drive Internally protected against short circuits, drive faults, fan failure, heat-sink temperature RS485 connection to micro-Controller for instrumentation and fault diagnosis
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9 Phase Digital Modulation
The 9 Modulation phases are separately synthesized digitally at 317 MSPS in the exciter FPGA. This results in very low quantization noise. Each Modulator phase samples the desired envelope at a rate of over 300 kHz. The 9 Phase process samples the envelope at over 2.7 million samples per second. This rate does not change with frequency. Distortion due to the modulation process is essentially eliminated. Reduced switching harmonic content allows for a very low Q modulation filter.
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Digital Modulation Technology
Separate power processing stages for Envelope Modulation and RF Amplification are Employed. This approach enjoys the following benefits: Optimal RF Transistor switching is maintained at all modulation levels. This cannot be achieved when the RF transistors must both convert DC to RF and vary the RF envelope. Digitally controlled RF Transistor switching has nearly eliminated switching loss across the AM band Allows for very high current capability transistors to reduce conduction loss Results in ultra high efficiency RF amplifier (up to 98% at 1710 kHz) Control of the RF amplifier DC supply input improves robustness into VSWR RF Amplifier DC Supply can be shut down during transient events to further improve robustness All amplifiers see the same load at all power and modulation levels
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Cooling 10 Ball Bearing DC Fans arranged in hot pluggable trays – two for each tray Typical fan life is over 11 years Fan speed is monitored by the power modules With 88% AC to RF efficiency, typically only 8kW of heat is produced, about the same as a typical 10kW FM transmitter Air is drawn in from the rear, and blown out the top at the front
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Main DC Power Supply AC input (at customer’s specified voltage) is converted to 250 VAC by the main transformer The 250V feed is rectified through a phase controlled three phase rectifier bank which provides regulation, voltage control and soft start Five large fuses protect banks of 4 modules. Individual fuses isolate faults at the 2500W power module level Filtering is done by choke input filters with a bank of electrolytic capacitors Supply Voltage is 400 VDC
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Low Voltage Power Supplies
Critical Low Voltage Supplies are Redundant LV Supplies Operate from Main AC Transformer Secondary for Improved Transient Suppression LV Supplies are fully regulated over a broad AC voltage range. This assures operation well outside normal mains fluctuations RF Drive Supply 15VDC Fan Supply 48 VDC (Switched) +5 and -15 VDC Logic Supply +12 VDC AUI +30 VDC Main Rectifier
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Combiner/Filter & Transient Protection
Series Resonant High Pass Section Voltage and Current Measurement for Active Protection Harmonic Filter Forward and Reflected Directional Coupler Ports for Spectrum Verification RF Amplifiers (x20) Adjustable Carbon Ball Gap Combiner Transformers (x20) Calibrated Fast Spark Gap (Pressurized Tritium) Frequency Agile: Harmonic Filter Re-Tune in a Few Hours
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Lightning/Transient/AC Protection
~10 uS Active VSWR Shutdown Series Capacitor/High Pass Stage Carbon Adjustable Ball Gap Static Discharge Choke Calibrated Fast Spark Gap (Pressurized Tritium) MOV Protection on Incoming AC Transient Attenuator Capacitors on AC Transformer Secondary (Common Mode) AC Surges Corrected by Phase Controlled B+ Rectifiers (Differential Mode)
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Control/Interface Card
AUI Interface Dual AES/EBU Inputs (Audio or I,Q) Balance Analog Audio Input Digital Modulation Outputs to Power Modules Ports for Parallel Remote Control Interface Backup Control Switches Dual (standard) DSP Exciter Slots Transmitter Master Control Processor
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Parallel Remote Interface
Darlington outputs for status Outputs have a configurable (red/green) LED for visual indication of status Optically Isolated Inputs for Control Inputs have a switch for testing or backup local control All inputs and outputs can be re-configured through the GUI Connections are made with pluggable terminal block connectors
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Embedded IBOC Generator
TCP/IP input stream from Exporter Digital I,Q output stream to NX exciter Fully embedded – no hard-drive or fans Operates from dual LVPS in NX transmitter No fans or hard disk
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Instrumentation NX User Interface includes extensive instrumentation features Modulation monitors Analog modulation: positive and negative peak and average levels Digital modulation: I and Q levels, peak and average Absolute modulation monitor 0 – 100% of peak envelope Spectrum analysis based on directional sample Real time impedance analysis Digital carrier demodulation Pre-correction curves Extensive internal metering of voltages, currents, drive levels at all levels including the power modules
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Real Time Network Analysis
Allows measurement of the impedance seen by the RF amplifiers while the transmitter is operating normally No tones are required, the measurement is made using the normal broadcast signal This information can be used to ensure impedance symmetry allowing optimal performance Nautel has filed a provisional patent application for this technology
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Exciter Section Synthesizes the RF Drive and 9 Phase Modulation Signals Digitizes RF combiner voltage and current samples for protection, instrumentation and linearization Corrects drive signals for amplifier and modulator non-linearities Receives analog and digital program inputs including I,Q baseband over AES/EBU Processes and generates components of signal such as: Audio filters and pre-emphasis Carrier control algorithms AM Stereo AMSS
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Exciter Block Diagram
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RF Phase Modulated Carrier 79.4 MSPS
FPGA Block Diagram 9 Phase PDM Generator I,Q to Mag Equalizer FIR – 64 Tap B+ Comp 8x FIR Interpolator AM/AM Correction 32x CIC Interpolator Gate 9 PDM 317 MSPS I,Q Ref 310 kSPS I,Q to Mag Phase Mag Phase To I 256x Interp FIR/CIC RF DAC Gate AM/PM Correction RF Phase Modulated Carrier 79.4 MSPS Carrier Frequency Complex Oscillator 256x Decimate FIR/CIC Combiner Current 310 kSPS High RF Current Sense 2fc IIR Filter RF ADC Combiner Current Gate Reflected Power Sense Scale & Phase Correction 256x Decimate FIR/CIC Combiner Voltage 310 kSPS 2fc IIR Filter RF ADC Combiner Voltage Gate
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Part 2 – Modulation and Pre-correction
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Theory of Operation X Envelope Elimination and Restoration
a.k.a. Kahn Technique Envelope Term RF Phase Term X
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AM Theory – General Signal
DSP Based Signal Generator Transmitter Power Stage Envelope Term Envelope Input RF Phase Term RF Input X Final Amplifier
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Envelope Bandwidth Expansion
Envelope Spectrum Undesirable bandwidth expansion of the envelope term is caused by the non-linear characteristic function: Linear distortion of the envelope causes non-linear distortion (IM) of the RF signal. The transmitter modulator bandwidth needs to be 2-3 times the RF signal bandwidth to avoid IM problems.
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Pre-Correction Principle
An amplifier characteristic g(x) may be corrected for with a complementary characteristic h(x) such that g(h(x)) = Gx For this to be true, G h(x) = g-1(x) Gx g(x) h(x) x h(x) Gx h g
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Pre-Correction Features
The FPGA has three correction sections in the forward path: Envelope equalization: Corrects for filtering effects in the modulator (envelope magnitude and phase response versus frequency) AM/AM Correction: Corrects for amplitude error in the modulator due to capacitive effects in the FET. (Essentially AM distortion) AM/PM Correction: Corrects for phase error in the RF amplifier due to capacitive effects in the RF FET. (IQM or IPM effects) Additionally it will be possible to correct for linear effects in the AM antenna system using a filter in the DSP
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Typical Correction Curves AM/AM
Gain Envelope Voltage
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Typical Correction Curves AM/PM
Phase Correction Envelope Voltage
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NX Modulator Response
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NX Modulator Response
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Antenna Considerations
-AM Antenna Systems, Especially Directional Arrays May Be Quite Narrow Band -Industry Standard Limits On The Sideband Are VSWR Better Than ±15khz Hermetian Symmetry Required With Tight Symmetry Limits Symmetry Measured At Amplifier Output – Transmitter RF Filter Must Be Considered Impedance Cusp Orientation Must Be Open To The Left (Constant Admittance) Well Behaved Impedance Cusp Correct Cusp Orientation
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Performance Results - AM
No visible distortion in the trough with AM to AM correction
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Performance Results - AM
Nearly total elimination of distortion at 25% modulation (Distortion approx %)
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Performance Results - IBOC
IBOC Performance into narrowband load 8 dB under the mask Front panel spectrum analysis
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Performance Results - DRM
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Thank You!
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