1. 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.

2. 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)

3. 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

4. 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)

5. 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

6. 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

7. 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.

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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)

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Exciter Block Diagram

22. 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

23. Part 2 – Modulation and Pre-correction

24. Theory of Operation X Envelope Elimination and Restoration
a.k.a. Kahn Technique
Envelope Term
RF Phase Term X

25. AM Theory – General Signal
DSP Based Signal Generator
Transmitter Power Stage
Envelope Term
Envelope Input
RF Phase Term
RF Input X
Final Amplifier

26. 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.

27. 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

28. 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

29. Typical Correction Curves AM/AM
Gain
Envelope Voltage

30. Typical Correction Curves AM/PM
Phase Correction
Envelope Voltage

31. NX Modulator Response

32. NX Modulator Response

33. 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

34. Performance Results - AM
No visible distortion in the trough with AM to AM correction

35. Performance Results - AM
Nearly total elimination of distortion at 25% modulation (Distortion approx %)

36. Performance Results - IBOC
IBOC Performance into narrowband load
8 dB under the mask
Front panel spectrum analysis

37. Performance Results - DRM

38. Thank You!