Nautel NX Series Installation guide

Document:NHB-NX400-OPS
Issue: 2.0 2020-09-09
Status: Standard

Nautel Limited
10089 Peggy’s Cove Road
Hackett’s Cove, NS Canada B3Z 3J4
Phone: [phone] or
Toll Free: +1.877.6NAUTEL (6628835) (Canada & USA only)
Fax: [phone]
Nautel Inc.
201 Target Industrial Circle
Bangor, Maine USA 04401
Phone: [phone]
Fax: [phone]
Customer Service (24 hour support)
+1.877.628.8353 (Canada & USA only)
+1.902.823.5100 (International)
Email: [email protected]
Web: www.nautel.com
The comparisons and other information provided in this document
have been prepared in good faith based on publicly available
information. The reader is encouraged to consult the respective
manufacturer's most recent published data for verification.
© Copyright 2020 NAUTEL. All rights reserved.

Issue Date Reason
2.0 2020-09-09 Second release of manual; supports hardware
NARA67A and software version NX SW 4.9.1.

This section provides a high-level description of the transmitter’s key sections. The transmitter
circuitry is subdivided into five basic stages.
•Ac-Dc Power Stage
•Exciter Stage - see page 1-4
•Control/Monitor Stage - see page 1-7
•RF Power Stage - see page 1-8
•RF Output Network - see page 1-9 (includes combiner and filter)
Refer to the functional block diagrams: see Figure 1.5 on page 1-13 and Figure 1.6 on page 1-14.
Some descriptions in this section refer to electrical schematics (SD-#s). These are located at the
end of the NX400 Troubleshooting Manual.
The NX400 features redundancy in all key systems:
• RF power modules
• Exciters
• Cooling fans
• Low voltage power supplies

See electrical schematics through . The ac/dc power supply stage contains the input power
transformer that receives the main ac input to the transmitter. It also contains the transmitter’s low
voltage power supplies and rack interface PWB. The ac/dc power supply stage also includes a three-
phase SCR rectifier assembly, B+ current sensor, B+ distribution assemblies, capacitor tray assembly
and arc detector assembly. With the exception of the power transformer, which has its own cabinet,
each cabinet in the transmitter system contains its own ac/dc power stage components.
The NX400’s power transformer can be set to use a range of input voltages. See Section 3, “Installing
the power transformer” on page 3-1Section 4, “Connecting transformer taps and secondary wiring”
on page 4-1 in the NX400 Installation Manual.
The secondary of the power transformer is applied to the SCR rectifier assembly to create the
transmitter’s B+ voltage (normally 400 Vdc). The output of the rectifier assembly is fed
through the B+ current sensor, which supplies a B+ current sample to the rack interface PWB.
The rectifier assembly output is applied to a choke input filter and the B+ distribution
assemblies, which then provide the B+ voltage to the RF power modules.
The B+ discharge relay, which is part of the capacitor assembly, opens when ac power is
applied and closes and ac power is removed, applying a high power discharge resistor across the
B+ bus to quickly discharge the voltage. When the discharge relay closes, a microswitch
ensures the SCR rectifier is immediately inhibited.
One phase of the power transformer’s secondary is also used to provide a source for the low
voltage dc power supplies and an ac sample for the rack interface PWB. The low voltage power
supply outputs (+48 V, +15 V and +12 V) are distributed throughout the transmitter via the
rack interface PWB. The +48 V power supply sources the transmitter’s cooling fans. An
inhibit signal, controlled by the rack interface PWB, is applied to the +48 V power supply to
turn the fans on and off. The +15 V power supply sources all of the low voltage control
circuitry. The +12V power supply sources the SBC and the touchscreen monitor.

The rack interface PWB provides various functions for the ac-dc power stage, including:
• Monitors the B+ voltage and regulates the SCR rectifier’s output via the B+ level
control signal applied to the rectifier’s SCR control board.
• Monitors the SCR rectifier’s fan speeds, temperature and phase loss indication as well
as the B+ current sample and provides this information, along with data received over
a serial bus from the RF power modules, to the transmitter controller.
• Monitors the arc detector assembly.
• Generates the +30 V dc supply used by the SCR rectifier’s controller and the -15 V dc
supply used by the Hall Effect B+ current sensor.
• Generates the +5V dc supply used by control logic throughout the transmitter.
• Accepts the +48V from the +48V power supply and distributes it to the fans and
power module interface PWBs.
• Accepts the fan tach signal from each fan in the RF Power Stage and distributes them
to the power module interface PWBs.

See Figure 1.1 and electrical schematic . The exciter stage consists of digital AM exciter PWBs A
(A11A2) and B (A11A3), RF drive distribution PWB (A13), and PDM distribution PWB (A14). The
dual digital AM exciter PWBs provide two independent exciter sections (A and B), which can be
selected automatically or by local or remote control. The control/interface PWB acts as an interface
point for audio inputs and RF drive and PDM outputs.
Digital AM exciter PWBs A (A11A2) and B (A11A3) accept audio signals for analog
modulation, I/Q over AES signals for DRM modulation and IBOC signals over I2S from an
integrated Exgine card, and generate fully digital RF drive and interphase PDM drive signals
for the power amplifiers and modulators in the RF power modules.
Audio signals for analog AM modulation can be applied as balanced analog (600 ohm
impedance) or digital (AES/EBU format). All audio inputs are digitized and sample rate
converted. A DSP provides initial data conditioning, including the initial filtering and
interpolation of incoming audio or digital radio data.
An FPGA generates the digital PDM signals and synthesizes the carrier frequency RF drive
signal. It also performs digital up-conversion, reverse path demodulation and down-conversion
and B+ voltage compensation.
Figure 1.1: Exciter Stage
+15 V dc
-15 V dc
+5 V dc
Analog/
Digital
Modulation
Inputs
Analog/
Digital Mod-
ulation
Inputs RF Drive (A)
RF Drive (B)
PDM (A)
PDM (B)
RF Drive 1
RF Drive 2
PDM 2/5/8
PDM 1/4/7

The digital PDM component consists of six phased PDM signals, each separated by 60 electrical
degrees. These PDM drive signals determine the transmitter output power level as well as the
output modulation level. Three of these phases, each separated by 120 electrical degrees (e.g., 1,
4 and 7), are applied to a given RF power module. To achieve optimal harmonic cancellation, the
RF power modules in cabinets 1 and 3 use one group of three phases (e.g. 1, 4, and 7) and the RF
power modules in cabinets 2 and 4 use a different group of three phases (e.g. 2, 5, and 8).
Samples of the RF output voltage, RF output current, SCR rectifier temperature and RF power
module temperature are monitored. If a parameter exceeds an acceptable limit, the active exciter
attempts to compensate by decreasing its output power to restore the parameter to an acceptable
level.
A sample of the B+ supply voltage is monitored. A B+ compensation circuit adjusts the PDM
duty cycle to compensate for variations in the B+ supply in order to maintain constant
transmitter output power and minimize hum.
A PDM inhibit (A or B) input is applied from the control/interface PWB to inhibit the PDM
drive for a specific exciter during certain alarm/fault conditions.
The control/interface PWB (A11A1) accepts the RF drive (+ and -) signals from the digital AM
exciter PWBs (A and B) and splits the active exciter’s RF drive 1 and 2 signals for application to each
cabinet’s RF drive distribution PWB (A13). The RF drive distribution PWB splits the signal from the
control/interface PWB and buffers the individual outputs that are provided to the RF power stage.
The control/interface PWB (A11A1) accepts the PDM signals from the digital AM exciter
PWBs (A and B) and splits the active exciter PDM signals for application to each cabinet’s PDM
distribution PWB (A14). The PDM distribution PWB accepts each signal from the control/
interface PWB and splits it into two (+ and -) differential outputs that are provided to the RF power
stage.

The exciter stage interfaces with the transmitter controller to perform other functions,
including:
• Provides an RF Drive Latch signal to the transmitter controller to inhibit the RF
drive for a set period of time during specific alarm/fault conditions.
• Provides a PM Enable signal for the RF power modules to monitor the PDM cable
status. This signal is held “low” (0 V) by the exciter.
• Provides real-time telemetry of exciter samples over a streaming bus to the
transmitter controller for use by various instruments on the AUI such as the
spectrum analyzer, Smith chart, signal constellation, audio levels, etc.
• Accepts 10 MHz, 1 PPS and 1 kHz synchronization signals from the selected
source (External 10 MHz, GPS Sync PWB or Combiner) for synchronizing the RF
carrier frequency and phase.

See electrical schematic .
The control/monitor stage monitors critical signal samples and status/alarm signals from the
exciter stage, RF power stage, and ac/dc power stage. It also provides customer interfaces for
monitoring transmitter status and accepts customer inputs for distribution to other system
components. The control/interface PWB (A4) is the primary component of the control/
monitor stage. It provides various functions, including:
• Accepts all analog and digital modulation inputs and synchronization signals (10 MHz,
1 PPS, 1 kHz) and distributes them, along with RF voltage and current samples, to the
exciter stage.
• Accepts +15 V, +5V and -15V from the rack interface to generate on-board power
supply requirements.
• Monitors the forward and reflected power samples from the directional coupler and
initiates a shutback when the reflected power exceeds the factory set threshold.
• Amplifies the forward and reflected power samples for the RF monitor output.
• Monitors shutback requests from the rack interface for arc detector and low B+ shut-
down events.
• In conjunction with the remote interface PWB, accepts customer connections for digi-
tal remote inputs, digital remote outputs and analog outputs.
• Accepts customer connections to complete the external interlock and transmitter
(PDM) inhibit circuits.
• Contains a microcontroller that: manages presets, scheduler and exciter changeover;
controls RF on/off status, controls RF symmetry, controls RF monitor level, stores
network settings; manages the transmitter’s response to certain alarm/fault conditions.
• Contains push-button switches that provide backup control for the RF on/off and
local/remote functions.

A 17-inch, colour LCD screen along with the Single Board Computer mounted on the front of
the control cabinet provides an advanced user interface (AUI) for the transmitter. The AUI can
be controlled by touch screen and is also available via the Ethernet connection on the SBC,
through a web browser on any web-interfaced PC or handheld device. See “Using the AUI” on
page 2-2 for detailed information on AUI functionality. The SBC also accepts customer USB
connections for audio playlist functionality.
The remote interface PWB provides customer connections for the discrete wire remote I/O
interface, including digital remote inputs, digital remote outputs and analog outputs. The
Remote Interface PWB also provides push-button control of digital remote inputs and LED
indication of digital remote outputs that can be used as a backup control/monitor interface.
See electrical schematics through . The RF power stage includes all of the transmitter’s 10
kW RF power blocks. The NX400 contains 40 RF power blocks - ten in each of its four cabinets.
Each RF power block contains RF power modules and associated relays, a fan tray, and connections
to the RF drive distribution PWB, PDM distribution PWB and rack interface PWB. Each RF power
module accepts RF drive from the RF drive distribution PWB through the power module interface
PWBs and PDM from the PDM distribution PWB. B+ and +15 V dc voltages are applied to the RF
power modules via the B+ distribution assembly, the rack interface PWB and power module interface
PWBs. +48 V dc voltage is applied to the fan trays via the rack interface PWB. The output of each RF
power module is applied to a primary winding of a series combining transformer. The resultant
combined output is applied to the RF output network.
Each RF power module also:
• monitors the tachometer signals from the two fans on its associated fan tray - applied
through the power module interface PWBs - and will shut down if one of the fans fail.
• controls its associated RF relay on the power module interface PWB, ensuring the
relay is closed when the RF power module is disabled, allowing combiner current to
continue flowing in the primary of its associated combiner transformer.
• provides status information to the rack interface PWB over a serial bus connection.

See electrical schematic and Figure 1.2 on page 1-9. The combined RF output is filtered
through an RF network consisting of two “T” networks with a shunt third harmonic trap, and then
provided to the antenna system. The RF output is monitored by an RF current probe, RF voltage
probe, and directional coupler. Samples from these probes are provided for control and monitoring
purposes (see “VSWR Protection” on page 1-10). A static drain choke provides a dc short to ground
to dissipate static build-up on the antenna system, and an adjustable spark gap provides protection
for the transmitter output from lightning events on the antenna.
Figure 1.2: RF Output Network
RF Pwr Mdl 1
RF 1 (+)
A3A67
Directional Coupler
RF Voltage Sample
Forward Power
RF Current Sample
RF 1 (-)
L
Shunt Capacitor Bank
A3A68
Static Drain
Parallel Capacitors
A3E1 Spark gap
RF Out
to Antenna
L
Series Combining Transformer (1-80)
Shunt Capacitor Bank
Sample
Reflected Power
Sample
A4A66
RF Current Probe
A2A65 Voltage Probe
RF Pwr Mdl 80
RF 80 (+)
RF 80 (-)
RF Pwr Mdl 81
RF 81 (+)
RF 81 (-)
RF Pwr Mdl 160
RF 160 (+)
RF 160 (-)
Series Combining Transformer (81-160)
L
A4A69
RF Current Probe
C
L

The transmitter uses an advanced DSP based VSWR protection system. Circuitry in the RF
output network (see Figure 1.2 on page 1-9) samples the RF voltage and RF current at the input to
the harmonic filter. These current (I) and voltage (V) samples are applied to ADCs on the digital AM
exciter PWBs. The digitized I and V signals are used to calculate the impedance (Z) at the combiner
output. An FPGA performs high-speed calculations, so there is minimal response delay.
There are several types of VSWR protection, which continuously operate:
Fast VSWR protection is used to detect transient faults when the reflected power quickly
increases due to arcing, lightning or short circuits. The protection circuit is set for a peak
reflected power equivalent to 1.5:1 at 400 kW plus 100% peak modulation using the reflected power
sample from the directional coupler (see Figure 1.2 on page 1-9), and triggers the controller's Fast
SWR Shutback alarm, designed to shutback PDM (reduce power to zero) and disable RF drive in less
than 100 ns. This protection ensures that the RF drive to the RF power modules is disabled in less
than one RF carrier period, preventing the amplifier from operating into a potentially harmful load.
The peak reflected power calculated by the FPGA is supplementary to the fast SWR
protection, allowing the exciter to detect and control the response to the event. The peak
reflected power limit - based on a VSWR of 1.5:1 at rated power plus 100% peak modulation - is
16 % of the transmitter's rated power (i.e. 64 kW). If this limit is exceeded, the transmitter’s output
power instantly reduces to 0 W. This triggers the exciter's SWR Shutback alarm.