NX50 Transmitter – Operations & Maintenance Study Notes

Section 1 – Transmitter Architecture, Signal Flow & Protection

• NX-50 is divided into 5 functional stages
• Ac–Dc Power Stage
• Exciter Stage
• Control / Monitor Stage
• RF Power Stage
• RF Output Network
• Refer to overall system block – Fig 1-6 (page 1-15)

Ac–Dc Power Stage

• 3-phase input transformer; taps selectable (see Installation Manual)
• SCR rectifier produces B+ (nominal 400\;\text{Vdc})
• Sub-assemblies: B+ current sensor, choke-input filter, capacitor tray, arc detector
• Electro-mechanical B+ discharge relay grounds bulk caps when ac removed; microswitch instantly inhibits SCR gate drive
• Low-voltage PSUs (+48 V fan rail; +15 V logic; +12 V SBC/touch-screen)
• Rack Interface PWB provides:
• regulation feedback to SCR controller
• monitors temps, fan tach, phase loss
• generates +30 V (SCR ctrl) & –15 V (Hall sensor) & +5 V logic
• distributes +48 V & fan tachs to modules

Exciter Stage (dual redundant A11A2 & A11A3)

• Inputs: balanced analog (600 Ω), AES/EBU, I/Q over AES (DRM), I2S (IBOC Exgine)
• DSP performs SRC, filtering, interpolation
• FPGA generates:
• Digital 6-phase inter-phase PDM (phases 1-8 @ 60° spacing)
• Digital RF drive (+/– differential)
• B+ compensation, reverse-path demod, up/down conversion
• Distribution PWBs:
• A7 RF Drive Distribution → splits to 5 RF blocks
• A8 PDM Distribution → differential PDM pairs
• Telemetry bus provides spectra, constellation, Smith chart, etc. to controller
• Sync inputs accepted: 10 MHz, 1 PPS, 1 kHz (external, GPS, or combiner)

Control / Monitor Stage (A4 Control/Interface PWB)

• Manages presets, scheduler, exciter changeover, alarm responses
• Interfaces: all audio/sync in; RF V-I samples; shutback lines; remote I/O
• Local HMI: 17-in colour LCD + SBC touch (Web-AUI also via Ethernet)
• Backup hardware pushbuttons for RF ON/OFF & Local/Remote

RF Power Stage

• 5 RF blocks × 4 modules = 20\,\text{modules} → 50\,\text{kW}
• Each module: accepts 3 PDM phases (e.g. 1,4,7) & ±RF drive; outputs into series-combining transformer primary
• Local fans (dual) monitored; module opens RF relay on fault to keep combiner current flowing
• Serial status to rack interface

RF Output Network

• Dual-T filter + 3rd-harmonic trap; static drain choke; spark-gap
• Sensing devices: RF current probe, RF voltage probe, directional coupler
• DSP-VSWR protection:
• Fast SWR (<100\,\text{ns}) uses peak reflected P{ref} > 1.5:1 (8 kW) • Calculated I/V reflected shutback • Average reflected fold-back keeps P{ref(avg)} < 0.04 P_{rated}
• Shutback cycle: zero W for 100\,\text{ms} then exponential ramp
• Cutback: 3 shutbacks in 15\,\text{s} → limit to 0.85 previous set-point, min 25 % rated

Section 2 – Operation & Advanced User Interface (AUI)

• AUI = HTML5 Web served from SBC; front-panel or remote (≤10 sessions)
• Initial login default: User “Nautel”, pwd blank; ports used 80,3501,22(ssh),7005(RDS),161(SNMP)

Home Page elements

• Top banner: Logo(logout), Date/Time, Scheduler toggle, Transmitter/Exciter summary, Preset quick-select
• Bottom banner: RF ON/OFF, Menu, Status, Logs, Local/Remote, User switch
• Task Area: up to 4 Instrument Panels + meter list

Instrument Panels (12)

Spectrum, EQ freq/impulse/delay, AM-AM, AM-PM, Signal constellation, Lissajous, Smith chart, Mod levels, Audio inputs, CCDF
— interactive cursors, RBW/span, gains etc.

Meter System

• Hierarchical tree (Controller, Exciters, Racks, PMs)
• Custom lists can be saved per user; green/yellow/red zones
• Remote disabled → read-only subset

Logs

• 20 pages × 100 items = 2000 events/batch × 5 batches
• Filter by device/date; copy to clipboard; statistics; legend for icons
• Three severity levels: ! (low), ! (medium), !! (high)

Presets (61 available)

• General tab: Output power, Overall Mode (Analog AM, DRM, DRM+AM, IBOC, IBOC+AM), Pre-correction Config
• Analog settings: Source, Format, Filter, NRSC-1 EQ, Dynamic Carrier Control (AMC/EAMC/DAM/DAM-Full \Delta C_{max}=3–4.44\,\text{dB}), AMSS
• Digital settings: DRM bandwidth, IBOC MA1/MA3, carrier differentials, analog gain, envelope conditioning
• Audio-Loss action: Alarm/Inhibit/Change-Preset + timeout
• Scheduler rules: up to 63; time-of-year, day mask, repeat, self-delete

Changeover Page

• Select main exciter, enable auto, declare if standby present
• Manual or auto transfer based on fault logic

System Settings

• Reboot AUI / active exciter
• Software upgrade (.tgz), history, manage files
• Screen config (local) – touch calibration, screensaver
• Exciter clock: internal / 10 MHz / GPS / Combiner (phase offset)
• RF monitor gain calibration
• Power lockouts (8) & thresholds (two low-fwd %)

User Settings

Network (DHCP/Static), Email (TLS SMTP, sender ID), Notifications (email/SMS per alarm list), Exgine IP, SNMP (read/write, traps), Critical-param capture, Time & NTP, Phone-Home cloud, Call-sign banner

Remote I/O

• 16 digital inputs: RF On/Off, Inc/Dec, presets, lockouts, scheduler etc. (edge or level logic)
• 16 digital outputs: alarms, active preset, remote enable, AUI watchdog (custom delay)
• Logic polarity configurable

Audio Player (USB soundcard req’d)

• Sources: Playlist (WAV/MP3 \ge 96\text{ kb/s}), Icecast/Shoutcast stream, Livewire
• Playlist jump markers, USB FAT32, manage via remote upload
• Use as backup via Audio-Loss or Scheduler presets

Section 3 – Routine Maintenance

• Visual & cleaning interval: ≈ 3 months (min)
• Vacuum only; never blow with air
• Air-filter wash & re-spray Handi-Koter; replace if damaged (RP #9151)
• Hardware torque yearly; pay attention to multi-strand terminations
• Replace Control/Interface PWB coin cell 3\;\text{V} annually to retain logs
• On-air QC: annually monitor audio, spectra, field pattern
• Lightning system inspect each 6 months (ground continuity, visual burn)

Section 4 – Non-Standard Maintenance

• Software upgrade sequence:
1 Check current versions (Menu → System Settings → Upgrade)
2 Download latest *.tgz and (if major) CF-image
3 Set static IP 10.10.10.2 on TX & 10.10.10.1 on laptop, upload file via Manage Files
4 RF OFF, Remote mode, Start Upgrade, wait 100 %
5 Cycle ac & restore network
• Compact-Flash replacement requires burning image (see IS13004) and may need BIOS boot-order tweak (IS18005)
• After new CF, perform touch-screen calibration & reset OS (root) password via SSH

Section 5 – Key Terms & Abbrev.

• ADC: Analog to Digital Converter.

• AES/EBU: Digital audio transfer standard using 3-pin XLR connectors, carrying both left and right-channel audio data.

• B+: The high voltage DC generated by the transmitter’s AC power supply, used by modulators and other circuitry.

• Cutback: A reduction in RF output power due to a total power limit fault or three shutbacks within a five-second period.

• DAC: Digital to Analog Converter.

• DAM: Dynamic Amplitude Modulation.

• DCC: Dynamic Carrier Control.

• DRM: Digital Radio Mondiale, digital audio broadcasting technologies for existing AM radio channels.

• DSP: Digital Signal Processing.

• FPGA: Field Programmable Gate Array.

• HD Radio: Another term for In Band On Channel (IBOC) technology.

• IBOC: Nautel’s In-Band-On-Channel technology, providing high-quality digital audio over existing AM radio channels.

• IPM: Incidental Phase Modulation.

• Latching alarm: An alarm that keeps the transmitter in an ‘RF inhibited’ state until manually or remotely reset (e.g., High SWR Shutdown).

• NE IBOC: Nautel’s In-Band-On-Channel signal generator, required for NX series IBOC installations.

• PDM: Pulse Duration Modulation.

• Preset: A setting controlling power level, active exciter, and power scheduler status based on time/date. Allows pre-programming multiple operating modes for exciters.

• Shutback: Complete loss of RF output power due to various faults such as high VSWR, low B+ voltage, high RF current, RF drive failure, external interlock, or spark gap.

• Surge protection board: An electrical panel protecting equipment from electrical surges in the AC power supply, antenna, or site ground, often caused by lightning strikes.

• VSWR: Voltage standing wave ratio, expressing the ratio of forward voltage to reverse voltage on the feedline and antenna system. An ideal VSWR of 1:1 provides maximum transmitter-antenna efficiency.

• VSWR, Cutback vs Shutback, Latching alarm, Phone-Home cloud etc.

Physical Safety – AC Disconnect & Keyed Access (optional)

• Service key ‘A’ interlocks ac breaker; key ‘B’ grounds antenna; two ‘C’ keys unlock rear and filter doors – sequence enforced to ensure ac off & antenna grounded before internal access

Key Numerical & Formulae Recap

• Nominal B+ \approx 400\,\text{Vdc} (limit 420)
• Fast SWR shutback threshold: P{ref,peak}=0.16 P{rated}=8\,\text{kW}
• Cutback floor \ge 0.25 P_{carrier}
• Filter BW examples: DRM {4.5,5,9,10,18,20}\,\text{kHz}
• PDM phases: six @ 60^{\circ} spacing; modules use 3 phases 120° apart