Description of Nautel NX 100 Transmitter:

PAKISTAN BROADCASTING CORPORATION Forums High Power Transmitters Multan HPT Description of Nautel NX 100 Transmitter:

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    Ch. Zeshan Yousaf
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    Description:

    The transmitter circuitry is subdivided into five basic stages.
    • Ac-Dc power stage
    • Exciter stage –
    • Control/monitor –
    • RF power stage –
    • RF output network – (includes combiner and filter)

    1- Ac-dc power stage

    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 include a three-phase SCR rectifier assembly, Hall Effect current sensor, and B+ distribution assemblies. 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 secondary of the power transformer is applied to the SCR rectifier assembly to create the transmitter’s B+ voltage. The output of the rectifier assembly is fed through the Hall Effect current sensor, which supplies a dc current signal to the rack interface PWB. The rectifier assembly output is applied to the B+ distribution assemblies, which then provide the B+ voltage to the RF power modules, and a B+ sample to the rack interface PWB.

    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 (+48V, +15 V and +12 V) are distributed throughout the transmitter via the rack interface PWB.

    2- Exciter stage

    The exciter stage consists of digital AM exciter PWBs A and B, RF drive distribution PWB , and PDM distribution PWB. 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

    Digital AM exciter PWBs A and B accept audio or IBOC inputs from an integrated DRM exciter or Exgine card and generate fully digital RF drive carrier frequency sources and interphase PDM drive signals for the power amplifiers and modulators in the RF power modules.
    Audio input can be applied as an analog (balanced or AES-EBU) or digital (I/Q in AES/EBU or CMOS format). All audio inputs are digitized, as necessary, 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.

    The digital PDM component consists of nine phased PDM signals, each separated by 40 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, three different phases (e.g., 2, 5 and 8) are applied to the adjacent module, and three other phases (e.g., 3, 6 and 9) are applied to the next module.

    Samples of the RF output voltage, RF output current 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 attempts to regulate the regulated B+ supply in order to hold the transmitter output power constant and mimimize distortion.

    An inhibit PDM input is applied from the control/display PWB to inhibit the PDM drive during certain alarm/fault conditions.

    RF drive distribution
    The control/interface PWB accepts the RF drive (+ and -) signals from the digital AM exciter PWBs (A and B) and splits the active exciter’s RF drive signal for application to the RF drive distribution PWB. 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.

    PDM distribution
    The control/interface PWB accepts the PDM (1 through 9) signals from the digital AM exciter PWBs (A and B) and splits the active exciter PDM signals for application to the PDM distribution PWB. The PDM distribution PWB accepts each signal from the control/interface PWB and splits it into two (+ and -) opposite logic outputs that are provided to the RF power stage.

    3- Control/monitor stage

    The control/monitor stage monitors critical signal samples and status/alarm signals from the exciter stage, RF power stage, and ac/dc power stage. For example, RF power monitoring and RF power stage status information is applied to the control/monitor stage. Based on the value and status of each input, the control/monitor stage produces the appropriate control signals for the exciter stage and the RF power stage to ensure the proper operation and protection of the transmitter.

    All analog and digital program inputs are applied to the transmitter via the control/interface PWB for distribution to the exciter stage.

    A 17-inch, colour LCD screen 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 as a flash graphic on any web-interfaced PC or handheld device via the internal NX web server.

    The control/interface PWB contains push-button switches that provide backup control for the RF on/off and local/remote functions.

    4- RF power stage

    The RF power stage includes all of the transmitter’s 10 kW RF power blocks. The NX100 contains 10 RF power blocks.

    Each RF power block contains four 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, PDM and control voltages from the exciter stage. B+, +48V and +15 V dc voltages are input from the B+ distribution assembly and 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.

    To facilitate “on-air” servicing, each module has an associated relay that forces a contact closure across the appropriate primary winding of the series combining transformer when an RF power module is removed from the transmitter. When the transmitter is shut down, all relay contacts close, isolating the RF power modules from the output network and improving their immunity to lightning.

    5- RF output network

    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 RF sample probe. Samples from these probes are provided for control and monitoring purposes.

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