Nov 1, 2008 12:00 PM, By Mike Woods
Nautel's design philosophy for RF transmitters has always been to use solid-state technology. The improved redundancy and safety associated with multiple parallel amplifiers is obvious; the primary challenge in designing solid-state products has always been economics-related because the cost of solid-state is proportional with power whereas as tubes are not. For many years, 10kW has been accepted as the practical economic threshold of solid state but the momentum of HD Radio at increased power levels enticed us to challenge the norms. We wanted to design a solid-state 40kW unit that would rival tube systems' prices but provide HD Radio readiness and all the redundancy associated with Nautel broadcast transmitters. Rack size, weight, cost and serviceability were just a few of the issues on the design considerations list as this product entered development.
Design of the Nautel NV40 transmitter began in 2006 and was completed in early 2008; the transmitter was introduced to the market at the 2008 NAB Show. The NV40 offers 44kW maximum power (analog). Its footprint is half that of comparable transmitters, while including an integral exciter with adaptive pre-correction. The unit's linear broadband design allows the exciter to select the required presets for operating frequency and output power level, enabling the transmitter's use anywhere in the FM band. The product also offers advanced instrumentation and management via a user-configurable front panel touchscreen. The transmitter is digital-ready, allowing a simple plug-in upgrade to the HD Radio Exgine.
A 40kW transmitter manufactured with old technology would be at least 120 inches wide; the NV40 is about half that at 65" wide. Our major goal was to make the technology cost go down to the end user while making the package the smallest in the industry. One major challenge in reducing the size was the combining and packaging of the cooling systems and amplifiers and putting them into a small enough space. To achieve this, we designed a power module with eight amplifiers, each providing about 375W, for 2,500W nominal power and 3,000W maximum power per module. The module is slim, providing a single RF input, a single RF output and cooling fans/heat sinks all in the same module. This transmitter has 16 RF power modules comprising a total of 128 amplifiers, for a total of 44kW in analog mode.
The combiner was another challenge for us. Rather than re-invent the technology, our innovation came in the execution of the combiner including the connection of the hybrid coupler devices, again with the goal of optimizing the overall size. Our engineers came up with a scalable combiner that combines power amplifiers and matches impedance (many 50O inputs to one 50O output) to a 10kW harmonic filter. We use four of these filters in the NV40 and can use them to easily create a variety of other power output levels. This streamlines things considerably in both engineering and production, with the benefit to the end user of helping to keep product costs attractive across the entire line.
The power supply was another way we improved this product. Tube transmitters use a single, heavy, iron power supply. In this transmitter, we can use switch-mode power supplies and scale them to the required level. This keeps the weight down and reduces shipping costs. All the low-voltage power supplies are fully redundant, and everything is hot pluggable, allowing for fast swap-out if needed. Two power supplies are provided for each RF module, with each supply operating four amplifiers in the module.
As the NV40 was being designed, we were also working on the new NX family of AM transmitters. This parallel design allowed us to create an advanced user interface (AUI) that could be used across both AM and FM platforms. Characterized by a large touch screen display and embedded instrumentation along with metering and status indication plus a TCP/IP interface, this GUI serves as the dashboard for the transmitter. One key feature is the built-in spectrum analyzer. It isn't the first time a manufacturer has added a spectrum analyzer to a product, but in addition to monitoring the exciter; the spectrum analyzer covers the entire transmitter. It's completely automated with a top-down interface; the user selects the desired mode to run and it pops up. Not only does the AUI offer the full-function spectrum analyzer, it provides comprehensive system monitoring and control down to the individual module level and also offers a constellation view of the HD Radio signal.
The transmitter's integral exciter does not have its own user interface; because it is integrated into the transmitter there are no pots or other adjustments and all exciter adjustments are done through the front-panel user interface. The exciter produces about 200W of RF power, which then goes to a 16-way splitter and is fed to the RF amplifiers, eliminating the need for an IPA module.
Above the RF Power Module section is the Control Board, which brings in all of the RF, dc and temperature probes; it does the appropriate monitoring and protection of the transmitter and it streams that data to the front-panel user interface. On the right hand side of that board is the Remote Interface board, which is completely user configurable. All of the telemetry, status and controls can be dictated by the end user so they can get exactly what they want in the way of monitoring. It can be adjusted for single ended or balanced control. On the Control Board are some rudimentary user controls should the front panel user interface become inoperable for some reason.
As with the Nautel V series transmitters, the NV40 is designed for digital operation, but now we have the capability of higher power levels with the 10dB change in injection levels. The transmitter can use Nautel's Power Boost technology in hybrid HD Radio systems to achieve higher digital power levels with greater transmitter efficiency. We also have the ability to equalize combiner systems with our adaptive equalization feature. This provides correction for phase and magnitude response in the combiner system, and it does it automatically.
Woods is the head of development for Nautel Limited.