10 years ago…RF discrete transistors integrated into pallets
Integration: Traditional designs in the military defense sector often utilized bipolar technology in a pallet form for radar and avionics applications. A pallet or module is a 50 Ω matched design. Most pallets that were requested were single transistor pallets with the input and output matching stages incorporated on a PCB substrate as in Figure 1. A pallet design offers an ease of manufacturability. The pallet manufacturer actually performs the physical mounting of the RF device to the metallic heatsink. This process assures a sufficient thermal transfer path with an adequate connection to RF ground. A sound thermal path to the heatsink eliminates overheating of the semiconductor material and ensures a long lifetime and reliable design. The pallet manufacturer also is responsible for the physical positioning of the device affecting RF performance with the x-axis determining phase performance and the y-axis affecting the impedance match and ultimately the RF performance parameters. With the RF design 100 percent RF tested before being shipped, all that is left to do at the customer end is to physically mount the pallet onto a heatsink with some metal screws and the design is complete. This shortens the design cycle, freeing up the end-user to concentrate on designing higher system level functionality.
Figure 1 Picture of a 25 W discrete transistor on a 50 Ω pallet.
A few years ago…several RF discrete transistors integrated into pallets
More integration: In 1997, Integra began its operations, introducing a single bipolar discrete transistor designed for S-Band Air Traffic Control Radars using patented technology. Market demand has led the company to double its pallet portfolio in the last two years. Pallets offer several key advantages over discrete devices. These pallets have higher levels of integration – multiple device stages for higher gain and/or multiple devices in parallel at the output for higher levels of power. Pallets allow the use of different semiconductor technologies with specific advantages to be integrated for overall performance optimization. Thus, companies knowledgeable in multiple technologies can offer high gain LDMOS and high power bipolar technologies for the best of both worlds. An example is shown in Figure 2. Additional integration of functionality is added for bias control, bias compensation or flattening the gain and/or output power across the operating frequency band.
Figure 2 Picture of a 2000 W avionics pallet with a LDMOS drive stage driving two high power bipolar devices in the output stage.
In the last year…RF pallets integrated into HPAs
Even higher integration: In 2011, the market has requested even more integration in the form of entire power amplifiers. Requests for high power amplifiers with at least 1 kW of output power are becoming more common. The request for amplifiers has come from sources with the same goal in mind – replacing TWT systems in the ISM and military radar industries. There will most likely be a place for TWTs as they produce very high power (MW) and very high gain. But more and more often, the driver stages are in the kW range, which can be handled with a solid-state solution. The ISM market, which includes industrial (frozen food heaters, laser cutters), scientific (linear particle accelerators) and medical (RF ablation), is heavily served with TWT solutions. Another market is military radar, traditionally served by tubes, which is now looking for solid-state systems for radars from L-, S-, C- and X-Band. A successful manufacturer needs the right portfolio with silicon solutions for the L/S bands and GaN for the higher frequency bands.
To meet this demand, senior engineers with design experience in this field are needed. A likely source for these engineers will be those who have worked for commercial cellular HPA developers. Already proficient with multiple stage pallets with additional functionality, migrating to HPA manufacturing is the next logical step. HPA design is not as trivial as placing a highly integrated pallet into an enclosure with a power supply and on/off switch. Instead, the complexity is another order of magnitude with mechanical concerns, environmental aspects and interference patterns all playing an important role in the final product. Also, the RF portion for HPAs have advanced circuitry in addition to the bias control and other circuitry – LEDs for on/off switch, over voltage protection, OVSWR alerts and other alarm functions.
Does this trend toward higher levels of integration speak to a lack of RF talent in the industry? Is the art of RF a dying breed of engineers as newer engineers focus more on digital/analog and only see RF PAs as a system block? Most new wireless designs focus on low power, such as cellular handsets and ZigBee, which surely sounds more exciting to the young professional. I prefer not to hazard a guess and leave this up to the reader to decide.
