HEMT devices give breakout performance for power amplifiers used to film aerial scenes in action movies
When Ray Breslin and Emil Rottmayer attempted a highly daring escape from a super-high security prison in this fall’s thriller Escape Plan, audiences got up close to see (spoiler alert!) our two heroes — played by Sylvester Stallone and Arnold Schwarzenegger, respectively — engage in a gunfight with the prison’s “crew” from a rescue helicopter all while Breslin dangles precariously above the prison from a rope ladder. But getting the audience so close to the action was no easy task, given the flight of the rescue helicopter and the remote location of the prison. To film action sequences like these, Hollywood studios deploy their own helicopter to film the scene and transmit the images back to the director, whose big chair is often miles away. Many of the high action scenes in Escape Plan, in fact, were filmed with both helicopters in free flight.
Of course, a major theatrical release is going to be filmed with state-of-the-art high-definition (HD) cameras. But even with a budget of $50 million (imdb.com), the studio can’t afford to have its content compromised by signal integrity issues when transmitting pictures back to terra firma. In fact, some scenes took place over open water, so a long signal range was essential to the studio. Filming helicopters typically carry camera and signal transmission equipment within a specially designed nose ball turret. The turret of the filming helicopter for Escape Plan used an S-band linear power amplifier that included a Gallium Nitride (GaN) high-electron-mobility transistor (HEMT). These devices ensured the director received very high quality, low distortion, low latency, high definition video transmitted over the longest possible remote distance.
“Putting cameras where no eye, or cable, can go”
Ball turret camera systems for wireless video transmission from aircraft are custom built for each filming project. RF Films, for instance, boasts on its web site that it offers its services to directors and special effects supervisors who are “regularly putting cameras where no eye, much less, cable, can go.” For Escape Plan and other projects such as aerial shots above high-profile sporting events, RF Films chose power amplifiers from Array Wireless. In years past, FM wireless systems were the industry standard for transmitting video signals from aircraft. However, FM systems were susceptible to multipath interference, as the world saw first-hand in 1994 during the infamous OJ Simpson “chase” where the live aerial image periodically browned out. In the late 1990s, coded orthogonal frequency division multiplexing (COFDM) technology came along, offering far better rejection of multipath. For helicopter deployments, COFDM technology overcomes the feedback from the Doppler effect reflecting off the rotors and maintains a clean signal.
High-definition wireless video service providers rely on power amplifiers that are highly linear. Power amplifiers with high linearity are desirable for multi-carrier modulations because of their ability to keep distortion to a minimum while exhibiting very high signal quality. Array Wireless’ power amplifiers present high linearity across a very wide band while minimizing DC power consumption. To achieve the very long-distance transmission required for aerial applications, Array’s power amplifiers employ GaN chips from Cree, Inc.
GaN transistors unlock high power output for HD video transmission
Gallium nitride is characterized as a wide bandgap semiconductor, and devices based on this technology offer the power density necessary to send clear signals farther and faster, which makes them desirable for digital broadcast applications. “Using a GaN HEMT devices you get probably an eight-times increase in the output power that’s based on a power density of 4 W/mm compared to silicon or gallium arsenide solutions,” says Simon Wood, RF Product Development Manager at Cree, explaining the company’s 28-V process.
Present bandwidths for digital broadcast applications are 8 MHz, but as bandwidths approach 60 MHz, designers need the high operating frequencies that today’s HEMT devices offer in order to meet the percent bandwidth requirements. Ed Takacs, President of Array Wireless notes, “I’ve personally been able to use half-micron GaN devices up through about 6.6 GHz in frequency. With (Cree’s) new quarter micron process, usable gain has pushed up past 8 GHz so it’s enabling us to come out with some products up to 7 GHz to 8 GHz power amplifiers.” Takacs also finds GaN-based RF transistors’ lower voltages on the drain — as low as 28 V — easier to use compared to other devices. “It’s a lot more difficult to generate 50 V at some sustainable large current than it is to generate the required 28 V.”
When selecting an RF transistor for HD or digital video equipment, be sure to tap into the vendor’s expertise. Reference designs make a good starting point because the designer can put the amplifier on the bench at their desired frequency range. “A lot of this work is in the 5.5-GHz type band,” notes Simon Wood. “We actually have reference designs already at 15-W and 30-W output power levels, and they’re ready to go for an engineer to plug in, turn on, and input their linear waveforms through those amplifiers.”
Designers implementing RF power amplifiers into HD video equipment must balance the longstanding customer requirements of higher power, higher linearity and lower distortion with the realities of getting the optimal architecture inside the hardware. Designers looking to reach the desired linearity requirement, for instance, may start with the largest available transistor and work down in size. Other methods include predistortion, envelope tracking, adaptive predistortion, and open loop predistortion. “What it boils down to is one way of reducing distortion does not necessarily work well or better at other frequencies and/or power levels,” says Takacs. Designers will also have to meet increasingly tighter rules from the FCC pertaining to distortion levels versus average output levels. Overseas standards, meanwhile, are even more stringent than the FCC’s requirements.
Future roles
Gallium Nitride HEMT transistors play an important role in helping power amplifiers enable wide bandwidth HD digital transmission performance for filming aboard motor and airborne vehicles as well as mounted to headgear and handheld devices. As these transistors continue to improve their high efficiency, low distortion, and light weight characteristics, expect more broadcasters and movie studios to take HD cameras and transmitters, along with the RF power amplifiers that they rely on, to even more challenging and remote filming locations.
