Drones Fly High with Reliable Signals Thanks to Efficient, Linear, High-Gain RF Amplifiers

The use of aerial drones continues to expand in applications ranging from infrastructure inspection and sports journalism to payload delivery and targeted crop spraying. In these and other applications, drone operators rely on robust communication between the drone and a base station to accurately control flight and transmit large volumes of data.

At the same time, drones need to balance signal strength with battery performance and operate in crowded radio frequency (RF) bands. A combination of mechanical hardware, solid-state electronics, and processing power is needed to address these challenges and ensure safe, efficient, and useful drone operation.

Reliable RF communication

With drones communicating in the sub-6 GHz range, and most signals falling in the ultra-high-frequency (UHF) band (0.3 GHz to 3 GHz), robust RF communication to and from drones is all about speed. Components in high-efficiency RF front ends—antennas, amplifiers, filters, and switches—must be able to react quickly to incoming and outgoing signals at high frequencies.

The fast-switching integrated circuits (ICs) needed for RF front ends are made of gallium arsenide (GaAs) and gallium nitride (GaN) semiconductors. These wide-band-gap materials have higher carrier mobility than silicon-based semiconductors, giving GaAs and GaN transistors a much higher switching frequency.

Fast-switching transistors support time division duplex (TDD) systems which rapidly alternate between transmission and reception over a single frequency. In drones, TDD reduces space required, weight, and power consumption by eliminating the duplexers and stringent shielding requirements of older frequency division duplex (FDD) systems. Even though TDD requires additional attention to timing and synchronization, its ability to dynamically adjust uplink and downlink durations optimizes data transmission. TDD systems also allow non-active receiving hardware to enter battery-efficient sleep mode.

Software-defined radios (SDRs) are also enabled by fast-switching GaAs and GaN ICs. SDRs let drone operators adjust many aspects of radio operations, such as frequency, transmission power, and coding rate, on a single set of hardware via a software link. SDRs support evolving bandwidth needs and allow drones to adapt to regional frequency standards.

Robust RF front-end solutions require agile electronics with fast-switching transistors for SDR and TDD, as well as support for higher-frequency transmissions while maintaining low signal-to-noise ratios and good linearity. RF front-end solutions from Qorvo use GaAs ICs in low-noise amplifiers and power amplifiers that ensure reliable drone operations through robust long-range communication.

Low-noise amplifiers boost incoming signals

In drone operations, incoming radio signals carry time-sensitive flight instructions or other mission information. However, these signals are relatively weak and can be difficult to separate from background noise. Low-noise amplifiers (LNAs) like the QPL9547TR7 (Figure 1) are placed directly after the receiving antenna in the signal chain to boost weak RF signals in the target frequency band.

Figure 1: QPL9547TR7 LNAs work over a wide frequency range, and the low parasitics of the 2 mm square package contribute to their linear, low-noise performance. (Image source: Qorvo)

Qorvo’s pseudomorphic High Electron Mobility Transistor (pHEMT) processes create solid-state transistors with 0.15 µm, 0.25 µm, or 0.5 µm gate lengths on GaAs semiconductor chips. Short gate lengths give electrons less distance to travel, allowing transistors to switch more quickly, which minimizes noise, increases gain, and supports wider bandwidth.

In QPL9547TR7 LNAs, the 0.1 GHz to 6 GHz frequency range supports RF front ends tuned to receive UHF drone telemetry, very high frequency (VHF) radio signals, LTE and 5G cellular signals, or Wi-Fi. After these signals are received by the antenna, a QPL9547TR7 LNA provides a typical gain of 19.5 dB at 1.9 GHz, resulting in an amplified signal up to 89 times stronger than the relatively weak incoming signal.

QPL9547TR7 LNAs also exhibit extremely low noise, as demonstrated by their 0.3 dB noise figure. With an output third-order intercept point (OIP3) of +39 dBm, these LNAs also have good output linearity, allowing them to boost the intended signal while ignoring third-order harmonics on nearby frequencies, even in a crowded frequency space.

The physical design of QPL9547TR7 LNAs helps preserve these performance metrics. Their eight-pin, 2 mm x 2 mm, dual flat, no-lead (DFN) surface mount technology (SMT) package uses short electrical paths with good grounding and a small footprint to minimize capacitance, inductance, and RF reflections that can add noise to weak signals. The ICs are self-contained and require minimal external components, preserving PCB space and reducing the need for capacitance matching.

QPL9547TR7 LNAs work at a 65 mA bias setting and draw relatively little current from drone batteries, extending operating life between charges. They use a single positive power supply between 3.3 and 5 VDC, so they can be powered by a single DC power rail. In TDD applications that divide time in a given frequency band between transmission and reception, QPL9547TR7 LNAs can be powered down during the transmission phase to further preserve battery life.

High-efficiency power amplifiers support high-bandwidth data

While drones need LNAs to ensure they continuously receive flight and mission instructions, they also need to transmit data back to the base station. This is where power amplifiers (PAs) such as Qorvo’s QPA9510TR7 (Figure 2) come in.

Figure 2: QPA9510TR7 PAs use the fast-switching capability of GaAs semiconductors to provide high-power, high-gain amplification in the UHF band. (Image source: Qorvo)

QPA9510TR7 PAs are placed in the signal chain directly before transmission antennas, where they boost signals at frequencies between 100 MHz and 1 GHz for long-range transmission. Drones transmitting in this range are most often connecting to hand-held equipment that uses the Global System for Mobile Communications (GSM) standard at around 900 MHz. However, the frequency range available with QPA9510TR7 PAs also allows for transmission of higher-end VHF and mid-band UHF signals.

With up to 34 dB of gain, QPA9510TR7 PAs boost signal strength by as much as 2,500 times with 55% energy efficiency. They have a peak linear output power (P1dB) of +35 dBm, or about 3.2 W, which enables robust long-distance RF communication. Gain can be adjusted over a 70 dB range via analog on-board power controllers.

The power controllers also help manage the QPA9510TR7 PAs’ usage of their drones’ onboard batteries. They can power down the amplifiers or place them in sleep or standby mode by sending a “low” logic signal. These capabilities help QPA9510TR7 PAs preserve drone battery life while drawing up to 208 mA from either a 2.8 V to 3.6 V or a 3.6 V to 5 V supply.

Because 45% of the electrical energy passing through QPA9510TR7 PAs is dissipated as heat, packaging is important. Their compact 3 mm x 3 mm quad flat no-lead (QFN) SMT package has short electrical leads and good grounding similar to that of the DFN package used for QPL9547TR7 LNAs. This helps maximize board space while minimizing noise and protecting linearity. QFN packages usually have an exposed pad that facilitates the dissipation of excess heat through metal embedded in the PCB. Passive heat dissipation also contributes to battery life by eliminating the weight and electrical requirements of fans or other cooling systems.

Conclusion

Power amplifiers and low-noise amplifiers in aerial drones handle UHF RF signals with high linearity and high gain while efficiently using battery power, space, and weight. ICs made using Qorvo’s expertise with wide-band-gap materials like GaAs meet these seemingly conflicting priorities. The result is drones with reliable RF connections over long distances that can perform tasks and supply data in agricultural, industrial, media, and commercial settings.