Dry goods! Core of antenna design for active phased array: T/R components

The core of the active phased array antenna design is the T/R component. The main factors considered in the T/R module design are: the number of different types of integrated circuits, the level of power output, the size of the received noise figure, and the accuracy of amplitude and phase control. At the same time, the design of the array of radiating elements is also crucial.

Ideally, the circuitry of all modules needs to be integrated on a single chip. In the past few decades, everyone has been working hard for this goal. However, due to the difference in requirements of different functional units in the system, the existing engineering technologies have made trade-off considerations in terms of system performance and difficulty of implementation. Therefore, it is common practice to classify circuits according to their functions and place them on different chips. , and then connected through a hybrid microcircuit, as shown.

The basic chip set of a T/R module includes three MMICs components and one digital large-scale integrated circuit (VLSI) as shown in the figure.

High Power Amplifier (MMIC)

Low Noise Amplifier Plus Protection Circuit (MMIC)

Adjustable Gain Amplifier and Adjustable Phase Shifter (MMIC)

Digital Control Circuit (VLSI)

According to different application requirements, the T/R module may also need some other circuits, such as the preamplifier circuit needs to amplify the input signal to meet the high peak power requirements.

Most X-band and above T/R components use GaAs-based MMICs. The disadvantage of this technology is its extremely low thermal conductivity, so GaAs-based circuits require thermal design.

The future direction of T/R components is based on the design process of GaN and SiGe.

GaN-based power amplifiers can achieve higher peak power output, thereby increasing the radar's sensitivity or detection range, and the output power is more than five times that of the GaAS process circuit. Although the SiGe process does not transmit as much power as GaAs, the material has a low cost and is suitable for the design of future low cost, low power density radar systems.

Normally, the average power output required by a radar system is basically determined after the caliber of a given array. The maximum achievable average power of the antenna is related to the output power of each TR module, the number of T/R modules, the efficiency of the T/R module, and heat dissipation. When the input power is determined, the higher the efficiency of the T/R module, the greater the corresponding output power.

In the design of high power amplifiers, the required peak power is an important indicator, defined as the average power divided by the minimum duty cycle. The peak power of the radar system is realized by the entire antenna array, that is, when the peak power is determined, the minimum number of T/R components required is also determined.

Radar system TR component design needs to consider factors such as antenna aperture, T/R module output power, and T/R component layout. For example, to achieve the same radar detection performance and the same number of T/R components, for a 4m2 aperture antenna, assume The output power of each T/R module is P. For a 2m2 aperture antenna, the output power of each T/R module is 2P, as shown in the figure.

In general, the radar system uses a central transmitter to operate, so it is necessary to reduce the emission-induced noise as much as possible. In active phased array antennas, the main source of noise is DC ripple or input voltage fluctuations. Due to the low voltage and high current of each T/R component, adaptive filtering of the input power is required.

The receiver noise figure is an important indicator for active ESA antennas. It is usually necessary to make the receive noise figure low to improve the radar performance. Under normal circumstances, the T/R component's receive noise figure refers to the entire module, including the noise factor of the LNA and the insertion loss caused by the preceding stage circuit (circulator, receive protection circuit, transmission line), as shown in the figure.

The accuracy of amplitude and phase control is related to the requirements of the radar system for the sidelobes of the entire antenna array. Assuming that the radar system needs the antenna to achieve low sidelobe, then the quantization step size of the phase and amplitude control circuit needs to be reduced, while the range of amplitude and phase control is increased to achieve the weighting of the true antenna array, and the amplitude and phase error are needed. Strict control.

The performance and cost design of an active ESA antenna is not only related to T/R components, but also closely related to the integrated design of the array.

In general, each antenna array radiating element must be precisely positioned in the array and mounted on a rigid backplane. When there is a reduction requirement for the antenna RCS, the random scattering will be enhanced after the deformation of the antenna array, and this effect cannot be eliminated.

Each T/R module is usually mounted on a back plate with heat sinks to dissipate the heat generated by the T/R components in a timely manner. For each phased array antenna, the specific T/R layout is different, and one of the common layout methods is to use the brick-type (sTIck) layout as shown in the figure.

Another type of active phased array antenna layout is a chip (TIle) structure as shown in the figure. Each T/R module is formed by vertically stacking three layers of circuit boards, and each circuit board includes four TR circuits. The heat generated by the circuit in the T/R assembly is transmitted through the circuit board to the surrounding metal structure for emission.

The phased array antenna using the chip T/R module also includes DC power, control signals, and coupling gaps of the RF signal, as shown in the figure.

For broadband or digital beamforming radar systems, it is often necessary to use an active phased array antenna with a subarray level layout. When the antenna adopts the sub-array layout method, the production and processing costs of the entire phased array antenna are greatly reduced, and the analog beam scanning capability is formed by adjusting the phase shifter at the rear end of each sub-array.

For analog radar systems, each subarray needs to pass through a time delay unit to scan the beam, as shown in the figure. For digital radar systems, the echo of each subarray is directly acquired by the receiver.