Millimeter wave (mmWave) imaging systems are becoming increasingly popular in security checks of public buildings, sports venues, and airports. These systems are capable of detecting metal and non-metal hazardous materials and reporting their location within the scanning area, thereby helping security professionals locate and identify suspicious items more quickly. This article will explore the fundamental principles of millimeter wave imaging, explain how the components in the millimeter wave solution designed by Analog Devices, Inc. (ADI) work together, and focus on the key role of edge processing technology in the iterative upgrade of the system.
Introduction to Millimeter Wave
In millimeter wave systems, the transmitter and receiver arrays are connected to a spatially distributed antenna array. At a specific point in time, one antenna in the array emits low-power, single frequency omnidirectional radio frequency (RF) signals, which are reflected by the target object (Figure 1). The backscattered signal generated by this reflection will be received by all antennas in the array, and the integrated circuit (IC) connecting the antennas obtains information by measuring the phase and amplitude of these backscattered signals.
Schematic diagram of millimeter wave system for transmitting antennas in sequence
Figure 1: In a millimeter wave system, the transmitting antenna sequentially broadcasts low-power, single frequency, and omnidirectional signals. Then, the receiving antenna measures the backscatter. (Image source: Analog Devices, Inc.)
Each transmitting antenna will send the same signal sequentially, and this measurement process will be repeated for each transmission. By repeating the entire process on multiple frequencies within the range of 10 GHz to 40 GHz, the system can capture the differences in penetration depth and signal reflection caused by frequency variations of different RF signals. The system resolution depends on the number of transmission and reception channels: for example, airport scanners have a large number of channels to meet the high resolution required for detecting small objects such as razors; For scenarios where weapons and explosives are the main monitoring targets, using fewer channels can both reduce costs and shorten scanning time.
The processor combines the backscatter information into a vector matrix. When these vectors are associated with frequency and spatial position, the generated multidimensional array can generate images that not only recognize metallic objects, but also detect non-metallic items hidden between and below clothing layers.
The scanning speed depends on the speed at which the system processes backscatter data, switches from transmitter to transmitter, and cyclically scans the required frequency. For example, a system with 500 components covering the range of 10 GHz to 40 GHz in increments of 50 MHz must undergo 300000 switches. The millimeter wave systems deployed today, with their fast switching capability, only require the scanned person to maintain a posture for a few seconds to generate effective images. As the switching speed becomes faster, in the future, millimeter wave systems can even recognize threatening objects when the subject passes through the detector on foot without stopping.
Building a millimeter wave system
In order to detect potential threats, achieve the required resolution, and facilitate rapid scanning, millimeter wave system designers must choose hardware that can work together. ADI's integrated millimeter wave system solution includes an ADF4368 microwave broadband synthesizer, multiple ADAR2001 transmitter ICs, multiple ADAR2004 receiver ICs, and an AD9083 analog-to-digital converter (ADC). Each device will be discussed in sequence below (Figure 2).
Millimeter wave system image integrator, transmitter, receiver, and ADC integrated (click to enlarge)
Figure 2: A complete millimeter wave system combines a synthesizer, transmitter, receiver, and ADC with power management, switches, and logic components. (Image source: Analog Devices, Inc.)
The signal chain starts from the ADF4368 microwave broadband phase-locked loop (PLL) synthesizer with an integrated voltage controlled oscillator (VCO) (Figure 3). ADF4368 can generate frequency steps in the range of 2.5 GHz to 10 GHz, with a step interval of 12.5 MHz, completely within its operating frequency band of 800 MHz to 12.8 GHz. The jitter of its continuous wave (CW) single ended RF signal is less than 30 fsecRMS.
Image of Analog Devices ADF4368 Microwave Broadband Synthesizer
Figure 3: The ADF4368 microwave broadband synthesizer with integrated VCO can provide low jitter CW RF output in the frequency range of 2.5-GHz to 10-GHz. (Image source: Analog Devices, Inc.)
The output signal power of ADF4368 is 9 dBm (7.94 mW). Due to the lower power required by the transmitter IC, the output of ADF4368 can be divided into seven channels, which can drive up to 128 4-channel transmitter ICs or 512 channels.
The ADAR2001 transmitter IC (Figure 4) receives input from ADF4368 and then multiplies, filters, attenuates, divides, and amplifies the signal to provide four antenna output channels with frequencies between 10 GHz and 40 GHz for each IC.