Applications such as intruder alarms, smart lighting schemes, traffic monitoring equipment and automatic doors need to detect motion and/or proximity. Commonly used infrared sensing techniques are limited in range and not able to determine object speeds.
Thus, radar-based systems are growing in popularity and expanding the application use cases. For designers looking to implement radar-based systems the challenges include identifying the best radar scheme for the job and implementing the application-specific algorithms needed to process the information.
In recent years, access to the 24GHz ISM frequency band and the development of supporting technologies and tools have provided new options to allow designers to address these challenges.
Radar for Motion/Proximity Detection
Radar (Radio Detection and Ranging) is no longer just a tool to support aerospace and maritime activities. Advanced Driver Assistance Systems (ADAS) have helped to bring radar from the skies to the streets. Now, innovations such as smart buildings, IoT-based remote monitoring, and advanced security systems are opening new opportunities for affordable radar-based sensing.
In today’s emerging generation of smart buildings, monitoring the location and movement patterns of occupants is critical for proper control of lighting and HVAC systems to ensure optimum comfort and energy efficiency. Similarly, advanced security requires accurate detection of potential intruders in all conditions especially when visibility is poor such as at night time. Typically, technologies such as passive infrared (PIR) have been used for room-occupancy sensing, and PIR or video in security systems. Automatic sliding doors in shops and offices are also often controlled using a PIR sensor for proximity detection.
PIR sensors and video cameras are available at relatively low cost and offer acceptable performance to support a basic level of automation. However, there are a number of limitations. PIR sensors, for example, have relatively short range of about 10 meters. Moreover, the target must be moving, and there must be a significant temperature difference between the target and the surrounding environment. There is a high probability both of false alarms and detection failures in unfavorable conditions, such as in hot weather when the ambient air temperature can be close to body temperature, or if the target is still or moving very slowly.
A radar-based occupancy sensor can have much greater range than PIR, up to 30-40 m depending on an object’s reflective properties, and is able to detect presence even when the target is standing still. Detection is not affected by environmental changes such as fluctuating ambient temperature. Moreover, radar-based sensing can be designed to calculate the position of an occupant, which gives the opportunity to enhance smart-building capabilities. Lamp dimming can be localized, for example, to provide optimum brightness for working while dimming other lights in the same room to maximize energy savings.
Radar can also detect the speed of a moving target and its direction of motion, which creates opportunities for many advanced building automation features. Unlike the sensors typically used to control sliding doors, for example, direction sensing can distinguish between passers-by and customers intending to enter the store, thereby helping avoid unwanted opening and closing of the doors. Speed and direction sensing also have important advantages for intruder detection, allowing equipment to become smarter and provide more detailed information for security staff.
Application-Ready Radar
Radar utilizes the principle of radio wave reflection – emitting radio waves in the direction of a target and monitoring the reflected signal to detect the presence of the target. A low-power Continuous Wave (CW) radar system is able to detect the presence of an object in the path of the radiated signal. A Frequency-Modulated CW (FMCW) system is able to measure the range of the detected object.
Using Doppler-based calculations, the system can detect movement, direction of movement and speed. A system comprising a single transmitter and receiver combination is not able to calculate the angle of the target’s position relative to the antenna. However, by introducing an additional receiver, the position of the target can be calculated.
When evaluating potential radar systems, designers must consider frequency range and the specific parameters of the target application.
The 24GHz ISM band is an ideal frequency range for commercial radar systems, as no license is required and systems can be used without interfering with other equipment such as Wi-Fi, smartphones or microwave ovens.
Highly integrated 24GHz radar transceiver ICs now available have a small PCB footprint and greatly simplify radar system design. By integrating all the circuitry for a given implementation in a single device, these transceivers eliminate design challenges such as matching external components and laying out RF transmission lines on the PCB.
Moreover, the high level of integration can save around 70% of the PCB area occupied by a comparable discrete solution. The ICs can be used with conventional PCB substrate material, and utilize a standard surface-mount package thereby allowing assembly on a conventional SMT production line.
A complete, compact radar system for domestic or commercial applications can be built by combining the radar ICs with a microcontroller to control the operation of the transceiver and perform the sampling and processing necessary to extract information from the reflected signal.
Developing Miniaturized, Low-Power Radar
Compared to traditional passive infrared sensing, a radar-based system can have relatively high power consumption if operated continuously. In practice, a battery-powered radar sensor can be implemented using a duty-cycling technique that activates the radar chip for the short period necessary to make an accurate speed measurement and quickly turn the device off until the beginning of the next measurement interval.
Figure 1 shows the Doppler shifts associated with various measured speeds, and the minimum measurement times necessary to calculate those speeds accurately. A measurement period of about 10ms enables accurate measurement down to about 2.5km/h.

A measurement update rate of 0.5 seconds allows detection of a target moving at up to about 20km/h. This is a suitable speed range for monitoring the movement of the human body. The measurement period and update interval can be adjusted to extend or reduce the speed range and alter power consumption. Activating the transceiver IC for 10ms in every 0.5 seconds effectively reduces the power consumption from 528mW in continuous mode (at 3.5V supply) to about 12mW.
To operate the transceiver in duty-cycle mode, the microcontroller can be used to gate the IC’s power supply by driving a power switch with a PWM signal. Figure 2 shows the ICs needed to implement the system and their internal functional blocks.
The microcontroller Digital-to-Analog Converter (DAC) is connected directly to the Voltage-Controlled Oscillator (VCO) that sets the transceiver’s operating frequency. In this system, only the In-phase component of the received signal is selected and fed to the microcontroller. This can help save cost and power in systems that do not need to detect the direction of movement.

When the transceiver is first turned on, the VCO is given time to settle. When the frequency is within the ISM band, the IC’s power amplifier is enabled and the received signal is sampled. When sampling is complete the transceiver is turned off and the microcontroller performs the necessary processing on the sample data before itself returning to standby mode. A timer then wakes the microcontroller to reactivate the transceiver at the beginning of the next measurement interval.
From the Bench to the Application – Support Tools
Reference design and development/evaluation kits are available to further simplify development of 24GHz radar applications, Figure 3 shows a single-board development kit that allows engineers to evaluate Doppler-based detection of movement and direction of movement, as well as Doppler based speed measurement. A FMCW option allows the sensor to measure distance of stationary and moving targets.
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