Analog Devices CN0566: Phased Array Development Platform

Introduction

Circuits from the Lab® reference designs, such as the CN0566, are engineered for quick system integration to address analog, mixed-signal, and RF design challenges. This document details the CN0566, a simplified phased array beamforming demonstration platform.

The CN0566 is designed to mount on a Raspberry Pi and utilizes the PlutoSDR module for digitizing intermediate frequency (IF) outputs. The software interface leverages the Linux industrial input/output (IIO) framework, supporting various development utilities and cross-platform APIs (Python, GNURadio, MATLAB). The system is powered by a single 5V, 3A USB-C adapter.

The platform features an 8-element linear array antenna operating from 10.0 GHz to 10.5 GHz (X band), enabling the use of common low-cost motion sensor modules as microwave sources.

Devices Connected/Referenced

Circuit Description

Phased array beamforming is a signal processing technique used in antenna arrays for radio communications, radar systems, and medical imaging. It allows antennas to be aimed directly at a target, reject interfering signals, and maintain links with moving objects. Beamforming involves adjusting the phase and gain of signals from multiple antennas before summing them.

The CN0566 implements an 8-element phased array, incorporating mixers, a local oscillator (LO), and digital control circuitry. It outputs two IF signals at 2.2 GHz, which are digitized by a PlutoSDR module.

Beamforming Fundamentals

Forming a beam pattern involves adjusting phase and gain for each antenna element to control the direction of the combined RF beam. This allows for real-time beam steering and reconfiguration without physical antenna movement.

In-Phase Combining: Figure 2 illustrates how inserting time delays can align wavefronts from a desired transmitter, causing signals to arrive in phase at the combiner, resulting in a larger output signal.

Out-of-Phase Combining: Figure 3 shows that applying the same delays to a wavefront from an undesired transmitter can misalign its phase, significantly reducing the combiner's output.

The time delay (Δt) required for beam steering is related to the incremental propagation distance (L) between elements and the speed of light (c), calculated as Δt = L/c. The propagation distance L is determined by the element spacing (d) and the electrical beam angle (θ), via L = d sin θ (Equation 1).

Time delay can be emulated with phase shift (ΔΦ), particularly for narrow bandwidth systems. The phase delay is computed as ΔΦ = 2πfL/λ = 2πfd sin θ/c (Equation 3).

The electrical steering angle (θ) can be found from the phase shift using θ = sin⁻¹(ΔΦc / (2πfd)). For example, with 14 mm element spacing and a 10.3 GHz wavefront at 30°, the optimal phase shift is 87.4°.

The antenna pattern is a combination of the element factor (pattern of a single element) and the array factor (electrically controlled beam pattern). The array factor for a uniform linear array is described by equations like Equation 4 and 5, which relate beam angle, element spacing, and phase shifts.

Figure 5 Description: An 8-element array factor plot showing beam steering at 30° with element spacing d = λ/2, illustrating the main beam and sidelobes.

RF Design

The CN0566 features an on-board 8-element patch antenna array operating from 10 GHz to 10.5 GHz. Each element uses sub-elements summed via PCB traces, with a shorting stub for ESD protection. These elements are coupled to ADL8107 low noise amplifiers (LNAs).

Figure 6 Description: Block diagram of the 8-element antenna patch and ADL8107 LNAs, showing connections via capacitors and optional SMP connectors.

Figure 7 Description: Plot of antenna gain versus frequency, showing a -3 dB bandwidth from 9.9 GHz to 10.8 GHz.

Beamformers: The core components are two ADAR1000 4-channel beamformers, offering per-channel 360° phase adjustment (2.8° resolution) and 31 dB gain adjustment (0.5 dB resolution). The CN0566 utilizes the receive paths of these devices.

Figure 8 Description: Diagram illustrating the ADAR1000 operation in receive mode, showing phase and amplitude control and signal summing.

Local Oscillator/Synthesizer: The ADF4159 PLL and HMC735 VCO create a frequency synthesizer for the 10.5 GHz to 12.7 GHz range, used to drive mixer LO ports. The LO frequency is typically 2.2 GHz above the desired signal, around 12 GHz to 12.7 GHz. The ADF4159 also supports FMCW ramps.

Figure 9 Description: Block diagram of the CN0566 synthesizer circuitry, showing the ADF4159 PLL, HMC735 VCO, and LO distribution.

Mixers and Filtering: The ADAR1000 RFIO output is filtered by a 10.6 GHz low pass filter before entering the LTC5548 mixer. The mixer outputs a 2.2 GHz IF, which is then filtered by a 2.5 GHz low pass filter.

Figure 10 Description: Block diagram of the CN0566 mixers and filtering path.

Figure 11 Description: Spurious Free Dynamic Range plot for the receive signal path, showing signal strength and spurious signals, with an SFDR of approximately 56 dBc.

Transmitter Signal Path

The CN0566 provides a transmit output for external antennas, useful for antenna pattern measurements or radar applications. The transmit frequency is derived from the on-board LO.

Figure 12 Description: Block diagram of the CN0566 transmitter signal path, showing the input SMA connector and outputs TX1 and TX2.

The transmit input signal (around 2.2 GHz) first passes through a low pass filter to mitigate harmonics from the LO. Figure 13 shows the raw transmit output, while Figure 14 shows the output after initial filtering. Figure 15 displays the final transmit output after amplification and bandpass filtering.

Figure 13 Description: Spectrum plot of the PlutoSDR transmit output for a 2.1 GHz tone without filtering.

Figure 14 Description: Spectrum plot of the transmit signal after initial filtering but before mixer filters.

Figure 15 Description: Spectrum plot of the final transmit output after amplification and bandpass filtering.

Virtual Arrays

The CN0566 supports virtual arrays, a technique common in radar. By toggling two transmitter outputs at different distances, a virtual array with twice the number of receive elements can be created, resulting in a narrower receive beam at the cost of longer data acquisition time.

Figure 16 Description: Diagram illustrating the concept of virtual arrays and transmit antenna switching.

Transmit antenna switching is controlled by the ADF4159's MUXOUT pin, which indicates the end of a PLL chirp. This signal, after level shifting, clocks a ripple counter that selects antenna toggle rates (2x to 128x chirps) via a multiplexer driven by Raspberry Pi GPIO signals.

Figure 17 Description: Block diagram of the transmit antenna switching circuit, showing the ADF4159 MUXOUT, level shifter, ripple counter, and multiplexer.

Digital Control and Level Shifting

A Raspberry Pi 4 board provides serial peripheral interface (SPI), I2C, and digital I/O control. Its 3.3V logic levels are used directly or level-shifted to 1.8V for interfacing with components like the ADAR1000 and ADF4159. Dual supply level translators ensure digital pins do not experience voltage mismatches.

The ADF4159 MUXOUT also serves as a PLL lock and end-of-ramp indicator for FMCW modes, with a level-shifted 3.3V output driving an LED indicator.

Power Architecture

The CN0566 is powered by a single USB-C receptacle (5V, 3A). This power is distributed to the Raspberry Pi and the board's power management system. Figure 18 illustrates the complete power tree.

Figure 18 Description: Diagram of the CN0566 power tree, detailing the voltage regulators and power distribution.

Key power components include the LTC4217 hot swap controller for safe insertion/removal, LT8609S DC/DC converter stepping 5V to 3.3V, ADP7158 LDO for 3.3V rails, ADM7150 LDO for 1.8V analog supply, ADM7170 LDO for the HMC735 VCO, and LT3460/ADP7118 for boosting to 15V/14V for the AD8065 amplifier.

System Monitoring and Control

The AD7291 8-channel, 12-bit SAR ADC monitors system voltages, VCO tuning voltage, and board temperature. Input ranges are scaled via resistor dividers. Table 1 lists the measured parameters and their scale factors.

Table 1 Description: Measured Parameters and Scale Factors for AD7291 inputs, showing voltage supplies and their corresponding divider scale factors.

System Performance

An ideal beamforming array produces a precise beam shape. Achieving this accuracy depends on precise gain and phase control for each element. The CN0566 accounts for potential errors such as element mismatches, ADAR1000 internal errors, and mismatches between components in the receive paths.

System Calibration: A calibration script is included to compensate for these errors. The process involves measuring and compensating for transmit channel mismatch, then residual element gain mismatches, and finally phase mismatches between adjacent elements.

Figure 19 Description: Plot of uncalibrated signal strength showing a maximum mismatch of 11.2 dB between elements, indicating two groups of four elements with differing strengths.

Figure 20 Description: Plot of calibrated signal strength showing a reduced maximum mismatch of 0.51 dB.

Figure 21 Description: Plot of gain versus phase difference for adjacent elements, illustrating phase sweeps and ideal nulls at ±180°.

Post-calibration, gain and phase accuracy approach the ADAR1000's resolution (better than 0.5 dB, 2.8°).

Common Variations

The CN0566 can be extended horizontally by stacking boards for narrower beams, requiring a common LO and synchronized SDR receivers. For applications not requiring monopulse tracking, multiple ADAR1000 outputs can be combined and digitized by a single ADC.

External 8-element antennas can be used via optional SMP connectors. Frequency plans within 8 GHz to 14 GHz can be implemented by modifying on-board filters.

For half-duplex operation, the ADTR1107 (6 GHz to 18 GHz) RF front-end is suitable.

Other Analog Devices phased array solutions include the ADAR1000EVAL1Z (32-channel), ADAR3000/ADAR3001 (16-channel, time delay beamformers), and ADAR4002 (single-channel, time delay beamformer).

Circuit Evaluation and Test

This section outlines the setup and procedures for evaluating the EVAL-CN0566-RPIZ. Refer to the CN0566 User Guide for complete details.

Equipment Needed:

• CN0566 Kit (EVAL-CN0566-RPIZ, Raspberry Pi 4, PlutoSDR)
• USB to micro-USB cable
• SD card with Analog Devices Kuiper Linux image
• 5V, 3A USB-C wall adapter
• 10 GHz microwave source (motion sensor)
• Tripod

For Running Scripts Locally on Raspberry Pi:

• Display monitor with HDMI
• Micro-HDMI to HDMI cable
• USB keyboard and mouse

For Running Scripts on a Remote Host Computer:

• Windows, Linux, or Mac computer with MATLAB or Python IDE
• Ethernet cable

Getting Started:

Configure the SD card with Kuiper Linux for the CN0566 as per the User Guide. Insert the SD card into the Raspberry Pi's slot.

Setup and Test: Connect the PlutoSDR to the Raspberry Pi, mount the tripod, and plug in the USB-C adapter. Power the microwave source and aim it at the antenna array.

Python Examples: Connect display, keyboard, and mouse. Open a terminal, run cn0566_find_hb100.py, and then cn0566_gui.py to observe the beam pattern.

MATLAB Examples: Connect the host computer via Ethernet. Run phaser_hb100_scan.m and phaser_rxtx.m in MATLAB to observe beam patterns.

Figure 22 Description: Connection diagram for setting up the CN0566 for testing.

Figure 23 Description: Typical beam pattern displayed in the GUI, showing the result with an HB100 sensor at mechanical boresight.

Learn More

Design Support Package:

• Keith Benson. 2019. "Phased Array Beamforming ICs Simplify Antenna Design." Analog Devices.
• Peter Delos, Sam Ringwood, and Michael Jones. "Hybrid Beamforming Receiver Dynamic Range Theory to Practice." Analog Devices.
• Peter Delos, Bob Broughton, and Jon Kraft. 2020. "Phased Array Antenna Patterns—Part 1: Linear Array Beam Characteristics and Array Factor." Analog Devices.

Data Sheets and Evaluation Boards:

Links to data sheets and evaluation boards for ADAR1000, ADF4159, ADRF5019, AD8065, ADL8107, LTC5548, LT8609S, AD7291, LT3460, LTC4217, ADP7118, ADP7158, ADM7150, ADM7170, HMC735, HMC652, HMC654 are available.

Revision History

04/2023 - Revision 0: Initial Version

ESD Caution

This product contains ESD-sensitive devices. Proper ESD precautions should be taken to avoid performance degradation or loss of functionality.