■ Design and Implementation of the Quad-Apollo MxFE™ X-Band Digital Beamforming Platform

This paper presents the design and implementation of the Quad-Apollo MxFE™ X-band digital beamforming platform, an advanced direct RF sampling reference architecture developed to demonstrate the full performance potential of Analog Devices' Apollo MxFE™ (mixed-signal front end) technology.

The platform integrates four transceivers with synchronized clocking, deterministic triggering, and phase-coherent timing to enable true digital beamforming for 16 transmit and 16 receive channels each at X-band frequencies.

This enables a scalable phase-coherent array capable of wideband operation and multi-beam support by implementing a comprehensive signal chain including a low-noise RF front-end, precision clock generation, multi-chip synchronization, and power distribution.

■ Provides a complete reference architecture for next-generation digital beamforming systems.

The Quad-Apollo MxFE™ X-band direct sampling digital beamforming platform (Figure 1) is an advanced solution that applies digital direct RF sampling to all elements, providing a complete reference architecture for next-generation digital beamforming systems.

Analog DevicesDesigned to highlight the capabilities of s)’s MxFE® technology, this platform integrates all key subsystems, including RF signal conditioning, precision clocking, multi-chip synchronization, and DC power management, into a single, high-performance environment, supporting phase coherence and scalable signal processing across multiple channels.


Each sub-array within the platform consists of 16 receive and transmit channels each configured as a uniform linear array, with the inter-element spacing designed to correspond to half a wavelength at 12 GHz.

This array architecture enables spatial sampling optimized for X-band operation while supporting full digital beamforming for all elements.

Additionally, this platform demonstrates the ability to seamlessly synchronize and clock multiple AD9084 Apollo MxFE data converters within a single phase-coherent architecture, providing a realistic example of system-level integration for multichannel RF applications.

Beyond its role as a reference design, the Quad-Apollo MxFE platform can be utilized as a general-purpose development and evaluation tool for advanced radar and communication systems.

This architecture supports multiple simultaneous beams, adaptive nulling, and reconfigurable beam patterns, providing the flexibility required by advanced phased array radar, electronic warfare (EW), and multi-mission sensing applications.

The system's deterministic timing framework, wideband front end, and scalable synchronization architecture are designed to accelerate innovation in high-speed, all-element digital array technology by implementing a complete signal chain from RF input to digital beamforming output.

The block diagram in Figure 2 shows the transmit and receive RF front-end signal chains of a single subarray, a data converter with enhanced digital signal processing capabilities, a multi-chip clocking solution, and a 12V DC power solution.

This solution combines commercial off-the-shelf (COTS) FPGA carrier cards with low-level open-source software drivers and MATLAB® toolboxes to provide a system platformSoftware control is possible.


■ RF front-end

The RF front-end of the Quad-Apollo MxFE system is intentionally designed with simplicity and modularity in mind, providing high flexibility, performance, and easy customization for a wide range of applications.

As shown in Figures 3 and 4, this architecture utilizes wideband yet low-complexity signal paths to achieve wide frequency coverage while maintaining high linearity and signal integrity throughout the entire signal chain.


A key design feature of this system is its adoption of a 2:1 wideband balun to connect the high-speed ADC and DAC to the rest of the analog signal chain.

Each transmit and receive channel integrates a digitally tunable X-band filter, ADMV8913, which provides precise frequency selection and out-of-band signal rejection.

This filter stage includes a board-level bypass path that can be activated via a capacitor rotation mechanism, allowing direct sampling at S-band, C-band, and Ku-band frequencies.

Bypassing the filter reduces the inherent anti-aliasing benefits of the ADMV8913, but provides greater frequency flexibility in broadband or wideband applications where filtering can be handled digitally.

In the receive path, each channel consists of a relatively simple but very linear wideband low-noise amplifier (LNA) and a wideband digital step attenuator (DSA) placed at the input.

The DSA enables fine-grained analog gain control over a wide range of signal conditions and is directly controlled via the data converter's GPIO interface, allowing fast and deterministic adjustments even during system operation.

This architecture supports rapid reconfiguration, making it particularly suitable for adaptive or real-time beamforming systems.

Receiver linearity is a special emphasis in this design.

In fully digital, all-element phased array systems, intermodulation distortion components tend to be spatially correlated across elements.

By prioritizing linearity, Quad-Apollo MxFE technology maintains stable dynamic range and spectral purity even in multi-channel phase coherent configurations.

For applications requiring additional frequency conversion, gain stages, or specialized signal conditioning, the platform supports integration of a plug-in personality card ahead of the front end.

This modular expansion capability gives system designers the flexibility to tailor the performance range to specific end-use requirements, including bandwidth, noise figure, and frequency coverage.

By keeping the subarray-level signal chain simple and wideband, the Quad-Apollo MxFE system provides a robust yet flexible foundation for developing scalable, high-performance RF systems.

■ Clocking architecture

Clocking plays a critical role in today's multi-converter systems, providing the foundation for multi-chip synchronization, uniform delay, and phase-coherent signal combining across large-scale distributed architectures.

Precise clock distribution ensures that all data converters, FPGAs, and analog front ends operate in perfect harmony, which is essential for applications such as phased array radars, multi-channel instrumentation systems, and advanced communications systems.

In these systems, synchronization based on the JESD204C standard is implemented through a combination of a dedicated clock generation block and a clock distribution block, and a bidirectional subclass 1 synchronizer.

This architecture ensures uniform delay across all JESD204C channels, enabling system designers to predict and control timing with nanosecond-level precision.

Additionally, this synchronization structure can be extended beyond a single subsystem, making it suitable for applications that must be implemented on a large scale.Even in this case, the delay can be kept uniform between multiple sub-arrays.

As shown in Figure 5, a single, highly stable reference clock forms the basis of the entire timing network.

This reference signal is distributed through an extremely low-jitter clock fanout buffer, providing a very clean, ultra-low phase noise sampling clock to the MxFE devices.

Each MxFE device uses a dedicated two-wire clock interface to maintain phase and frequency alignment and compensate for drift due to temperature changes or long-term aging.


The same master reference clock generates digital clocks for the FPGA fabric and also drives a secondary clock generation stage that provides a precision reference signal to the synchronizer.

A 10-channel precision synchronizer forms the core of this coordination mechanism, providing bidirectional low-frequency timing signals that align all data converters and FPGAs to a common timing reference.

Additionally, the synchronizer supports per-channel propagation delay trimming, allowing engineers to fine-tune channel alignment and phase relationships to achieve optimal performance.

Depending on the overall array architecture, synchronizers can be deployed in a daisy-chain or fan-out configuration, ensuring scalable synchronization across multiple subarrays or system partitions.

Even at extremely high sampling rates of up to 20 GSPS, this clocking infrastructure maintains uniform alignment, preserving phase coherence and timing integrity throughout the system.

By combining low-jitter clocking, a hierarchical distribution architecture, and JESD204C subclass 1 synchronization, this multi-converter platform implements an integrated, phase-stable timing framework that supports high-speed data acquisition and signal processing performance.


Table 1 summarizes three representative operating configurations of a Quad-Apollo MxFE system driven by a 400 MHz reference clock.

These settings illustrate the tradeoffs between sampling rate, digital decimation, and data throughput, and suggest ways to optimize bandwidth and interface performance for different system requirements.

■ Implementation and simulation of next-generation radar and electronic warfare architecture

System-level verification of the Quad-Apollo MxFE X-band digital beamforming platform demonstrated phase-coherent operation across all channels, achieving sub-degree phase alignment accuracy and sub-picosecond timing accuracy.

The performance demonstrated in adaptive beam steering, spatial nulling, and multi-beam transmission demonstrates that the platform is ready to implement and simulate next-generation radar and electronic warfare architectures.

These results represent a significant milestone toward realizing scalable, all-component digital beamforming systems.

※ About the author
Siddhartha Das is a systems applications engineer in the Subsystems and Sensors team within the Aerospace, Defense, and Communications business unit at Analog Devices, located in Durham, North Carolina. He received his bachelor's degree in electrical engineering from Virginia Tech in 2023 and is currently pursuing his master's degree in electrical engineering there. At ADI, Siddhartha focuses on the development, integration, and characterization of advanced phased array subsystems. His work spans high-performance RF signal chains, digital beamforming platforms, and multi-chip synchronization technologies, with a focus on supporting next-generation radar and communications systems.