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X-Microwave: A Better Way to Prototype RF Designs

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by Jacob Ciolfi, Field Applications Engineer, Analog Devices

Evaluation board-based prototyping for an RF design takes a significant amount of engineering time and resources, and the result may still lack the performance of the final system once it’s built on a single board. However, prototyping RF designs with the X-Microwave modular system can dramatically reduce the time and resources needed to test an RF signal chain. It enables modifiable, near-PCB prototypes through 60 GHz to be built and tested in a single afternoon. This article provides an overview of the X-Microwave platform and its advantages, as well as a step-by-step guide to getting started.

The typical prototyping experience for an RF design includes purchasing an evaluation board for each component in the signal chain and using RF cables to string the boards together. The result is a rough approximation of how the signal chain would perform if it were built on a single production PCB after being properly laid out.

Unfortunately, this method can accrue significant insertion loss from long evaluation board PCB traces and extensive cabling and connectors, and it can be time consuming to bring online because of the specific voltage requirements of each board. It’s also common for an RF component to require multiple voltages with specific power rail sequencing, which if violated could destroy the component. Just the power and RF wires alone can create a rat’s nest and if any board needs digital control, things get further complicated. If the system doesn’t work the first time you turn it on, debugging quickly degrades into an exercise in patience and perseverance.

X-Microwave’s approach is faster, easier, and more accurate. For example, after planning your RF signal chain, you head to the lab, grab the parts, and have a prototype constructed on the bench in an hour. You connect a single 12 VDC supply, a signal generator, and a spectrum analyzer, and you’re taking measurements of PCB-like performance within a decibel of your simulations the first time you power everything up. Ten minutes with a hex key and you have it swapped out and are testing your updated design.

Figure 1: A full X-Microwave prototype, including power and digital control, consisting of an FMC-XMW bridge board, an X-Microwave signal chain, and a Raspberry Pi

This is the prototyping experience offered by X-Microwave’s modular RF prototyping platform that allows easily modifiable signal chains to be built without specialized tools. The signal chains are composed of X-Microwave blocks, which are connectable single-IC RF boards, and components in the ecosystem support frequencies up to 60 GHz. The RF connections, solderless contacts secured by hex screws, are robust and simple to install. The signal chain is much easier to power and digitally control than evaluation boards because a single 12 VDC supply controls the board and either a Raspberry Pi, FPGA, or other driver—your choice (Figure 1). This design approach enables fast signal chain edits, reduces debug time significantly, and keeps the prototype compact, clean, and portable.

The X-Microwave Solution

An X-Microwave prototype consists of small, single-IC blocks that can be strung together to create a signal chain. From amplifiers to mixers, switches, PLLs, and VCOs, the ecosystem has thousands of RF blocks available to support a variety of full signal chains. Each individual RF block comprises a single RFIC, either a packaged part or die, with the surrounding passives required for optimal function and matching.

X-Microwave has developed its solution to ensure the RF layout and design performs as close to data sheet specifications as possible. On each RF block, grounded coplanar waveguide traces run from the IC to launches on the edges of the block. RF connections are made from these launches to the neighboring block using solderless ground-signal-ground (GSG) interconnects.

These interconnects closely resemble continuous PCB traces, allowing the prototype’s overall performance to represent final system performance much more accurately than a large connection of evaluation boards. The GSG jumper connections have an insertion loss of fractions of a decibel, and as the number of components in the signal chain increases and more interconnects are needed, the difference in insertion loss between X-Microwave and SMA-linked evaluation boards becomes even more pronounced.

Figure 2: An X-Microwave signal chain

The RF blocks are mounted together on a protoplate (Figure 2), with SMA probe blocks attached to the ends of the signal chain to get the RF signal into and out of the board. X-Microwave also has walls and lids available to enclose RF blocks, allowing you to simulate cavity effects.

Figure 3: A bias and control block (bottom) connecting to an RF block (top). A protoplate is not pictured.

Dedicated bias and control boards mounted on the bottom of the protoplate provide power and control signals (Figure 3). Each active RF block pairs with a dedicated bias and control board that has the circuitry needed to supply the regulated voltages, power sequencing, and digital control required by the component. The bias and control board attaches to the bottom of the protoplate directly underneath the RF board it is supporting, making an electrical connection to the RF board above with spring pins. With the power sequencing and bias taken care of by these dedicated blocks, the designer is free to focus on optimizing RF performance.

Prototyping your RF Signal Chain

Creating an RF design with X-Microwave is like designing any other RF signal chain. To quickly find the X-Microwave block you need, X-Microwave has a component search function that can filter by type, specification, and manufacturer (Figure 4). Once you have selected the parts, the next step is simulating your proposed signal chain. Keysight’s Genesys® software, an RF simulation tool, has a built-in library with X-Microwave models.

Figure 4: The X-Microwave banner on the HMC8402-DIE web page

These models simulate the RF X-Microwave blocks from launch to launch without de-embedding the traces, improving the simulation accuracy of the board compared with de-embedded IC simulations. The extensive X-Microwave library provides models for many parts that don’t have Genesys models directly from the manufacturer.

After running simulations and reaching a desired level of performance in Genesys, X-Microwave’s layout tool then allows you to complete an entire RF layout including DC power, and it can be accessed online on the company’s web site. The layout tool can be used to plan a placement map of the X-Microwave blocks in the signal chain on the protoplate (Figure 5). Once you place the RF blocks, you can add the bias and control boards automatically with a single click. All components used in the signal chain update live in the bill of materials in the upper right, where you can also find an Export CSV button. The .csv file contains a bill of materials you can send to X-Microwave for a formal quote when you’re ready to move on to ordering.

Figure 5. A sample design in X-Microwave’s layout tool showing a planned signal chain and highlighting the part selector and BOM functionality

In addition to the RF blocks, bias, and control boards needed to make up the signal chain, a few additional parts are required to electrically connect and mechanically mount the blocks. The protoplate is sold in two sizes, 32×32 and 16×16, which refers to the grid units or spacing between the screw holes on the board. You will also need GSG jumpers and anchors, which are small, flexible, rectangular circuits that are placed across the launches of bordering RF blocks to form an RF connection. Anchors are screwed in across the GSG jumpers to secure them to the RF blocks and ensure a continuous electrical connection (Figure 6).

Figure 6: A GSG placement procedure (top) and GSG and anchor (bottom)

Connecting an external RF signal source into the signal chain requires an X-Microwave probe. There are two different probes available, 2.92 mm and 1.85 mm, depending on the frequency. The 2.92 mm is advertised as good through 50 GHz, while the 1.85 mm is advertised as performing above and beyond X-Microwave’s 67 GHz testing ceiling. You will also need screws to attach everything to the protoplate. Up to seven different screw lengths may be used, from the shortest for attaching bias and control boards to the longest for attaching X-Microwave wall edges with a lid on top.

The tools required for connecting all the pieces include a 1/16 in. hex key to tighten the screws and tweezers to place small things in tiny spots. Once your RF and bias and control blocks arrive, follow the map you made with the X-Microwave online layout tool to put the board together.

Before you begin testing, first power the boards and hook up digital control. To connect power and digital control to the bias and control boards, the AD-FMCXMWBR1-EBZ bridge board is your best option (Figure 7). It provides up to eight GPIO lines, two full SPI buses with eight chip select lines each, and two full I2C buses. The bridge board also has two modes of digital control: a Raspberry Pi connected directly to the bridge board driving the chain with something as simple as a few lines of Python script, or an FPGA interfaced with the X-Microwave signal chain through the FMC connector on the bridge board. This allows the development and testing of near-production software alongside the hardware prototype.

Figure 7: An FMC-XMW bridge board, the Analog Devices AD-FMCXMWBR1-EBZ

The 12 VDC supply connected to the bridge board provides seven voltage rails for the signal chain, three of which are adjustable using potentiometers. A few other settings, including bridge board level shifters, can be selected using jumpers. Finally, the bridge board connects to the X-Microwave prototype with only two cables, resulting in a distinct lack of clutter on the RF lab bench—a stark contrast to its usual state as a maze of cables and alligator clips.

The bridge board is a terrific solution for digital control and power as it has minimal hardware and is portable, which makes it well-suited for demonstrations and travel. The only additional equipment required is an RF source and an RF measurement tool. A clean lab bench and modular ecosystem enable faster and more productive debugging, getting you to production with less headaches and fewer engineering hours.

Conclusion

X-Microwave is the solution to (almost) everything painful about conventional evaluation board RF prototyping, an art that has traditionally taken more than its fair share of engineering hours and frustration. When used with ADI’s FMC-X-Microwave bridge board that requires a single 12 VDC supply for power and a Raspberry Pi for digital control, each signal chain demo can fit in a small shoebox and be set up in less time than it takes a slide deck to load.

You might assume that this level of performance is more expensive than traditional prototyping practices, but except for the one-time start-up costs, prototyping with X-Microwave is often comparable in cost to that of building the system with eval boards. In fact, some X-MWblocks are actually less expensive even before considering the benefits that reduced engineering time brings to your bottom line.

Acknowledgements

The author acknowledges the contributions of people who helped with this project. Steve Ruscak and Jeff Stevens provided guidance for the project and assisted with the writing process. In addition, my RF mentors provided invaluable help in the lab, Wesley Harris and Sydney Wells for their suggestions early on in shaping the article, and Carolyn Reistad for her valued advice and recommendations.

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