New mezzanine standard supports multi-GHz signal acquisition

continue to offer the ideal solution to the front end of embedded subsystem designs. They are used for a wide variety of applications ranging from the attachment of multiple I/O channels, often requiring continuous multi-GHz sampling rates, to pre- or post-processing of video or protocol stacks. However, this broad range of function and performance presents vendors of off-the-shelf products with the challenge of providing widespread flexibility with the fewest possible design variations. The FPGA Mezzanine Card (FMC) standard, ANSI/VITA 57, bridges the external world to an FPGA's I/O capability via advanced connector and signaling capabilities aligned with future I/O bandwidth expectations.

FPGAs and various applications

While FPGAs are now used for in many very diverse applications, it is applications with many parallel and repetitive algorithms that really benefit. Surveillance, Signals Intelligence (SIGINT), and Electronic Counter Measures (ECM) are examples requiring the high channel count, I/O bandwidth, high performance, and low latency that FPGA-based technologies can provide. The latest generation of devices has the speed and complexity needed for continuous sampling of emissions within a baseband. To achieve this requires multi-GBps bandwidth between the sampling A to D and FPGA. The L band, which lies between 1 and 2 GHz, is an example of such a band that might be of interest, requiring a minimum of 3G samples at 8-bit resolution, equating to 3 GBps input bandwidth to cover the band completely. Low latency is also critical to these types of applications, either to switch to higher resolutions to refine a search or to react rapidly to certain external events, for example an ECM system's response to an emitter type.

FMC specifications

The FMC mezzanine has been developed to provide this high-bandwidth, close coupling between sensor and FPGA while also providing an added degree of I/O flexibility in a very small space envelope. Adopted by a number of COTS embedded computing vendors, FMC is a small form factor mezzanine that can be implemented in single or double widths, with the single width module measuring only 69 mm x 76.5 mm (2.72" x 3"). It uses a high-density I/O connector of 160 or, optionally, 400 pins between the FMC and its basecard. The connector supports single-ended or differential signaling at up to 2 Gbps plus a number of industry-standard 10 Gbps Multi-Gigabit Transceivers (MGTs). The smaller connector option supports one MGT pair while the 400-pin High Pin Count (HPC) option supports 10 pairs. The MGTs are intended for high-speed serial transmission links with protocol support provided by the FPGA. In this role, an FMC offers the ideal form factor for conversion between copper and fiber.

High-bandwidth paths from an FMC to its basecard make use of differential pairs. The transmission distance between an FMC and its associated FPGA on the basecard is very short, making it practical to use both edges of a 1 GHz clock to transfer data, giving each differential pair a bandwidth of 2 Gbps. Clocking only 16 pairs at 1 GHz provides the 4 GBps required by many baseband applications, still leaving many pairs free for additional functions or to provide similar high-speed data pipes to more than one FPGA. The FMC is compatible with many modular board-level standards such as VME, , , or . Figure 1 shows an FMC module and one 3U and one 6U VPX board, each with one or two FMC module sites as offered by Curtiss-Wright Controls Embedded Computing (CWCEC). An FMC can be air- or conduction-cooled to suit a wide variety of environmental conditions. Similar to the XMC/ standard, FMC can have I/O signals routed via a front panel bezel or through the basecard and backplane for enhanced maintainability.

The FMC and XMC/PMC, while apparently similar, are complementary in their application of FPGA technology to a DSP requirement. An XMC/PMC module will generally include both A to D and FPGA on a single mezzanine, using high-speed serial links such as PCI Express or Serial to transfer results to the baseboard for additional processing or distribution. However, in some cases, FPGA-based applications are reaching beyond the capability of XMC/PMC to support increased I/O bandwidth, system latency, or power dissipation/cooling requirements. An FMC is better suited to applications such as SIGINT that require multiple FPGAs mounted on the baseboard with more than one multi-GHz path to distribute data, or other applications where more than one I/O function must be accommodated. An example is future multimode, multispectral sensors.

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Figure 1: The FMC (pictured) is compatible with many modular board-level standards such as VME, CompactPCI, AdvancedTCA, or 3U and 6U VPX (also pictured).