RTC Magazine

Many communications systems digitize incoming analog signal information with a high-speed A/D converter and then route the digitized information between system elements for processing and analysis. Typically, the digitized information is intermediate frequency (IF) data sent from a radio frequency (RF) downconverter to digital signal processing equipment or sent from digital signal processing equipment to an RF upconverter.

Until now, the interface for transmitting the digitized IF data stream between system elements over the communications link has been application- and/or equipment-specific. Often, it has also been proprietary: the system’s digitizing source packages the IF data into a unique, proprietary format, which the signal processing destination must know how to unpack (Figure 1).

As a result, every time a source or destination component changes, the interface for passing digitized data between them also changes, and new software must be written to achieve or restore interoperability.

Standardizing the Digital IF Interface

Standardizing the digital IF interface across receiver/transmitter equipment, signal digitization and conversion equipment and signal processing equipment would clearly benefit both OEMs and vendors. System manufacturers would no longer have to rework their systems each time they upgraded a component.

In addition, instead of being locked in to a particular vendor, OEMs could pick and choose the best component for the application at hand from a marketplace of interoperable products and essentially “plug and play” those components. Vendors would no longer be required to rewrite their digital IF interconnect logic to yet another data format, saving resources and increasing time-to-market. Standardizing digital IF could make system deployment faster and technology refresh easier for both system manufacturers and their vendors.

In 2004, Mercury Computer Systems and DRS Signal Solutions (DRS-SS) formed an informal industry group that solicited participation and input from the signal acquisition and processing community and its OEM customers for the development of such a standard. The informal industry group voted to associate with the VITA Standards Organization (VSO), and the VITA 49 Working Group was created to design a digital IF interface standard for adoption by VITA.

The Digital IF Data Representation

The Digital IF Data Representation (VITA 49) defines a data structure for the transmission of digital IF data between one or more sources and one or more destinations for both the receive and transmit paths. The Digital IF Data Representation is link-agnostic: it defines a methodology for representing digital IF data that can be layered on top of any transport protocol and any physical communication link (Figure 2).

The goal of the Digital IF Data Representation is to define a data structure that can be used by a sensor source to transmit digitized data to a signal processing destination, or by a signal processing source to transmit digital data to an emitter destination. Initially, it is focused on radio IF to convey digitized analog radio signals between RF communication receivers/transmitters and digital processing devices (Figure 3).

Although the Digital IF Data Representation is intended for use in both military and commercial applications, it is particularly targeted toward beam-forming and direction-finding signal-processing systems, as well as communications and signal intelligence (SIGINT) systems. Any communications system that needs to change an analog signal to digital information and then send it on for processing—such as police and fire department communications systems—is a candidate for the Digital IF Data Representation. Designers of electronic intelligence (ELINT) systems and software defined radio (SDR) systems may also find this standard useful.

Digital IF Data Representation Packet Types

The Digital IF Data Representation uses a packet-oriented approach to specifying a data representation standard. The working group has discussed the need for three packet types. A streaming data packet defines the base structure for representing digital IF data, a source characteristics packet enables components in a distributed system to communicate their capabilities to each other, and a status change packet conveys changes in the system’s state.