Home Brew USB Radio Interface

USB Rig Interface In Use

A USB Radio Interface is often desirable in modern amateur radio operations for connecting a personal computer to a radio transceiver. On the computer side, the USB port is generally the most convenient and widely used interface medium. Things are not quite as settled on the radio side, as the major radio manufacturers have defined their own control methods and interface standards.

Nevertheless, the various radio interface standards do have much in common, enough so that USB radio interfaces can be designed to be compatible with virtually all popular radio transceivers.

USB radio interfaces are available for purchase from a number of manufacturers with various features, form factors, and sizes. My own needs were for a compact and reliable USB radio interface for use in portable or mobile operations. A commercial unit with minimal capability was purchased and tried, but it was plagued with RFI-susceptibility problems.

After performing market research and not finding anything meeting my exact desires, a decision was made to design and build my own home brew USB Radio Interface. The interface described here is the result of that effort. This interface has the following features:

  • Fully compatible with all major amateur radio software applications.
  • No special software drivers are needed as the USB hub, dual UART, and audio CODEC integrated circuits are already supported by the major operating systems. These same components are used in many common computer peripherals.
  • Uses the proven FTDI dual UART component that does not have the timing and control issues exhibited by certain similar devices.
  • Only a single USB cable connects to the computer. The interface is completely powered from computer over the USB port. A soundcard capability is built in so a separate audio cable is not needed.
  • All non-USB radio connections are made via a single 25-pin D-SUB connector. This connector and its pinouts are compatible with those of the RigExpert and Navigator interfaces.
  • Small size: enclosure 2.6 x 3.6 x 1.1 inches; printed circuit board 2.2 x 3.2 x 0.7 inches.
  • CAT interface signal levels include RS-232, 5V-TTL, and Icom CI-V, and are compatible with virtually all radios.
  • Direct PTT transmitter keying control is used, not a VOX circuit.
  • CW keying is via an optically-isolated solid state relay compatible with both modern solid state radios and older tube radios that use grid-block keying.
  • Transformer-isolated transmit and receive audio signal paths.
  • A spare USB port is provided.
  • EMI resistance, ESD protection, signal integrity, and low-noise features include four-layer PCB with internal power and ground planes, isolation of digital and analog audio circuits including usage of split power and ground planes, dedicated linear voltage regulator for analog audio circuits, USB common mode EMI chokes, and transient protection devices for both USB connectors.

USB Radio Interface Bare Printed Circuit Board.

USB Radio Interface Bare PCB

PCB Bottom View After Component Assembly

USB Radio Interface PCB assembled

Assembled PCB Installed In Its Enclosure.

USB Radio Interface Assembled

Assembled USB Radio Interface

USB Radio Interface Assembled (box open)

The free manual contains the project’s history, interface circuit descriptions, additional photographs, schematic diagram, and bill of materials. The manual is in PDF format and is available for download here: USB_Radio_Interface_Manual.pdf (2MB).

Revised 12-27-2013. In addition to writing for Direct Dirt, David has his own site where you can find updates to this article and other technology, radio, and motorcycle oriented articles.