FSK Interface Page

N6TV “Serial Box”

Bob, N6TV, offers a very nice interface with a host of features. It is a Serial Port Splitter / Amplifier Interface / FSK, CW, and PTT keying interface / Breakout Box.

The Serial Box (or “S-BOX”) makes it easy to connect multiple devices to any transceiver with a serial port, including:

  • Any control device with a serial port or USB-to-Serial adapter (PC, MAC, Linux, microHAM, RemoteRig)
  • Any automatic amplifier designed to monitor serial port traffic (Elecraft, ACOM, SPE)
  • Any automatic antenna controller that monitors serial port traffic (SteppIR, KRC2)
  • Any other device designed to track frequency changes by monitoring the transceiver’s serial port (e.g. RFSpace SDR-IQ)

Check out the web page here.

Photo 1 – The “Serial Box” by Bob, N6TV

TinyFSK & Mortty

TinyFSK is an ingenious FSK interface created by Andrew Flowers, K0SM, utilizing Arduino or Arduino-compatible boards and supported by major logging programs like WriteLog, N1MM, and DXLabs. Mortty is a kit created by Steve Smith, N8AR, and Larry Gauthier, K8UT, that utilizes TinyFSK. You can buy the kit and build the interface. It’s not difficult. Mortty connects to your computer’s USB port and provides outputs for FSK/CW and PTT. For more information, see the Mortty website here. Information on interfacing Tinyfsk & Mortty with MMTTY here.

Photo 1 – Mortty is a CW/RTTY interface designed by N8AR & K8UT.


Simple Homebrew Transistor Interface

If you are handy with a soldering iron, you can build your own simple transistor interface. These are the exact circuits I (AA5AU) use for FSK, PTT and CW keying. These circuits are connected to the same DB9 connector which allows all three circuits to be controlled by the same serial port (DB9 pin-out shown here).

DB9 Connector pin-out
Diagram 1 – DB9 Connector pin-out

The transistors can be just about any general purpose NPN transistor. I use an ECG 123 (same as NTE 123). The 2N3904 transistor works fine too. The resistor value can be just about anything from 500 to 2k ohm or possibly lower or higher. I use 1k ohm resistors.

Simple FSK, PTT & CW transistor circuits.
Diagram 2 – Simple FSK, PTT & CW transistor circuits.

These simple transistor interfaces will work for most radios that are capable of transmitting FSK.  If you find your FSK transmit signal is inverted, you can normally change the polarity from a menu option in your transceiver.  Some radios, such as the Ten-Tec Omni Series and Kenwood TS-930, do not have the capability of changing FSK polarity.  These radios require either a PNP transistor or straight through keying circuit.

W3YY Interface

The W3YY interface is no longer available. I am leaving a photo of the interface board here to honor Bob for his contribution to RTTY.

Photo 3 – W3YY Interface with components installed

Simple Transistor Interface for Kenwood TS-930

The FSK circuit for the TS-930 is the same as shown in Diagram 2 except replace the NPN transistor with a PNP.

Simple Transistor Interface for Ten-Tec Omni Radios

Ten-Tec radios Omni 5, Omni 6 and Omni 6+ can key FSK simply by coming off pin 3 of the DB9 serial connector through a 1k ohm resistor (see circuit below). Please note that the Ten-Tec Orion and Orion II both need a transistor or opto isolator circuit and a straight resistor should not be used (Thanks Marsh KA5M). If you are using a Rigblaster with the Omni 5, Omni 6 or Omni 6+, try this circuit from K3DI here.

Direct connect FSK for some Ten-Tec radios.

ND5S Interface Box

Ron, ND5S, built his interface into a very nice looking box (shown below) with a short pigtail coming out to the DB 9 connector (which is female).

The box is Radio Shack p/n 270-1802. He used 3 chassis mounted RCA jacks.

Says Ron “The label was made on PowerPoint on plain paper affixed with two way carpet tape and covered with clear contact paper.”

Ron, ND5S, built his interface into a very nice looking box.
Photo 4 – Ron, ND5S, built his interface into a very nice looking box.

Ron’s PowerPoint file is available for download. The template is pictured below. Download here.

ND5S template
Photo 5 – ND5S template

Modifying the Signalink USB Interface for FSK

Sometimes you can modify your existing interface for FSK. Frank, K7SFN, has an excellent write-up on modifying a Signalink USB for FSK transmission.  Check out this great page!


K5WW Nomic Modification for FSK

Erik, K5WW, shows how to build an FSK interface inside a RigBlaster Nomic – details here!