Simple Interfaces for FSK, PTT & CW Keying

These simple transistor interfaces will work for most radios that are capable of transmitting FSK.  If you find your transmit is inverted, you can normally change the polarity of transmit FSK with 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.  These circuits are shown at the bottom of this page.

DB9 Connector Pin 3 (TXD)

FSK to Radio

FSK Circuit

Ground is on the DB 9 Connector Pin 5


DB9 Connector Pin 7 (RTS

PTT to Radio

PTT Circuit

Ground is on the DB 9  Connector Pin 5


DB9 Connector Pin 4 (DTR

CW to Radio

CW Circuit

Ground is on the DB 9 Connector Pin 5

These are the exact circuits I use for FSK, PTT and CW keying with WriteLog, but they should work for any other program as well. The PTT & FSK circuits work great with MMTTY.

These circuits are connected to the same DB 9 connector to the same serial COM port on the PC.

The transistor can be just about any general purpose NPN transistor.  I use the ECG 123 (same as NTE 123).  The 2N3904 works fine too.

The resistor value can be just about anything from 500 to 2k ohm or possibly lower or higher.  1k ohm worked great for me.

Pin 5 on the DB 9 connector is common to all circuits.  However, you may not need all three interfaces.  You can build them as single units or two in one, it doesn't matter.  For instance, for WriteLog, I don't need PTT because it is done by computer control of the radio.





PNP Transistor FSK Circuit

An FSK circuit employing a PNP transistor (diagram to the left) is used when the FSK circuit using an NPN transistor causes your transmit FSK signal to be inverted and you do not have a menu option in your radio to reverse the polarity of the FSK.  The Kenwood TS-930 requires this circuit to be used.

Ten-Tec Omni Series FSK Circuit

The Ten-Tec Omni 5, Omni 6 and Omni 6+ radios can key FSK simply by coming off pin 3 of the DB 9 serial connector through a 1k ohm resistor.