An Updated CW Hardware Interface for FlDigi

Matt Roberts - matt-at-kk5jy-dot-net

Published: 2018-05-31

Updated: 2019-08-27


The Fldigi digital mode software suite supports a number of modems, including CW (Morse code telegraphy) and RTTY.  The Fldigi decoders are some of the best available, making the software an excellent option for on-air digital operation.


Figure 1: CW Interface Circuit
(click to enlarge)
One of the main challenges when using any digimode software package for CW is that most HF transcievers only support CW features (e.g., narrow filters, specialized noise reduction, etc.) when they are used in a dedicated CW mode.  For some radios, this is also true for RTTY operation.  In such modes, the radio typically expects that transmitted elements be supplied via hardware key closures.  Since multipurpose computers tend to not be real-time, generating a reliable hardware keying waveform from a PC can be a challenege, both in terms of supporting hardware interfaces, and in generating accurate element timing with that hardware.

The Fldigi manual describes a simple hardware circuit that can easily solve this challenge by converting sound card CW tones into key closure signals that can be used to drive a typical CW transmitter.  Using newer versions of Fldigi, the same circuit can be used to support CW keying, PTT control, and FSK "shift" signals.  The design is solid, but some of the recommended parts—namely the recommended transformer and germanium diodes—are increasingly difficult to find.

Figure 1 shows an updated version of the circuit using current-production parts.  This circuit has been built and tested using Fldigi, for both CW and PTT purposes, performing well in a contesting environment.

While similar to the original circuit, there are some key improvements:
  1. The transformer recommended in the manual was replaced with one made by Xicon, with part #42TL016-RC.  The center-tap pins on each side of the transformer were left disconnected.  There are a number of active-production components that could be used as a replacement part, but this is one of the least expensive I have found.  Any transformer with reasonable impedance windings, and with a high-end frequency range of at least 3kHz will work.
  2. The fixed voltage divider has been replaced by a 10kΩ linear-taper potentiometer, which allows the signal level provided to the NPN transistor to be adjusted as needed.  If an extra degree of safety is desired for the base of the transistor, the 100Ω resistor, marked with an asterisk (*), can be placed above the 10kΩ potentiometer, as shown.
  3. The diodes were all replaced with 1N5817 Schottky diodes.  This is a common part, available from many suppliers via that part number.  This diode has a VF of ~400mV, making it a great replacement for the diodes in the original circuit, both for the germanium rectifier diodes, and for the reverse-polarity protection diodes on either side of the BJT.
The updated circuit otherwise functions just like the original.  It is essentially a voltage multiplier circuit, converting the AC signal from the sound card into a larger DC signal for driving the 2N2222 NPN small-signal BJT, which then keys the radio.

This type of circuit is useful for more than just CW, and with more software than just FlDigi.  There are many soundcard mode software packages that support PTT, CW or RTTY keying using an audio tone, and any of these can be made to control a hardware input using this device.


Copyright (C) 2018-2019 by Matt Roberts, KK5JY.
All Rights Reserved.