Here s a simple circuit for interfacing CW Decoders with audio coming out from the transceiver speaker for decoding of CW signals. The audio from the transceiver speaker contains a large amount of noise besides the intended CW signal. This circuit filters out the unwanted noise and boosts the desired tone of the CW signal. This results in a vastly improved signal presented to CW Decoder input resulting in accurate decoding.

 Fig. 1

This is a tuned (rather peaking) op amp circuit having a very narrow bandwidth at the tuned frequency. I had noted this unique peaking circuit for some other application from a published literature (sorry for not noting down the orignal reference!); but realized that it can very effectively be used as an audio interface circuit for CW Decoders. Fig. 2 & 3 show the simulated frequency response of the peaking circuit. The 3db bandwidth is around 35Hz only! This is a very good performance for a simple single op amp circuit!

The above circuit operates from a 9V battery. The circuit acts as a tuned amplifier ( with a voltage gain of about 5 and BW of about 35Hz) for the signals received from the elektret mike. The output is connected to decoder input through a shielded audio cable. Input signals can be tuned over a frequency range of 600Hz to about 1KHz. 

 Fig.6

Fig.6 below shows the complete layout of the interface circuit. A generous amount of hot glue is used for fixing PCB, Mike and wires.

 Fig.7

Here is the system integration of elektret mike audio interface to transceiver speaker and PC based CW Decoder.

 The calibration of the tuning knob of the interface unit can be done using an audio signal generator and an oscilloscope. The signal from the generator is fed at the input of elektret mike. The output amplitude is observed on the oscilloscope. 

Feed a 600 Hz signal from the generator. Maximize the output amplitude of the signal on the scope while adjusting the tuning knob. Mark the 600Hz position on the box. Next feed a signal of 650Hz at the input and follow the same procedure. Likewise calibrate all the knob positions for the complete range of frequencies.

Alternatively, you can use the interface unit without calibrating it from the signal generator and oscilloscope. A simple procedure for this is given on the right hand side column.

The fig. 2 below shows the frequency response of the op amp based sharply tuned band pass filter. Notice the 3db bandwidth is only about 35Hz. (The figure can be zoomed in by pressing the + sign in the lower right hand side of the figure)

Fig. 2

Fig. 3

Fig. 3 shows the multiple frequency responses of the filter as it is tuned across the frequency band. The audio tones of the CW signals lie normally between 700 to 800 Hz.

 Fig.4

The circuit has been assembled on a general purpose PCB and housed inside a small tin box.

Fig.5

Picture below shows the bottom view of the box. Notice the elektert mike on the left hand side of the box. A small piece of Velcro is pasted on the bottom side for attaching this box near the  transceiver speaker. Velcro also dampens and isolates the mechanical vibrations emanating from fan motor and the speaker itself.

Steps for using the elektret mike audio interface for (say) 700Hz side tone frequency of your transceiver:

1. Keep the tuning knob at midway position of the potentiometer.

2.Switch on the interface unit.

3. Press the morse key / keyer to generate the side tone (of 700Hz).

4. Adjust the tuning knob so that the side tone signal displayed on the decoder spectrum display is maximum. 

5. During receive mode you"ll notice only the signal at & in the vicinity of 700Hz peaking. All other noise/signals away from center frequency of 700Hz will be attenuated significantly.

6. Adjust other settings as per decoder software usage.

7. And enjoy accurate decoding of CW signals. Happy CWing!!

(Do not forget to switch off the interface unit after you are through with DXing!!)