A Sinewave Oscillator for modulating
FM Transmitters with click-free Morse Code

For transmitting MCW using an FM transceiver, this simple circuit has proven most satisfactory.   The Twin-T sinewave oscillator runs continuously. With key-up, the MOSFET pulse shaper is shorting the sinewave output of the oscillator to ground. 

With key-down, the  4.7 uF capacitor is 'slowly' discharged, closing the MOSFET N-channel allowing the tone to rise to full amplitude over two or three audio cycles.  

At key-up, the capacitor charges again and 'slowly' opens the MOSFET gate over two-or three audio cycles.  The result is a nicely shaped, click-free envelope.  
The 10 k potentiometer with steering diodes sets symmetrical slopes on the leading and lagging edges of each morse 'bit'. This refinement is needed because of innate manufacturing spreads in MOSFET characteristics.

The output can plug directly into the line input of a laptop computer for use as an AF amplifier. 
An outboard amplifier could of course be employed to drive a bit of tone to an adjacent microphone, or one could get all techie about it and interface to the a HT microphone input, with appropriate level correction if required,  and taking care not to create any earth loops.

Sample of the audio from a 2 m transceiver while receiving morse
generated by the Code Practice Oscillator shown above.

It is considered 'bad practice' to use a keyed square wave tone for Morse practice, and especially so through a repeater.  The noise generated by a 555 timer-type oscillator not only sounds sharp and unpleasant, but because of its innate harmonic-rich content, generates harsh key clicks.  An oscilloscope trace clearly shows there are no actual electrical clicks.  The human ear interprets the violent burst of harmonic energy within the leading and lagging edges of each morse bit as a click.   In addition to sounding terrible, depending upon the audio filtering in the repeater, key clicks may generate unexpected, undesirable outputs.   Some would have you believe it is 'bad practice' to send morse through a repeater, even when the bits sinewave and nicely shaped; their motives can be discussed at another time.    If in doubt, seek the repeater trustee's permission before using it for MCW QSOs. 

With the microphone placed at a suitable distance from the CPO's speaker,  VOX (if available with an appropriate delay, and not too sensitive), a timer circuit, a separate toggle switch or least recommended the standard microphone pressel switch, may be used to activate the local transmitter.  However, it's always best to have both hands free rather than one hand tied up holding the pressel.

For code practice classes or personal code practice simply feed the oscillator's AF output to an appropriate audio amplifier and distribute the signal to headphones or speakers. It is amazing how few (new) CW operators critique their sending, and how many so-called experienced straight key operators have developed bad habits, which could be usefully improved or eliminated with a CPO and recorder to play back their code occasionally. 

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