Sinewave Oscillator for driving FM Transmitters
with a click-free Audio Tone

For transmitting MCW using an FM transceiver, this simple circuit has proven most satisfactory.   The Twin-T sinewave oscillator runs continuously drawing ~ 150 uA in key-up condition.     With key-up, the MOSFET pulse shaper is shorting the sinewave output of the oscillator to ground. 

With key-down, the current drawn increases to ~ 260 uA and the 
4.7 uF capacitor is discharged, 'slowly' 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 sets symmetrical slopes on the leading and lagging edges of each 'bit'.It is needed because of innate manufacturing spreads in MOSFETS characteristics.

The output plugs directly into the line input of a laptop computer as the 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 microphone input, however, handhelds don't always take kindly to external inputs.

The AF output is simply fed to the transceiver's microphone input, or to its physical microphone, and the level adjusted to produce the misnamed yet descriptive, modulated continuous wave mode (MCW).

It's considered 'bad practice' to use a keyed square wave tone through a repeater;  it not only sounds sharp and unpleasant, but by its innate harmonic-rich content, generates harsh key clicks. In addition to sounding horrible, depending upon the audio filtering in the repeater, key clicks may generate undesirable outputs.    VOX (if available), a timer circuit, or even a separate toggle switch may be used to activate the local transmitter while sending.   Some would have you believe it is 'bad practice' to send morse through a repeater at all.    Like water off a long-run roof.

For code classes or personal code practice simply feed the oscillator's AF output to an appropriate audio amplifier and distribute the signal to phones or speakers. 

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