Smooth Tone Clickless CW Sidetone Generator

Published:2013/3/7 3:08:00 Author:Ecco | Keyword: Smooth Tone, Clickless, CW Sidetone Generator | From:SeekIC

This circuit is essentially a minimal version of an analog music synthesizer that is hard-wired for the purpose of making morse code tones. The oscillator section produces a fixed frequency triangle wave on LM324 pin 14, that is fed into a low pass filter to get a sine wave on LM324 pin 8. A triangle wave can be thought of as a series of sine waves with decreasing amplitudes as the frequency increases. The low pass filter simply removes the upper harmonics and passes the fundamental sine wave frequency through. The sine wave is sent to a voltage controlled amplifier made from three 2N3904 transistors. The upper two 2N3904 transistors should be a matched-gain pair in order to minimize thumping sounds. The VCA is modulated with a keying waveform to produce the resulting modulated waveform. The keying waveform is generated by smoothing out the square wave keyed signal on the 4N32 emitter with the 220K/220K/20n (.02uF) capacitor/resistor low pass filter on the base of the lower transistor. The two 220K resistors in series with the 4N35 output transistor act as a voltage divider, this causes the the keying envelope to swing between 0V and 6V, which is a good range for controlling the VCA. Keying of the circuit is performed either by the manual key input, or via the keyer input, which can be used to connect the circut to a morse code keying chip. To use the keying input, an active low open collector driver should be used. The audio signal comes out of LM324 pin 7, it connects to a 10K volume potentiometer and an attenuator made with two resistors. The attenuated audio is then fed to the LM386 audio amplifier.



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