Aircraft Audio Interface - Nav Radio

Hi,
I have never really fiddled with audio signals and have the need to generate morse coded audio signals into an aircraft intercom system. I have prototyped a circuit which uses 555 timers to generate an appropriate frequency (say 1020 Hz). I use a microcontroller to modulate the audio tone to generate the morse code output. Looking at the specs for some of the nav radios that normally attach to the intercom system, I have the output isolated from my circuit by a 600 to 600 ohm audio transformer. After testing, I am seeing the my square wave outputs are distorted (from the 555) when connected to the transformers. Where to go next? I built a test box with a 150 ohm speaker, which I connected directlyt to the output of the transformer. The output tones seem might distorted. Cheap speaker (yes)? Overdriving? Helpful links or suggestions? Thanks for looking at my post.

You really want a ~1000Hz sine wave, keyed on/off by your PIC. One way is to generate a square wave, and then filter the hell out of it using a low pass filter with cutoff frequency of slightly higher than 1000Hz. Another way is to build a poor man's DAC using four PIC ports and binary-weighted resistors, and then generate the 1000Hz tone by updating the port values at 16Khz. That way, there is minimum external analog wiring outside the PIC.

Does it matter if the square wave is distorted? You could be overdriving the transformer.

Rather than using a 555 would it be easier to use a keyed colpitts oscillator? Something like a single transistor colpitts oscillator similar to this one. Possibly configure your PIC to key +V. Long ago I built variations of these for use as code practice oscillators, we just keyed +V.

The circuit found here is another interesting example that used the inductance of the headphones. A 2K isolation transformer should work there and the capacitive divider chosen for 1 KHz. I don't know, just some ideas to get you a 1 KHz sine wave.

Ron