This is the most complicated place to upload an image to. I give up.
Anyhow, I made the duty ratio of the waveform to be 10%-90% and independent of the frequency. The frequency is calculated taking 6 cylinders to cause 3 pulses, so a 600-3000 rpm which is 10-50 rev/sec or 30-150 Hz.
If the pcm only needs the average O2 sensor voltage, then there is no need for the square wave and a variable voltage between .2v (200mv) to .8v (800mv) should suffice to aid in the diagnosis of an aging upstream sensor (original factory, 12 years, 190 kMiles). If the test with my little square wave fails, then I'll have to add a microcontroller to the circuit so that the frequency (and may be other parameters) change with the rpm by getting another wire from another sensor or the pcm connector
