> W.r.t. early homebrew encoders, I never really liked the 555 version > because of the non-sinewave output. Since the encoder need to cover > more than an octave, fixed filtering of any kind couldn't be used to > clean it up.
You can find the 555 used as a pretty good tone generator in many a circuit, including a fair number of dtmf pads. If you look at the circuit on the sonic web page you'll notice the low pass filter, which works pretty well. Even with the values shown I found the described circuit puts out a lot more audio than required for a typical radio so after all the filtering you still had a high value resistor in series to knock the level down. > So I went with the XR2206 function generator which put out a nice > clean sine wave. Only other problem was frequency stability: Advantage to the 555, which by nature of design and operation is relatively immune to voltage and temp drift with more than a reasonable amount of change. > only many years later after spending much time & effort looking > for the most stable Rs & Cs did I discover when trying to build > a 1000 Hz sine generator that the chip temperature affects the > operating frequency. Never had much of a a problem with the 555 and the circuit is dirt cheap to build. cheers, s.
