At 8/28/2007 04:27, you wrote: > > 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.
Still, you can't build an effective LPF for it. Best you could do would be to put a 6 dB/octave LPF that broke around 2 x 67 Hz, & let the higher tone frequencies get rolled off. Either way, you still get buzzzzzzzzz... > > 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. I never found the 555 to be very stable; the XR2206 always did better. Bob NO6B
