> 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...
Do you assume multiple frequency operation? I set the oscillator to one frequency and park it inside the radio. Place the sonic page described or similar rc filter in line and try the circuit. You'll find it works pretty well for what it is... the only buzzzzz that makes it to and through most modulators is pretty much only the desired ctcss frequency. I've never experienced a case where the delivered audio was objectionable. > I never found the 555 to be very stable; the XR2206 always > did better. > Bob NO6B But the 2206 tends to be much less tolerant of voltage and temp changes as found in 99% plus mobile radio operations. Otherwise a great chip for what it is... I wouldn't have put the 555 ctcss encoder circuit on the sonic web page unless I was sure it was a well tested design that worked really well. I haven't had to build one in a number of years but we cranked out a few hundred back in the day... never a failure or complaint. Sure there are better ctcss generator alternatives but probably not for the price and performance of the described 555 encoder circuit. cheers, skipp
