At 8/28/2007 09:26 AM, you wrote: > > 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?
Yes. > 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. Well, I did in every case. Not surprising, as there's a lot of odd harmonic content within a square wave; I suppose the lack of a 2nd harmonic helps your LPF design a little. Plus if you're feeding a phase modulator, you've got another 6 dB/octave working against you. Perhaps you were feeding direct FM modulators & using higher CTCSS freqs.? Perhaps it's a difference of perception. Back in the day, there was a rather high emphasis (pun quasi-intended) on PL tone purity. If your encoder had any harmonic content others let you know. In addition, some of the repeaters around here seemed to 'emphasize' the PL harmonics for some reason, as they sounded worse through the repeater than when heard directly. > > 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... From the respective data sheets: drift w/temperature (typical): LM555: 150 PPM/°C XR2206: 20 PPM/°C drift w/supply LM555: 0.3%/V XR2206: 0.01%/V Looking at the drift specs, it looks like all my drift problems with the XR2206 were likely limited to the Rs & Cs used, as even a 50 °C change would only result in a typical 0.1 Hz drift @ 100.0 Hz (due only to the chip). Bob NO6B
