Hi Bob, Cut a long story short .. looks like I did not actually finish the circuit which with my memory is no surprise.
While working on the second build have found that the 1500 caps between pins 1 and 5 are missing as are the connections to pins 4 and 11. I think the reason was that I did not know what the values were exactly. Sorry about this but could you tell me what the value is. Regards, Doug - GM7SVK --- In [email protected], n...@... wrote: > > At 3/19/2009 08:18, you wrote: > >Hi Doug, > > > >Many years ago I had a similar situation. Instead of using the N06B filter > >I used the built in hi-pass filter in a Comm Spec PL encoder/decoder > >board. It is very similar to the NO6B filter. > > Actually the reason I developed my own HPF was because of the poor > performance of the filter in the CommSpec board. It's transient response > is poor, causing a ringing "boxy" sound around 400 Hz. > > My design is an adaptation of the HPF used in the Sigtone C1116 CTCSS > board. All I did was move the 3 dB point down from 300 Hz to ~210 Hz, > since I don't use any of the higher CTCSS tones & didn't want to filter the > audio any more than necessary, since some of the outgoing paths end up > being HPF'd again downstream by 300 Hz cutoff HPFs. Even at 151.4 Hz the > attenuation is about 20 dB - good enough for me. > > > What was happening was that the filter was introducing "twist" (subtle > > distortion) to the DTMF waveform. > > IIRC both the CommSpec & Sigtone designs are flat once you're well within > the voice band (600 Hz & higher). But if 2 or more CommSpec filters are > cascaded in a system the high pass rolloff may be high enough to attenuate > some of the low DTMF tones. But that's not your problem if you're using my > design. I suggest looking at the output of the filter with a scope to see > if there's any ultrasonic oscillation that could be confusing the DTMF > decoder downstream. You do have that 27k or some value of resistor >= 1k > on the output op amp (pin 8), right? An output isolation resistance is > required to prevent external capacitive loading from causing that op amp to > oscillate. > > Bob NO6B >
