Thank you both. Yes Bob have the 27k but seem to have made a mistake in the build somewhere in that the scope shows CTCSS still present on the output. Looks like there is some attenuation taking place, reducing all audio passing through by about a third. Cannot as yet see my error - will try a new build and advise.
Best regards, Doug - GM7SVK --- In Repeater-Builder@yahoogroups.com, 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 >
