DTMF Filtering

Christopher41877

Hi everyone,
I have a problem with DTMF decoding.· I am sending DTMF tones to an MT8880 from a BS2.· Everything works ok when I send it directly from the pic to the 8880 but it has to be wireless.· I had two T5620's (Motorola two-way radios)and only when the volume was turned all the way down did it work.· For some reason if it was turned up a little bit it would not work.· I can't use those radios so I bought another pair, T4525's.· I can't get the volume down as low as the T5620's.· My question is, is there a way to filter the incoming DTMF tones to an acceptable level so the 8880 can decode them?
Thanks,
Chris

ccevt

I don't quite get the whole picture from your description, but if it is levels you are having a problem with, it sounds like it is attenuation, not filtering that you need. If you are taking receive audio out of the radio, place a resistive divider between the radio and the decoder. Design it to pass about 20 to 30 % of the incoming level and that should allow you go run the volume control in more of a mid range position. Resistor values depend upon the relative impedances of receiver and decoder, but usually it will be about 4 to 32 ohms receiver output impedance and 10Kor more input to the decoder. Try something like 330 ohms in series with 100 ohms to ground forming a simple L-Network attenuator. Hope this helps.

Horace