where is the frequecy of dtmf telephone??

[buts101]

i tried to measure the frequency of corresponding number pressed in dialpad of dtmf telephone by tapping the wire....but i couldn't get the frequecies while pressing..only displays 0hz...what happened...how can i measure frequecies...? do i have to open the telephone cover...?plz help me which terminal should i check to get frequencies of numbers?

 

[jallem]

in reality every time you push any button you
are going to get more two tones. the combination
on them wil give you the button pushed. You
need to measure in vocal frequencies, I dont have
the table with me but I think is the order of 1 to 2KHz.

 

[flatulent]

https://www.commlinx.com.au/DTMF_frequencies.htm shows the frequencies. Note that there are two frequency bands that are less than an octave wide. Also no frequency is a harmonic of another frequency. What is usually done is the signal is separated by a high and a low pass filter. Then the frequency in each output is measured.

 

[aldevan]

for 1 digit is two frequency, see the table blow
--------1209Hz 1336Hz 1477Hz 1633Hz
697Hz ----1--------2--------3------- A
770Hz ----4--------5--------6--------B
852Hz-----7--------8--------9--------C
941Hz-----*--------0--------#--------D

 

[Externet]

Hi.
You tried to measure the frequencies by tapping the wire? Which wire?
From the keypad? from the telephone line?

You cannot probe the telephone line, you must use a transformer in SERIES with ONE of the telephone line wires to be able to extract or sample the audio or tones.
A frequency counter will read erratic anyway, as DTMF is Dual Tone... two simoultaneous tones as listed above. Only pressing two numbers at the same time will output a single tone.

Miguel

 

[rogerliu]

the waveform in telephone line is base on 3~12v DC with <1Vpp AC signal(dual tone), so applying an DC couple capacitor is need. the best choose to detect which
key pressed, using DTMF Decoder can do it well.

see attached file

 

[platonas]

Why do you need to measure the frequency?
Anyway, you can easyly do it with an oscilloscope connected between the wires of the telephone line. The frequency meter can not measure two simultanious frequencies like in the DTMF phone.