Question: 
How do I measure DTMF tone frequencies?  

Answer: 

The tones produced when you press the key on a phone keypad are commonly referred to as Dual-Tone Multi-Frequency (DTMF) tone pairs. Each key on the phone has unique upper frequency and lower frequency tone combinations that are created simultaneously when the key is pressed. These tones are decoded for a variety of uses, from understanding the phone number you entered to guiding you through voice mail systems. The accuracy of these tone pairs is critical so that they may be correctly decoded by the receiving device.

The table below lists the commonly used frequencies for DTMF operation for each key. For example, pressing the 3 key on the phone causes the 1477 Hz and 697 Hz tones to be created.

Commonly Used DTMF Tone Pair Combinations
  1209.0 Hz 1336.0 Hz 1477.0 Hz 1673.0 Hz
697.0 Hz 1 2 3 A
770.0 Hz 4 5 6 B
852.0 Hz 7 8 9 C
941.0 Hz * 0 # D

The Agilent E1961A AMPS/136 Mobile Test Application running on the Agilent 8960 Series 10 Wireless Communications Test Set can measure the frequency of mobile phone DTMF tones using its audio analyzer.

DTMF Measurement Procedure

  1. Connect the phone to the test set's RF IN/OUT port.
  2. Bring the phone up on an analog voice channel. This procedure is explained in the test set's Manual Operation: Getting Started Guide .
  3. Press the Measurement selection key and select Frequency Modulation .
  4. Press the F1 (Frequency Mod Setup).
  5. Change the Modulation Frequency State setting to On .
  6. Change the Filter Type setting to 100 Hz BW BPF .
  7. Change the 100 Hz BW BPF Center Frequency setting to match the upper or lower frequency for one of the mobile's keys. (For example, set this field to 697 Hz to check the lower frequency for the " 1 " key).
  8. Hold down the appropriate key on the phone to send the DTMF that you want to test.
  9. Read the Modulation Frequency to see if it is correct for the key you pressed. For example, if you entered 697 Hz in step 7, and you are pressing the " 1 " key on your mobile, the modulation frequency should read very close to 697 Hz.
  10. Check the upper and lower DTMF tones for each key by changing the 100 Hz BW BPF Center Frequency setting appropriately and pressing the corresponding key on the mobile.