Circuit Notebook 105 - DTMF Level Tester

In addition to telephone dialling, DTMF (Dual Tone Multi-frequency) control is frequently used for the control of various functions in the amateur radio/TV shack/studio.

It is often used to control several functions in ATV repeaters. My local ATV repeater GB3TM uses a DTMF tone on the input channel for switching between analogue and digital output and for switching to a local test card.

DTMF tones are set into two ranges, a low group 697 to 941 Hz and a high group 1209 to 1633 Hz. This frequency range does not present any problems for the normal audio channel and, generally speaking, DTMF signals are very tolerant to variations in audio signal level. However, if the tone amplitude is too high and audio distortion occurs, particularly after passing through a digital signal chain, then DTMF signalling may refuse to work.

Click to view full size in a separate window With this in mind, I built a simple DTMF generator, shown in Fig.1. This is essentially a telephone DTMF signaller and an attenuator housed in a separate box. The signaller has a small loudspeaker on the back and would normally be placed over a telephone mouthpiece to provide DTMF facilities for a 'phone which does not have a 'touch-tone' keypad. You may sometimes find these signallers at radio rallies. It is also possible to extract the keypad and dialler from a suitably equipped hand-microphone.

Circuit

The circuit is shown in Fig.2. No battery switch is required as the dialler only draws current when a key is pressed. The output is taken from across the internal loudspeaker. With a fresh battery the maximum signal level from across the loudspeaker of my signaller was -18dBu (97.5 mV). This signal is taken to a string of resistors which are wired around a single pole 12 way rotary switch (Maplin N89BX). A 220R pot at the top of the chain allows the -20 dBu output to be set correctly at 77.5 mV. I used a digital multimeter on the 200 mV A.C. range and keyed '1' for the lowest frequencies. The inside of the case is shown in Fig.3.

Click to view full size in a separate window The level of the output signal will be affected as the battery discharges, but as the drain is only momentary when a button is pressed, many months of operation should only reduce the battery voltage by a small amount. The battery consists of three AA cells which are housed in a black battery box, visible in Fig.3.

Outputs

Two identical outputs are provided, one through a BNC socket and the other through a phono socket. The output is not buffered, but the output impedance is reasonably low, just a few hundred ohms, so will be suitable for most purposes. In my unit, the signaller is connected to the attenuator box using a DIN connector and cable. This is to allow the signaller to be used for other purposes.

Using The Tester

As an example of its use, I fed the DTMF signal from the tester into the audio input of my 23cms analogue transmitter. I monitored the return signal locally, using an F.M. satellite receiver with an oscilloscope connected across the audio output. I keyed a tone and increased the level until distortion could be heard on the receiver and was visible on the 'scope. I noted the level and reduced this by 10 dB to allow adequate headroom. I then repeated the process by transmitting through the repeater and confirmed a DTMF signal level which was free from distortion by 10 dB through the complete system. This signal level should provide reliable DTMF control and will be used in the future.