Measurements on commercial hardware

A project log for Pulse to Tone Converter

Converts pulse dialing of your POTS telephone to DTMF dialing allowing you to use it with your VOIP's FXS-port.

Christoph TackChristoph Tack 12/24/2017 at 19:310 Comments

Test environment

As the design process will require measurements and manipulations on a potential high voltage telephone line, we can make it easy for ourselves by making a complete test setup on our test table.  You probably don't want to destroy the SLIC in your VOIP Box, so we'll be using a separate SLIC-module.

The Silvertel AG1171 is a very simple to use SLIC-module.  It might be hard to get as a hobbyist.  The R-Tone KS0835F is fully compatible and available from the usual Chinese channels.

Modular jack RJ11

Info can be found on Wikipedia

So much for the standard, the Bbox2 VOIP-gateway uses reverse polarity.

Connecting the SLIC

For the KS0835F this becomes:

Let's do some basic measurements first on this slic.

Injecting audio in the SLIC

Differential mode input impedance

Our signal will be injected as a differential voltage to the SLIC.  An audio transformer is used to converted the single ended test signal to a balanced signal.  This signal is then AC-coupled to TIP-RING.  By inserting an 22Kohm sense resistor, we can measure the input impedance of the SLIC.  It appears to be close to 15Kohm.  This impedance is constant to at least 22KHz.


The SLIC provides constant current to the load and measures the voltage over that load.  The way to inject data is to modulate the load.  The question is how much do we need to vary the load to get a certain output voltage?

Measurements have shown that a 30mA current change in the loop leads to a 1V change in the audio output.

Audio output

The audio output voltage is the simply the difference of the TIP and RING voltage, transferred to a 5VDC-offset.

Measuring DTMF signals of a Philips D200 DECT-base station

Let's see what COTS-devices output when they're generating DTMF-signals.  In order to do that, we connect the Philips D200 DECT-base station to the SLIC's tip&ring.  The oscilloscope is connected to the audio out of the SLIC.

The screenshot shows a screen capture of the SLIC's audio out, while the Philips D200 is dialing "1".  The FFT-shows energy on 700Hz (actually 697Hz) and 1200Hz (actually 1209Hz).

This screenshot is measured on the same signal.  This time, the phone is dialing "12".

The SLIC audio conversion relative gain is 0dB, so it doesn't change the amplitude of the audio on the telephone line.  

The Philips D200 generates it's DTMF which complies to the standard.

Bbox2 VOIP Box FXS-port measurements

Measurements done with POTS connected.