Thursday, December 10, 2009

Telephone signals

Took the Rigol DS1102E scope and connected the probe to my home telephone line. I obtained the following readings. Click on the image to see a more legible version. Do keep in mind that dial tone, ringing, busy tone, and off-hook warning frequencies (and cadences) vary from country to country. So do voltages and other parameters. Seems like a free for all.

1. On-hook, telephone on standby. When the phone is not being used, the line is at 48 volts DC.

2. On-hook, phone ringing.

In the screen shot above the cursors are on and I've adjusted them to measure the DC voltage right before the ringing signal. You can see at the top right inset (delta Y) that the voltage has jumped to 54 Volts right before ringing.

The amplitude of the ringing signal itself is shown below measured. The value (delta Y) is 132 Volts peak to peak. That's around 47 Volts RMS (132/2/√2) riding on top of the 50 or so Volts DC.

I switched to split screen mode and both ringing signal and its frequency spectrum as obtained through the fast Fourier transform (FFT) function are shown below.

The same in full screen mode:

As you can see cursor A reads 20.8Hz. That's the ringing signal frequency

3. Off-hook, dial tone:

Like the ringing signal the dial tone is also a sine wave, but this one has high frequency noise riding on it. The cursors are on and you can read off the peak-to-peak amplitude which is around 350mV. The FFT function was turned on when I took the above reading and it is shown below.

I changed the resolution from 500Hz to 50Hz per division to get a better view of the peak frequency:

The dial tone is basically a single tone at 420Hz, although during other times I've measured it at 440 to 450Hz. The drift could be at the telephone company's end or somewhere along the transmission.

4. Off-hook, busy signal. I picked up the phone and just waited until the dial tone gave way to the busy signal. That took several seconds. Here's the waveform I got.

The low level high frequency noise to the left of the busy signal is the lull between the busy tone pulses. The purple curve at the bottom half is the frequency spectrum provided by the FFT. You can already see three or four significant blips there. Below are measurements of those frequencies using the cursors.

And a more detailed look at those:

So what I got were prominent signals at 480Hz and 620Hz with smaller signals at 1.9KHz and 2.0KHz. The latter two may or may not be actual components of the busy signal.

5. Off-hook alert. After several more seconds the busy signal gives way to the loud off-hook warning tone, which is screaming, "Hang up, you idiot!"

As you can see the maximum peak-to-peak voltage is approximately 460mV. The waveform along with the frequency spectrum:

Frequency spectrum measurements:

A closer look at those frequencies:

The most significant frequencies are 1.4KHz and 1.8KHz. The amplitudes of the 2.0, 3.2 and 3.6KHz are rather small. These may or may not be part of the off-hook alert signal.


  1. Thank you for this cool work!! Very usefull for me as i'm designing a circuit to detect line voltage without being affected by the ring signal. I was making a mistake by using 140V without dividing it by 2! :P

  2. Good work, good presentation...Greatly appreciated.