PRINCIPLES OF TELECOMMUNICATIONS

LAB # 1

THE DTMF TELEPHONE

OBJECTIVES:

  1. To measure the ON HOOK /OFF HOOK loop current of the telephone system.
  2. To view and analyze various telephone system signaling frequencies
  3. To study and analyze the frequencies produced by the touch tone DTMF telephone set

EQUIPMENT:

Bell 2500 Dual Tone Multiple Frequency.

Breadboard, Telephone box

1- 470 ohm resistor, 1W

1- 2.2Kohm resistor

1- 33nF capacitor

Variable voltage power supply

Telephone connector box

Oscilloscope

2-Oscilloscope leads, 2 sets of power supply leads, a bunch of alligator clips, connecting wires

[1]

INTRODUCTION :

The Touch Tone telephone is also known as Duel Tone Multiple Frequency or DTMF telephone. Originally it was developed to speed up the dialing process ( from the older rotary phones ) but in recent years a number of other communications and control applications have taken advantage of the DTMF standards, such as stock market quotes, telephone banking, airliner reservations, remote video surveillance and controlling home appliances etc.

The proper operation of these telephones depends on applying the correct voltages to the telephone’s TIP and RING terminals at – 48 Volts ( However in this experiment we shall apply approximately 24 volt. The method for generating the tones for these phones has changed; in today’s more modern telephone system, Digital Integrated Circuits ( ICs ) are used to produce the different tones

One important measurement associated with this style of Telephones is the subscriber loop current. It is the current that flows when the telephone is OFF HOOK. The subscriber loop is then connected to the central office. A –48 V battery at the central office circulates off-hook current in the closed loop. When the handset is ON-HOOK, the loop is disconnected from the LO and current is zero. Placing a known resister in series with the loop and measuring the voltage drop across it can measure the ON-HOOK current. By ohm’s Law loop current = V/R

Fig below shows the DTMF keypad. The diagram beside it shows the waveforms generated when one of the key is pressed. The DTMF dialing was discussed in class. Please refer to your class notes for details of the keypad.

PROCEDURE:

PART 1

  1. Obtain a touch tone telephone and set and write down the manufacturer and model number of the phone.

Manufacturer: ______Model # : ______

  1. Connect the Telephone box to the end of the Telephone cord. The coloured wires should now be accessible to make your external connections
  2. Build the circuit as shown below.

VR

470 ohm Resistor

TIP (GREEN)

0-25 VTelephone

RING(RED)

GND

  1. Turn on the power supply and adjust the voltage to 10 V. With the Telephone ON-HOOK and OFF –HOOK, measure the voltage drop across 470 ohm resistor. ______
  2. Repeat thee measurements with the input voltage at 15 V and 20V
  3. Calculate the loop current for all three measurements

QUESTIONS:

With the telephone in the ON-HOOK position and the voltage set to

(a)20 V, what is the subscriber loop current? ______

(b)15 V, what is the subscriber loop current? ______

(c)10 V, what is the loop current?______

With the telephone on the OFF-HOOK position and the voltage set to

(a)20 V, what is the loop current? ______

(b)15 V, what is the loop current? ______

(c)10 V, what is the loop current? ______

How does the local office know, that your telephone is OFF-HOOK?______

PART 2 : Listening to tones

Take the phone OFF-HOOK and depress any of the keys. A DTMF tone should be heard in the earpiece. Try another key and listen to change in the pitch.

Was a different tone observed? Yes? ______No? ______

PART 3 :Adding a low pass filter to the circuit

A low pass filter allows only low frequencies to pass and blocks higher frequencies. The exact cut-off point is determined by the values of capacitor and resistor. Build the circuit shown below.

TIPRING2.2 K

CH 1CH 2

0- 25VTELEPHONE33nF

470

GND

(a)Connect channel 1 of the scope across 470 ohm resistor and channel 2 across the capacitor.

(b)Turn on the power supply and adjust it to give 20 volts

(c)Lift the phone off –hook and press the key that will produce the lowest frequency from each group.

Key that produces the lowest frequency is______

Sketch an accurate waveform observed for the lowest frequency. Make sure to indicate the time and amplitude.

CH1 CH 2

(d)Select the key that will produce the highest frequency and repeat the measurements of step c above. Which key produced the highest frequency ______

CH 1

CH 2

(e)Press simultaneously two keys in the top row of the keypad. What does the scope show? what is the difference the waveform on ch 1 and ch 2. Sketch the waveforms

CH1

CH 2

(f)Very the power supply between 15 and 25 volts and observe the effect on the signal on CH 1 and CH 2. Sketch as accurately as you can the waveforms on Ch 1 and CH 2 at 15 volts and 20 Volts

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[1] Hyder Khoja: Aug20/03