Anatomy of a Cordless Telephone

To illustrate the parts of a cordless telephone, we will
show you the inside of this one from General Electric (GE).
It was made in 1993 and operated in the 43-50 MHz range.

As mentioned above, all cordless phones have a base and
a handset. Let's look at these parts individually.

Base

The base unit of the cordless phone is plugged into
the telephone jack on your wall.

If you open up the base and expose the circuit board, you see
several components that carry out the functions of the base:


* phone line interface - receives and sends telephone
signals through the phone line.

* radio
o amplifies signals to and from phone-line
interface, user controls and speaker phone (if present).
o broadcasts and receives radio signals to and from
the handset.

* power - supplies low voltage power to the circuits and
recharges the battery of the handset.

Phone Line Interface

Phone line interface components do two things. First, they
send the ringer signal to the bell (if it's on the base) or
to the radio components for broadcast to the handset. This
lets you know that you have an incoming call. Second, they
receive and send small changes in the phone line's electrical
current to and from the radio components of the base. When you
talk, you cause small changes in the electrical current of
the phone line, and these changes get sent to your caller.
The same happens when the caller talks to you.

Radio Components


The radio components receive the electrical signals from the
phone line interface and user controls (keypads, buttons).
The radio components convert the signals to radio waves and
broadcast them via the antenna. Radio components use quartz
crystals to set the radio frequencies for sending and
receiving. There are two quartz crystals, one for sending
signals and one for receiving signals. Remember that the base
and handset operate on a frequency pair that allows you to
talk and listen at the same time (duplex). The radio
components include an audio amplifier that increases the
strength of the incoming electrical signals.

Power Components

A DC power cube transformer supplies the low voltage required
by the electrical components on the circuit board. The power
components on the circuit board work with the power cube to
supply electrical current to re-charge the battery of the
handset.

In addition to the above components, some bases also have
audio amplifiers to drive speakers for speaker phone features,
keypads for dialing, liquid crystal displays (LCDs) for
caller ID, light-emitting diodes (LEDs) for power/charging
indicators, and solid state memory for answering machine or
call-back features.

Handset

You can carry the handset with you throughout the house or
outside within the range of the base transmitter. The handset
has all of the equipment of a standard telephone (speaker,
microphone, dialing keypad), plus the equipment of an FM
radio transmitter/receiver.

When you open up the handset, you see these components:

* speaker - converts electrical signals into the sound
that you hear.
* microphone - picks up your voice and changes it to
electrical signals.
* keypad - input for dialing
* buzzer or ringer - lets you know that you have an
incoming call.
* radio components
o amplify electrical signals to and from microphone
and speakers.
o send and receive FM radio frequencies
* LCD or LED displays - indicator lights.
* re-chargeable battery - supplies electrical power to
handset.

Speaker

The speaker receives the electrical signals from the audio
amplifier in the radio components and converts them into
sound. When you remove the cover from the speaker, you see
a large round permanent magnet with a hole in the middle
and a deep groove surrounding the hole. Within this deep
groove is a coil of fine copper wire that is attached to
a thin plastic membrane. The plastic membrane covers the
magnet and coil.

To hear sounds, the following events happen:

1. Electrical signals come from the radio components.
2. The electrical signals travel in the coil of copper
wire.
3. The electrical signals induce magnetic currents in the
coil of wire, thereby making it an electromagnet.
4. The electromagnetic coil moves in and out of the groove
within the permanent magnet.
5. The coil moves the attached plastic membrane in and out
at the same frequencies as the changes in electric currents.
6. The movements of the membrane move air at the same
frequencies, thereby creating sound waves that you can hear.

Microphone

The microphone changes the sound waves from your voice into
electrical signals that are sent to the audio amplifier of
the radio components. A microphone is essentially a speaker
that works in reverse. When sound waves from your voice move
the membrane, they make tiny electric currents either by
moving a coil of wire within a magnet or by compressing the
membrane against carbon dust.

Keypad

The keypad allows you to dial a number. It transfers the
pressure from your fingertip on the appropriate key into an
electrical signal that it sends to the radio components.
Below the rubber keypad is a circuit board with black
conductive material under each button (shown above).
The keypad works like a remote control. When you press
a button, it makes a contact with the black material and
changes its electrical conductance. The conductance sends
an electrical signal to the radio components indicating that
you have selected that number.

Buzzer or Ringer

When the radio components of the handset receive the ringer
signal from the base, they send electrical signals to the
buzzer. The buzzer changes those electrical signals into
sound much like the speaker does. You hear the buzzer sound
and know that someone is calling you. In some phones, the
speaker is used to make the ringer sound and there is no need
for a separate ringer.

Radio Components

The radio components of the handset are like those of
the base -- they convert electrical signals from the
microphone into FM radio signals and broadcast them at the
same frequency as the receiving crystal of the base unit.
The radio components also receive radio signals at the same
frequency as the broadcasting crystal from the base, convert
them to electrical signals and send them to the speaker
and/or buzzer (ringer).

Remember that the base and handset operate on a duplex
frequency pair that allows you to talk and listen at the
same time.


Duplex Example
Base:

* 44.32 MHz transmitter
* 49.28 MHz receiver

Handset:

* 49.28 MHz transmitter
* 44.32 MHz receiver

LCD or LED Displays

Most handsets have one or more light-emitting diodes (LED)
that indicate various things, such as when the phone has an
open line or when the battery is low.

Some handsets have an LCD that can display numbers for caller
ID features, similar to a cell phone. The LCD may be
reflective or backlit so that you can see it when the room l
ight is low.

Battery

The handset's battery supplies the power for all of the
electrical components in the handset. All cordless phone
handsets have a rechargeable battery (nickel-cadmium,
nickel-metal hydride or lithium). When the battery runs low,
an indicator light on the handset usually lights up or
flashes. In some phones, a "beeping" sound may also indicate
a low battery. You then recharge the battery on the base of
the cordless phone.

The GE cordless phone that we dissected was from 1993.
Modern cordless phones have the same functions and much of
the same hardware. However, many of the electronic circuits
that were once achieved with transistors, resistors and
capacitors have been replaced with integrated circuits.
This advancement allows the handset to be either smaller with
the same functions or the same size with more functions.

In summary, a cordless phone is basically a combination of
a telephone and an FM radio transmitter/receiver. Because
it is a radio transmitter, it broadcasts signals over the
open airways rather than specifically between the base and
handset.

Because of this open broadcast, It is possible for other
people to listen to your phone conversation by using
a radio scanner. So an important issue and feature to look
for in a cordless phone is security -- DSS offers the best
protection against eavesdropping.