BT HAWK (CORDLESS TELEPHONE)


Introduced in 1983 the HAWK (illustrated opposite) is British Telecom's first cordless telephone. Available in ivory and brown it consists of 2 parts,

a. A free standing or wall mounted BASE UNIT which is connected to the telephone line using Phone Socket, and......

b. A portable HANDSET that can be carried around the customers premises and will give the user the same facilities as a conventional telephone. When not in use the handset is placed upon the base unit for recharging.

Speech and signalling is transmitted between the handset and base unit using 2 low-power radio links, with a range under ideal conditions of up to 100 metres. Walls and other large obstructions, particularly metal, will reduce the usable range.

Because only 8 pairs of radio channels have been provided for the whole of the UK, then it is possible that HAWK customers living near to each other and using the same radio channels may experience overhearing or other operational difficulties. This is unavoidable, but since the range of the telephone is only 100 metres it is envisaged that this will only happen occasionally.

Although difficulties may be experienced, there is only a slight chance that two adjacent customers will be able to make calls on each others line, even though they may use the same radio channels. This is because during manufacture, a security code is allocated to each base unit and handset so that they become a matched pair. Whenever calls are made or received, the security code is exchanged between them, and the call only allowed to mature if the exchange of code is correct.

Because of the security code arrangements it is not possible to use the handset with other than the base unit supplied.

When received, the Hawk handset will require charging for approx. 24 hours before it is installed, otherwise it will not be possible to test it on site. Read through this guide note fully before attempting to charge the handset. After charging, repack Hawk carefully for transportation to the customers premises.

The main features of the base unit are:-

  • A TELESCOPIC AERIAL which receives signals transmitted from the handset.
  • A 1.5m long WIRE AERIAL used to transmit signals to the handset.
    The positioning of these aerials and the siting of the base unit is important if maximum range is to be achieved.
  • A CALL BUTTON which enables the handset to be called with a distinctive warbling tone. The call button has been provided so that calls answered on a cord telephone on the installation, can be passed to the portable handset.
  • A POWER LAMP which is lit whenever power is connected to the base unit. Power in the form of a low voltage ac. is supplied to the base unit for rectification and regulation from a double insulated plug mounted transformer. The 3m long output cable of the transformer plugs directly into the base unit.
  • An IN USE LAMP which is lit whenever the portable handset is in use or when the call button is pressed. Maximum brightness of the lamp occurs when the transmit aerial wire is correctly positioned.
  • A CHARGE LAMP which is lit whenever the handset batteries are being recharged. A current regulated output is supplied to the handset to recharge its cells from two contacts situated in the handset cradle.

The main features of the handset are:

  • A KEYPAD with last number re-dial (LR) and scratch-pad (P) facilities.
  • A FLEXIBLE AERIAL which folds into the side of the handset when not in use. This aerial is used to transmit signals to the base unit.
  • A FERRITE ROD AERIAL contained within the handset body, which receives signals from the base unit.
  • STANDBY /TALK SWITCH. When in the standby position and switched 'on' the handset is ready to receive calls. Switching to 'talk' activates the handset and answers incoming calls. If an outgoing call is to be made, switching to 'talk' initiates the sending of the security code which ensures that the call will not be originated via someone else's base unit.
  • OFF/ON /HI SWITCH. The handset should be switched 'off' when not in use, or when it is being recharged. The 'on' position is used in conjunction with the 'standby' switch when calls are originated or received. The 'hi' position is used to amplify incoming speech.
  • A TALK LAMP which is lit whenever the handset is switched to TALK.
  • A LOW BATT LAMP which is lit whenever the handsets Ni-cad batteries need recharging. The capacity of the cells is sufficient for 4 hours continuous use, or 12 hours intermittent use. The two contacts situated just above the microphone, engage with those in the handset cradle on the base unit to provide the charging circuit. The 2 sprung clips situated in the handset cradle of the base unit, engage 2 slots on the sides of the handsets switch panel, to help retain the handset in position when Hawk is wall mounted.
  • A BELT CLIP to facilitate carrying the handset about. Note the correct position for the clip on the illustration.
  • The EARPIECE also doubles as the tone caller. The incoming ringing gradually increases in intensity in case the handset is in the vicinity of the users ear.

Base Unit

The base unit has a 47.5MHz RECEIVER. A 4.5kHz detector within the receiver, continually monitors the presence of a4.5kHzsubcarrierwhich is transmitted by the handset to indicate that a call is in progress. Dialling information is transmitted from the handset by interrupting the same carrier at 1Opps.

The LINE INTERFACE electrically isolates the telephone line from the rest of the base unit circuitry, and performs the electrical functions of a normal telephone, i.e. impedance matching, regulation, ringing detection, dialling (by relay) and speech transfer to and from the line.

The CONTROL CIRCUIT utilises information from the receiver and line interface to control the transmitter and access to line. A microprocessor and a read only memory (ROM) programmed with the security code are used in this control. Elements of the microprocessor are also used to shape the dialling pulses from the handset.

The 1.7MHz TRANSMITTER develops some 50OmW of power. The electromagnetic field near to the output coil which is connected to the transmitters wire aerial, is sensed and then amplified to drive the 'in use' lamp. Maximum brightness of the lamp indicates that the aerial is correctly aligned.

The Handset

An electronic microphone is used to feed speech to the 47.5MHz TRANSMITTER, whose output is connected to the flexible fold down aerial on the handsets side. The 4.5kHz sub carrier used for call control and dialling is generated within the transmitter and combined with the speech and security code signals before modulation and final amplification. The transmitter develops some 10mW of power, with the output being carefully filtered to remove harmonics, particularly those at 95MHz which may cause interference with FM broadcast reception.

Like the base unit, the CONTROL circuitry uses a microprocessor and associated Read Only Memory to send and receive the security code and also to generate the calling tones for the earpiece. The control circuit takes the number information from the keypad and arranged for the 4.5kHz sub carrier to be pulsed at the required rate.

The 1.7MHz RECEIVER utilises a ferrite rod aerial similar to those found in medium /long wave broadcast receivers.

 
 
 
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Last revised: December 18, 2010

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