GB2154100A

GB2154100A - A coupler for a cordless telephone

Info

Publication number: GB2154100A
Application number: GB08505442A
Authority: GB
Inventors: Thomas H Schiller; Aubert E Bennett
Original assignee: VIKING PHONE CO Inc
Current assignee: VIKING PHONE CO Inc
Priority date: 1982-01-25
Filing date: 1985-03-04
Publication date: 1985-08-29
Also published as: GB2120905A; DE3301778A1; GB2120905B; JPS5915340A; GB8505442D0

Abstract

A coupler for coupling a base unit of a cordless telephone system, including a base unit and a remote unit, to an alternating current power supply includes a transformer (68), having a primary winding for coupling to a power outlet (67) and a centre-tapped secondary winding connected via diodes (75,76) to a connector (35) for supplying direct current for said base unit, a first line (69) for transmitting an RF signal from said base unit and coupled via an inductor (77) to the centre tap of said secondary winding and via a pair of capacitors (72,73) to said primary winding, and a second line (70) for coupling the ground terminal of said power outlet (67) to said base unit and the centre tap of said secondary winding. The coupler enables both the receiving of power for the base unit and the transmission of a radio frequency signal through the power lines whereby the wiring system of a building serves as an antenna. <IMAGE>

Description

1

GB 2 154 100 A 1

SPECIFICATION

An improved coupler for a cordless telephone

The invention relates to the field of cordless telephones.

5 Cordless telephone systems are well-known in the prior art. They typically include a base unit which connects to a standard alternating current (AC) power outlet and a telephone line. This base unit communicates through radio transmissions with a 10 remote unit. The remote unit is generally a handheld memberthrough which calls are received and placed via the base unit. Within the confines of allowable transmission power, ranges of 500 feet or so are achieved. Transmissions from the remote 15 unit to the base unit are made at aproximately 49 mHz. This Federal Communications Commission (FCC) certified link includes five frequency modulated (FM) channels each spaced-apart by 30 kHz. The base unit transmits at approximately 1.7 20 mHz to the remote unit. This lower frequency signal is generally coupled into the AC wiring of the house or other building, using the wiring as an antenna.

According to the present invention there is provided in a cordless telephone system which 25 includes a base unit and a remote unit, an improved coupler for coupling said base unit to an alternating current power outlet comprising: a transformer having a primary winding for coupling to said power outlet and a center tapped secondary winding; 30 diodes coupled to said secondary winding for providing a direct current potential for said base unit; a first line fortransmitting a radio frequency signal from said base unit; an inductor coupling said first line to the center tap of said secondary winding; 35 a pair of capacitors for coupling said first line to said primary winding of said transformer; a second line for coupling to the grounding terminal of said power outlet, said second line being coupled to said center tap of said secondary winding, whereby a highly 40 efficient power adapter is realized.

An improved power adapter is described which couples the base unit to a power outlet both for purposes of receiving power and for the transmission of a radio frequency signal through the 45 power lines.

The present invention will be described further, by way of example, with reference to the accompanying drawings, in which:—

Fig. 1 is a block diagram of the base unit of a 50 cordless telephone system;

Fig. 2 is a block diagram of the remote unit of the cordless telephone system;

Fig. 3 is an electrical schematic of the power adapter used to interconnect the base unit with a 55 power outlet;

Fig. 4 is an electrical schematic of the filter and tone decoder used in the base unit;

Fig. 5 is an electrical schematic of the tone encoder used in the remote unit;

50 Fig. 6 is an electrical schematic of the modulator and oscillator circuits used in the remote unit; and

Fig. 7 is a cross-sectional side view of the remote unit.

A cordless telephone apparatus which comprises a base unit and a remote unit is described. In the following description, numerous specific details are set forth such as specific frequencies, etc., in order to provide a thorough understanding of the apparatus. However, it will be obvious to one skilled in the artthat-the apparatus may be practiced without these specific details. In other instances, well-known circuits are shown in block diagram form in order not to obscure the description with unnecessary detail.

First referring to Fig. 1, the block diagram of the base unit includes a receiver section, a transmitter section and a remaining section which provides control functions and coupling to the telephone line 25.

The receiver section includes an antenna 10 which receives the signal transmitted by the remote unit (one of five channels at approximately 49 mHz). The RF signal is amplified by amplifier 11 and mixed in a mixer 12 with a signal from local oscillator 13. An IF amplifier, detector and filter which includes a squelch circuit shown as block 14 is coupled to receive the output of the mixer 12. The amplifier 11, mixer 12, oscillator 13 and the circuitry shown within block Mare ordinary well-known circuits commercially available.

The audio output from the block 14 (line 15) is coupled to an audio-out filter 19 and also to a tone filter 16. As will be described, the signal transmitted by the remote unit is encoded by frequency modulating the carrier with one of a plurality of predetermined tones which are at, or near, the lower limit of the audio range (hereinafter sometimes referred to as "subaudio tones"). The tone filter 16 and subaudio tone detector 17 detect the presence of one of the predetermined tones. The specific circuit usedforfilter 16 and decoder 17 shall be described in detail in conjunction with Fig. 4.

If a certain one of the predetermined tones is present, a signal is communicated via line 18 to the logic control means 29. The logic control means 29 then allow coupling of the telephone line 25 through the telephone line control means 26 to permit signals from the telephone line hybrid circuit 23 to be coupled to the telephone line 25. The logic control means 29 also enables the transmitter voltage controller 37 thereby permitting power to be supplied to the transmitter section. A ringing oscillator 27 receives the ringing signal from the telephone line control means 26 and communicates the ringing signal on line 28 to the modulator31.

The audio signal from line 15 is filtered by the filter 19. This filter is a highpass filter which passes frequencies above 120Hz and thus removes the subaudio tones, thereby preventing this transmission into the telephone line. The output of the filter 19 is coupled to the touch-tone decoder, controller and auxilliary output circuit means 20. The output of this block is coupled (as indicated by line 29) to the telephone line control means 26 for coupling to line 25. The touch-tone decoder controller converts the touch-tone sounds from the remote unit into the "dialer" signals in those cases where the telephone line 25 is not adaptable to receive touch-tone signals, that is where the Central

65

70

75

80

85

90

95

100

105

110

115

120

125

2

GB 2 154 100 A 2

Office operates with manual dialers. The block 20 is optional and not generally required. In most cases, the audio signal from filter 19 whether it be those generated by voice or a touch-tone dialer are 5 coupled directly to the telephone line control means

26. The telephone line control means 26, oscillator

27, controller 37, hybrid circuit 23, touch-tone decoder controller 20 and filter 19 are ordinary circuits known in the art.

10 The incoming audio signal from the telephone line hybrid circuit is coupled to highpass filter and amplifierto generate the modulating signal (block 31). The output of block 31 frequency modulates the approximately 1.7 mHz RF signal generated by 15 oscillator 32. Oscillator 32 is a self-excited (non-crystal controlled) oscillator. The output of oscillator 32 is coupled to driver 33 and then into a final amplifier 34. The signal is then coupled through the connector 35 to the AC power line. The specific 20 adapter 36 will be described in detail in conjunction with Fig. 3.

With reference to Fig. 2, a block diagram forthe remote unit's receiver section is disclosed which includes an antenna 41 which receives the signal 25 from the base unit. An RF amplifier 42 amplifies this signal and couples it to the mixer 43. An IF amplifier, FM detector and filter are used as indicated by block 44 to provide an audio signal on line 46. The circuits shown by block 44 also include a squelch circuit 30 which provides a signal on line 45 when an RF signal is being detected. An audio amplifier 47 amplifies the audio signal on line 46, and couples it to a speaker 48. When no signal is present on line 45 a portion of the audio amplifier 47 is disconnected 35 from its battery power supply, thereby reducing the power consumed. Obviously, when an RF signal is received, the signal on line 45 causes the entire audio amplifier 47 to be activated. The receiver section of the remote unit may be fabricated using 40 well-known components.

As mentioned, a single oscillator 53 is used to provide a frequency reference for mixer 43 and also to provide a radio frequency reference forthe signal transmitted by the remote unit to the base unit. The 45 triple input audio mixer/modulator 53 modulates the RF signal with the audio signal from microphone 49, the audio frequency signals from the touch-tone keyboard or the subaudio tones from atone generator shown as the subaudio encoder 52. A 50 portion of the modulator 50 and the entire oscillator 53 are shown in detail in Fig. 6. The subaudio tone encoder 52 is shown in detail in Fig. 5.

The modulated output from the oscillator 53 is coupled through a driver 54 and an amplifier 55 to 55 provide the RF signal at antenna 56.

The remote unit also includes a battery pack 58 and other miscellaneous controls well-known in the art.

Referring now to Fig. 7, the remote unit is a hand-60 holdable member having the speaker 48 at one end and a microphone 49 at the other end. One surface of the unit includes a touch tone keyboard 51 and the switch 59. Another switch is mounted on the unit although not visible in this view. One switch is an 65 on/off switch while the other is a standby/talk switch. When this latter switch is in the "talk" position it is equivalent to having an off-hook condition.

Two circuit boards are mounted within the remote unit. One circuit board 62 includes the transmitter section such as the oscillator 53, driver 54 and final amplifier 55 of Fig. 2. The other circuit board 63 includes the RF amplifier 42, mixer 43 and the other circuitry shown in the receiver section of Fig. 2. The RF signal from the transmitter is coupled directly to the adjacent, telescoping antenna 56 while the receiver mounted on the circuit board 63 is coupled to the ferrite antenna 41. This antenna is disposed within the housing of the remote unit.

In many prior art remote units, interference occurs between the receiver and the transmitter. This causes noise problems in addition to degrading the overall performance of the system. As shown in Fig. 7, the transmitter and receiver sections are spaced-apart and are mounted as far apart as possible within the unit's housing. Also, the printed circuit boards 62 and 63 are mounted adjacent to their respective antennas, thereby eliminating long internal radio frequency paths. Additionally,

antenna 56 is perpendicularto antenna 41, reducing pick-up by antenna 41 of the transmitted signal.

Referring now to Fig. 3, the illustrated adapter is used to couple the RF signals from the base unit into the power circuits and to couple power into the base unit. The connector 35 includes 3 lines, 69,70 and 71. The RF signal is coupled to the power adapter on line 69. Line 70 is a ground line; a positive potential is coupled to the base unit on line 71. The power adapter includes a transformer 69 with its primary winding 68 coupled to an ordinary AC power plug 67. The center tap 74 of the secondary winding is coupled to the ground line 70; line 70 is coupled to the grounding terminal of plug 67. This line is also coupled to line 69 through an inductor 77. The secondary winding is coupled through diodes 75 and 76 to line 71 to provide the positive potential on line 71. The RF line 69 is coupled to the primary windings 68 through capacitors 72 and 73.

Capacitors 72 and 73 (.002 mfd in the presently preferred embodiment) provide substantial impedance at 60 Hz, hence, power from the primary winding is not coupled into line 69. If one of the capacitors 72 or 73 fails (shorts), the inductor 77 prevents line 69 from rising in potential since this inductor effectively provides a short circuit at 60 Hz. In the presently preferred embodiment, the inductor 77 has 33 pH of inductance.

As mentioned, the signal transmitted between the remote unit and base unit is modulated with a low frequency (subaudio) signal. Unless a certain frequency is present, the base unit will not, for example, allow placement of a call. The circuit which provides this low frequency tone is shown in Fig. 5. The circuit is a somewhat standard oscillator powered from line 80 and employing transistor 81. The low frequency tone is generated online 82. The oscillator includes a resistor 83 and a capacitor 84 identified as Rx and Cx, respectively. A table is shown to the right of the oscillator circuit with the presently preferred values for Rx and Cx. If, for

70

75

80

85

90

95

100

105

110

115

120

125

130

3

GB 2 154 100 A 3

example, the particular system is to be encoded with a tone of 40 Hz, then Rx is 10k and Cxis 0.1 mfd. When the system is fabricated, one of the five tones is selected for both the remote unit and base unit.

5 Also, one of the five FCC assigned channels is selected. With the selection of one of the five channels and one of the five tones, 25 combinations are possible. Thus, in theory only every 25th remote unit and base unit will be compatible. This 10 minimizes the possibility of unauthorized or accidental access to the base unit.

The circuit for detecting the subaudio tone is shown in Fig. 4. It includes a lowpass filter which has a cutoff frequency of approximately 120 Hz. The 15 input audio signal (line 89) is coupled through capacitor 86 to the filter associated with the transistor 87. The low frequency signal is then coupled through line 88 to the tone decoder. Although in the presently preferred embodiment a 20 low frequency tone is used to control access to the base unit, it will be appreciated by one skilled in the art that non-audible high frequency signals may also be used to achieve substantially the same result.

25 The tone decoder comprises a phase-lock loop contained within the integrated circuit 91. In the presently preferred embodiment, a commercial phase-lock loop integrated circuit (Part No. 567CN) is utilized. The frequency of this loop is determined 30 by resistor 92 (Rx). As mentioned, a tone is selected to match that transmitted by the remote unit. If the frequency of the signal on line 88 matches the frequency for which the phase-lock loop has been set, a signal appears on line 93 allowing the base 35 unit, for instance, to place a call. Powerforthe circuit of Fig. 4 is provided on line 90.

Referring now to Fig 6, a crystal 96 provides a frequency standard for the modulator and oscillator of Fig. 6. This crystal, in conjunction with the 40 variable capacitor 100, resistor 101, inductor 102 and capacitor 103 provide the fundamental frequency of approximately 12.5 mHz to the base terminal of the transistor 97. (Crystals of frequency equal to 12.457 or 12.472 mHz are used). 45 The audio signal on line 95 frequency modulates the signal applied to the transistor 97 because of capacitor 100. Relatively small amounts of modulation are required.

The output of the transistor 97 is coupled through 50 line 98 to provide a local oscillator signal for th mixer 43 of Fig. 2. Note that this local oscillator signal may be modulated by the audio signal, however, after the mixing operation and the detection of the audio signal received by the 55 antenna 41 of Fig. 2, substantially no audible signal is heard. The output from the transistor 97 is frequency multiplied by 4 to provide a signal on line 99 which after being amplified is transmitted. The inductor 102 and capacitor 103 are used to provide 60 this multiplied signal. The effect of this multiplication is to increase the modulation, thus a larger amount of energy associated with the audio signal is transmitted.

Thus, a cordless telephone has been described 65 including the use of a single oscillator in the remote unit, subaudio tone encoding to prevent unauthorized access of the base unit and an improved layout in the remote unit which reduces interference between the transmitted and received 70 signals.

The present application has been divided-out of our co-pending U.K. Patent Application No. 8229466 in which there is described and claimed an improved remote unit in a cordless telephone 75 system which includes a base unit.

Claims

CLAIM

In a cordless telephone system which includes a base unit and a remote unit, an improved coupler for coupling said base unit to an alternating current 80 power outlet comprising: a transformer having a primary winding for coupling to said power outlet and a center tapped secondary winding; diodes coupled to said secondary winding for providing a direct current potential for said base unit; a first line 85 for transmitting a radio frequency signal from said base unit; an inductor coupling said first line to the center tap of said secondary winding; a pair of capacitors for coupling said first line to said primary winding of said transformer; a second line for go coupling to the grounding terminal of said power outlet, said second line being coupled to said center tap of said secondary winding, whereby a highly efficient power adapter is realized.

Printed for Her Majesty's Stationery Office by Courier Press, Leamington Spa. 8/1985. Demand No. 8817443. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.