METHOD OF GENERATING METERING PULSE, AND METERING PULSE TRANSMITTER

Abstract: The invention relates to a method for generating a metering pulse and a metering pulse transmitter used for generating the metering pulse to a subscriber wire. The metering pulse transmitter comprises means (11) for generating a signal, means (14) for generating a virtual ground by means of which the level of the signal is shifted, a first amplifier element (13) and a second amplifier element (15), the output of said first amplifier element (13) being operatively connected with the input of the second amplifier element (15), and separation means (16) connected with the first amplifier element (13) and the second amplifier element (15). The amplifier elements (13, 15) generate opposite-phase signals which are substantially equal in strength and which are combined in the separation means (16) to form the metering pulse, which is transmitted to the subscriber wire.

Description: METHOD OF GENERATING METERING PULSE, AND METERING PULSE TRANSMITTER FIELD OF THE INVENTION The invention relates to a method of generating a metering pulse, said method being used in a metering pulse transmitter; in said method is generated a signal which operates at an operating level, and for said signal is generated a virtual ground by means of which the level of the signal operating at the operating level is shifted and in which said signal is amplified. The invention further relates to a metering pulse transmitter which is used for generating a metering pulse in a subscriber wire, said metering pulse transmitter comprising means for generating a signal, means for generating a virtual ground by means of which the level of the signal is shifted, a first amplifier element and a second amplifier element, the output of said first amplifier element being operatively connected with the input of the second amplifier element, and separation means connected with the first amplifier and the second amplifier. DESCRIPTION OF THE PRIOR ART A metering pulse transmitter is used in telecommunication applications in which metering pulses, for instance, are supplied to a subscriber wire. The metering pulse transmitter generates a high level signal and transmits the signal to the subscriber wire. The metering pulse transmitter is used particularly in connection with a subscriber interface unit in which a 50 Hz signal, for instance, is transmitted to subscriber wires a and b. The metering pulse generated by the metering pulse transmitter is used in the charge control of sub- scriber terminals, such as coin-operated telephones or the like. If the metering pulse transmitter is used for the above purpose, the subscriber terminals must have necessary metering means. The metering pulse is transmitted to a subscriber loop during a call, so the metering pulse must be transmitted without disturbing the call. In the prior art, generating the metering pulse has been based on a transformer coupling. To the transformer coupling has been introduced a signal which has been modified so as to achieve a metering pulse of a desired strength to a subscriber wire. However, there are several problems in the metering pulse transmitter of the prior art. Generating metering pulse has re- quired a space-taking transformer coupling. Owing to the usage of transform- ers, the metering pulse transmitter has become relatively heavy. The transformers have also disturbed the equipment and components near the metering pulse transmitter. The extent of losses has also been a problem in the transformers. The transformers have had high tolerances which have further made it more difficult to generate the metering pulse. BRIEF DESCRIPTION OF THE INVENTION The object of the present invention is to generate a metering pulse so as to avoid the problems caused by the use of transformers. This is achieved by a method described in the introduction, the method being characterized in that two opposite-phase signals of substantially equal strength are formed from the amplified signal and combined to form the metering pulse, which is transmitted to subscriber wires. The metering pulse transmitter of the invention is characterized in that amplifier elements generate opposite-phase signals of substantially equal strength, said signals being combined in separation means to form a metering pulse, which is transmitted to subscriber wires. Considerable advantages are achieved by the solution of the invention. In the solution the signal used in the metering can be generated by smaller switched-mode transformers, for instance. The metering pulse trans- mitter of the solution is considerably smaller and lighter, since no space-taking and heavy transformers are used in generating the signal. The metering pulse transmitter does not disturb the operation of the nearby components and electronic equipment. Furthermore, the metering pulse transmitters of the solution are more rapid and cost-effective to manufacture. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in greater detail in the following with reference to the examples illustrated in the attached drawings, in which Figure 1 illustrates a general block diagram of the metering pulse transmitter of the invention, Figure 2 shows some signals of the metering pulse transmitter of the invention. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a general block diagram of a metering pulse transmitter. The metering pulse transmitter shown in the figure generates a meter- ing pulse. Subsequently, the metering pulse is transmitted to subscriber wires a and b. In each subscriber wire a and b a metering pulse is generated in the same manner. Merely the metering pulse transmitted to subscriber wire a will be described in the following. The subscriber wires a and b provide a channel in which the signal which sets up a speech connection and a metering pulse are transmitted during a call. The metering pulse transmitter of the figure comprises means 11 (gen) for generating the signal frequency, an attenuator element 12 (att), a first amplifier element 13 (amp) and a second amplifier element 15 (amp). The metering pulse transmitter further comprises means 14 (vgnd) for generating a virtual ground. The metering pulse transmitter further comprises separation means 16 (sep) for separating the metering pulse from the subscriber wire. The means 11 generate the frequency used in the metering pulse to each subscriber wire a and b. An analogue signal setting up the call is transmitted in the subscriber wires. The subscriber wires form part of a subscriber loop to which a terminal is connected. The terminal can be a telephone, for example. The means 11 are connected to the first amplifier element 13 through the attenuator element 12. The means 14 are connected with the attenuator element 12, the first amplifier element 13 and the second amplifier element 15. The means 14 generate the virtual ground required in the above elements 12, 13, 15. From the first amplifier element 13 the signal is supplied to the second amplifier element 15. In the solution in accordance with the figure, the first amplifier element 13 has considerably stronger amplification than the second amplifier element 15. Each amplifier element 13, 15 is connected to the subscriber wire through the separation circuit 16. In the block diagram in accordance with Figure 1 , a sinusoidal signal whose frequency is 50 Hz, for example, is advantageously generated by the means 11. The sinusoidal signal generated by the means 11 is used in generating the metering pulse transmitted to each subscriber wire a and b. The frequency of the signal generated by the means 11 is identical with the frequency of the metering pulse. The sinusoidal signal generated by the means 11 is connected to the attenuator element 12, in which the signal is shifted to operate at a desired operating level. The attenuator element 12 is advantageously implemented by operational amplifier circuitry, for instance. In the solution in accordance with the figure the attenuation factor of the attenuator element 12 has been set to be ten, for example. The attenuator element 12 attenuates the signal received from the means 11. Subsequent to attenuation, the signal is connected to the first amplifier element 13, which amplifies the signal received from the attenuator element 12. The amplification factor of the first amplification element 13 is one hundred, for instance. On the other hand, the amplification of the second amplifier element 15 is one, for instance. The amplification factors are set in advance. The high feedback factor of the first amplifier element 13 enables the output impedance of the amplifier element 13 to be low. The problems caused by the amplifier element 13 in its output signal can be minimized owing to the low output impedance. The amplifier elements 13, 15 are implemented in practice by operational amplifier circuitry, for example. The amplification factor of the amplifier element 13, 15 is accurately set on a desired value by an operational amplifier. The means 11 comprise several channels to which the signal is being supplied simultaneously. Since the means comprise several channels, the attenuator element 12 adapts the signals generated by the means 11 to a level which enables several channels to be supplied simultaneously. The amplifier elements 13, 15 generate an amplifier stage which supplies its signals to the subscriber wire a. The amplifier elements 23, 25 generate an amplifier stage which supplies its signals to the subscriber wire b. In the solution in accordance with the figure the operating voltages of the different amplifier element stages are advantageously separated from each other galvanically. The galvanically separated operating voltages enable inter-channel crosstalk to be minimized. The amplifier elements 13, 15 modify the voltage level of the wires according to the metering pulses. A virtual ground is generated between the operating voltages of the amplifier element 13 by operational amplifier circuitry. The signal received by the amplifier element 13 is set to operate in a proper operation range by means of the virtual ground. The first amplifier element 13 is a non-inverting amplifier, whereby the outgoing signal supplied from the amplifier element 13 is in the same phase with the incoming signal received by the amplifier element 13 from the attenuator element 12. The signal level of the outgoing signal of the first amplifier element 13 is ± 30V for instance. Part of the outgoing signal of the amplifier 13 is supplied to the second amplifier element 15. Part of the outgoing signal of the amplifier 13 is supplied to the separation means The amplification factor of the second amplifier element 15 is set to be considerably lower in comparison with the amplification of the first amplifier element 13. In the solution in accordance with the figure the amplification of the second amplifier element 15 is one. The amplifier element 15 is advanta- geously implemented by operational amplification circuitry which is in principle similar to that of the first amplifier element 13, whereby the amplification factor of the amplifier element 15 can be set on a desired value as accurately as possible. A virtual ground is also generated between the operation voltages of the second amplifier element 15. The signal received by the amplifier element 15 is set to operate in a proper working range by means of the virtual ground. The amplifier element 15 operates as an inverting amplifier. The amplifier element 15 inverts the phase of the signal received by the amplifier 15 from the amplifier element 13 in such a manner that it is of the opposite phase, in other words the amplifier element 15 inverts the phase of the signal by 180 degrees. The outgoing signals of the amplifier elements 13, 15 are, however, identical in strength. The outgoing signal of the amplifier elements 13, 15 is supplied to the separation means 16 for combining. The separation means 16 separate the metering pulse generated in the amplifier elements 13 and 15 from the subscriber wire a, if the metering pulse is not transmitted to the subscriber wire. The separation element allows the voltage traveling in the subscriber loop to pass through when the metering pulse transmitter is in a non-active state. The power supply of the metering pulse transmitter is thus not overloaded. The separation means 16 is in practice implemented by a transistor-diode connection with a feedback loop, for instance. Since the separation means 16 may have a voltage loss of one volt, for instance, a voltage equal to the voltage loss of the separation element 16 can be introduced into the feedback loop of the amplifier element 15. The signals of opposite phases generated in the amplifier elements 13 and 15 are combined in the separation means 16 to form a metering pulse in such a man- ner that the root mean square value of the signal generated is 45 Vrms, for example. The frequency of the generated metering pulse is 50 Hz, for example. The metering pulse comprises pulses whose duration ranges for example between 90 ms and 160 ms. The pulses are usually transmitted at intervals of over 150 ms. Figure 2a shows a signal supplied to a first amplifier element 13. The signal is sinusoidal and its peak value is at a voltage level 1. Figure 2b shows a signal outputted from the first amplifier element 13. Figure 2b shows that the amplifier element 13 has amplified the signal by an amplification factor one hundred. The phases of the signals are, however, identical in the signal received by and transmitted from the amplifier element 13. The outgoing signal of the amplifier element 13 is supplied to an amplifier element 15 and separation means 16. A signal outputted from the second amplifier element 15 is shown in Figure 2c. It is shown in Figures 2b and 2c that the phases of the signals received by and transmitted from the amplifier element 13 are opposite. This means that the amplifier element 15 has inverted the signal supplied to the amplifier element 15 by 180 degrees. The levels of the incoming and outgoing signals of the amplifier element 15 are, however, identical. This means that the amplification factor of the amplifier element 15 is one. The output signal of the amplifier element 13 and the output signal of the amplifier element 15 are combined in the separation means 16. Said signals of opposite phases are absolutely combined. In absolute combining the ground used is a virtual ground. The signal received as a result of the combining is shown in Figure 2d. Figure 2d also shows that the levels of the signals described in Figures 2b and 2c are each half the level of the signal in Figure 2d. The opposite phase signals of the same level, described in Figures 2b and 2c, are combined in such a manner that the strength of the signal increases in the combining. The combining of the signals results in a sinusoid signal which is used as a metering pulse. The generated metering pulse is supplied to a subscriber wire. The root mean square value of the metering pulse is 45 Vrms, for instance. The metering pulse is advantageously transmitted in common mode to each subscriber wire. The signal transmitted in common mode does not disturb the speech signal passing transversely in the wires. The metering pulse can be transmitted during on-hook and off-hook states. Even though the invention has been described above with reference to the examples illustrated in the accompanying drawings, it is to be understood that the invention is not restricted to them but it can be modified in many ways within the scope of the inventive idea disclosed in the appended claims.

Claims: CLAIMS 1. A method of generating a metering pulse, said method being used in a metering pulse transmitter; in said method is generated a signal which operates at an operating level, and for said signal is generated a virtual ground by means of which the level of the signal operating at the operating level is shifted and in which said signal is amplified, characterized in that two opposite phase signals of substantially equal strength are formed from the amplified signal and combined to form the metering pulse, which is transmitted to subscriber wires. 2. A method as claimed in claim ^characterized in that the metering pulses pass in the subscriber wires in common mode. 3. A method as claimed in claim ^characterized in that the opposite phase signals are absolutely combined in such a manner that the strength of the signal increases in the combining. 4. A method as claimed in claim 1, characterized in that subsequent to the amplification of the signal the level of the signal is substantially half the level of the metering pulse. 5. A method as claimed in claim ^characterized in that the transmission of the metering pulse to the subscriber wire is inhibited when the metering pulse transmitter is not in use. 6. A method as claimed in claim 1, characterized in that a generator supplying several channels is used in the method, said generator generating a signal which is adapted to a level which enables several channels to be supplied simultaneously. 7. A method as claimed in claim 1, characterized in that floating voltages are used in the method, said voltages being galvanically separated from each other. 8. A metering pulse transmitter for generating a metering pulse in a subscriber wire, said metering pulse transmitter comprising means (11) for generating a signal, means (14) for generating a virtual ground by which the level of the signal is shifted, a first amplifier element (13) and a second amplifier element (15), the output of said first amplifier element (13) being opera- tively connected with the input of the second amplifier element (15), and separation means (16) connected with the first amplifier (13) and the second ampli- fier (15), characterized in that the amplifier elements (13, 15) generate signals which are substantially of equal strength and of opposite phases, and which are combined in the separation means (16) to form a metering pulse, which is transmitted to the subscriber wires. 9. A metering pulse transmitter as claimed in claim 8, charac- terized in that the separation means (16) absolutely combines the opposite phase signals in such a manner that the strength of the signal increases in the combining. 10. A metering pulse transmitter as claimed in claim 8, characterized in that the means (14) generate a virtual ground for the amplifier elements (13, 15) receiving a signal by whose virtual ground the level of the signals received by the amplifier elements (13, 15) is shifted. 11. A metering pulse transmitter as claimed in claim 8, c h a r a c - terized in that the levels of the opposite-phase signals generated by the first amplifier element (13) and the second amplifier element (15) are substan- tially half of the level of the metering pulse. 12. A metering pulse transmitter as claimed in claim 8, c h a r a c - terized in that the separation means (16) separate the metering pulse from the subscriber wire, if necessary. 13. A metering pulse transmitter as claimed in claim 8, charac- terized in that the metering pulse transmitter comprises amplifier stages (13, 15), (23, 25) which are substantially identical in their functions and in which the operating voltages used are galvanically separated from each other. 14. A metering pulse transmitter as claimed in claim 8, c h a r a c - terized in that the first and the second amplifier elements (13, 15) use the same, floating operating voltage. 15. A metering pulse transmitter as claimed in claim 8, c h a r a c - terized in that the amplifier elements (13, 15) transmit the metering pulse to the subscriber wires in common mode. 16. A metering pulse transmitter as claimed in claim 8, c h a r a c - terized in that the means (11 ) generate a frequency used in the metering pulse. 17. A metering pulse transmitter as claimed in claim 8, character i z e d in that the separation means (16) are implemented by transistor- diode circuitry. 18. A metering pulse transmitter as claimed in claim 8, characterized in that the metering pulse transmitter comprises an attenuator element (12) connected with the means (11). 19. A metering pulse transmitter as claimed in claim 8, characterized in that when the means (11) comprise several channels, the attenuator element (12) adapts the signal generated by the means (11) to a level which enables the means (11 ) to supply said channels simultaneously.

Inventors: PARKKINEN JUKKA, SAERKELAE PETRI, SALO ARTO, VARANKA KAUKO, PARKKINEN, JUKKA, SAERKELAE, PETRI, SALO, ARTO, VARANKA, KAUKO
Applicants: NOKIA TELECOMMUNICATIONS OY, PARKKINEN JUKKA, SAERKELAE PETRI, SALO ARTO, VARANKA KAUKO, PARKKINEN, JUKKA, SAERKELAE, PETRI, SALO, ARTO, VARANKA, KAUKO
Application year: 1998
Main classification: H04M15
IPC codes: H04M15