Method for the Elimination of Transients From the Operating Voltage of TDMA System

This invention relates to a method of eliminating transient in an apparatus comprising a current source and several functional blocks, at least one of which consumes a pulsating current which produces transients in the operating voltage.

In apparatuses in which the consumption of current by different blocks varies greatly and in which the consumption of current may be pulse-like, transients are formed in the operating voltage. When included in the operating voltage, transients are detrimental to those functional blocks which require a very stable operating voltage to function properly. For example, an especially stable operating voltage is required in digital radio telephones having a digital system based on time division multiplexing (TDMA). In this system, voltage-controlled oscillators (VCO) are used, for example, in the SAT detector and also in other blocks. As a result any transients in the supply voltage will cause great frequency deviations in the various circuits. Therefore, very strict specifications have been set for voltages in the TDMA system.

Usually transients are eliminated by means of various filters or a voltage regulator. When a transient is very sharp, a conventional regulator is not capable of following it and, thus, the transient is seen in the regulator output as a greater-than-allowed voltage variation. It is, therefore, difficult to eliminate transients completely by known methods.

By the method according to the present invention, a transient caused by the pulse-like current intake of some block to the other blocks is denied access. The method is characterized in that a switch in the apparatus separates the current source and the transient-causing block from the other blocks during the time that transients are produced. The switch connects the current source and the transient-producing block to the other blocks and to the buffer during the time that no transients are produced.

Thus, in this method, the current source is continuously connected to the block or blocks which take in a current pulse with an abrupt leading edge. The current pulse will produce transients in the terminals of the current source. While the current pulse is being taken in, the switch separates the current source and the transient-producing blocks entirely from the rest of the system. During this time, the rest of the system receives its operating energy from a separate buffer. When no transients are being produced, the switch closes and connects the current source and the transient-producing blocks to the rest of the system and to the buffer. The blocks of the system now receive their operating energy from the current source, which at the same time charges the buffer.

The method is now described in greater detail with reference to the accompanying figures, in which:

FIG. 1 is an illustration of the principle of the method, and

FIG. 2 is an example of the implementation of the switch.

In FIG. 1, block 2 represents a functional block or several functional blocks of a digital radio telephone taking in a high pulse-like current with an abrurt leading edge. The transmitter part of a radio telephone is such a block. Current is supplied to block 2, as it is to the entire system, by a current source 5, which may be, for example, a battery or an external source of voltage. Block 1 stands for the other functional parts of the radio telephone, such as the receiver, the synthesizer, and the logic unit. The linear regulator 3 regulates voltage to block 1. According to the invention, the system includes a switch 4 and a buffer 6, which serves as a place for storing energy. The buffer 6 may be any means in which energy can be stored and from which energy can be discharged. Preferably the buffer 6 is a battery, but depending on the application, it may also be a capacitor.

The operation is as follows: In a radio telephone having a digital system based on time division multiplexing (TDMA), the transmitter of the radio telephone operates for a time interval of 1/8 T, T being the total time of the cycle. When the transmitter switches on, block 2 in FIG. 1 takes in a high pulse-like current having a very abrupt leading edge. Since the switch control 7 of the radio telephone is the microprocessor of the logic unit and the telephone operator based on TDMA, it knows when the transmitter will be turned on and off, so it causes the switch 4 to open at the same moment that the operation of block 2, i.e., the transmitter, begins and close when it ends. Thereupon, the transients produced by block 2 are seen at the terminals of the current source 5, but they cannot proceed to block 1, which requires a stable operating voltage. During this time, block 1 receives its operating voltage from the buffer 6. When the current-taking by block 2 ends, the control closes the switch 4, whereupon block 1 takes in current from the current source 5. The current source 5 at the same time charges the buffer 6, the voltage of which is somewhat lower than the voltage of the current source 5. The charging will continue until the switch 4 opens and a new current pulse enters block 2. The closing of the switch 4 produces a rapidly rising step voltage in block 1. If the switch 4 is implemented as an ideal switch, the leading edge of the step voltage is vertical. In this case the linear regulator 3 is not capable of following the voltage, resulting in an undesirable voltage spike at its output. For this reason, a low-pass filter (shown by dashed lines in FIG. 1) is added between the switch 4 and the regulator 3. The low-pass filter delays the voltage and rounds out its leading edge, allowing the regulator 3 sufficient time to follow the voltage. When the switch 4 includes a transistor, for example a MOSFET transistor, it is advantageous to regulate the operation of the switch by means of a round-edged control pulse, in which case the rising rate of the voltage at the time of the closing of the switch 4 becomes more gradual and the regulator 3 is capable of correcting the voltage so as to meet the requirements of block 1. The on-resistance of the switch 4 adjusts the amplitude of the charging current of the buffer 6. It is, of course, also possible to use a separate resistance in addition to the resistance of the switch itself, if a lower charging current is desired.

One practical embodiment of the switch 4 is shown in FIG. 2. The control for the switch is obtained from the microprocessor of the logic unit of the telephone. Just before the current pulse arrives at block 2, the control pulse transmitted by the microprocessor drops, whereupon the Darlington-coupled transistor 9 is no longer conductive and the lattice voltage of MOSFET 10 rises and the p-channel closes, i.e. the switch 4 is open. Respectively, when block 2 is no longer taking in current, the control pulse transmitted by the microprocessor opens the MOSFET channel, i.e. the switch 4 closes.

In a radio telephone, the method according to the invention effectively protects those functional blocks which require a stable voltage from transients caused by other blocks. Although a radio telephone has been discussed above, the invention can also be applied to other electronic apparatuses. In practice, the device according to the invention can also be implemented in some manner other than that which has been described above, while keeping within the protective scope of the claims.