D/A converter offset correction

Abstract: An offset voltage zeroing method for a D/A converter, in which oscillations of the output voltage are radically reduced. An output signal of the converter is applied to a first terminal of a capacitor (5). A second terminal of the capacitor is coupled to a reference voltage (V ref ) by a controllable switch (6) as long as there are no pulses from the output of the converter. As a result, an offset-corrected output signal is obtained from the second terminal.

Description: This invention relates to offset voltage zeroing in which output voltage variations of a digital-analogue (D/A) converter may be substantially reduced, these variations being caused by temperature induced changes in the offset voltage of the converter. Especially in D/A converters using CMOS technology, problems arise due to variations of the output voltage at different temperatures, these being on the one hand dependent on variations in the reference voltage and on the other hand, on the drifting of the offset voltage of the operational amplifiers used in the implementation of converters. The reference voltage could, in principle, be provided using circuits external to the converter, but that would lead to an increase in the number of components and in additional costs. Moreover, changes in the offset voltage would still cause problems. In digital radio telephones which use a time division multiple access (TDMA) system, transmission is carried out in the form of bursts so that a signal to be transmitted consists of subsequent pulses. A rise and decay of a transmission pulse should not be stepwise but the rise and decay are expected to be controllable so that the transmission spectrum would not spread to be too wide; nevertheless, the rise and decay times should be as short as possible. The cos 2 waveform has often been considered to be an appropriate rising and decaying pulse shape. The timing of a transmission pulse should also be independent of the power level of the transmitter. In European Patent Application EP-A-0,434,294 and corresponding US-A-5,101,175 (which claim priority from Finnish patent application No. 896266 entitled "A power control method for a voltage controlled power amplifier and circuitry used in the same") there is disclosed a power control method for a transmitter based on the use of two control signals TXC and TXP. The block diagram of the transmitter of a digital radio telephone according to said method is presented in Figure 1. In the circuit the input square wave pulses P in are amplified as desired and cos 2 -transformed, as to the rise and decay, into output powered pulses P out in a multistage power amplifier 1. The output power is measured with the aid of a directional coupler 2 and a power sensor 3 from which a voltage V1 indicative of the power is obtained. Said voltage is carried to a control amplifier 4 being e.g. an operational amplifier, and a control pulse TXC enters a second input of the control amplifier 4 from a D/A converter. The control loop includes resistors R1, R2 and R3. With the output voltage of the control amplifier 4 a square wave pulse TXP is summed after the resistor R3, the height of said pulse being approximately the same as the threshold voltage above which the power amplifier 1 starts to produce power. The square wave pulse starts substantially at the same moment as the control pulse TXC starts to rise. Now the capacitor C1 is charged through the resistor R4 to approach rapidly the threshold level, and the control loop of the power amplifier 1 is operative as soon as there is power at the output of the power amplifier 1. If the actual control signal TXC affecting the output power has been produced e.g. with an ordinary inexpensive CMOS D/A converter, the output power varies with changing temperature. This is due to changes of the reference voltage and offset voltage in the D/A converter described above. Very few means for eliminating this problem are available in the prior art. An individual temperature compensation can be carried out in every radio telephone, which causes extra work. Alternatively, high-standard and more expensive D/A converters may also be used. The object of the present invention is to provide an offset voltage zeroing method for a D/A-converter whereby the above described problems can be solved. The present invention is defined in the appended claims and in one embodiment variations of the offset voltage are substantially eliminated from a TXC-D/A signal by adding a serial capacitor and by keeping the terminal of the capacitor opposing the TXC circuit with the aid of a switch at a reference voltage V ref as long as there is no pulse in the TXC-D/A signal. An embodiment of the invention is described below in detail referring to the accompanying Figures, in which:- Figure 1 shows schematically a known power control method of a transmitter based on the use of two control signals TXC and TXP, Figure 2 is a circuit diagram illustrating the operation principle of the offset voltage zeroing method in accordance with the invention, Figure 3 shows the curve shapes of the signals used in the present method, and Figure 4 is a diagram of a circuit in accordance with the invention. Figure 1 is described above. An embodiment of the invention is described below with reference to Figures 2 to 4. Figure 2 shows the operation principle of an offset voltage zeroing system of a D/A converter in accordance with the invention. A TXC-D/A signal is a pulse (burst) derived from a D/A converter, being in shape cos 2 as regards its rise and decay, intended for controlling the transmitter 1 shown in Figure 1, and the TXC signal refers to a pulse obtained from a TXC-D/A signal to be carried to the power control loop of a transmitter, and being cos 2 -shaped regarding the rise and decay, wherefrom the variations of the offset voltage have been removed. The TXP signal is a square wave pulse used in accelerating the power control circuit in the manner described in the aforementioned EP-A-0,434,294. The changes in the output voltage TXC-D/A of a D/A converter, caused by variations in the offset voltage, can be removed from the TXC-D/A signal by adding a serial capacitor 5 and by maintaining the terminal of the capacitor 5 facing the TXC terminal with the aid of a switch 6 at the reference voltage V ref as long as there are no pulses in the TXC-D/A signal. As soon as a pulse TXC-DA enters, the switch opens and V ref is switched off from the TXC terminal of the capacitor 5. The pulse may now travel unchanged through the capacitor 5, and after the pulse ends, the voltage of the TXC terminal is switched back to the level of the reference voltage. The switch 6 is controlled via an inverter 7 with a TXP pulse. Figures 2 to 4 concern an embodiment of the invention for controlling the transmitter of a radio telephone. If the switch 6 was not used, the voltage level would glide after the capacitor 5 up and down, being dependent, as regards the rise and decay, on the level of the cos 2 shaped pulse. If the switch 6 is zeroactive in operation, no inverter 7 is needed. No inverter 7 is needed, either, if an inverted TXP pulse is used in the power control loop. The same offset voltage zeroing principle may be used in applications using the time-sharing principle, since the offset voltage can be zeroed without being included in the actual operational cycle. Figure 3 shows the curve shapes of the signals. The dashed line presents the variation of the TXC-D/A output voltage 8 of a D/A converter caused by the offset voltage. The TXP voltage is indicated by numeral 9 and the TXC voltage by numeral 10. Figure 4 shows a circuit implementation in accordance with the invention. For the inverter 7, a transistor 11 is used and for the switch 6, a transistor 12. The TXP square wave pulse has been applied to the base of the transistor 11 through a resistor 13. The base of the transistor 12 has been coupled to the positive operating voltage through a resistor 14. The reference voltage has been produced from the operating voltage with the aid of voltage divider resistors 15 and 16. The reference voltage may also be zero, resulting in omission of the resistors 15 and 16, and in grounding the emitter of the transistor 12. With the aid of the invention, D/A converters of prior art technology can be used. Only a few additional components are needed in the design, and it removes the need of tuning the stages controlled with a D/A converter. No individual temperature compensations of radio telephones are needed and also advantages related to production are achieved, such as improvement in the gain. Most part of the offset voltage zeroing coupling can be integrated within a D/A converter circuit. Compensation of the offset voltage of a D/A converter is described above with an example concerning the control pulse of a radio phone transmitter implemented by means of the digital time sharing principle. It is obvious that this principle can be used in any embodiment in which one wishes to eliminate the effect of the offset voltage from the output voltage of a D/A converter. The only prerequisite for such an application is to find out when a pulse from the converter starts and ends and that this information can be used for controlling the switch. Said information can be obtained from another pulse which preferably starts just before the start of a pulse from the converter and ends simultanously therewith. The invention is particularly appropriate for use in a radio telephone operating on the time division principle. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention, which is defined by the appended claims.

Claims: A radiotelephone transmitter that outputs a time division multiplex signal comprising a power amplifier; a D/A converter having an output signal (TXC-D/A) used as a control signal for the power amplifier; and offset correction circuitry for correcting the effect of offset voltages of the D/A converter, wherein the output of the converter is coupled to a first terminal of a capacitor (5), the second terminal of the capacitor (5) being coupled to a controllable switch (6) which, in the closed position couples the second terminal with a reference voltage (V ref ) and in the open position disconnects the reference voltage from the second terminal such that a corrected output signal is obtained from the second terminal of the capacitor the controllable switch being controlled by a control signal comprising a square wave pulse. A transmitter according to claim 1, characterized in that the control voltage of the switch is pulse-like and it starts and ends substantially simultaneously with the pulse of an A/D converter. A transmitter according to claim 2, characterized in that the controllable switch is closed as long as there are no pulses from the output of the converter. A transmitter according to claim 3, characterized in that the controllable switch (12) couples the second terminal of the capacitor (5) and the reference voltage (V ref ) in response to a positive control voltage. A transmitter according to claim 4, characterized in that a control voltage pulse is applied to the switch (12) when the voltage value of the pulse is substantially zero or provided with an opposite sign relative to the pulse of the A/D converter. A transmitter according to claim 4, characterized in that a control voltage pulse is applied inverted to the switch (12) when the voltage value of said pulse is of equal sign as the pulse of the A/D converter.

Inventors: SARASMO JUKKA, VAEISAENEN RISTO, SARASMO, JUKKA, VAEISAENEN, RISTO, Väisänen, Risto
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LTD.
Application year: 1992
Main classification: H03G3
IPC codes: H03M1, H03G3