Telecommunications – Transmitter – With feedback of modulated output signal
Reexamination Certificate
1999-06-22
2002-04-09
Bost, Dwayne (Department: 2681)
Telecommunications
Transmitter
With feedback of modulated output signal
C455S115200, C455S127500
Reexamination Certificate
active
06370364
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to radiotelephones and, in particular, to radiotelephones, mobile stations and wireless communicators, including those capable of operation with a cellular network, that employ a closed loop transmitter power control system.
BACKGROUND OF THE INVENTION
The control of the RF transmitter power in a mobile station is an important consideration, as mobile stations are expected to operate at or very close to prescribed RF transmitter power levels. Furthermore, during a call the mobile station may be commanded by a base station to adjust the RF transmitter power. For example, if a mobile station in a vehicle begins a call with a relatively high power level at the edge of a particular cell, and then approaches the center of the cell, the mobile station will likely be required to decrease its RF transmitter power. Failure to properly control the transmitter power can result in, for example, an occurrence of dropped calls and/or the generation of interference that adversely affects other users. Transmitter power control is important in all types of wireless communication systems, including FM (analog) systems, as well as digital TDMA and CDMA systems.
Another important aspect of RF power control is an ability to completely turn off the RF transmitter when commanded or desired, thereby outputting, ideally, zero transmitted RF power.
FIG. 3
shows a simplified diagram of a conventional closed loop transmitter power control system of a type that is typically found in mobile stations. The closed loop transmitter power control system feeds an antenna through a directional coupled from an RF power amplifier. The RF power amplifier amplifies an input RF signal, which is modulated in some way with appropriate signalling information and, during a voice call, the user's voice. Data calls can be made in the same way (e.g., when a computer modem connects to a data communications network, such as the Internet). The purpose of the coupler
32
is to extract a small part of the transmitted RF energy and provide this energy to a detector, which functions to convert the RF output of the coupler to a DC signal having a magnitude that is a function of the output RF power level. The detector output signal is applied to one input of a closed loop error amplifier, which receives a power control voltage at another input. The power control voltage, which may be set by a controller (a microprocessor) using a digital to analog converter (DAC), is representative of a desired output RF power level. The output of the error amplifier is a difference between (i.e., the error) in the commanded RF output power level and the actual output RF power level, as reflected by the DC signal that is output from the detector. The error signal from the closed loop error amplifier is then used to control the gain of the RF amplifier, thereby closing the power control loop. When the output RF power level at the input to the antenna, as indicated at the output of the detector, equals the commanded RF power level, as indicated by the power control voltage output from the DAC, the closed loop has obtained equilibrium by setting the output of the error amplifier to a proper potential.
General reference in this regard can be made to the following commonly assigned U.S. Patents: U.S. Pat. No. 5,230,091 issued Jul. 20, 1993, entitled “Method And Apparatus For Tuning And Compensating Power Levels In A Radio Telephone”, by Risto Vaisanen; U.S. Pat. No. 5,276,917 issued Jan. 4, 1994, entitled “Transmitter Switch-On In A Dual-Mode Mobile Phone” by Petteri Vanhanen et al.; U.S. Pat. No. 5,548,616, issued Aug. 20, 1996, entitled “Spread Spectrum Radiotelephone Having Adaptive Transmitter Gain Control” by Lars H. Mucke et al.; and U.S. Pat. No. 5,697,074 issued Dec. 9, 1997, entitled “Dual Rate Power Control Loop For A Transmitter” by Eero Makikallio et al.
Based on the foregoing discussion it can be appreciated that the overall calibration or alignment of the RF power control loop is an important consideration, particularly when making factory and field office calibrations. Typically expensive and complex test equipment, such as RF power meters and the like which also require periodic calibration, are required to actually measure the RF power output at various levels. The measured RF power is then compared to the expected (commanded) power, and calibration constants are derived and stored in a memory of the mobile station for later use. The calibration constants are used to null out any offsets in the RF power control loop circuitry, such as those exhibited by the DAC. In that the gain of the error amplifier is typically in the range of about 10 to 100,000, it can be appreciated that even a small offset voltage in the DAC's output can result in a large shift in the output of the closed loop error amplifier, thereby causing the actual RF transmitter power to deviate from the desired value.
Attention to other drift sources, namely the error amplifier and the power detector, can be considered as well. Additionally, the calibration constants are used to provide the desired output power levels. Typically, one of the constants indicates a “knee value” level that just sets the loop at equilibrium, but does not increase the RF output power. The other constants indicate the various desired output power levels. For a power ramp the output of the DAC is ideally ramped from the knee value level to the desired output power value, and then back to the knee value level for ramp down. The knee value, ideally, compensates the various loop offsets, while the other constants are usually defined with reference to the knee value. The knee value may thus also be referred to as a “base value”.
As such, it can be appreciated that finding the knee value simplifies the task of determining the power level dedicated constants, as the latter can be simply added to the knee value. The power level dedicated constants then depend on the conversion coefficient of the DAC, the coupling factor and the impedance of the directional coupler, and the signal envelope sensitivity of the power detector. If these parameters are sufficiently constant, the calibration of the power level dedicated constants may not be required (depending, of course, on the accuracy requirement for the output power). In this case a calibration of the knee value may suffice, and predetermined values for the “add on” power level constants then applied. It can thus be appreciated that the knee value is a very sensitive parameter, and important in the sense that all power levels use it as a starting point for a power ramp.
With DACs incorporating some kind of level shifting circuitry, the output voltage drift with reference to a given input code may be considered as an offset drift of the DAC. More generally, however, the DAC output may be considered as simply drifting (not an offset drift per se) with temperature and aging. From the power control loop point of view this drift appears as an offset drift around the control voltage knee value, i.e., the value needed to set the loop just at equilibrium without causing an increase in the RF output power.
A small offset (or drift) in any of the relevant loop parameters near to the power control voltage knee value (such as at the start of a power ramp up or when using a small output power level) is detrimental. This is due to the typically very small detector voltage obtained under these conditions (for zero RF output power the envelope voltage component of the detector is zero, and increases with increasing RF power). In general, the directional coupler is a linear device and does not incorporate an offset or nonlinearity error.
In order to be able to shut the RF power down completely, a “zero code” output of the DAC must be below a level defined by the detector bias voltage and the input offset of the loop error amplifier. The offset alignment procedure is run in order to determine a value for a digital control word that just balances the RF control loop, while not generating any significant RF out
Bost Dwayne
Harrington & Smith ,LLP
Nokia Mobile Phones Ltd.
Trinh Sonny
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