"The combination of logarithmic amplifier and temperature sensor is a feasible design temperature compensation scheme to significantly reduce the role of two main error factors in RF power management - temperature and manufacturing process changes. In some cases, the temperature compensation hardware is integrated into the power detection chip.
RF power management overview
Accurate base station RF power management is very important. If the driving capacity of the transmit power amplifier exceeds the required output power level, it will lose a lot. Excessive current consumption will not only increase the cost, but also cause the heat dissipation problem that needs to increase heat dissipation measures. In extreme cases, power amplifier overdrive can lead to reliability problems caused by burnout failure.
Another benefit of accurate base station RF power management also exceeds mobile transmitters because they have the same requirements. With the ability to accurately control the output power, mobile devices can minimize the cost of power supply current. For example, RF power management allows the transmitted power to be precisely limited to the minimum required power level, thereby reducing the battery current. Precise power control extends talk time while allowing mobile transmitters to meet cellular standards.
Fig. 1 shows a block diagram of a typical RF power management circuit. The transmission signal channel consists of three coherent units: baseband, radio frequency (RF) transmission and power amplifier. Before the transmission signal reaches the antenna, a part of the transmission signal is sampled by the bidirectional coupler. The sampled RF power is sent to the power detector, where it is converted to DC voltage. Then the output voltage of the power detector is digitized and sent to the digital signal processor (DSP) or microcontroller (MCU). Once the digital power measurement value is obtained, a decision can be made according to the relationship between the measured output power and the required output power. MCU can use digital to analog converter (DAC) and variable gain amplifier (VGA) to adjust the output power to drive the power control of signal channel - whether baseband signal, RF signal or power amplifier. Once the measured output power is balanced with the required output power, the RF power management loop will reach steady state. At the same time, the temperature sensor is introduced as the input of MCU to increase the temperature compensation ability. A similar RF power management loop can be realized by using only analog circuits in the transmitter.
Figure 1. RF power management circuit uses logarithmic amplifier to make full use of its wide detection range which is linear in dB
Previously, diode detectors have been used in RF power management circuits to adjust transmission power. They provide good temperature performance at high input power values, but deteriorate at low input power. Even using the temperature compensation circuit, because the temperature performance of the diode detector deteriorates under low input power, it can only provide a very small detection range. A popular method to replace the diode detector is to demodulate the logarithmic amplifier. The logarithmic amplifier provides an easy to use RF power detection response that is linear in dB and has a wide dynamic range.
Fig. 2 shows a progressive compression logarithmic amplifier. In this example, four 10 dB cascaded limiting amplifiers form a step-by-step compression chain. Five full wave rectifier detector units convert RF signal voltage into current - one detector unit is at the RF input and the other four are at the output of the amplifier stage. The current generated by the detector unit is proportional to the amplitude of the voltage signal, and these currents are approximated by a logarithmic function. A high gain stage is used to convert the sum of the incoming current into voltage. Five detector units across four 10 dB amplifiers allow the logarithmic amplifier to have a 50 dB detection range.
Figure 2. Five detectors across four 10 dB amplifiers allow progressive compression of logarithmic amplifiers to reach a 50 dB detection range
Fig. 3 shows the transfer function of a 60 dB dynamic range 1 MHz to 8 GHz bandwidth logarithmic amplifier at 2.2g Hz. There is a linear relationship between RF output power and its output voltage, that is, when the input power increases, the corresponding output voltage increases linearly in dB. The figure also includes a logarithmic consistency error curve. This logarithmic consistency error curve is used to further check the performance of logarithmic amplifier. In the linear area of the detection range represented by the gray bright line, the slope of the transfer function and its intercept with the x-axis can be calculated. This information provides a simple ideal model for comparison with the actual response of the logarithmic amplifier. The ideal linear reference model is represented by a dotted line in the figure. The ideal linear model is compared with the actual response curve to produce a logarithmic consistency error curve (in DB).
Figure 3. Comparison of the ideal reference model calculated in the linear region of the detection range of the logarithmic amplifier with its actual response curve. The comparison results produce a logarithmic consistency error curve.
The method of calculating the consistency error of logarithmic amplifier is similar to the two-point calibration method used in the calibration of RF power management system. During product testing, two known RF signal strengths are selected within the linear range of the detector. Using the output voltage, the slope and intercept response characteristics can be calculated and stored in nonvolatile memory in order to establish a simple linear formula. Using the linear function relationship in dB and the measured detector voltage, it is easy to calculate the field transmission power. The important advantage of using two-point calibration is to reduce the cost and shorten the test time. However, this calibration method is only possible due to the linear performance of logarithmic amplifier.
Because calibration is usually done at one temperature, the quantitative effect of temperature on the detector is very important. The change of logarithmic detector accuracy with temperature can be expressed by consistency error. Fig. 4 shows the transfer function of a 45 dB logarithmic amplifier operating at frequencies up to 3.5 GHz at 900 MHz. The figure includes the transfer function at - 40C and + 85C and the relationship curve of logarithmic consistency error with temperature. Because of the so-called two-point calibration, three linear consistency error curves are generated with the same 25C linear reference.
Fig. 4. The log consistency error of a single device at 900 MHz shows an accuracy of ± 0.5 dB in the operating temperature range.
The transfer function of the logarithmic amplifier at an ambient temperature of 25 ° C has a slope of 50.25 dB / V and an intercept of - 51.6 DBM (the intersection of the extension of the linear reference line and the x-axis). The curve at 25 ° C fluctuates around the 0 dB error line, however, with a small slope and intercept offset at the temperatures at both ends. Within the operating temperature range and 40 dB detection range, the logarithmic consistency error of a single device shall be kept within ± 0.5 dB. The temperature drift at + 85 ° C is the limit of the dynamic range. Although a single device may have good accuracy in the operating temperature range, the inherent small differences between devices caused by semiconductor manufacturing process can prove to be an obstacle to accurate RF power management.
Fig. 5 shows the distribution of log consistency errors for 70 devices. Sampling over a wide range of devices to demonstrate deviations caused by the manufacturing process. Each device has three temperature curves calibrated against a linear reference value of 25 ° C. Although there are obvious deviations between devices, their distribution values are very close. The overall distribution curve of the device has an accuracy of ± 1 dB in the operating temperature range and the detection range greater than 40 dB. Due to the repeatable drift between devices, temperature compensation is introduced.
Figure 5. The log consistency error between devices is obviously different, but its overall distribution is very close.
Generally, wireless communication standards require that the transmission power detection scheme has the accuracy of ± 1-dB and ± 2-dB, while the limit is relaxed at extreme temperatures. The initial accuracy of logarithmic amplifier can meet most standards without fine adjustment. Nevertheless, logarithmic amplifiers have many obvious advantages, which exceed the RF power management requirements determined by different standards.
How can MCU compensate errors
As discussed earlier, MCU can effectively adjust the transmission power by using the bias voltage of the transmission signal channel. By adding temperature sensors, MCU can further improve the accuracy of RF power management system. As long as the detector has repeatable temperature drift, error compensation for some measured values can be realized. The compensation algorithm program considering environmental changes can be integrated into the decision program of MCU to significantly reduce or eliminate manufacturing process and temperature changes. For example, if a power detector has a repeatable temperature drift, a compensation algorithm can be used to eliminate the expected error at a known temperature.
Fig. 6 shows a log consistency error curve for many log amplifiers. At 3.5 GHz, the temperature drift extends from + 1 dB to - 4 dB. The overall distribution curve of the device at - 40 ° C follows the curve at 25 ° C. On the contrary, the distribution curve at + 85 ° C moves by 2.5 dB and is no longer parallel to the distribution curve at 25 ° C. Although the temperature drift at this frequency is large, the distribution at each specific temperature remains very close. Due to the repeatability of these drifts, a compensation scheme can be realized to significantly improve the accuracy.
Figure 6. The temperature drift distribution curve at + 85 ° C at 3.5 GHz moves and is no longer parallel to the distribution curve at 25 ° C.
The error model for this temperature is represented by the trend line in the linear region of the log consistency curve at + 85 ° C.
Temperature drift is caused by the change of slope and intercept with temperature. In view of this understanding, an error model can be summarized by analyzing the overall distribution curve of the device. The error expression of the distribution curve moving with temperature can be established, as shown in Figure 6. The trend line error line drawn in the figure through the linear area of the log consistency curve at + 85 ° C represents the error model at + 85 ° C. Using the slope and intercept characteristics of the error line, this temperature change can be offset by using the compensation function relationship. Nevertheless, the error model only describes the error caused by temperature drift at + 85 ° C.
Most of the temperature drift occurs between + 25 ° C and + 85 ° C. For the error function that is generally applicable to all temperatures, a temperature scale factor K (T) can be used to establish the functional relationship of various temperature ranges, where K (T) is a function of temperature. Combining the compensation error function and the temperature scale factor function, the combination result is shown in Figure 7. When the temperature rises, the scale factor will change, so that the error caused by the rise of temperature drift can be eliminated.
Fig. 7 shows the log consistency distribution of ad8312 using the above error compensation method. Before error compensation, the log consistency error is 5 dB. After error compensation, the logarithmic consistency error is increased to about ± 0.5 dB over the entire operating temperature range from - 30 DBM to 0 DBM power input range. The achievable accuracy of this RF power management system is determined by the overall distribution curve of the device. The same results can also be applied to low temperature and low frequency cases where the temperature drift is not significant.
During the semiconductor manufacturing process, some parameters are changing, such as thin layer resistance, capacitance and capacitance β Value. All these parameters will affect the slope, intercept and temperature performance of the logarithmic amplifier. One way to reduce the influence caused by the change of manufacturing process parameters is to use laser fine-tuning logarithmic amplifier. Fig. 8 shows the logarithmic consistency error distribution curve of a laser fine tuned 60 dB logarithmic amplifier at 1.9 GHz. The device does not use digital compensation, but analog compensation, that is, the built-in temperature circuit and external resistor are used to optimize the temperature performance. The resistance value depends on the value required by the correction factor. The function of this analog compensation circuit can make the measurement result deviate from the center value of the overall distribution curve by ± 0.5-db.
conclusion
Using precise RF power management, base stations and mobile phone transmitters can benefit from power amplifier protection and reduce power consumption, which far exceeds the requirements of cellular standards. Using stable logarithmic amplifier and temperature sensor, MCU can compensate temperature drift error to improve the overall accuracy of RF power management system. Logarithmic amplifiers are closely related to temperature distribution, so simple error compensation is allowed. Two point calibration for moderate temperature drift enables accurate RF power management with an accuracy of ± 0.5-db over the entire temperature range.
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