Analog Microcontroller Forms Heart of Low-Cost, High-Efficiency PA Monitor
ADI has three options
for implementing a PA
monitor:1 a discrete solution, an integrated
solution based on the AD72942
This article describes a reference design—based on the ADuC7026—that realizes the functions of monitoring and controlling the power amplifier, including configuring the output power, monitoring the voltage standing wave ratio (VSWR), monitoring the laterally diffused metal oxide semiconductor (LDMOS) drain current and temperature, and signaling an alarm when a parameter is over a predefined threshold.
Figure 1. System block diagram.
PA Monitor Module
Figure 2 shows the functional block diagram of the temperature monitor, which uses the ADT75 digital temperature sensor to monitor the temperature of the two LDMOS stages. Available in 8-lead MSOP and SOIC packages, the ADT75 digitizes the temperature to a resolution of 0.0625°C. Its shutdown mode reduces the typical supply current to 3 µA.
Figure 2. Temperature monitor functional block diagram.
Figure 3 shows the flow chart for the temperature monitoring routine. After receiving the temperature detection command, the ADuC7026 MCU sets the temperature detection flag. Next, it reads the temperature data from the ADT75 via the I2C® bus and transmits the data to the PC. Then, it looks at the ADT75’s overtemperature pin (OS/ALERT) and turns on an LED if the temperature is over the threshold. If the ADuC7026 receives the configure temperature threshold command, it reads the configuration data from the PC and writes the threshold temperature to the ADT75 via the I2C bus. If the microcontroller receives the read temperature threshold command, it reads the threshold temperature from the ADT75 and transmits it to the PC.
Figure 3. Flow chart for temperature monitoring routine.
Current Monitoring: The PA performance can be improved by keeping the LDMOS drain current stable over time and temperature; this also maintains the output power in the expected range. The temperature and high-voltage supply influence the drain current; the high-voltage supply also influences the drain voltage. The LDMOS drain current can be measured with a high-voltage current-shunt monitor. If the drain current is continuously monitored, an operator can readjust the gate voltage when a voltage surge occurs on the high-voltage supply to keep the LDMOS working at the best operating point.
Figure 4 shows the functional block diagram of the current monitor, which uses the AD8211 precision high-voltage current-shunt amplifier to sample the drain current of the two LDMOS stages in the PA module. The AD8211 features a fixed gain of 20 V/V, with a typical ±0.5% gain error over the entire temperature range. The buffered output voltage, which directly interfaces with typical analog-to-digital converters, is sampled by the ADuC7026’s on-chip ADC. The drain current threshold is set by the AD5243 digital potentiometer, which is controlled by the ADuC7026 via the I2C bus. The output of the ADCMP600 comparator determines when the drain current is over or under the threshold. The system lights the corresponding LED to alert the operator if the drain current is over the threshold.
Figure 4. Current monitor functional block diagram.
VSWR Monitoring: VSWR, a key parameter in antenna systems, provides a measure of the match between all of the elements in the antenna system. Reverse power influences the PA’s output power, and the transmitted signal is distorted if it is too high. So, it is necessary to monitor VSWR to maintain optimal performance in base stations.
Figure 5 shows the functional block diagram of the VSWR monitor, which uses a bidirectional coupler and the AD8364 dual TruPwrTM detector to measure forward- and reverse power. The AD8364 two-channel, true-rms-responding RF power-measurement subsystem precisely measures and controls the signal power. Its flexibility allows easy monitoring and control of RF power amplifiers, radio transceiver AGC circuits, and other communications systems. The AD8364 output can be used to calculate VSWR and monitor the match on the transmission line. A large VSWR indicates a problem with the antenna, so the operator should protect the system by adjusting the PA gain or the power supply voltage.
Figure 5. VSWR monitor functional block diagram.
Autopower Control: The transmitter must ensure that the power amplifier can satisfy the system requirements and hold the output power at the desired value—high enough to cover the expected area but low enough to keep from influencing another base station. It should provide overpower protection to ensure that the power amplifier does not operate in an overpower condition, which can cause the power amplifier to saturate and the signal to have nonlinear distortion. For these reasons, the output power must be tested and controlled to make sure it remains steady.
Figure 6 shows the functional block diagram of the autopower control loop, which comprises the bidirectional coupler, TruPwr detector, microcontroller, and voltage-variable attenuator. The bidirectional coupler transfers the forward power to the TruPwr detector, which tracks the change in signal amplitude. The ADuC7026’s on-chip ADC samples the output. The microcontroller compares the actual output power with the expected power and uses a proportional-integral-derivative (PID) algorithm to adjust the control voltage error, making the power amplifier operate at the point of best performance.
Figure 6. Autopower control functional block diagram.
Figure 7 shows a flow chart for the PID algorithm. First, the program initializes the control parameter Kp, Ki, and Kd and sets the expected output power. Next, the ADC samples the output of the AD8364. The sampled data is then filtered and converted to power. Then, the difference between the expected output power and the actual output power, the next expected sample value, and the control voltage are calculated according to the system transfer function, and the DAC registers are configured. This completes one cycle of the sample and control process, which then continues in a circular manner.
Figure 7. Flow chart for PID algorithm.
Figure 8. Flow chart for controlling the UI.
Figure 9 shows the temperature test result. A user can change the high- and low-temperature thresholds at any time. In the example, the high threshold is changed from 35°C to 31°C. When the ambient temperature rises above the new threshold, the overtemperature alarm light turns red and the PC sounds a continuous alarm.
Figure 9. Interface for showing temperature test result.
Figure 10. Hardware connection for PA monitor demo.
2Data sheets and additional product information on all ADI products can be found at www.analog.com.
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