Digital Closed-Loop Power Amplifier Control System Based on DSP

This paper proposes a digital closed-loop power amplifier control system based on DSP. It determines the best step for the output power to rise or fall by detecting the positive and negative power voltages, increases or decreases the output power of the power amplifier according to the step, and detects whether the output power exceeds the rated output power at any time. If it exceeds the rated output power, it makes corresponding adjustments according to the maximum drop step, and reduces the output power of the power amplifier to the rated power in time, thus forming a closed-loop control system. Since DSP is used as the processing core, the processing speed and calculation accuracy are guaranteed. This can not only ensure the safety and reliability of the power amplifier, but also make the power amplifier output the maximum power that can be transmitted as much as possible.

　　System hardware structure

　　The whole system is composed of C5509A, AD9857 and AD7655. The system block diagram is shown in Figure 1.

　　Figure 1 System structure diagram

　　As can be seen from Figure 1, DSP is the core control unit; AD9857 is used to transmit data, perform up-conversion, and output analog data to the power amplifier, which is then transmitted through the antenna. The control signal of AD9857 is realized by the SPI interface of DSP. AD7655 is responsible for collecting the forward and reverse voltage values ​​of the power amplifier and sending the voltage values ​​back to DSP. DSP controls the power amplifier according to the voltage values. SRAM stores the transmitted baseband data and the intermediate variables of the transmitted baseband data calculated in real time. Flash saves the programs required by DSP for DSP to call when it is powered on; HPI port is used between DSP and PC, and PCI bridge chip is used to realize the connection with PC. DSP connects AD9857, AD7655, SRAM and Flash through DSP's EMIF interface. EMIF interface is the external memory interface (External Memory Interface) of DSP, which can be easily connected to external Flash, asynchronous SRAM and other devices. The data communication rate of external devices in this system is relatively low, with the highest data transmission rate of 48Mb/s of AD9857. The EMIF interface of C5509A is suitable to meet its speed requirements.

　　Closed-loop control algorithm

　　The typical control method for the transmitter power amplifier output power is to adjust the transmitter output power according to the standing wave ratio. The numerical relationship between the transmitter's forward and reverse power detection voltages and the power amplifier output power and standing wave ratio is as follows.

Forward power detection voltage:

　　(1)

　　Reverse power detection voltage:

　　(2)

　　VSWR:

　　(3)

　　Wherein, VF_Full represents the forward power detection voltage corresponding to the maximum rated power output of the power amplifier, PFWD represents the forward power output of the power amplifier, PREV represents the reverse power, and PF_Full represents the maximum rated power output of the power amplifier.

　　The maximum increase or decrease step allowed for the power amplifier output power is calculated based on the forward and reverse power detection voltages to control the power amplifier output power.

　　Assume PR_Full is the maximum reverse power that the power amplifier can withstand, and PR_Full=b (constant); POUT is the output power when the reverse voltage reaches VR_Limit; the corresponding forward power detection voltage VF_Full=a (constant) when the rated power is output; PF_Full/PR_Full=n (constant). These three constants are determined by the indicators of the power amplifier.

　　According to formula (3), when the reverse voltage reaches VR_Limit, the standing wave ratio is as follows:

　　(4)

　　(5)

　　Since the maximum value of POUT is b, we have:

　　(6)

　　when

, that is to say, the rated power is output at this time.

　　According to the above discussion, the rated power output of the power amplifier is limited by the ratio n of the power amplifier's PF_Full/PR_Full. Under a certain standing wave ratio, the power amplifier cannot output the rated power. For example, when the standing wave ratio is 3 and n is 3, the rated power cannot be output. At this time, the power that the power amplifier can output will be less than the rated power, but what is the maximum power that can be output? According to formula (5), Figure 2 draws a power control curve. The output rated power is set to 0dB in the figure. For comparison, the figure shows the curves of n=9 and n=5 and the linear control method.

　　Figure 2 Power control curve

The linear control method is a linear proportional control method directly based on the standing wave ratio. Linear control does not consider the situation of the power amplifier itself, so its control is simple and relatively easy to implement. However, the control is rough, and for a good quality power amplifier, its performance is not fully utilized, and for a poor quality power amplifier, it is easy to burn the power amplifier.

　　As shown in Figure 2, when n=9, the power amplifier has poor indicators and cannot be used under normal circumstances. Especially when using the linear control method, the power amplifier will burn out if it works for a long time. This is because the linear control method outputs the rated power when the standing wave ratio is equal to 2. After using the closed-loop control algorithm, since n=9, the rated power cannot be output when the standing wave ratio is equal to 2, ensuring that the power amplifier will not burn out.

　　As shown in Figure 2, when n=5, the power amplifier has better indicators. Using the linear control method, it still strictly abides by the oblique line in the figure. The power output of the closed-loop control method is obviously larger, which fully utilizes the characteristics of the power amplifier.

　　In actual working conditions, when the standing wave ratio is greater than 4, both the power amplifier and the antenna will deteriorate significantly. At this time, in order to ensure the stability of the system, the curve will be directly set to -12dB to maintain the output of the power amplifier at a lower level.

　　experiment

　　In order to verify the effectiveness and practicality of the method proposed in this paper, the method is applied to shortwave and ultra-shortwave radio stations, and the power control convergence curve is shown in Figure 3. It can be seen from the figure that the power control reaches a stable state after about 10 steps. After stabilization, the curve fluctuates less and the power output is almost constant.

　　Figure 3 Power control convergence curve

　　Summarize

　　This article introduces the closed-loop control algorithm for controlling the output power of the transmitter power amplifier, as well as the system composition. The output power of the power amplifier is controlled by calculating the forward and reverse power detection voltages and determining the maximum step size of the power amplifier output power increase or decrease. The entire system is implemented using DSP and AD9857. After shortwave and ultra-shortwave system tests, the power amplifier output efficiency based on the closed-loop algorithm is significantly higher than other typical linear control algorithms. While ensuring the safety of the power amplifier, it has practical value in improving the efficiency of the power amplifier.