When designing a wireless transmitter, the measurement and control of RF power is a key consideration. High-power RF amplifiers (PA) rarely work in open loop mode, that is, the power sent to the antenna port is not monitored in some way. However, regulations such as transmission power, network robustness, and coexistence with other wireless networks require strict control of transmission power. In addition to these external requirements, precise RF power control can improve spectrum performance and save the cost and power consumption of transmitter power amplifiers. In order to adjust the transmission power, it may be necessary to perform some form of calibration on the output power of the power amplifier at the factory. For different complexity and effectiveness, the calibration algorithm varies greatly. This application note describes how to implement a typical RF power control scheme and compares the effectiveness and efficiency of various factory calibration algorithms.
Typical wireless transmitter with integrated power control
As shown in Figure 1, this is a typical wireless transmitter block diagram that integrates transmit power measurement and control functions. By using a directional coupler, a small part of the signal of the PA is fed back to the RF detector. In this case, the location of the coupler is generally close to the antenna, behind the duplexer and isolator, so the power loss associated with these devices needs to be considered during the calibration process. The typical value of the coupling coefficient of the directional coupler is 20 dB~30 dB, so the feedback signal of the coupler is 20 dB~30 dB lower than the signal sent to the antenna port. Coupling signal power in this way will result in power loss in the transmit path, which is usually a few tenths of a decibel. In wireless infrastructure applications, the typical range of maximum transmit power is 30 dBm~50 dBm (1W~100W). For RF detectors that measure transmit power, the signal of the directional coupler is still too strong. Therefore, additional signal attenuation is required between the coupler and the RF detector. The power detection range of modern rms and non-rms response RF detectors is about 30dB to 100dB, and the output is stable relative to temperature and frequency changes. In most applications, the output of the detector is converted into a digital quantity through an analog-to-digital converter (ADC), and the code obtained from the ADC is converted into a reading of the transmission power using the calibration coefficient stored in the non-volatile memory (EEPROM) . Compare this power reading with the setpoint power level, and if there is a difference between the setpoint power and the measured power, then a power adjustment should be made. This power adjustment can be done at any one of multiple points in the signal chain. Such as adjusting the amplitude of the baseband data, adjusting the variable gain amplifier (at the IF or RF side), or changing the gain of the PA. In this way, the gain control loop adjusts itself and keeps the transmit power within the required range. It is important to point out that the gain control transfer functions of VVA and PA are often non-linear. Therefore, the actual gain change obtained by a given gain adjustment is uncertain, so a control loop is needed that can provide information about the execution The adjustment feedback information, and the guidance information for the subsequent repeated operation process.
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