2.5G and 3G base station power amplifier monitoring

Cellular communications program development and advanced modulation closer ties. In the latest generation (2.5G and 3G) base stations, the design strategy including the achievement of high linearity to minimize power consumption while the method. For example, by monitoring the performance of base station power amplifier, power amplifier can (PA) output power maximization, and the best linearity and efficiency. Fortunately, the use of tailor-made for this purpose discrete integrated circuit (IC), can be very simple to monitor the PA's output level.

Wireless base station power consumption, linearity, efficiency and cost performance depends primarily on PA and the signal chain. Silicon laterally diffused metal oxide semiconductor (LDMOS) transistors have the advantage of low cost and high power, very suitable for the design of modern cellular base station PA. On the linearity, efficiency and gain a comprehensive trade-offs can best maintain the LDMOS PA transistor bias conditions.

For environmental reasons, the base station power efficiency optimization of telecommunications companies are also major considerations. To reduce the base station's total energy consumption, reduce their impact on the environment, the industry is doing a lot of effort. The daily operating cost base is mainly derived from electricity consumption, which, PA power consumption may account for more than half, therefore, optimize the power efficiency of PA can improve the operating performance of the base station, environmental protection and help reduce the cost.

Control the drain bias current, so that changes in temperature and time constant, which can significantly enhance the PA's overall performance, while ensuring that the output power level to keep within the specified scope. Control gate bias current is one way to put it together with a fixed resistive divider in the testing / evaluation phase of optimization of gate voltage.

While such a fixed gate voltage Jiejue quite cost-effective program, but it has a major drawback, that is, not taking into account the environment 變化, manufacturing Dikuozhanhuo power Dianya variation. Using a high-resolution analog converter (DAC) or a low-resolution digital sub-pressure meter to dynamically control the PA gate voltage, output power can be better controlled. Use of user-programmable-gate voltage, even if voltage, temperature and other environmental parameters change, PA is also able to maintain its optimum bias conditions.

Affect PA drain bias current of the two major factors that PA's high-voltage power line changes and on-chip temperature. PA transistor's drain voltage is vulnerable to high voltage power line change. By using a high current (I) sense amplifier to accurately measure high-voltage power line current, we can monitor the PA's drain voltage of the transistor. Full scale reading from a external sense resistor (R) set. In monitoring high current applications, the sense resistor's power dissipation must be I2R. If the resistance exceeds the rated power, its value may be offset or completely wrong, resulting in termination device on the differential voltage exceeds the absolute maximum ratings.

The measured voltage to the output of current sensor that can be more input to the ADC (ADC) in order to generate the necessary control of digital information. Be taken to ensure that current sensor output voltage as close as possible the maximum analog input range of ADC. Through continuous monitoring of high voltage lines, power supply line when the sensing voltage surge occurs, the power amplifier can be re-regulate their gate voltage, thereby maintaining an optimum bias conditions.

LDMOS transistor's drain-source current IDS, there are two items related to temperature, that is, effective electron mobility μ and threshold voltage Vth:

2.5G and 3G base station power amplifier control

Threshold voltage and effective electron mobility decreases with increasing temperature. Therefore, the temperature change will lead to changes in output power. Through the use of one or more discrete temperature sensor to measure the temperature of the PA, you can monitor the board temperature. To meet the system requirements, from analog voltage output temperature sensor to a single digital output temperature sensor, Inter-IC bus (I2C), and even the serial peripheral interface (SPI) control, with a broad range of discrete temperature sensor available .

The temperature sensor's output over input to the ADC in to the temperature data is converted to digital data used for monitoring (Figure 1). According to the system configuration, the board may need to use a large number of temperature sensors. For example, if the PA were used more than one or more pre-drive front-end needs so that each amplifier with a temperature sensor can provide more control on the system. In this case, requires a multi-channel ADC to convert the analog output temperature sensor. At present, various types of ADC has a built-in overflow (out-of-range) warning function, when the input exceeds the time limit has been set will give a warning. Link in the PA signal, this function of temperature and current sensors for monitoring readings of great significance. High limit and low limits can be set in advance, and only when exceeding these limits produce warning messages. Such designs also generally register with the lag. If the limit exceeded, the warning mark this register determine the reset point. Delay registers to prevent temperature or current sensor readings trigger the warning mark row. For example, Analog Devices Inc. AD7992, AD7994 and the AD7998 12-bit low-power ADC I2C interface to carry the overflow limit on the indicator, also each offer 2,4 and 8 channel power handling.

Use of control logic circuit, can be from the current mix of sensors and temperature sensors for continuous monitoring of digital information. Number of partial pressure by meter or DAC to dynamically control the PA gate voltage, while monitoring the sensor readings, can maintain an optimal bias conditions. DAC's resolution will be needed to control the gate voltage level to decide. In the base station design, telecommunications companies generally adopt a number of PA (Figure 2), because then, for each radio frequency (RF) carrier selection when PA greater flexibility. For each PA can be optimized to a particular modulation scheme. Then PA in parallel together, we can provide higher linearity and higher overall efficiency. In this case, PA may require multiple cascaded gain stages, including the variable gain amplifier (VGA) and the front drive-level, to meet the gain and efficiency requirements. Multi-channel DAC module to meet these different level settings (level-setting) and gain control requirements.

2.5G and 3G base station power amplifier monitoring

PA in order to achieve accurate gate control, DAC, such as Analog Devices Inc. AD5321, AD5627 and AD5625 on each with 12-bit, single, double and four outputs. These components have a very good source / sink function (source and sink capabilities), in most applications can be no output buffer. Low-power, monotonicity and stability to ensure shorter time to edge, to achieve accurate level setting applications.

If precision is not the main specification, and can accept 8-bit resolution, then the total number of partial pressure is more cost-effective choice. Partial pressure meter with digital and mechanical divider or variable resistor the same electronic adjustment function, but also provides higher resolution, solid-state reliability, and excellent temperature performance. Non-volatile, one-time programmable (OTP) Digital meter is suitable for partial pressure of time division duplex (TDD) RF applications at this time, PA during the TDD receiver off, the launch of a fixed gate voltage during the conduction through. This pre-programmed start-up voltage in the PA transistor can be reduced into the launch phase of conduction delay and improve efficiency. PA during the shutdown at the receiving capacity of the transistor to avoid firing received signal noise. This technology can also improve the overall efficiency of PA. According to the channel number, interface type, resolution, and non-volatile memory requirements, a large number of total number of partial pressure of choice for such applications. 256-slip point, one-time programmable, dual-channel I2C partial pressure meter, such as Analog Devices's AD5172, very suitable for RF amplifiers in level set applications.

By PA on the complex RF output signal power level of accurate measurement of amplifier gain can be better controlled to optimize the device efficiency and linearity. Using root mean square (RMS) power detector, available from WCDMA, EDGE and UMTS cellular base station RF signal to extract precise rms power levels.

Figure 3 shows a simple control loop, in which the output power detector and the PA gain control terminal connected. Based on output voltage VOUT and the RF input signal is set between the relationship between the VOUT voltage regulator power detector (here, VOUT is the error amplifier's output), until the RF input level and the load control voltage VSET match. With ADC, a complete feedback loop, which can track the power detector output, and adjust its VSET input. This method can be used to gain control voltage variable amplifier (VVA) and VGA, which mixed-signal links with the number in front of class. To measure the transmission and reception power, two power detectors can be used simultaneously measure two complex input signals. In the VGA or the pre-driver amplifier in the PA system before, only need a power detector. At this time, a device gain is fixed, and VOUT to provide control input to another device.

2.5G and 3G base station power amplifier monitoring

In the high-voltage power supply line to the sensor to voltage spikes, or excessive current circumstances, some applications may not be approaching digital control loop to prevent damage to the device. Digital control loop including: use of current sensing, analog-digital conversion to high-current sensing, as well as by external control logic processing digital data. Determine if the outlet on the current loop is too large, it will send a command to the DAC to reduce the gate voltage or power off the part.

Can use the analog comparator through a RF switch to control the RF signal input to the PA (Figure 4). If the power supply line to the high-current sensing, RF signal can be shutdown to prevent damage to PA. Analog comparator means does not require digital processing technology, the control loop is much faster. Sensing the output voltage can be set directly with the DAC to compare a fixed voltage. When the voltage is high, in sensing the output of the generated fixed voltage, the comparator can trigger the RF switch on a control pin, almost immediately cut off the gate of the RF input signal to the PA.

2.5G and 3G base station power amplifier control

Figure 5 shows a typical use of discrete components PA control structure. The only monitoring the amplifier is PA itself; However, the signal chain of any one amplifier can be handled in this way. All of these discrete components are out of work with a data bus, here is the I2C data bus, and through a host controller to be controlled to minimize the number of components, complexity and cost.

2.5G and 3G base station power amplifier monitoring

From the design point of view, using discrete components to monitor the base station is the main advantage of PA custom large selection of products. PA PA front-end vendors link design more complex, contains a variety of gain and control technology. Existing multi-channel ADC and DAC are very suitable for dealing with different cellular base station system and architecture division, so that the base station designers to implement cost-effective distributed control.