The demand for high data rates is driving the migration of mobile communication systems from 2G to 3G. The higher data rates in these systems add more performance and specifications to the mobile phone RF design.
In order to achieve the highest bandwidth efficiency in the occupied frequency band, the third generation mobile communication system adopts a linear modulation method that improves spectrum utilization efficiency, such as quadrature phase shift keying, 8-phase shift keying, and quadrature amplitude modulation.
These non-stationary envelope modulation methods require the use of a linear RF power amplifier in the transmission path to maintain good adjacent channel power rejection ratio (ACPR) and error vector modulation (EVM) characteristics. A typical linear RF power amplifier used in CDMA uses a Class A structure to meet linearity requirements. Class A amplifiers in textbooks have an efficiency of approximately 30% at 1dB compression point. Class A amplifiers operate at less than 1dB compression point and their power efficiency decreases.
In IS-95 and CDMA2000 systems, RF power amplifiers typically operate at a backoff of 6dB to 40dB from peak power or 1dB compression point. (This means it operates from 6dB to 40dB below the 1dB compression point.) Therefore, RF power amplifiers are in very low efficiency most of the time. However, RF power amplifiers are the largest power consuming component in mobile phones. Studies have shown that in conventional mobile phone applications, the RF power amplifier consumes 20% to 40% of the total power consumption.
Therefore, we know the extreme importance of reducing RF power consumption, so that we can achieve longer mobile phone battery life, or longer talk time.
This paper presents a simple power tracking technique to improve the efficiency of RF power amplifiers. The technology uses a dB linear RF power detector, LMV225, and a DC-DC converter switch. This improved method switches the DC supply voltage (VCC) of the RF power amplifier at two different output power levels through a DC-DC converter. The RF power detector LMV225 determines the supply voltage of the RF power amplifier. Off-the-shelf CDMA2000 RF power amplifiers can use this technology to increase the energy efficiency of mobile phones.
RF power amplifierRF power amplifiers are at the heart of such applications. The RF power amplifier together with the DC-DC converter constitutes an efficiency improvement circuit for the power amplifier. The SKY77152 is a very popular CDMA2000 RF power amplifier product on the market. According to its product specification, it can increase power efficiency by more than 40% near the 1dB compression point, PAE.
CDMA RF power amplifiers typically have two supply voltage pins, VCC and VBIAS, as shown in Figure 1. There is also a reference voltage pin, commonly referred to as VREF. In any case, VREF must be 2.85V. The power amplifier can be turned off by setting VREF to ground. Most CDMA RF power amplifiers have two modes of operation: high power mode and low power mode. The VCONT pin can be used to set the operating mode of the power amplifier. When the RF output signal is high, the CDMA RF power amplifier needs to operate in high power mode to maintain proper distortion performance. If the output signal level is relatively low, the CDMA RF power amplifier can be switched to low power mode. However, this switching has the side effect that the phase shifts of the two signal paths are different. This can cause problems with baseband processing and correction.
Figure 2 shows the typical POUT and PIN characteristics of a CDMA RF power amplifier when both DC supply voltages VCC and VBIAS are reduced. The figure shows that the output RF power can be obtained by reducing the DC supply voltage of the RF power amplifier.
Power increase efficiencyDC to RF efficiency (or power increase efficiency, PAE) is defined as follows:
Although all RF power amplifier manufacturers use the power amplifier's peak DC to RF efficiency for maximum output power, in reality the RF power amplifier itself rarely operates at this peak power level. In mobile applications, high peak power plays an important role in dissipating heat. On the other hand, when the output RF power is low, the PAE of the RF power amplifier also drops.
In battery-powered handsets, the probability distribution of the output RF power (shown in Figure 3) should be used to estimate the average efficiency of the mobile system. This average efficiency represents the ability of the mobile system to convert battery energy to available transmit power in actual operation.
As shown in Figure 3, most of the time, the output power of the RF power amplifier of the IS-95 mobile phone is less than POUT = +15dBm. Therefore, it makes sense to increase the PAE of the RF power amplifier at small signal levels.
Equation 1 and Equation 2 show an idea that the DC power consumption PDC can be reduced by lowering the supply voltage VCC of the RF power amplifier.
It seems that improving the PAE of the RF power amplifier seems to be a very simple matter, but there are several important specifications to consider when reducing the RF power amplifier supply voltage. They include ACPR, EVM, and conversion time from one supply voltage to another.
Adjacent channel power suppression ratioThe adjacent channel power rejection ratio (ie, ACPR) is defined as the ratio of the average power of a certain offset frequency to the average power of the transmission frequency. Table 1 shows the performance requirements of CDMA2000. Although the empty intermediaries of IS-95 or IS-98 do not have the formal requirements of ACPR as CDMA2000, mobile RF designers are advised to check whether their designs meet the specifications in Table 1. A bad ACPR value indicates that the transmission path is not linear enough, so the RF signal is distorted before entering the receiver of the base station.
Table 1: Adjacent Channel Power Rejection Ratio (ACPR)
Empty intermediary
frequency
bandwidth
Offset frequency @ ACPR1
Offset frequency @ ACPR2
Measurement ResoluTIon Bandwidth
IS-95
824-849MHz
1.25MHz
±885KHz
±1.98MHz
30KHz
PCS
1850-1910MHz
1.25MHz
±1.25MHz
±1.98MHz
30KHz
ACPR1=-42dBc And ACPR2=-54dBc
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