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Large signal polar modulation is only one approach used in EDGE systems today. EDGE modulation contains amplitude and phase information with the amplitude having a 3.2 dB peak to average ratio and a 14 dB valley. This requires the use of either a linear PA, which may not be desired due to current drain on the system or a large signal polar modulator system as described below. The basic idea of this architecture is to use the PA as an amplitude modulator in the system. Figure 1 is a diagram of a polar modulation system taken from the RFMD RF6003 datasheet.

The cordic separates the I and Q information into amplitude and phase components. These two components are then corrected to account for the distortion in the phase locked loop (PLL) and mainly the distortion in the PA. The information is then adjusted in time to align the amplitude and phase information so that the modulation is correct at the output of the PA. The success of this technique is largely dependent on the distortion of the PA and how this distortion is then used to pre-distort the amplitude and phase information in the system. The major advantage of this type of system is that a highly efficient saturated PA can be used to transmit both EDGE and Gaussian minimum shift keying (GMSK) modulation. In other types of systems, a linear PA is used to transmit the EDGE modulation due to the 3.2 dB peak that is required, and often the global system for mobile communications (GSM) efficiency is compromised. Lower efficiencies result in more current drain and less battery life to the user, which is undesirable. Also, linear architectures require filtering to reduce the intermodulation products that often occur in traditional mixing of signals. Since the PA is used as the modulator in a large signal polar modulation system, a brief introduction to the power control architecture will be discussed next.

Voltage control power control allows for the PA to be used as a modulator due to the linear transfer function of the input voltage to the RF output voltage. To accomplish this, the voltage applied to the collectors of the RF transistors is supplied by regulators. This allows for the power to be accurately controlled by regulating the collector voltage of the PA. Figure 2 is a simplified schematic of the PA that is used in a large signal polar modulation application.

Equation 1 demonstrates that the power can be effectively controlled by regulating V_CC to the collectors of the PA.

In this type of complex system there are several factors that can affect the overall system performance. Outside of the normal PA specifications such as efficiency, power, stability and other considerations, the designer has other requirements that are unique to large signal polar modulation. One of these requirements is group delay of the PA. If the reader recalls from Figure 1, the amplitude and phase information must be time aligned in order for the modulation to be accurately combined at the PA output. Group delay of the PA is the dominant factor in this area of distortion. The group delay of the PA is the time delay between the input control voltage of the PA to the RF output over the modulation frequency range. This time alignment has to be constant over temp, voltage, process and more for the amplitude and phase information to be combined accurately. Any misalignment that may occur will cause degradation in output RF spectrum performance (ORFS).

Other requirements that are unique to large signal polar modulation are AM/AM and AM/PM distortion. AM/AM distortion is described as the non-linear transfer function between a control voltage input in volts vs. the output RF voltage. As mentioned earlier, voltage control architecture allows for a very linear response that is easily curve fit, which allows for easy pre-distortion in the system. Figure 3 is an AM/AM plot of an amplifier with this form of power control.

Probably the most difficult distortion in PAs for designers is the AM/PM distortion. Saturated PAs are very non-linear due to their low bias points and higher efficiencies, which cause AM/PM distortion in the amplifier. AM/PM is described as the S21 phase vs. input control voltage of the PA. Figure 4 is a plot of the AM/PM curve for an RFMD RF 3169 PA over frequency.

The AM/PM response is different from the linear AM/AM curves in Figure 3. This is a problem that designers struggle with in polar modulation systems. Depending on the system and the amount of memory available for pre-distorting the amplitude and phase information, the curve shape will result in degradation in pre-distortion error. This problem can be either alleviated or compounded depending on the system and the amount of memory dedicated for the pre-distortion. In this particular system from RFMD, a polynomial is used to curve fitthe distortion in the PA. The AM/PM curve has to be designed to fit the system's pre-distortion capabilities. The EDGE modulation has a 14 dB valley and a 3.2 dB peak Per the modulation standard, if the transmitter is operated at 26 dBm in high band, the modulation will need to be tracked from 29.2 dBm down to 12 dBm. Looking at the AM/AM and AM/PM curve, this will place the valley of the modulation to the left of the inflection point of the AM/PM curve. This can cause error in the system, as the AM/PM curve may not accurately be fitted by the polynomial. The design challenge is to modify the AM/PM curve so that it will work in the given system with little or no degradation in ORFS due to error in the pre-distortion. In order to accomplish this, the number of inflection points must be decreased. Since the area of concern is at lower power levels the focus needs to be on the first and second stage of the PA as these two stages have the most effect at lower power levels. One aspect that greatly affects the AM/PM curve is by modifying the V_CC collector voltages that are applied from the regulators. The AM/PM curve in Figure 4 was generated using the V_CC profiles from the regulators in Figure 5. This is a plot of output voltage vs. input voltage to the collectors of the PA.

Since the focus of this experiment is on the first and second stage the designer has flexibility in the responses that are applied to the collectors. For example, the first stage is not limited by voltage on the collector due to the lower power levels until V_CC1 is below 1.8 V, therefore, voltages above 1.8 V do not affect the output power of the first stage. Since there is no need to reach more than 2.0 V on the Q1 collector, the ramp gain from the regulators can be modified to have lower gain or higher gain in the op-amps. The goal of the AM/PM curve is to decrease the number of inflection points and to move the main inflection point at lower Vramp values to the left thus pushing the inflection point out of the modulation during the highest PCL. Modifying the V_CC profiles effects the AM/AM and AM/PM curve in such a way that the designer has control over what power this inflection point occurs. To conduct this experiment a prototype board with external op-amps and potentiometers were built to modify the collector voltages on Q1 and Q2. A schematic of a simple experimental board is illustrated in Figure 6.

Using a network analyzer and sweeping the input voltage to the PA, the pot can be adjusted for ramp gain, turn-on voltage, etc., to optimize the AM/PM curve shape. With this configuration, the AM/AM and AM/PM curve shape can be changed, therefore, optimization can be done in real time by looking at the network analyzer and adjusting each collector profile. In this example, Figure 7 shows the Q1 and Q2 profiles that were determined to offer advantages in the AM/PM curve shape.

Using the modified collector profiles in Figure 7, the AM/PM curve shape is plotted in Figure 8.

If the reader compares Figure 8 with Figure 4, you can see how the inflection point is changed from a sharp corner to a soft, rounded shape. Figure 8 is a much easier curve to fit and accurately pre-distort the data, which will result in much better system performance. To prove the concept, system simulations can be run to predict output RF spectrum performance (ORFS) based on curve fit error associated with the AM/AM and AM/PM distortion of the PA alone. Figures 9 and 10 show the performance of the PA alone and the effectiveness of the pre-distortion based on curve shapes alone. The curves used in this simulation are the original curve shapes illustrated earlier in Figure 4 at 1910 MHz.

The data in Figures 9, 10 and Table 1 show the performance of the PA before any modifications were made to the design. Figures 11, 12 and Table 2 illustrate the PA performance with the modified collector profiles.

Comparing the data of the PA before optimizing the collector profiles reveals the delta in performance that can be achieved due to the PA. An improvement of 5 dB on the ORFS due to modulation and 3 dB to 4 dB in the switching spectrum by optimizing the collector profiles. This same approach can also be used for over-temperature performance as well. When the PA's AM/AM and AM/PM curve changes over temperature, then ORFS degradation due to error in the pre-distortion will occur. Using the same principles outlined above, the collector profiles can be modified over temperature to keep the AM/AM and AM/PM curves more constant over temperature. This will result in minimal or no degradation over temperature, therefore allowing for more total system margin to the specifications.

AM/AM and AM/PM distortion is a major design challenge for PA designers who are tasked with designing PAs to be used in open loop large signal polar modulation systems. The methods presented here are effective in improving AM/PM curve shapes without sacrificing normal PA specifications. EDGE modulation is gaining in popularity and will be a part of the cellularindustry for quite some time. Understanding the techniques to improve PA design for open loop large signal polar modulation will always be a challenge when designing saturated PAs while at the same time trying to improve AM/PM distortion.

Jackie Johnson has been with RFMD since January of 2000. He received his B.S. degree from North Carolina Agricultural and State University in December of 1999. He is currently an RF engineering group leader focusing on integration of power amplifiers into handset designs.

Table 1. Tabular data for ORFS due to modulation of the original PA.

Frequency	Modulation Spectrum	ETSI Limits
-600 kHz	-62.18 dB	-60dB
-400 kHz	-61.49 dB	-54dB
+ 400 kHz	-63.05 dB	-54dB
+ 600 kHz	-65.65 dB	-60dB
RMS EVM	0.43 pct
Peak EVM	1.33 pct
Average Power	28.08 dBm

Table 2. Tabular data for ORFS due to modulation with modified collector profile.

Frequency	Modulation Spectrum	ETSI Limit
-600 kHz	-66.84 dB	60dB
-400 kHz	-66.92 dB	-54dB
+400 kHz	-68.22 dB	-54dB
+600 kHz	-70.18 dB	-60dB
RMS EVM	0.30 pct
Peak EVM	0.52 pct
Average Power	28.11 dBm

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