Paper Review

To investigate the relationship between noise from the DC-DC converter for PAs and the RF signal quality, we built an evaluation board that had a DC-DC converter for PAs and a PA module installed. The appearance and circuit diagram of this manufactured evaluation board is shown in Fig. 1. The DC-DC converter used in the evaluation board uses a switching frequency of 6MHz for the PA, and the PA module used was compliant with the W-CDMA^*2 and LTE^*3 shared Band I specifications. Also, all peripheral components used for the DC-DC converter for PAs and the PA module (input/output capacitor, power inductor, etc.) used the components recommended by each manufacturer.

As shown in Fig. 2, the evaluation system of the RF signal quality uses a signal analyzer that has a signal generator function, and the evaluation system was set up so that an RF signal was input to RF IN of the PA module on the evaluation board where it was amplified and output by the PA module, and then the RF signal was returned to the signal analyzer. Also, the evaluation index for the RF signal quality was based on the widely-used ACLR (Adjacent Channel Leakage Ratio) evaluation at maximum output.

Under these conditions, the effect on the RF signal quality by the noise from the DC-DC converter for PAs was investigated by evaluating the RF signal quality for both the case when power was supplied to the PA module from a stabilized DC power supply with a low noise level and the case when power was supplied from a DC-DC converter for PAs.

As a result, when compared to the case when power was supplied to the PA module from a stabilized DC power supply, the case when power was supplied from the DC-DC converter for PAs showed noticeable deterioration of the RF signal quality in the L1 and U1 bands, which are the closest to the RF signal, and this confirmed that noise from the DC-DC converter for PAs had an impact on the RF signal quality. Figure 3 shows the RF signal quality for both the case when power was supplied to the PA module from a stabilized DC power supply and the case when power was supplied to the PA module from the DC-DC converter for PAs.

Fig. 3 RF Signal Quality When Power Was Supplied from Stabilized DC Power Supply and When Power Was Supplied from DC-DC Converter for PAs

From our research results, we surmised that suppressing the noise from the DC-DC converter for PAs that electrically connects the power line could improve the RF signal quality. Therefore, we inserted a noise filter immediately behind the power inductor and capacitor for output of the DC-DC converter for PAs, and we evaluated the RF signal quality for W-CDMA and LTE.

As a result, we were able to verify that inserting a high-frequency coil (Murata part number LQW15CN series) or chip ferrite beads (Murata part number BLM15PX121) as a noise filter enabled an improvement in the RF signal quality. Figure 4 shows the insertion position of the noise filter, and Fig. 5 shows the noise filter conditions and evaluation results of the RF signal quality.

Fig. 4 Noise Filter Insertion Position

Fig. 5 Noise Filter Conditions and RF Signal Quality Evaluation Results

Effect on Power Conversion Efficiency of DC-DC Converter for PAs by the Noise Filter

During mobile phone communication, the PA module is the circuit with the largest power consumption and is the part that most heavily impacts battery life. For this reason, we examined how the noise filter used for noise suppression affects the power conversion efficiency of the DC-DC converter for PAs. These results are shown in Fig. 6.

Cases (1) to (4) in Fig. 6 are the noise filter conditions shown in Fig. 5. The difference in the power conversion efficiency was 0.23% when a high-frequency coil with the largest Rdc (DC resistance) at 96nH (noise filter condition (3) in Fig. 5) was inserted, which is the case that produces the maximum effect on the power conversion efficiency, compared to the case when no noise filter was inserted (noise filter condition (1) in Fig. 5).

We considered the mechanism for how noise from the DC-DC converter for PAs affects the RF signal quality. The ideal PA characteristics are linear without any second-order distortion. However, actual PAs include nonlinear characteristics, and so signal-generated spurious^*4 emissions (F2-F1, F2+F1) occur due to the sum and difference of the two signal types (F1, F2) in the second-order distortion. If these are placed in the PA circuit of the mobile phone so that the frequency of the noise of the PA power line is F1 and the frequency of the RF signal is F2, the noise of the power line appears as spurious emissions on the low-frequency side and high-frequency side of the RF signal. Figure 7 shows the mechanism by which noise from the DC-DC converter for PAs affects the RF signal quality. For example, the ACLR evaluation frequency range for W-CDMA is 25MHz with the RF signal as the center frequency, and so this means that low-frequency noise at 12.5MHz or less in the power line, that is, the switching noise of the DC-DC converter for PAs, is affecting the RF signal quality.

To verify this mechanism, the filter configurations shown in Table 1 were used in the power line between the DC-DC converter for PAs and the PA module in the evaluation board, and the switching noise level of the DC-DC converter was changed. Then, we examined the relationship between the switching noise level at 6MHz and the RF signal quality evaluation results in these states. The results are shown in Fig. 8. This enabled us to confirm a correlation between lower switching noise levels at 6MHz and better RF signal quality. In this way, we were able to verify the mechanism by which noise affects the RF signal quality.

Fig. 6 Effect on Power Conversion Efficiency of DC-DC Converter for PAs

Table 1 Filter Configurations

Fig. 7 Mechanism Where RF Signal Quality Is Affected by Noise from DC-DC Converter for PAs

Fig. 8 Relationship Between Switching Noise Level and RF Signal Quality

The power line of the PA module must always have a power inductor and capacitor for output of the DC-DC converter for PAs and a bypass capacitor for the PA module. For this reason, any examination must include not only the insertion loss of the individual noise filter elements, but also the insertion loss that includes the power inductor and capacitor for output of the DC-DC converter for PAs and the bypass capacitor for the PA module. We therefore examined the relationship between the RF signal quality and the insertion loss (see Fig. 9(a)) in the state that includes the power inductor and capacitor for output of the DC-DC converter for PAs and the bypass capacitor for the PA module in the filter configurations shown in Table 1.

As a result, we were able to confirm a correlation between the size of the insertion loss at 6MHz in the filter configurations and the RF signal quality evaluation results (see Fig. 9(b)). Also, as shown in Fig. 9(b), we could not verify any improvement in the RF signal quality in filter configuration (5) where the L value of the power inductor was doubled or filter configuration (6) where the noise filter was inserted immediately behind the power inductor compared to filter configuration (1).

Fig. 9 Relationship Between Insertion Loss and RF Signal Quality in Various Filter Configurations