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.