Power line noise countermeasures for improving RF signal quality
In the Power Amplifier (PA) of a mobile wireless terminal including a smartphone, in order to suppress unnecessary radiation (out-of-band & spurious emission), there are many examples of improving the power quality (PI: power integrity) of the PA. In wireless communication, the International Standard (ITU), 3GPP (Wireless Communication Standards Agency), and various operators have set strict standards for the range of unnecessary radiation. Therefore, it is necessary to improve the RF signal quality through the PA's power line noise countermeasures. In order to improve the signal quality of RF (unnecessary radiation outside the band), this paper introduces the noise countermeasure method of the PA power line of the mobile terminal using the chip ferrite bead and the chip inductor.
Countermeasures for improving signal quality through wired connection evaluation
Among the evaluation items of RF signal quality by wired connection, there are ACLR*1 (temporary leakage power ratio) and SEM*2 (spectral emission module). These are the parasitic (unnecessary radiation) in the vicinity of the test RF signal.
In this article, we introduce the method for improving the RF signal quality when a DC/DC converter is used in the power supply of the PA.
1. Adjacent channel leakage power ratio (ACLR): The degree of signal leakage in adjacent signal channels.
2. Spectrum emission template (SEM): Permitted permissible value for out-of-band leakage.
Mechanism of RF signal quality degradation (unnecessary radiation outside the band) (example)
The noise transmitted by the PA's power line touches the power of the PA, which appears in the PA output power, which affects the RF signal.
The noise (F1) transmitted by the power line appears in a stray form on both sides of the carrier (F2-F1 and F2+F1) due to the 2nd distortion characteristic of the PA.
Power quality and noise issues for PAs powered by DC/DC converters (2nd order distortion)
Focus of countermeasures
The frequency of the power supply noise that affects the quality of the RF signal can be determined to find a countermeasure.
For example, in the RF signal quality evaluation, the general ACLR evaluation frequency bandwidth is centered on the carrier frequency (F2), W-CDMA is 25 MHz, and LTE is 50 MHz. If only one side is considered, the amplitude of the evaluation becomes 12.5 MHz for W-CDMA and 25 MHz for LTE compared to the carrier frequency (F2). Therefore, in ACLR, frequency noise (F1) with W-CDMA of 12.5 MHz and LTE of 25 MHz or less has an impact on RF signal quality. The low-band noise is the main cause of the noise emitted by the switching of the DC-DC converter (the switching frequency of a general DC/DC converter for PA is 2 to 10 MHz). Therefore, in the countermeasure against the power line of the PA, it is necessary to suppress the level of switching noise of the DC/DC converter.
In order to suppress the level of switching noise of the DC/DC converter, the countermeasures shown in the following figure are effective. The output power LC (power inductor and output capacitor) of the DC/DC converter is directly inserted into the chip inductor LQW15CN series or the chip ferrite bead BLM15PX121.
Countermeasure result
The aforementioned countermeasure results are confirmed by evaluating the board.
It is investigated by inserting a filter whether the loss of the original has a bad influence on the power conversion power of the DC/DC converter. Because of the ultra-low DC impedance characteristics of the LQW15CN series and BLM15PX121, the impact on power conversion efficiency should be at a level that is not problematic.
Countermeasure examples in actual equipment
The following is a countermeasure example in the actual device.
In the smartphone, the output power LC (power inductor and output capacitor) of the DC/DC converter is directly inserted into the LQW15CN18N to confirm the improvement of the mid-spurs in the vicinity of the RF signal.
Conclusion
Among the power lines of the PA of the mobile wireless terminal, countermeasures against improved signal quality are considered by wired connection evaluation.
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