In order to suppress unwanted emissions (out-of-band & spurious emissions) in Power Amplifiers (PA) of wireless mobile terminals including smart phones, there are cases where improvement of the power integrity (PI) of the PA is required. The allowable value of the unwanted emissions in wireless communications are strictly regulated in the International Standard (ITU), 3GPP (a wireless communication standards organization) and each carrier.
Therefore, it is necessary to improve the RF signal integrity by performing noise suppression of the PA power lines.
This page introduces the noise suppression methods of the PA power lines of mobile terminals utilizing chip ferrite beads and chip inductors, for the purpose of improving the RF signal integrity (improvement of the unwanted emissions out-of-band) .
Noise Suppression to Improve RF Signal Integrity by Wired Connection Evaluation
There are ACLR (Adjacent Channel Leakage Ratio) and SEM (Spurious Emissions Mask) evaluation items for the RF signal integrity in wired connections. These are the tests items to evaluate the spurious (unwanted) near the RF signal.
This page introduces the improvement method of the RF signal integrity, when a DC/DC converter is used for the power supply of a PA.
Wired Connection Evaluation System
Since the PA power oscillates by noise transmitted through the PA power line, this noise appears in the PA output and affects the RF signal.
The noise (1) transmitted through the power lines appear on both sides of the carrier (F2-F1 or F2+F1) as spurious by the secondary distortion of the PA.
Improvements can be performed by specifying the frequency of the power supply noise which affects the RF signal integrity.
For example, in the RF signal integrity evaluation, the general ACLR evaluation frequency bandwidth is 25MHz for W-CDMA and 50MHz for LTE centering on the carrier frequency (F2) . When considering one side only, a width of 12.5MHz will be evaluated for W-CDMA from the carrier frequency (F2) , and 25MHz for LTE. Therefore, in ACLR, a frequency noise (F1) below 12.5MHz for W-CDMA and 25MHz for LTE will affect the RF signal integrity. Low frequency noise is mainly caused by the switching noise of DC/DC converters (the switching frequency of a DC/DC converter for general purpose PA is 2-10MHz) .
The switching noise of the DC/DC converter affects the RF signal integrity.
In the improvements for PA power lines, the switching noise of DC/DC converters must be suppressed.
In order to suppress the level of switching noise of DC/DC converters, the improvements shown below are effective.
The chip inductor LQW15 series, or chip ferrite beadsBLM15PX121SN1 is inserted immediately after the output LC (power inductor and output capacitor) of a DC/DC converter.(Click here for the selection of filters.)
Note: When there is no bypass capacitor of the PA module, or the impedance of the power line to the bypass capacitor of the PA module is high, a capacitor of 0.1 uF or more may be required immediately after the filter (position of ●).
Selection of filters to be used
The results of the improvements mentioned above were confirmed by an evaluation board.
In the W-CDMA and LTE, it was confirmed that the ACLR improved by insertingLQW15CN series or BLM15PX121SN1 immediately after the output LC (power inductor and output capacitor) of the DC/DC converter.
We have investigated whether the loss of this component would affect the power exchange efficiency of a DC/DC converter by inserting a filter.
Since the LQW15CN series and BLM15PX121SN1have ultra-low DC resistance, the results were at a level where there was no effect on the power exchange efficiency.