RF InductorProposal of the Noise Suppression Technology in the PA and Power Line

-Example of Using Power Inductors for Chokes-

Introduction

- Background -

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.

  • Definition of Unwanted Emissions/Definition of Unwanted Emissions/W-CDMA ACLR
  • Definition of Unwanted Emissions/Definition of Unwanted Emissions/W-CDMA ACLR

    W-CDMA ACLR

Definition of Unwanted Emissions/Definition of Unwanted Emissions/W-CDMA ACLR

W-CDMA ACLR

- Outline -

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 Open in New Window

  • PA Power Lines of Mobile Terminals

    PA Power Lines of Mobile Terminals

  • ACLR Evaluation Screen

    ACLR Evaluation Screen

- Mechanism of RF Signal Integrity Deterioration (Out-of-band & Spurious Emissions) (Example) -

Power integrity of the PA when the power is supplied from DC/DC converter and noise problems (Secondary distortion)

PPower integrity of the PA when the power is supplied from DC/DC converter and noise problems (Secondary distortion)

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.

- Points of Improvements -

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.

  • ACLR Evaluation Screen

    ACLR Evaluation Screen

  • Points of Improvements

- PA Power Line Improvement Method -

In order to suppress the level of switching noise of DC/DC converters, the improvements shown below are effective.

Swipe left or right to change screens View in landscape mode.

The chip inductor LQW15 series Open in New Window , or chip ferrite beadsBLM15PX121SN1 Open in New Window 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 Open in New Window

- Improvement Results -

The results of the improvements mentioned above were confirmed by an evaluation board.

ACLR Evaluation

In the W-CDMA and LTE, it was confirmed that the ACLR improved by insertingLQW15CN series Open in New Window or BLM15PX121SN1 Open in New Window immediately after the output LC (power inductor and output capacitor) of the DC/DC converter.

- Confirmation of Side Effects (Power Loss) of Improvements -

We have investigated whether the loss of this component would affect the power exchange efficiency of a DC/DC converter by inserting a filter.

Confirmation of Side Effects (Power Loss) of Improvements

Since the LQW15CN series Open in New Window and BLM15PX121SN1 Open in New Window have ultra-low DC resistance, the results were at a level where there was no effect on the power exchange efficiency.

Improvements for RF Signal Integrity Deterioration by Power Supply Noise

- Example in Actual System -

An example of the improvements in an actual system is shown in the following.

Example in Actual System

In smart phones, it was confirmed that the spurious of the frequency band near the RF signal improved by insertingLQW15CN18N Open in New Window immediately after the output LC (power inductor and output capacitor) of the DC/DC converter.

Summary

- Example of Improvements in Actual System -

In the PA power lines of wireless mobile terminals, the method to improve the RF signal integrity was examined by wired connection evaluation.

Conclusion:
Improvements for RF Signal Integrity by Wired Connection Evaluation

The low frequency noise (switching noise) of a DC/DC converter could be suppressed by inserting theLQW15CN series Open in New Window and BLM15PX121SN1 Open in New Window immediately after the output LC (power inductor and output capacitor) of the DC/DC converter. As a result, the RF signal integrity was improved.

Other Examples

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