From Power Amplifier Design: A Collection from Applied Microwave & Wireless

A novel design allows a single detector to be used at different frequencies

By Alan Rixon and Raymond Waugh



This article discusses a novel detector for power control loops. Its design combines circuitry to suppress the second harmonics generated at the input with provisions for temperature compensation.

The Problem

The increase in demand for dual band mobile phones for use on GSM900 and GSM1800 or GSM1900 systems has given designers a considerable challenge how to fit the additional circuitry required into the same or smaller space as the existing single band phones. The RF output power monitoring part of the power control loop is one area where designers would like to save space. In a single band phone, this is often designed with a directional coupler, either a printed or a surface mount component, and the detector normally based around a temperature compensated Schottky circuit [1].

For a dual band phone, the coupler would ideally be placed in the path to the antenna. However, this would not increase the component count needed. Unfortunately, a conventional detector circuit generates harmonics of the fundamental frequency. This is normally not a problem in a single band phone, because there is sufficient low-pass filtering between the coupler and the antenna.

In a dual band phone, however, the same antenna is used for both bands, and therefore low-pass filtering is not possible. This means that the second harmonic generated by the detector when...

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