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METHOD OF USING A CIRCULATOR FOR RF POWER AMPLIFIER HIGH VSWR PROTECTION

IP.com Disclosure Number: IPCOM000009274D
Original Publication Date: 1999-Jun-01
Included in the Prior Art Database: 2002-Aug-14
Document File: 5 page(s) / 356K

Publishing Venue

Motorola

Related People

Sayed Merzaban: AUTHOR [+2]

Abstract

A method is presented to provide protection for an RF power amplifier module against high voltage standing wave ratio (VSWR) in a UHF transmitter and to reduce the transmitter intermodulation in the presence of co-located transmitters.

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m MOTOROLA Technical Developments

 METHOD OF USING A CIRCULATOR FOR RF POWER AMPLIFIER HIGH VSWR PROTECTION

by Sayed Merzaban and Bruce Barlow

  A method is presented to provide protection for an RF power amplifier module against high voltage standing wave ratio (VSWR) in a UHF transmitter and to reduce the transmitter intermodulation in the presence of co-located transmitters.

INTRODUCTION

  A power amplifier operating in a communica- tion device such as a base station, mobile radio, etc. typically utilizes one of two methods to provide protection against high VSWR. The fast method is to use a circulator between the PA and antenna, with a large 50 ohm load connected to port 3. When there is high VSWR at the antenna port the power will be reflected to port 3 of the circulator and absorbed in the load which is usually equipped with a fan for cooling.

  The second method is to use a bi- directional coupler between the PA and antenna. The directional coupler is used for sampling the forward power to control the output power of the amplifier module and the reverse power to detect high VSWR.

  In this paper we will discuss a new method com- bining a directional coupler for sampling the for- ward power for power control and a circulator for sampling the reverse power for VSWR protection. We will also show that the combination is superior solution for both level and high VSWR control and protection.

DESIGN CONSIDERATIONS

  Several factors need to be considered when designing the protection circuitry for the power amplifier (PA) used in transceivers:

1. Power output
2. VSWR
3. Detection and Control

POWER OUTPUT

  The need to control output power is dependant on the application of the transmitter. It is generally not a requirement in personal portable equipment. For mobile and fixed transmitters operating over 2 to 3 watts in extreme environments, it is desireable to reduce the power to avoid catastrophic failure in the transmitter tinal amplifier stages when operating into load mismatch caused by damage or degrada- tion in the antenna system. This is a most important consideration in public networks where on site maintenance is difficult and degraded service may be acceptable until service is completed.

VSWR

  The relationship between VSWR and output power protection will be primarily determined by the safe operating limits of the transmitter final amplifier devices under the specified environmental extremes. This requires the design of a detection cir- cuit that lowers the PA output when VSWR increas- es above a limit that provides a safe operating mar- gin for the PA over temperature extremes. On the other hand, the PA may be designed to be suffcient- ly robust to deliver the output power irrespective of the value of VSWR. However, this generally wastes space and power and requires more expensive amplifer devices.

DETECTION AND CONTROL

  The use of a Bi-Directional coupler is a com- monly encountered solution for controlling forward power and detecting...