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Browse Prior Art Database

AM SUPPRESSION FOR CONSTANT ENVELOPE TRANSMITTERS

IP.com Disclosure Number: IPCOM000007650D
Original Publication Date: 1996-May-01
Included in the Prior Art Database: 2002-Apr-11
Document File: 4 page(s) / 128K

Publishing Venue

Motorola

Related People

Zalman Schwartzman: AUTHOR [+2]

Abstract

Transmitters of phase and frequency modulation type are designed to generate a constant amplitude RF output. Only the phase of the RF signal carries information. Unfortunately, mobile radios face fast variations of the DC supply voltage, in form of rip- ple, caused by the alternator charge current.

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MOIVROLA Technical Developments

AM SUPPRESSION FOR CONSTANT ENVELOPE TRANSMITTERS

by Zalman Schwartzman and Eyal Fayneh

BACKGROUND

  Transmitters of phase and frequency modulation type are designed to generate a constant amplitude RF output. Only the phase of the RF signal carries information. Unfortunately, mobile radios face fast variations of the DC supply voltage, in form of rip- ple, caused by the alternator charge current.

Most RF power amplifiers will yield a change of the output amplitude because ofthe DC variation.

  In many cases, the amplitude modulated RF field can be induced on sensitive parts ofthe synthesizer, which may cause frequency variations.

  These variations are undesirable. They may com- bine with the desired modulation signal, and be detected at the receiving end as interference.

  "Alternator whine" is a very well known exam- ple of such a mechanism in many currently manu- factured mobile radios. One way to solve this prob- lem is to heavily shield the power amplifier and the sensitive parts of the frequency generating circuits. This is a costly method that may not be consistent due to mechanical variations that affect the shielding efhciency.

  The subject ofthis paper is a method to regulate any fast amplitude variations ofthe RF output, that are caused by ripple on the DC supply.

  This method will reduce the RF interfering sig- nal substantially at the source, without using costly shielding.

  The loop does not have any effect on the steady state amplitude of the RF power that is controlled by the microprocessor, therefore all the protection functions on the PA are unaffected.

  The circuit described is designed to track the supply voltage. This guaranties it will be continuously active and will not run out of dynamic range even in extreme conditions ofDC supply or antenna load.

CIRCUIT DESCRIPTION

The operational amplifier Ul is used as a buffer to prevent loading ofthe directional coupler.

  The operational amplifier U2 monitors the AC content of the directional couplers output. The ampli- tied AC signal which indicates the AM content of the output RF is then amplified and AC coupled into the base of an emitter follower transistor (Q2, Q3) in negative feedback. The output of the emitter follower is connected...