Browse Prior Art Database

VOLTAGE CONTROLLED HARMONIC FILTER

IP.com Disclosure Number: IPCOM000008268D
Original Publication Date: 1997-Sep-01
Included in the Prior Art Database: 2002-May-31
Document File: 4 page(s) / 142K

Publishing Venue

Motorola

Related People

Jochen Karasch: AUTHOR [+2]

Abstract

Harmonic filters are used in two-way commtmi- cations technology to reject spurious emissions/ harmonics (harmonics are always the multiple of the carrier frequency). The spurious emissions occur with the generation of the transmitter frequency. These filters are built up in-between the transmitter output stage and the antenna or the antenna connection, respectively. In general, many harmonics are generated, which have different amplitudes caused by the various transmitter stages. As the limit regarding the spurious emissions/ harmonics that are allowed to be radiated by the radio is existing (e.g. -1000 MHz < -36 dBm > 1000 MHz > -30 dBm, see ETS 300 086) appropriate measurements have to be taken to assure a sufkient suppression of such unwanted signals. The filter must therefore fulfill a high number of require- ments. It has to be broadbanded, has to have a low insertion loss and should be linear.

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MO-LA Technical Developments

VOLTAGE CONTROLLED HARMONIC FILTER

by Jochen Karasch and Werner Zundel

INTRODUCTION

  Harmonic filters are used in two-way commtmi- cations technology to reject spurious emissions/ harmonics (harmonics are always the multiple of the carrier frequency). The spurious emissions occur with the generation of the transmitter frequency. These filters are built up in-between the transmitter output stage and the antenna or the antenna connection, respectively. In general, many harmonics are generated, which have different amplitudes caused by the various transmitter stages. As the limit regarding the spurious emissions/ harmonics that are allowed to be radiated by the radio is existing (e.g. -1000 MHz < -36 dBm > 1000 MHz > -30 dBm, see ETS 300 086) appropriate measurements have to be taken to assure a sufkient suppression of such unwanted signals. The filter must therefore fulfill a high number of require- ments. It has to be broadbanded, has to have a low insertion loss and should be linear.

PROBLEM TO BE SOLVED

  Usually fixed filters with low-pass characteristics are used as harmonic filters. These filters have to have a sufficient attenuation throughout the frequency range to be measured as well as a low ripple. This is difftcult for radios with broadband characteristic, 136 MHz -174 MHz or 403-470 MHz, as the first harmonic is between 272 MHz and 348 MHz in the VHF band and the filter is having a falling characteristic at the cutoff frequencies. This slope which is falling towards the lower frequencies can, for example, influence the futtdamen- tal wave signal at I74 MHz. This is a power reduc-

tion of this frequency that is higher than the limit of - 1.5 dB. Therefore the lower filter curve must have a steep slope. This is causing the occurrence of a filter ripple across the frequency. Thus harmonics, which are located above the transmitter switching range in a ripple, can be falling outside of the required limits.

PROPOSED SOLUTION OF THE PROBLEM

  Our suggestion for improvement starts at this point. We suggest to use a harmonic filter which is adjustable in a certain range dependent on the trans- mitter frequency by the use of varicaps. The single filters are notch filters, which are adjusted to the respective critical harmonic with their tuning diodes.

  The changes of the capacities and therefore also of the filter frequency is...