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ECONOMICAL HIGH ISOLATION FILTER BANK SWITCHING

IP.com Disclosure Number: IPCOM000008763D
Original Publication Date: 1998-Jun-01
Included in the Prior Art Database: 2002-Jul-10
Document File: 4 page(s) / 193K

Publishing Venue

Motorola

Related People

Tal Mor: AUTHOR

Abstract

Transmission lines are used to rotate the out of band reactance of a filter in a switched filter configuration. This improves the isolation of the switches and reduces the number of switching elements required.

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Page 1 of 4

MO-LA Technical Developments

8

ECONOMICAL HIGH ISOLATION FILTER BANK SWITCHING

by Tal Mor

ABSTRACT

  Transmission lines are used to rotate the out of band reactance of a filter in a switched filter configuration. This improves the isolation of the switches and reduces the number of switching elements required.

INTRODUCTION

  This paper describes an economical way to switch between two filters, with a minimal number of switching elements, (such as PIN diodes), while maintaining high isolation between the filters.

PROBLEM(S) TO BE SOLVED

With radio equipment that needs to cover more than one band, there is often a need for switchable

filters. Examples for this are "talkaround" mode in tanking radio or dual band cellular radios.

  For the filters to perform at their optimum, the switch isolation must be greater than the rejection of each filter at the other's passband. Otherwise, the coupled power through the switch will pass through the unselected filter and effectively reduce the selected filter isolation. For example, when switching between two 800 MHz filters, at the trunked receive and transmit band, isolation of better than 30 dB is required.

  The simple way to switch two filters is to use a switching element (such as a PIN diode), at the input and output of each filter as described in Figure 1. While the diodes connected to the Fl filter are on, the signal will pass through this filter, so that the output spectrum is around F 1.

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t

out

Fig. 1

n Motorola. I"C. ,998 184 June 1998

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Page 2 of 4

0 M M-LA

Technical Developments

  However, the isolation of the two off state A common way to achieve better isolation is to diodes cannot be added, so we effectively isolate F2 use compound series/shunt switches as described in by something in between one diode isolation and Figure 2. This arrangement can yield as much as 30 two times as much. Typical figures at 800 MHz are dB per series/shunt pair.
lo-15 dB for one series diode. A simple arrange-
ment such as in Figure 1 gives 15-20 dB, depending Naturally, this will require more switching on the insertion phase of the filter. elements and thus be more costly.

out

In

F =

Fig. 2

PROPOS...