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FREQUENCY SYMMETRICAL IMPEDANCE INVERTER

IP.com Disclosure Number: IPCOM000005414D
Original Publication Date: 1980-Aug-01
Included in the Prior Art Database: 2001-Oct-10
Document File: 1 page(s) / 58K

Publishing Venue

Motorola

Related People

Robert O. Loving: AUTHOR

Abstract

An impedance inverting circuit is commonly used to couple the output of an active stage to the-input of a crystal filter in order to obviate a reduction in the output intercept point of the active stage from the high, off frequency impedance of the crystal filter. Cost effective monolithic crystals usually have the electrode frequencies below the design center frequency of the required filter. The input impedance characteristic of the resulting inverterlcrystal filter structure is such that the peaks of the "flybacks" are assymmetrically placed about the design center frequency. An inverter circuit is described in which substantially geometric symmetry may be obtained in the flybacks.

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m MOTOROLA Technical Disclosure Bulletin Vol. 1 No. 1 August 1980

FREQUENCY SYMMETRICAL IMPEDANCE INVERTER By Robert 0. Loving

ABSTRACT

   An impedance inverting circuit is commonly used to couple the output of an active stage to the-input of a crystal filter in order to obviate a reduction in the output intercept point of the active stage from the high, off frequency impedance of the crystal filter. Cost effective monolithic crystals usually have the electrode frequencies below the design center frequency of the required filter. The input impedance characteristic of the resulting inverterlcrystal filter structure is such that the peaks of the "flybacks" are assymmetrically placed about the design center frequency. An inverter circuit is described in which substantially geometric symmetry may be obtained in the flybacks.

DESCRIPTION OF THE SYSTEM

   A typical impedance inverter suitable for coupling the output of a mixer to the input of a monolithic crystal filter is shown in Figure 1. The characteristic impedance,+, of the inverter is the geometric mean of the desired load impedances for the mixer M and the crystal filter F. The reactances of the inverter net- work are such that XCIZ = t, and the equivalent reactances of the parallel tuned circuits formed by LG and L&are equal to + t at the design center frequency, fc:, of the filter. The low cost monolithic crystals of the filter F normally have the electrodes at the same frequency which generally is not equal to f,....