Twin-T dual notch filter
AssigneeMatsushita Electric Industrial Co., Ltd.
InventorNigel J. Tolson
Life of this patent
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Abstract
A communication filter circuit useful in a dual-mode receiver comprises a cascade of an active twin-T filter and a passive twin-T filter sections defining sharp notches at the center of second adjacent and of third adjacent channels, respectively, and specifically providing most attenuation at 60 kHz and at 90 kHz, for operation in the ISM136 band which has 30 kHz channel spacing. Since communication is channelized and limited in bandwidth, an active/passive twin-T notch filter structure is effective for signal attenuation. In a circuit in which real and imaginary signal components are processed, active-passive twin-T filters are provided in each of four signal paths, namely I and −I, Q and −Q.
Claims10
A filter circuit apparatus for suppression of spurious signals in a superheterodyne circuit for receiving communication in channels comprising:
- a first analog active twin-T filter in a first signal path defining a first sharp notch at the center a second adjacent channel;
- and
- a first analog passive twin-T filter section coupled to receive output of the first analog active twin-T filter, defining a second sharp notch at the center of a next adjacent channel, to suppress spurious signals at frequencies of modulation product.
A method for processing multiple signal modes according to different radio standards of a received RF signal, comprising:
- performing downconversion of the received RF signal to produce analog I and Q signals;
- and for each of the analog I signal and the analog Q signal, filtering out unwanted signals by:
- for a first standard, processing the analog signal using a first passive notch filter to produce a first filtered signal;
- and
- for a second standard, processing the analog signal using an active notch filter to produce a second filtered signal;
- wherein the active notch filter exhibits smaller group delay than the passive notch filter.
A dual mode communications receiver, comprising:
- a demodulator operable to provide in-phase and quadrature-phase base band signals;
- a first active twin-T filter having an input coupled to an in-phase output of said demodulator;
- a first passive notch filter network having an input coupled to the in-phase output of said demodulator;
- a second active twin-T filter having an input coupled to a quadrature-phase output of said demodulator;
- and
- a second passive notch filter network having an input coupled to the quadrature-phase output of said demodulator.
Dependencies are inferred from the claim text.