Wave transmission lines and networks – Coupling networks – Frequency domain filters utilizing only lumped parameters
Reexamination Certificate
1999-05-12
2001-02-20
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Coupling networks
Frequency domain filters utilizing only lumped parameters
C333S08100R
Reexamination Certificate
active
06191665
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to radiofrequency (RF) receivers of communication devices. In particular, this invention relates to, but is not necessarily limited to, a coupling circuit to improve intermodulation in such receivers.
BACKGROUND OF THE INVENTION
In radiofrequency (RF) receivers of communication devices, a carrier signal is typically received via an antenna and then processed by circuitry to obtain a lower frequency baseband signal. Such circuitry have different circuit stages or circuit elements to process the carrier signal. Processing the carrier signal includes amplification, mixing to convert the carrier signal to the baseband signal and filtering of noise or undesired harmonics resulting from the amplication or the mixing.
Performance of the stages or elements can be characterised with parameters such as, for example, a second or a third order intercept point (IP2&IP3). IP2 and IP3 indicates how well such stages or elements perform in the presence of strong nearby signals including noise and undesired harmonics. Such nearby signals cause intermodulation distortion, which affects desired signals that are processed by the stages or elements. In a communication device such as, for example, a two way radio, an overall or system IP2 or IP3 of the (RF) receiver within the two way radio can be degraded by the IP3 of an individual stage or element. One technique to overcome or alleviate degradation of the system IP2 or IP3 is to couple the stages or elements of the (RF) receiver with low impedance stages. Hence, as a low impedance stage, a diplexer typically couples a mixer to a crystal filter in conventional (RF) receivers. Such a diplexer preceeds the crystal filter to thereby provide a low impedance load for the mixer.
However, diplexers do not provide a complete solution to the degradation of the system IP2 or IP3 of (RF) receivers. This is because different types of mixers or crystal filters require different circuitry to be configured for the diplexers. Furthermore, diplexers can be complex and require many discrete elements which, therefore, increase the cost of (RF) receivers.
SUMMARY OF THE INVENTION
It is an object of this invention to overcome or at least alleviate at least one of the problems associated with coupling of signals between circuit stages or elements of (RF) receivers of communication devices.
According to one aspect of the invention, there is provided a coupling circuit for coupling a frequency signal between circuit stages, said coupling circuit comprising:
a resonator having at least two impedance elements; and
an attenuator having at least two resistors, said attenuator being connected to said circuit stages and said resonator;
wherein, when said resonator resonates with said frequency signal, at least one of said resistors is isolated from said circuit stages to thereby alleviate attenuation of said frequency signal between said circuit stages.
Preferably, said attenuator can be a pi-network formed with said resistors.
Alternatively, said attenuator can be a tee-network formed with said resistors.
Suitably, said impedance elements can comprise at least one capacitor and at least one inductor, said impedance elements being connected in parallel relative to each other.
Preferably, said circuit stages can include a mixer.
Suitably, said circuit stages can include a crystal filter.
According to another aspect of the invention, there is provided a coupling circuit comprising:
an attenuator having at least two resistors, said attenuator being connected to circuit stages, said circuit stages being configured to process a frequency signal; and
a resonator having at least two impedance elements, said impedance elements being connected in parallel relative to each other,
wherein, when said resonator resonates with said frequency signal, at least one of said resistors is isolated from said circuit stages to thereby alleviate attenuation of said frequency signal between said circuit stages.
Preferably, said attenuator can be a pi-network formed with said resistors.
Alternatively, said attenuator can be a tee-network formed with said resistors.
Suitably, said impedance elements can comprise at least one capacitor and at least one inductor.
Preferably, said circuit stages can include a mixer.
Suitably, said circuit stages can include a crystal filter.
REFERENCES:
patent: 3868604 (1975-02-01), Tongue
patent: 5187457 (1993-02-01), Chawla et al.
patent: 5796286 (1998-08-01), Otaka
Ross, Richard A. “CQ Reviews: The Healthkit SB-104 SSB/CW Transceiver Kit” Aug., 1975 Radio Almanac (USA), vol. 31, No. 8, pp 28-32 & 68-73.
Chan Sek Loon
Cheah Sin Hooi
Bettendorf Justin P.
Doutre Barbara R.
Motorola Inc.
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