Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
1998-06-02
2002-08-20
Eisenzopf, Reinhard J. (Department: 2745)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S323000, C327S359000
Reexamination Certificate
active
06438365
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to radio communication devices and systems and specifically to mixers in radio communications devices.
BACKGROUND OF THE INVENTION
Balanced mixers are used in radio receivers for transforming incoming radio frequency (RF), containing numerous frequency signals, to an intermediate frequency (IF). A mixer is a non-linear device that receives two signals, one from an (RF) antenna that is amplified by a Low Noise Amplifier (LNA), and the other from an internal local oscillator (Lo), and produces a third output signal. The resulting output signal can be demodulated using a variety of demodulation schemes and presented to a user. Since the output signal is a host of sum and difference terms, the mixer can be used to step up the frequency of the input RF signal.
Balanced mixers have been used extensively, together with other configurations, in various radio systems.
Monolithic double balanced mixers are generally based on structures known as Gilbert cell, first described by Gilbert in 1969. Although the Gilbert type mixer has excellent carrier suppression and low second order distortion, it suffers from a high noise figure and intermodulation distortion.
FIG. 1
presents a typical structure for a Gilbert cell mixer known in the prior art. The mixer core includes four transistors Q
1
, Q
2
, Q
3
, and Q
4
that are coupled to differential local oscillator inputs Lo+ and Lo−. The RF signal is present at the inputs RF+ and RF− as a differential (balanced) signal and is coupled to the mixer core using transistors Q
5
and Q
6
. The RF signal is amplified and converted from voltage to current by transistor pair Q
5
and Q
6
and is switched by the mixer core at the rate of the local oscillator. The switched current produces an intermediate frequency signal IF, at the outputs Out− and Out+, having a frequency described by the following equation:
Frequency (IF)=Frequency (Lo)−Frequency (RF)
The resistors Rc
1
and Rc
2
are used for converting current information into voltage information. A degeneration resistor Re couples the emitters of transistors Q
5
and Q
6
to the current sources Io
1
and Io
2
. Degeneration resistor Re improves the input third order intercept point (IIP
3
) for the incoming RF signals. However, increasing the value of degeneration resistor Re will degrade the noise figure and the mixer gain. Input impedance, provided by the input differential transistor pair Q
5
and Q
6
with emitter degeneration, will be high due to series feedback, and will require additional circuitry for input impedance matching.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide a balanced mixer which obviates and mitigates from the disadvantages of the prior art and improves the overall performance of the mixer circuit input third order intercept point (IIP
3
), noise figure, and input impedance matching ability.
According to the present invention, there is provided a balanced mixer comprising: a first differential input port to provide input radio frequency to the balanced mixer; a second differential input port to provide local oscillator frequency to the balanced mixer; a transconductance amplifier means connected to the first input port; a splitter circuit means connected to the amplifier means for splitting the input radio frequency; a mixer means connected to the splitter means and to the second input port for receiving the split input radio frequency and the local oscillator frequency; a feedback means connected between the splitter means and the first input port to provide feedback current to the first input port; and an output port connected to the mixer means.
One advantage of the present invention is that the feedback means improves the overall performance of the mixer circuit input third order intercept point (IIP
3
), noise figure, and input impedance matching ability.
Other advantages, objects and features of the present invention will be readily apparent to those skilled in the art from a review of the following detailed descriptions of a preferred embodiment in conjunction with the accompanying drawings and claims.
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patent: 5548840 (1996-08-01), Heck
patent: 5589791 (1996-12-01), Gilbert
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patent: 5630228 (1997-05-01), Mittel
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patent: 5966645 (1999-10-01), Davis
patent: 5974301 (1999-10-01), Palmer et al.
Trask, Chris, “Feedback technique improves active mixer performance”, RF Design Engineering Principles and Practices, Sep. 1997.
Morin, Marc A., “A 1.6 Gb/s Delta-Sigma Modulator withintegrated Wideband Mixer for RF Applications”, Proceedings of the 1998 Bipolar/BiCMOS Circuits and Technology Meeting, Sep. 27-29, 1998, Minneapolis, Minnesota, pp. 148-151.
Appiah Charles N.
Baker Harold C.
Eisenzopf Reinhard J.
Hendry Robert G.
Philsar Semiconductor Inc.
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