Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
1998-12-31
2001-03-20
Cumming, William (Department: 2684)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S317000, C455S319000, C455S313000, C455S323000, C455S118000
Reexamination Certificate
active
06205325
ABSTRACT:
The present invention relates to a mixer circuit with feedback that provides improved linearity over a wider dynamic range. Furthermore, the structure of the present invention enables a low noise transconductance stage to drive a low capacitance mixing stage with improved switching speed, higher frequency operation, and reduced local oscillator drive.
BACKGROUND OF THE INVENTION
The use of multi-user, radio communication systems has achieved wide popularity in recent years. Advancements in communication technologies have permitted the implementation of such radio communication systems to be afforded by a large number of users.
In a radio communication system, a communication channel connects a sending and a receiving station. A communication channel uses a portion of the electromagnetic spectrum to create a wireless connection, replacing wire line connections. Communication by way of a radio communication system provides the particular advantage of untethered communications.
A sending station of a radio communication system converts information which is to be communicated to the receiving station into a signal with characteristics that permit its propagation over the communication channel. To convert the information into a communication signal, the sending station modulates the information onto a carrier wave with an assigned frequency.
Mixer circuits typically form a portion of a sending station. The information to be communicated by the sending station is initially at baseband frequency. The mixer circuit of the sending station up-converts in frequency the baseband information. A sending station sometimes includes more than one mixer circuit. When multiple mixing stages are utilized, an IF (intermediate frequency) signal is formed at the first mixer stage and a RF (radio frequency) signal is formed at the final mixing stage of the sending station.
A typical receiving station receives a radio frequency communication signal by way of a communication channel and analogously converts the radio frequency signal into a baseband signal. Thereafter, the information of the received communication signal is recovered. The receiving station utilizes one or more mixer circuits to down-covert in frequency the received communication signal into a baseband signal. When multiple mixing stages are utilized, an IF signal is formed at the first mixer stage and a baseband signal is formed at the final mixing stage.
The performance of the receiving station is defined, amongst other things, by its sensitivity and selectivity. Sensitivity is the ability of the receiving station to recover information in the presence of noise and depends on various factors, including noise figure, information bandwidth, and carrier-to-noise ratio requirements. The noise figure is set by the front-end down converter of the receiving station. Selectivity is the ability of the receiving station to isolate information amongst interfering signals. The selectivity of the receiving station is determined by channel filtering and signal handling characteristics.
The mixer circuit plays a big part in determining the performance of the receiving station. A typical front-end down converter of the receiving station includes a low noise amplifier, an image rejection filter, and a mixer circuit. A conventional mixer circuit is generally categorized to be either a passive circuit or an active circuit.
Passive mixer circuits generally exhibit a low noise figure and are operable over a wide dynamic range. However, passive mixer circuits generally require relatively high local oscillator drive and low noise IF (intermediate frequency) amplification to compensate for relatively high insertion loss. When power considerations are significant, such as when the mixer circuit is to form a portion of a low power integrated circuit, powered by a portable power supply, the relatively high power requirements associated with passive mixer circuits are disadvantageous.
Conversely, active mixer circuits exhibit higher noise figures and operate over narrower dynamic ranges. However, active mixer circuits use less power because they require less IF amplification and lower local oscillator drive. The reduced power requirements of active mixer circuits are generally preferred in applications related to low-power, integrated electronics. These active mixer circuits are especially advantageous for mobile stations used in radio communication systems, such as cellular communication systems. However, for these requirements, standard active mixer circuits do not offer high enough performance and improved active mixer circuits are needed.
An active mixer exhibiting improved dynamic range and ease of design would be advantageous. Such an active mixer could form a portion of a mobile station operable in a cellular, or other radio, communication system.
It is in light of this background information related to mixer circuits that the significant improvements of the present invention have evolved.
SUMMARY OF THE INVENTION
The present invention provides a mixer circuit that operates over a wider dynamic range than permitted by conventional active mixers.
The active mixer circuit includes a transconductance stage which is coupled to receive an input signal. The transconductance stage includes a pair of transconductance elements, each coupled to receive the input signal. A feedback element is coupled from the output of the first of the transconductance elements to the received input signal. The output of the second transconductance element is coupled to a mixing stage, which forms a portion of the mixer circuit. The mixing stage convolves the output of the second transconductance element with a mixing signal.
Feedback in a conventional mixer is not practical because the output of the mixer is at a frequency that is different from the frequency of the input signal. Feedback is effective in the present invention because the output of the first transconductance element is at the same frequency as the input to the transconductance stage. Furthermore, it is the transconductance stage that is the largest contributor to distortion in conventional mixer circuits. Such feedback extends the linearity and, thereby, the dynamic range of the mixer circuit.
Another advantage of the present invention is that the transconductance stage is realized with parallel or multiple elements. Here, the bias current and composite geometries can be chosen to minimize the noise figure. Typically, a large geometry device is used, which can be realized by multiple devices in parallel. A large geometry transistor or multiple devices require sizeable current to realize peak operating frequency. In a standard mixer circuit, the bias current of the transconductance stage feeds directly to the switching transistors of the mixer circuit. These switching transistor elements are sized to handle all the bias current. These transistors are consequently large geometry, possessing high capacitance values, which limits the high frequency performance of the active mixer and burdens the LO buffer amplifiers. In the present invention, only a portion of the input signal bias current is provided to the mixing stage and the switching transistors can be sized smaller than required of conventional mixer circuits. Such smaller sizes reduce the parasitic capacitances of the switch transistors and also reduce the time required for their switching operation. The multi-element construction of the transconductance stage permits the geometry of the switch transistors of the mixing stage to be optimized, independent of the geometry of the transconductance stage and associated bias current.
A mixer circuit embodiment of the present invention forms a portion of the front-end, down converter section of a radio receiver, such as the radio receiver utilized in a cellular communication system. RF signals representative of the received signal are applied to the mixer circuit. Down-mixing local oscillator signals are also applied to the mixing circuit, which produces a down-converted signal. The IF signal is then provided to an I/Q
Cumming William
Gantt Alan T.
Nokia Mobile Phones Limited
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