Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
2000-11-20
2004-03-23
Nguyen, Duc M. (Department: 2685)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S326000, C455S317000, C455S296000, C375S328000, C375S324000
Reexamination Certificate
active
06711397
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to wireless radio receiver technology and, more specifically, to improved circuits and methods for the direct conversion of radio frequency modulated signals to baseband signals without requiring conversion through an intermediate frequency.
2. The Prior State of the Art
Electrical signals have proven to be an effective means of conveying data from one location to another. The further a signal is transmitted, however, the greater the decay in the signal and the greater the chance for irreversible loss in the data represented by the signal. In order to guard against this signal decay, the core electrical signal that represents the data (i.e., the baseband signal) may be modulated or superimposed on a carrier wave in the Radio Frequency (RF) frequency spectrum.
In order to properly interpret the signal, conventional RF receivers demodulate the baseband signal from the received signal. The data represented by the extracted baseband signal may then be interpreted by other downstream circuitry.
In order to perform this demodulation, typical receivers include circuitry which first converts the received radio frequency modulated signal into an intermediate frequency (“IF”) signal. This intermediate frequency signal is then converted into the baseband signal for further data processing. Receiver architectures that convert through the intermediate frequency are often called “heterodyne” receiver architectures. Naturally, circuit elements (called “IF components”) are required in order to deal with the intermediate conversion to and from the intermediate frequency.
It is desirable to reduce the cost, size, and power consumption of a particular receiver architecture design for strategic marketing of the receiver. This is particularly true of wireless RF receivers since those receivers are often portable and run on battery power.
One technology developed in order to reduce RF receiver cost, size, and power consumption is called “direct conversion.” Direct conversion refers to the direct conversion of RF modulated signals into corresponding baseband signals without requiring conversion through the intermediate frequency. Such direct conversion receiver architectures are often called “zero-IF,” “synchrodyne,” or “homodyne” receiver architectures.
FIG. 1
illustrates a conventional direct conversion circuit
100
in accordance with the prior art. The circuit
100
includes an antenna
101
which receives the RF modulated signal. The antenna then provides the received signal to an amplifier
102
which amplifies the signal for further processing. The amplifier
102
may be, for example, an RF low noise amplifier.
The amplified signal is then split into two branches, an “in-phase” branch
110
, and a “quadrature-phase” branch
120
. Each branch includes a mixer that initially receives the amplified signal. For instance, the in-phase branch
110
includes an in-phase mixer
111
, and the quadrature-phase branch
120
includes a quadrature-phase mixer
121
. A local oscillator
130
provides a sine or square wave signal as a control signal to each of the mixers. Each mixer is configured to nonlinearly process the amplified signal and control signal, resulting in output signal components at frequencies equal to the sum and difference of amplified signal and control signal frequencies, plus higher-order components at other frequencies. The circuit includes a 90-degree phase shifter
131
which causes the control signal for the quadrature-phase mixer
121
to be 90 degrees out of phase with the control signal for the in-phase mixer
111
.
The signal from the in-phase mixer
111
is then passed through a low pass filter
112
to a baseband amplifier
113
to complete the extraction of the baseband (difference frequency) signal from the received signal as far as the in-phase branch
110
is concerned. Likewise, the signal from the quadrature-phase mixer
121
is passed through a low pass filter
122
to a baseband amplifier
123
to complete the extraction of the baseband (difference frequency) signal as far as the quadrature-phase branch is concerned. The quadrature baseband signals are then processed by signal processing circuitry
150
.
The direct conversion circuit of
FIG. 1
does not convert through an intermediate frequency and thus there are no IF components needed to deal with an intermediate conversion. Consequently, the direct conversion circuit of
FIG. 1
is smaller, and requires less power than conventional heterodyne receiver architectures, which perform intermediate conversion.
However, there are some performance issues for the direct conversion circuit of
FIG. 1
that limit its practical implementation. First, there is often local oscillator leakage to the antenna
101
, to the amplifier
102
input and to the mixer inputs. This results in local oscillator Direct Current (DC) self-mixing products that can overpower and degrade the baseband signals. Second, the antenna may radiate the local oscillator leakage causing an interference problem for other nearby receivers. This may also cause a time-varying self-mixing product due to radiated RF leakage reflecting off nearby objects, possible in motion, and being received back at the same antenna, then adding to the leakage component present at the antenna terminal. Third, there is a lack of RF selectivity, combined with amplifier and mixer limited dynamic range, resulting in direct AM detection of high level, in-band or adjacent channel interference. The net result of these performance degradations is that there may be some loss of data in the baseband signals and some interference with the operation of nearby receivers.
What is therefore desired are circuits and methods for direct conversion of RF modulated signals directly into baseband signals without intermediate conversion through and intermediate frequency while reducing the above-described problems related to oscillator leakage, interference with surrounding antennas, self-mixing products at the antenna, low RF selectivity, and limited dynamic range.
SUMMARY OF THE INVENTION
A direct conversion receiver converts a radio frequency modulated (RF) signal into a corresponding baseband signal without requiring conversion through an intermediate frequency. The direct conversion receiver abates local oscillator leakage, increases dynamic range and increases RF selectivity as compared to conventional direct conversion circuits.
After being received and amplified as necessary, the RF signal is provided to two branches of the direct conversion circuit, an “in-phase” branch and a “quadrature-phase” branch.” However, instead of the conventional one mixer per branch, each branch includes two mixers. These four mixers periodically pass on the RF signal if the corresponding control signal provided to the mixer is high. A local oscillator provides the controls signals in the form of binary waves which have the same period, nominally, as the carrier period of the RF signal. Each mixer is provided with a primary binary control signal having a duty cycle of approximately 25 percent in the logic “high” state and a corresponding binary complement signal having a duty cycle of approximately 75 percent in the logic “high” state. The quarter-period “high” states of the primary control signals are time-shifted, from one control signal to the next, to produce quadrature-phased, primary binary control signals with relative phases of approximately 0, 90, 180, and 270 degrees.
As for the in-phase branch, a first mixer has an output terminal coupled to the positive terminal of the operational amplifier. This first mixer is provided with a corresponding first primary binary control signal. A second mixer in the in-phase branch has an output terminal that is coupled to the negative terminal of the operational amplifier. This second mixer is provided with a second primary control signal that is 180 degrees out of phase as compared to the first primary control signal. An operational amplifier receives the signal passed by the t
Christensen Craig L.
Petrov Andrei R.
Reinhard Kenneth L.
AMI Semiconductor Inc.
Nguyen Duc M.
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