Apparatus and method for integration of tuner functions in a digital receiver

A receiver to process a RF input signal having a plurality of channels includes a direct down conversion circuit, a demodulation circuit, and a local oscillator circuit. The direct down conversion circuit provides a downconverted signal based on the RF input signal and a local oscillator signal. The demodulation circuit receives the downconverted signal and provides a demodulated signal. The local oscillator circuit sets a frequency of the local oscillator signal based on a selected channel of the plurality of channels.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to digital receivers, and more specifically to digital receivers capable of processing both analog and digital signals.

2. Background Art

Television signals are transmitted at radio frequencies (RF) using terrestrial, cable, or satellite transmission schemes. Terrestrial and cable TV signals are typically transmitted at frequencies of approximately 57 to 860 MHZ, with 6 MHZ channel spacing in the United States and 8 MHz channel spacing in Europe. Satellite TV signals are typically transmitted at frequencies of approximately 980 to 2180 MHz.

Regardless of the transmission scheme, a tuner is utilized to select and down-convert a desired channel from the TV signal to an intermediate frequency (IF) signal or a baseband signal, which is suitable for processing and display on a TV or computer screen. The tuner should provide sufficient image rejection and channel selection during down-conversion as is necessary for the specific application. The National Television Standards Committee (NTSC) sets standards for television signal transmission, reception, and display. To process a NTSC signal, it is preferable that the tuner have a high-level of image rejection. However, less image rejection is acceptable for non-NTSC signals depending on the specific application and the corresponding display requirements.

After the tuner down-converts the desired channel from the TV signal, the resulting IF or baseband signal is typically converted into a digital signal to be processed by a digital receiver. However, placing an analog signal, such as the desired channel from the TV signal, in close proximity with a digital signal can cause interference between the signals. Thus, the tuner circuitry and the digital receiver circuitry are often separated in traditional communication systems.

Separating the tuner circuitry and the digital receiver circuitry has several disadvantages. For example, more circuit area is needed for separate tuner and digital circuits, which leads to higher cost.

What is needed is a method or apparatus for integrating tuner functions in a digital receiver.

BRIEF SUMMARY OF THE INVENTION

The present invention is an apparatus and method for integration of tuner functions in a digital receiver. For example, a receiver includes a direct down conversion circuit, a demodulation circuit, and a local oscillator circuit. The receiver receives a RF input signal having a plurality of channels. The receiver down-converts a selected channel of the plurality of channels to provide a baseband signal or an IF signal.

According to an embodiment, the direct down conversion circuit includes mixers and a low pass filter coupled to the output of the mixers. The direct down conversion circuit provides a downconverted signal based on the RF input signal and a local oscillator (LO) signal. The RF input signal can include first and second quadrature components, and/or the LO signal can include first and second LO quadrature components.

If both the RF input signal and the LO signal include quadrature components, then four mixers are generally used for downconversion. If either the RF input signal or the LO signal includes quadrature components, then two mixers are generally used for downconversion. For example, during operation of the direct down conversion circuit in the latter scenario, the first mixer can combine the RF input signal and the first LO quadrature component to provide a first downconverted quadrature component. The second mixer can combine the RF input signal and the second LO quadrature component to provide a second downconverted quadrature component. In another example, the first mixer can combine a first quadrature component of the RF input signal and the LO signal. The second mixer can combine a second quadrature component of the RF input signal and the LO signal.

The first and second downconverted quadrature components are multiplexed and passed through at least one analog-to-digital converter to provide a digital signal. The digital signal passes through a demultiplexer and is provided to the demodulation circuit. The demodulation circuit provides a demodulated signal to the local oscillator circuit. The local oscillator circuit sets a frequency of the local oscillator signal based on a selected channel of the plurality of channels. At least one digital-to-analog converter (DAC) receives a digital representation of the local oscillator signal from the local oscillator circuit and converts the digital representation into an analog local oscillator signal. A narrow band filter provides the local oscillator signal to the direct down conversion circuit.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates a communication system according to an embodiment of the present invention. The communication system100includes a channel selector110, a receiver120, and a display device130. For example, a user selects a desired channel using the channel selector110. The channel selector110transmits a signal104including information associated with the desired channel to the receiver120. In an embodiment, the channel selector110transmits an infrared signal indicating the desired channel to the receiver120. It is understood in the art that the channel selector110is physically coupled to the receiver120in some embodiments.

The receiver120receives a radio frequency (RF) input signal102and the signal104from the channel selector110. The RF input signal102typically includes multiple channels. The receiver120uses the signal104from the channel selector110to determine which of the channels of the RF input signal102to transmit to the display device130.

The display device130can be a cathode ray tube (CRT) display device, a liquid crystal display (LCD) device, a plasma display device, or an image projection device, to provide some examples. The display device130provides a pictorial representation of the selected channel. In an embodiment, the display device130is capable of accepting a signal having a higher resolution than a standard National Television Standards Committee (NTSC) signal. For example, the display device130can be capable of accepting an enhanced-definition television (EVTV) signal or a high-definition television (HDTV) signal.

The operation of the receiver120is described as follows and in reference toFIG. 2, whereFIG. 2represents the frequency spectrum of the particular signals that are received and/or generated by the receiver120. As shown inFIG. 2, in one embodiment, the RF input signal102can include channels within a frequency range from 57 to 860 MHz. For example, the RF input signal102can be a cable television signal having a channel spacing of 6 or 8 MHz, although the scope of the present invention is not limited in this respect. As shown inFIG. 2, a channel at 585 MHz can be selected as an example. Upon selection of the channel, the RF input signal102is generally downconverted to provide an IF signal or a baseband signal. For example, the 585 MHz channel can be downconverted to facilitate processing of the channel prior to its display. As shown inFIG. 2, the 585 MHz channel can be downconverted to 3 or 4 MHz in an embodiment.

The downconverted signal is typically generated by combining a local oscillator signal and the selected channel of the RF input signal102in a receiver120. For example, a mixer can provide a downconverted signal having a frequency based on the frequency of the local oscillator signal. In a first embodiment, the local oscillator signal is a quadrature signal having first and second LO quadrature components. Generally, quadrature components are substantially the same in amplitude and frequency; however, the two components are typically 90° out of phase with each other. In a second embodiment, the RF input signal is a quadrature signal having first and second RF quadrature components. In a third embodiment, the local oscillator signal and the RF input signal each have quadrature components.

With respect to the third embodiment, each of the RF quadrature components can be combined with each of the LO quadrature components. The LO quadrature components can be referred to as LOiand LOq. The RF quadrature components can be referred to as RFiand RFq. For example, mixing the RF quadrature components and the LO quadrature components can provide quadrature signals defined by the following equations:
IFi=LOi*RFi+LOq*RFq
IFq=LOi*RFq−LOq*RFi.

To simplify the discussion, the first embodiment is described with reference toFIGS. 3-5, which provide some examples of receivers that utilize a local oscillator having first and second quadrature components to provide the downconverted signal, according to embodiments of the present invention.

FIG. 3illustrates a receiver in which quadrature components of a local oscillator signal are independently generated according to an embodiment of the present invention. The receiver300includes a direct down conversion circuit310, a demodulation circuit360, and a local oscillator circuit370. The direct down conversion circuit310receives a RF input signal102. The RF input signal102is generally amplified by a low-noise amplifier312to amplify the RF input signal102to an amplitude above the noise floor of the receiver300. According to an embodiment, the RF input signal102is amplified before being received by the direct down conversion circuit310. For instance, a discrete low-noise amplifier, such as Broadcom part number BCM3405, can be coupled to the input of the direct down conversion circuit310. In an embodiment, the RF input signal102is amplified by the direct down conversion circuit310. For example, low-noise amplifiers312can amplify the RF input signal102before the RF input signal102is passed to mixers314.

The mixers314mix the RF input signal102and a local oscillator signal to provide a downconverted signal. As shown inFIG. 3, mixer314amixes the RF input signal102and a first quadrature component of the local oscillator signal to provide a first downconverted quadrature component. Mixer314bmixes the RF input signal102and a second quadrature component of the local oscillator signal to provide a second downconverted quadrature component. For instance, the downconverted quadrature components can include unwanted adjacent channel energy. One or more low pass filters (LPFs)316can eliminate or reduce the unwanted energy.

A multiplexer320can be included to select the first downconverted quadrature component or the second downconverted quadrature component to be sent to at least one analog-to-digital converter (ADC)330. InFIG. 3, the receiver300includes a single ADC330for illustrative purposes, though the scope of the present invention is not limited in this respect. For instance, a single ADC can be used to reduce gain and/or linearity mismatches between the quadrature components. In an embodiment, using a single ADC reduces the size of the receiver300.

The multiplexer320can interleave samples of the first downconverted quadrature component and the second downconverted quadrature component to provide an interleaved sample of the downconverted quadrature components to the ADC330. In one embodiment, the multiplexer320toggles at a rate equal to at least twice the effective sampling rate of the ADC330. For example, sampling at this rate can facilitate accurate conversion of the downconverted quadrature components by the ADC330.

The ADC330converts the interleaved sampling of the downconverted quadrature components into a digital signal. According to an embodiment, the sampling rate of the ADC330equals the interleaving rate of the multiplexer320plus an over sampling ratio. For instance, basing the sampling rate of the ADC330on the over sampling ratio can extend the noise performance of the ADC330and/or reduce the number of bits required by the ADC330.

A demultiplexer340de-interleaves the digital samples of the downconverted quadrature components provided by the ADC330. In an embodiment, the demultiplexer340toggles at a rate equal to the toggle rate of the multiplexer320. The de-interleaved samples of the downconverted quadrature components can be frequency shifted or time shifted to restore quadrature alignment and/or quadrature time alignment, although the scope of the present invention is not limited in this respect. For example, mixers350can introduce a frequency offset to the de-interleaved samples of at least one of the downconverted quadrature components to provide frequency-corrected samples to the demodulation circuit360.

The demodulation circuit360provides a demodulated signal to the local oscillator circuit370. In an embodiment, the demodulation circuit360is a quadrature amplitude modulation (QAM) demodulation circuit. For example, the demodulation circuit360can include a Nyquist filter, a variable rate symbol demodulator, an equalizer, and a carrier recovery loop. According to an embodiment, QAM improves the data transmission rate of the receiver300without degrading the bit error rate (BER) of the receiver300.

The local oscillator circuit370sets the frequency of the local oscillator signal based on the selected channel of the RF input signal102. For example, the local oscillator circuit370can receive information regarding the desired channel from the channel selector110shown inFIG. 1and set the frequency of the local oscillator signal based on that information.

Receivers typically include at least one voltage controlled oscillator (VCO) that generates a signal having a frequency based on the input voltage of the VCO. According to an embodiment, the local oscillator circuit370includes a VCO. For example, each of the channels of the RF input signal can be associated with a particular LO frequency needed to downconvert the selected channel. The VCO can receive an input voltage based on the desired channel and set the frequency of the local oscillator signal based on the input voltage.

The local oscillator circuit370digitally generates the local oscillator signal according to an embodiment. For instance, the local oscillator circuit370typically generates a digital representation of the local oscillator signal. The receiver300often includes a memory372to store a read-only memory (ROM) lookup table. The ROM lookup table can include a plurality of entries. According to a first embodiment, each entry represents a phase of the local oscillator signal or a sine or cosine thereof. The local oscillator circuit370can retrieve an entry from the ROM lookup table at each cycle or half-cycle of the VCO clock, for example, to provide the digital representation of the local oscillator signal.

According to another embodiment, the ROM lookup table stores an offset value. For example, the offset value can indicate a difference between the actual frequency of the local oscillator signal and the desired frequency of the local oscillator signal. The frequency of the local oscillator signal can be set based on the offset value. For instance, the offset value can be combined with the local oscillator signal to provide a frequency-shifted local oscillator signal.

In another example, the offset value can indicate a difference between the actual phase of the local oscillator signal and the desired phase of the local oscillator signal. The phase of the local oscillator signal can be set based on the offset value. For instance, the offset value can be combined with the local oscillator signal to provide a phase-shifted local oscillator signal. Basing the frequency or the phase of the local oscillator signal on the offset value can save time, as compared to accessing the ROM lookup table in successive cycles of the local oscillator circuit370.

Digitally generating the local oscillator signal can enable a reduction in the number of VCOs needed in the receiver300. For instance, a reduction in the number of VCOs can provide a reduction in the size of the receiver300. Including fewer VCOs in the receiver300can result in a lower cost of the receiver300.

According to an embodiment, the local oscillator circuit370is a direct digital frequency synthesizer (DDFS). The DDFS digitally converts phase information relating to the local oscillator signal to a digitized sinusoidal waveform. The DDFS can receive the phase information from the ROM lookup table or from the demodulated signal received from the demodulation circuit360, to provide some examples. The DDFS can provide faster frequency switching, lower phase noise, and/or higher frequency resolution, as compared to standard phase-locked loop (PLL) frequency synthesizers.

The DDFS typically includes a phase accumulator374to receive phase information relating to the local oscillator signal with each successive clock cycle of the local oscillator circuit370. For example, the phase accumulator374can receive first phase information during a first clock cycle, second phase information during a second clock cycle, and so on.

The DDFS can further include a phase-to-sine converter376to convert phase information received from the memory372into a digitized sinusoidal waveform. For example, the phase-to-sine converter376can provide a first waveform representing the sine of the phase information and a second waveform representing the cosine of the phase information. In an embodiment, the first waveform is a first quadrature component of the local oscillator signal, and the second waveform is a second quadrature component of the local oscillator signal.

The memory372typically stores information relating to time-independent variations between the quadrature components of the local oscillator signal. The DDFS generally monitors time-dependent variations between the quadrature components. For instance, the DDFS can monitor the quadrature components of the local oscillator signal in the analog domain. This can reduce the size and/or number of components needed in the receiver300.

Quadrature components of the local oscillator signal can be generated independently in accordance with the embodiment shown inFIG. 3. According to an embodiment, the local oscillator circuit370reduces a gain mismatch or a phase mismatch between the quadrature components. For example, the local oscillator circuit370can access the ROM lookup table to determine a phase offset or a frequency offset to be applied to one of the quadrature components.

The offset value stored in the ROM lookup table can indicate a phase difference between quadrature components of the local oscillator signal, for example. The offset value can be used to adjust the phase of at least one of the quadrature components of the local oscillator signal. Utilizing the offset value to correct the phase difference between the quadrature components of the local oscillator signal can eliminate the need for other quadrature correcting circuitry or software. For example, correcting the quadrature of the local oscillator signal using the local oscillator circuit370can reduce the number of components needed in the receiver300, thereby reducing the cost of the receiver in an embodiment.

The frequency of the local oscillator signal can be based on a frequency control word associated with the local oscillator signal. For instance, a clock signal can be multiplied by the frequency control word to calculate the frequency of the local oscillator signal. The offset value stored in the ROM lookup table can be used to calculate the frequency control word associated with the local oscillator signal. In an embodiment, the offset value is used to set the frequency of at least one of the quadrature components of the local oscillator signal.

According to an embodiment, the receiver300includes two DDFSs. For instance, a first DDFS can be used to convert phase information relating to a first quadrature component of the local oscillator signal to a first digitized sinusoidal waveform. The second DDFS can be used to convert phase information relating to a second quadrature component of the local oscillator signal to a second digitized sinusoidal waveform.

As shown inFIG. 3, digital representations of the local oscillator quadrature components are provided to digital-to-analog converters (DACs)380. The DACs380can convert the digital representations into analog local oscillator signals. For instance, the DACs380can directly generate the analog local oscillator signals. Alternatively, the DACs380can generate reference signals, which can be used by phase-locked loops (PLLs), such as PLLs392, to generate the analog local oscillator signals. According to an embodiment, the DACs380reduce jitter of the analog local oscillator signals.

Passing the local oscillator signal through a filter390can eliminate or reduce energy at frequencies outside the passband of the filter390. The filter390can be a low pass filter or a bandpass filter, to provide some examples. According to an embodiment, the filter390is a narrow-band filter. InFIG. 3, the filter390can be set at a particular frequency or range of frequencies that represents the desired channel of the RF input signal. In a first embodiment, the passband of the filter390is set based on the frequency of the local oscillator signal set by the local oscillator circuit370. In a second embodiment, the passband of the filter390is set at a predetermined frequency or range of frequencies, and the local oscillator circuit370manipulates the frequency of the local oscillator signal to be within the passband of the filter390. For example, the local oscillator circuit370can multiply the frequency of the local oscillator signal by a factor based on the selected channel of the RF input signal102.

The filter390generally includes at least one phase-locked loop (PLL)392. The PLLs392can provide the quadrature components of the local oscillator signal to the direct down conversion circuit310to be mixed with the RF input signal102. As shown inFIG. 3, a PLL392can be included for each quadrature component of the local oscillator signal. However, a single PLL can be used to filter both quadrature components.

The PLL392often manipulates the frequency of the local oscillator signal by a predetermined factor. According to an embodiment, the PLL392multiplies the frequency of the local oscillator signal by a factor in a range from approximately two to approximately thirty. The PLL392can increase the frequency of the local oscillator signal by a factor of six in a cable modem system, for example. The PLL392can increase the frequency of the local oscillator signal by a factor of twelve in a satellite communication system, to provide another example.

Using the PLL392to multiply the frequency of the local oscillator signal by a fixed value can allow the DAC380to sample at a lower rate. For example, the sampling rate of the DAC380is decreased by a factor that is proportional to the factor by which the frequency of the local oscillator is multiplied in an embodiment. Using the PLL392to multiply the frequency of the local oscillator signal by a fixed value can enable the size of the PLL392to be reduced, as compared to the situation in which the PLL392is used to multiply the frequency of the local oscillator by a variable factor to generate the frequency of the local oscillator signal.

According to an embodiment of the present invention, the direct down conversion circuit310, the demodulation circuit360, and the local oscillator circuit370are on a common substrate. One or more of the multiplexer320, the ADC330, the demultiplexer340, the DAC380, and the filter390can be on the common substrate, as well. Combining elements, such as those mentioned above, on a common substrate can reduce the cumulative circuit area required by the elements. Reducing the circuit area reduces the cost of the elements in an embodiment.

FIG. 4illustrates a receiver in which quadrature components of a local oscillator signal are generated using an oscillator according to an embodiment of the present invention. For example, the local oscillator circuit370can provide a digital representation of a local oscillator signal that does not include quadrature components. The digital representation can be received by a single DAC410, as shown inFIG. 4, though the scope of the invention is not limited in this respect. The DAC410converts the digital representation of the local oscillator signal to an analog reference signal. The reference signal often includes quantization noise and/or images, which can be related to the finite sampling rate of the DAC410, for example. The PLL420can filter the reference signal to reduce or eliminate the quantization noise and/or images. The PLL420generates the local oscillator signal based on the reference signal. The oscillator430generates quadrature components of the local oscillator signal to be provided to the direct down conversion circuit310. According to an embodiment, the PLL420includes the oscillator430. For instance, the oscillator430can be embedded in the PLL420.

The receiver400can include feedback440between the oscillator430and the PLL420. For example, the PLL420can use information received from the oscillator430via the feedback440to increase the frequency of the local oscillator signal. According to an embodiment, the PLL420generates the local oscillator signal having a frequency that is based on the reference signal and the information received from the oscillator430via the feedback440.

The oscillator430can be one or more ring oscillators or inductor-capacitor (LC) oscillators, to provide some examples. A ring oscillator generally has a greater bandwidth than a single LC oscillator and requires less circuit area than multiple LC oscillators. A ring oscillator typically includes a plurality of inverters. For example, the ring oscillator can include n inverters. Each inverter can have an input and an output. The inverters can be coupled, such that the output of a first inverter is coupled to the input of a second inverter, and the output of the second inverter is coupled to the input of a third inverter, etc. For instance, the output of the nth inverter can be coupled to the input of the first inverter. In an embodiment, the first mixer314aof the direct down conversion circuit310is coupled to a particular inverter. The second mixer314bcan be coupled to another inverter to enable the signal received by the first mixer314ato be 90° out of phase with the signal received by the second mixer314b.

An image filter can be coupled between the oscillator430and the direct down conversion circuit310. For instance, the image filter can filter the quadrature components of the local oscillator signal before passing the quadrature components of the local oscillator signal to the mixers314. The DAC410, the PLL420, the oscillator430, and/or the image filter can be disposed on a common substrate with the direct down conversion circuit310, the demodulation circuit360, and the local oscillator circuit370.

FIG. 5illustrates a receiver in which quadrature components of a local oscillator signal are generated using dividers according to an embodiment of the present invention. The oscillator430can oscillate at a frequency that is a multiple of the local oscillator (LO) signal frequency. The dividers510can divide the frequency of the signal provided by the oscillator430by a factor to provide the LO quadrature components at a particular frequency.

For example, the oscillator430can oscillate at a frequency twelve times the LO signal frequency. The dividers510can be divide-by-two dividers. In this example, the divide-by-two dividers can divide the frequency of the signal that is provided by the oscillator430by two to provide quadrature components having a frequency of six times the LO signal frequency.

The dividers510can be initialized one-half of an input cycle apart, for example. The dividers510can be triggered on alternating edges of the signal provided by the oscillator430. The first divider510acan be triggered on a rising edge of the signal provided by the oscillator430, and the second divider510can be triggered on a falling edge of the signal, or vice versa. The resulting LO quadrature components are typically 90° out of phase with each other. The oscillator430can be a differential oscillator to provide a signal having symmetrical rising and falling edges.

According to an embodiment, the dividers510are coupled between the PLL420and the oscillator430. For example, the dividers510can reduce the frequency of the signal provided by the PLL420before passing the signal to the oscillator430. In an embodiment, a single divider is coupled between the PLL420and the oscillator430.

FIG. 6illustrates a receiver in which quadrature components of a local oscillator signal are generated using a filter according to an embodiment of the present invention. The filter610generally removes jitter from the local oscillator signal. The filter610can be capable of accommodating a range of local oscillator frequencies.

The filter610can be an image filter. The filter610can be a low pass filter or a bandpass filter, to provide some examples. In an embodiment, the filter610is a poly-phase filter. The poly-phase filter generally includes a capacitor-resistor (CR) high pass filter portion and a resistor-capacitor (RC) low pass filter portion. LO quadrature components can be provided respectively by the two filter portions. At the 3 dB point, for example, the magnitude of the LO quadrature components is approximately the same, and phase of the two components differs by approximately 90°. The filter610can be adjustable to accommodate particular local oscillator frequencies. For instance, the frequency response of the filter620can be digitally programmed to accommodate a range of local oscillator frequencies.

Referring toFIG. 7, mixers350as shown inFIGS. 3-6are not necessarily needed according to an embodiment of the present invention. For instance, the local oscillator circuit370can set the frequency of the local oscillator sufficiently, so that a frequency offset need not be provided to the downconverted signal. In another example, the local oscillator circuit370and/or the ADC330reduce a gain mismatch or a linearity mismatch between quadrature components to a degree that mixers in the digital domain of the receiver300,400,500, or600are not necessary to adjust the frequency difference between quadrature components.

Although the receivers300,400,500,600, and700ofFIGS. 3-7, respectively, include a single ADC330for illustrative purposes, the scope of the present invention is not limited in this respect. Referring toFIG. 8, multiple ADCs830can be used to convert the downconverted quadrature components into digital signals. For instance, a first ADC830acan convert the first quadrature component into a first digital signal, and a second ADC830bcan convert the second quadrature component into a second digital signal. Using a different ADC830for each downconverted quadrature component eliminates the need to have the multiplexer320and the demultiplexer340, according to an embodiment.

FIG. 9illustrates a receiver having a baseband equalizer according to an embodiment of the present invention. Quadrature paths of the receiver900are generally not completely isolated from each other. For instance, a first quadrature component traveling along a first path910acan include information from a second quadrature component traveling along a second path910b, and vice versa. The baseband equalizer920can determine how much information from one quadrature component is included in the other quadrature component, and vice versa. The baseband equalizer920generally subtracts the second quadrature component information or a portion thereof from the first quadrature component. The baseband equalizer920typically subtracts the first quadrature component information or a portion thereof from the second quadrature component.

The baseband equalizer920can provide quadrature phase correction of the digitized downconverted signal. For instance, one of the demultiplexed quadrature components received from the demultiplexer340can be frequency shifted or phase shifted with respect to the other demultiplexed quadrature component. The baseband equalizer920can reduce or eliminate the difference in frequency and/or phase between the demultiplexed quadrature components.

The baseband equalizer920generally includes a phase detector and an amplitude detector. The phase detector can detect a difference of phase between quadrature components. The amplitude detector can detect a difference of amplitude between the quadrature components.

FIG. 10illustrates a flow chart of a method of processing a RF input signal102having a plurality of channels according to an embodiment of the present invention. A local oscillator circuit370can set the frequency of a local oscillator at block1010based on a selected channel of the plurality of channels. For instance, the frequency of the local oscillator can be modified using the channel selector110as shown inFIG. 1. A direct down conversion circuit310can directly downconvert the RF input signal102at block1020to provide a downconverted signal based on the local oscillator signal. For example, at least one mixer314can mix the local oscillator with the RF input signal102to provide the downconverted signal. In an embodiment, information not associated with the selected channel is removed from the downconverted signal. For instance, at least one low pass filter316can low pass filter the downconverted signal to remove unwanted harmonics. A demodulation circuit demodulates the downconverted signal at block1030to provide a demodulated signal.

Referring toFIG. 11, a low noise amplifier (LNA) amplifies the RF input signal102at block1105to ensure that its amplitude is above the noise floor of the receiver300,400, or500. Mixers314mix the RF input signal with local oscillator (LO) quadrature components at block1110to provide downconverted quadrature components. For example, the direct downconversion circuit310can downconvert the RF quadrature components to an IF frequency or to baseband. At least one low pass filter (LPF) filters the downconverted quadrature components at block1115to remove unwanted harmonics. A multiplexer320can multiplex the downconverted quadrature components at block1120at a multiplexing rate of at least twice the frequency of the downconverted quadrature components to provide a multiplexed signal. An analog-to-digital converter (ADC)330converts the multiplexed signal at block1125into a digital signal, which is demultiplexed at block1130by a demultiplexer340to provide digital quadrature components. In an embodiment, a demultiplexer340demultiplexes the digital signal at the multiplexing rate.

Mixers350can combine a frequency offset with the digital quadrature components at block1135to center the digital components in the Nyquist filter bandwidth. A demodulation circuit360demodulates the digital quadrature components at block1140, so that they can be provided to a symbol mapper or a forward error correction (FEC) circuit, to provide some examples. A local oscillator circuit370can use the demodulated quadrature components to set the frequency of the LO quadrature components at block1145. In an embodiment, setting the frequency of the local oscillator signal eliminates the need to combine the frequency offset with the digital quadrature components at block1135.

Mismatches can occur between quadrature components. For example, the phase of one quadrature component can shift with respect to the other quadrature component as the two components travel along their quadrature paths. A mismatch, such as the phase mismatch just described, can be corrected by adjusting the phase difference between the LO quadrature components at block1150. For example, the local oscillator circuit370can set the phase difference between LO quadrature components at a value different than 90° to take into consideration the mismatch. In an embodiment, setting the frequency of the LO quadrature components, as set forth at block1145, includes adjusting the phase difference between the LO quadrature components, as set forth at block1150.

Mixers350correct imbalances between quadrature components of the downconverted signal before the demodulator360demodulates the downconverted signal, according to an embodiment. The local oscillator signal is generally based on the demodulated signal. For instance, the local oscillator signal can be based on a difference between quadrature components of the downconverted signal.

At least one digital-to-analog converter (DAC)380converts the LO quadrature components to analog signals at block1155. A filter390can filter the LO quadrature components at block1160using at least one phase-locked loop (PLL)392, for example. The local oscillator circuit370generally performs operations using a digital representation of the LO signal, and the filter390typically performs operations using the analog LO signal provided by the DAC380. If a RF input signal is detected, as determined at diamond1165, processing the RF input signal continues with mixers314mixing the LO quadrature components and the RF input signal, as set forth at block1110. If no RF input signal is detected, processing the RF input signal ends.

FIG. 12illustrates a flow chart of a method of setting a frequency of a local oscillator signal according to an embodiment of the present invention. The local oscillator circuit370, for example, can read or sample the frequency, phase, or amplitude of the local oscillator signal to determine whether the phase or frequency of the local oscillator signal should be modified. The local oscillator circuit370can read or sample characteristics of the local oscillator signal using digital and/or analog representations of the local oscillator signal. For example, the local oscillator circuit370can track time-dependent errors in the receiver300,400,500,600,700,800, or900.

A memory372can store the difference between the phase or frequency of the local oscillator signal and the desired phase or frequency. The local oscillator circuit370can set the phase or frequency of the local oscillator based on the difference that is stored in memory372. The memory372, for example, can store a read-only memory (ROM) lookup table at block1210. The ROM lookup table can include an offset value. The offset value can be based on a difference between the actual phase or frequency of the local oscillator and a desired phase or frequency of the local oscillator signal. The local oscillator circuit370, for example, can compare the phase or frequency of the local oscillator signal to the desired phase or frequency to calculate the offset value. The local oscillator circuit370can retrieve the offset value from the ROM lookup table at block1220. At block1230, the local oscillator circuit370sets the frequency of the local oscillator signal based on the offset value.

Referring toFIG. 13, the ROM lookup table can include phase information relating to the local oscillator signal. For example, the phase information can include a plurality of values, with each value representing a desired phase of the local oscillator signal for an associated clock cycle. The local oscillator circuit370can retrieve the phase information at block1320. In an embodiment, the local oscillator circuit370retrieves a value at each successive clock cycle. A particular value can be associated with more than one clock cycle. The local oscillator signal is generated at block1330based on the phase information. The sine or cosine of the values can be associated with successive clock cycles, such that a discrete or digitized sinusoidal waveform is provided in an embodiment. For instance, the local oscillator circuit370can generate a digital representation of the local oscillator signal, and the DAC(s) can convert the digital representation to the analog local oscillator signal.

In an embodiment, the local oscillator circuit370combines a phase offset with the value of the phase retrieved from the ROM lookup table. For example, manipulating the phase of the local oscillator signal can account for a phase shift that occurs during processing of the RF input signal. The phase of one quadrature component can shift more or less than the phase of the other quadrature component in some instances. For instance, differences in the quadrature paths can create a phase shift between the quadrature components. The phase difference between LO quadrature components can be adjusted using the phase offset to account for this phase shift between quadrature components. In an embodiment, the local oscillator circuit370can use the phase offset to adjust the quadrature between the LO quadrature components to be a value other than 90°.

CONCLUSION