Direct conversion turner

A direct conversion tuner down-converts television signals, cable signals, or other signals directly from an RF frequency to an IF frequency and/or baseband, without an intermediate up-conversion step for image rejection. The direct conversion tuner includes a pre-select filter, an amplifier, an image reject mixer, and a poly-phase filter. The pre-select filter, amplifier, and the image reject mixer can be calibrated to provide sufficient image rejection to meet the NTSC requirements for TV signals. The entire direct conversion tuner can be fabricated on a single semiconductor substrate without requiring any off-chip components. The tuner configuration described herein is not limited to processing TV signals, and can be utilized to down-convert other RF signals to an IF frequency or baseband.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to television tuner circuits, and more specifically to a direct conversion television tuner for processing broadcast, cable, and satellite television signals.

2. Related 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 spacings. Satellite TV signals are typically transmitted at frequencies of approximately 980 to 2180 MHz.

Regardless of the transmission scheme, a tuner is utilized to down-convert the received RF 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 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 depending on the specific application and the corresponding display requirements.

To achieve a high level of image rejection, traditional TV tuners utilize a dual-conversion architecture having two mixers and a surface acoustic wave (SAW) filter. The first mixer up-converts the received RF signal to a first IF frequency (e.g. 1200 MHZ) that is fixed above the RF signal band, using a variable local oscillator (LO) signal. A SAW filter, centered at the first IF, selects the channel of interest and provides the necessary image rejection to prevent signal interference. The second mixer then down-converts the first IF to a lower frequency second IF, using a second fixed frequency LO signal. The second IF output is at baseband for a NTSC compatible signal. Alternatively, the second IF is at 36 or 44 MHZ for a cable system output that is fed into a set-top box or a cable modem. Channel selection is realized by adjusting the first LO signal so that the desired down-converted channel(s) falls in the narrow passband of the SAW filter. The remaining channels are rejected by the SAW filter.

Due to advances in silicon integrated circuit (IC) technology, most of the tuner components (i.e. mixers, local oscillators, etc.) can be fabricated on a single silicon IC, with the exception of the SAW filter. The SAW filter is a mechanically resonant device that is typically fabricated on a ceramic substrate, and therefore cannot be integrated on-chip with the other tuner components. As such, the SAW device remains a discrete component in the TV tuner design, which prevents the TV tuner from being fabricated on a single silicon substrate.

A single chip solution is highly desirable for TV tuners and cable modems. The single chip solution will reduce or eliminate component adjustment during manufacturing, and therefore lead to a reduction in manufacturing time and cost. The single chip solution will likely improve electrical performance of the tuner because there are parasitics associated with driving a signal off-chip for processing. Additionally, the single chip solution will reduce the size of the tuners, which becomes more critical for non-TV set applications. Therefore, what is required is a TV tuner architecture that can be implemented on a single semiconductor substrate.

Furthermore, a TV tuner with low power requirements is also desired. Cable TV operators plan to offer voice telephone service and/or Internet access to their customers over currently installed cable lines. The telephone traffic will be transmitted over the cable lines at RF frequencies, along with the TV programming. As such, the TV tuner will likely be utilized to down-convert the telephone traffic to baseband for user consumption. In the current POTS (plain old telephone service) system, government regulations require that the phone companies supply sufficient power over telephone lines to operate the customer's telephone. This allows the customer to utilize the telephone during a power outage, to call the electric company for example. If a similar requirement were placed on voice-over-cable service, then the cable company would have to supply enough power over the cable lines to operate the TV tuner.

The conventional dual conversion tuner architecture requires a few watts of power for operation, which is a relatively high power requirement. The high power drain occurs because the dual conversion scheme requires two mixers that are driven by two local oscillators operating at RF frequencies. If voice-over-cable is to be realizable, then the tuner power requirements should be reduced to approximately 0.5 watt of power dissipation.

Therefore, what is needed is a tuner architecture that has good electrical characteristics (e.g. high image rejection, and good linearity), low power requirements, and which can be implemented on a single semiconductor substrate.

SUMMARY OF THE INVENTION

The present invention is directed to a direct conversion television tuner for processing television or cable signals. The direct conversion television tuner down-converts a selected channel directly from an RF frequency to an IF frequency and/or baseband, without performing an intermediate up-conversion frequency translation as in conventional tuners.

The direct conversion TV tuner includes a pre-select filter, an amplifier, an image reject mixer, and an IF filter. The pre-select filter receives an RF signal having multiple TV channels. The image reject mixer down-converts a selected channel to an IF frequency that is within the passband of the IF filter. Channel selection is performed by tuning the frequency of a local oscillator signal that drives the image reject mixer, and thereby tuning the channel that is translated into the passband of the IF filter. The frequency of the IF output signal is flexible, and can be set so it is compatible with conventional set-top box and cable modem applications. Additionally, the IF output signal can be converted to a digital format using an A/D converter. In embodiments, the IF output signal is further processed to generate a NTSC compatible signal that can directly drive a television set without any further processing.

In embodiments, the pre-select filter, amplifier, and the image reject mixer are individually calibrated to improve image rejection at the selected channel frequency. The calibration is performed by injecting a test signal into the tuner input, where the frequency of the test signal is at the selected channel frequency. After which, the I/Q balance of the pre-filter is separately determined based on the test signal, bypassing the effects of the amplifier and the image reject mixer. Any I/Q imbalance in the pre-select filter is corrected by adjusting filter parameters. Once the filter is calibrated, the I/Q balance of the amplifier is determined based on the test signal input, bypassing the effects of the image reject mixer. Any I/Q imbalance in the amplifier is corrected by adjusting the amplifier parameters. Once the amplifier is calibrated, the I/Q balance of the image reject mixer is determined based on the test signal input. Any I/Q imbalance in the image reject mixer is corrected by adjusting parameters of the image reject mixer, and/or by adjusting the I/Q balance of the local oscillator that drives the image reject mixer.

The calibration procedure can be repeated for each new channel selection to compensate for I/Q imbalances that vary over frequency. Additionally, the calibration procedure can be repeated for the same channel after a defined time delay to compensate for temperature effects or other miscellaneous time sensitive effects. The result of the calibration procedure is that tuner image rejection preferably meets or exceeds corresponding requirements over the input frequency band of interest.

An advantage of the present invention is that the direct conversion tuner has a single conversion architecture. More specifically, the selected TV channel is directly down-converted to an IF frequency in a single frequency translation by the image reject mixer. In contrast, conventional tuners have a dual conversion architecture, which performs two frequency translations (one up-conversion and one down-conversion), along with off-chip filtering. The direct conversion tuner has lower power requirements because the single frequency conversion only requires one high frequency local oscillator and one mixer, compared with two of each for the conventional dual conversion tuner.

Additionally, the direct conversion tuner can be fabricated on a single semiconductor substrate, which does not require any off-chip filtering. This is possible because the calibrated image reject mixer provides high image rejection, without using an off-chip SAW filter. As a result, the direct conversion tuner enables a single-chip solution for TV tuner and cable modem applications.

DETAILED DESCRIPTION OF THE EMBODIMENTS

1. Terrestrial, Cable, and TV Satellite Services

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 spacings. Satellite TV signals are typically transmitted at frequencies of approximately 980 to 2180 MHz. Regardless of the transmission scheme, a tuner is utilized to down-convert the received RF signal to an IF signal or a baseband signal, which is suitable for processing and display on a TV or computer screen. The desired tuner output can vary depending on the specific transmission scheme, as described below.

Terrestrial TV service is transmitted over the air from ground-based (as opposed to satellite-based) antennas. Terrestrial TV service can be subdivided into conventional analog TV and digital TV ( including DTV and HDTV).

FIG. 1Aillustrates a conventional analog TV environment100having a TV tuner106that receives an RF TV signal102from an antenna104. RF TV signal102carriers conventional analog TV programming. The analog tuner106down-converts the TV signal102to a NTSC compatible signal108. NTSC stands for National Television Standards Committee, and the NTSC standard specifies the parameters for a baseband TV video/audio signal that drives a conventional raster TV monitor. As such, the NTSC baseband signal108can be directly coupled to the TV monitor110.FIG. 1Billustrate the frequency spectrum of a NTSC signal108. As shown, the NTSC signal108has a frequency spectrum at baseband with a 6 MHZ bandwidth.

FIG. 2illustrates a digital TV environment200having an antenna204, a tuner206, a digital processor208, and a TV monitor210. The antenna204receives a digital TV signal202, which can be a high definition TV signal (HDTV), for example. The tuner206down-converts the signal202to an intermediate frequency (IF) signal207. The processor208further processes the IF signal207, for display on the TV monitor210. In embodiments, the processor208converts the IF signal207to a NTSC compatible signal to drive the TV monitor210.

Cable TV service is carried to the customer using coaxial cables (or the equivalent), where the cables are typically buried underground. Traditional cable TV carries analog TV programming. Modern cable TV service can include digital cable TV, Internet access, and even telephone service. The telephone service is typically provided using a packet-based communications protocol, such as Internet protocol (IP).

FIG. 3Aillustrates an analog cable TV environment300including a cable302, a set top box305having a tuner306and a set-top box processor310, and a TV monitor312. The cable302carries a cable signal304having analog cable TV programming. The tuner306down-converts the cable signal304to produce an analog IF signal308carrying analog TV signals, and a NTSC signal309. The NTSC signal309exists at baseband and can be routed directly to the TV monitor312as described above. The analog IF signal308carries additional TV features (e.g. on-screen program information) that are not carried by the NTSC signal309. The analog IF signal308is sent to the set-top processor310for further processing in order to generate a signal suitable to drive the TV monitor312.

For comparison purposes,FIG. 3Billustrates the frequency spectrum of the analog IF signal308and the NTSC signal309. As described above, the NTSC signal has a 6 MHZ bandwidth at baseband. The analog IF signal308has a 6-8 MHZ bandwidth at 36 MHZ, or 44 MHZ, or some other useful IF frequency. In other words, the IF frequency of the signal308can be selected so as to be compatible with the processor310. In embodiments, the IF signal308is modulated according to quadrature amplitude modulation (QAM), which is a multi-level multibit signal format that is well known to those skilled in the arts.

FIG. 4illustrates a digital cable TV environment400having a cable402, a tuner406, a processor410, a telephone412, a computer414, and a TV monitor416. The cable402carries a cable signal404that includes one or more of the following: digital cable TV signals, Internet access, and telephone traffic. The tuner406down-converts the cable signal404to produce an IF signal408carrying digital data, and a NTSC signal409. The NTSC signal409can be sent directly to the TV monitor416for display as described above. The digital IF signal408carries the digital cable TV, Internet access, and telephone traffic. In embodiments, the digital IF signal408is a QAM signal having a bandwidth of 6-8 MHZ and existing at 36 MHZ or 44 MHZ, and having a frequency spectrum that is similar to the frequency spectrum of the analog IF signal308(in FIG.3B).

Still referring toFIG. 4, a processor410separates the content of the digital IF signal408, and provides any necessary post-down-conversion processing. After processing, telephone traffic is sent to the telephone412, Internet traffic is sent to the computer414, and TV programming in sent to the TV monitor416. The digital cable TV signals may be converted to the NTSC format prior to display.

The processor410is meant to be a functional representation and is not meant to be limiting. In other words, the processor410can include one or more distinct processors to separately process the voice, Internet, and TV programming signals, as per the specific application and signal protocols. When the cable signal404is providing Internet access, then the tuner406and processor410are often referred to as being part of a cable modem, as will be understood by those skilled in the arts.

FIG. 5illustrates a satellite TV environment500having an antenna504, a satellite tuner506, a processor510, and a TV monitor512. The antenna504receives a satellite TV signal502and sends it to the tuner506. The tuner506down-converts the signal502to an IF signal508. For most satellite TV systems, the IF signal508is a digital signal. The processor510further processes the IF signal508, for display on the TV monitor512. In embodiments, the processor510converts the IF signal508to a NTSC compatible signal that drives the TV512.

To summarize the TV delivery systems inFIGS. 1-5, the tuner output is either a NTSC signal at baseband, or an IF signal. The NTSC signal can directly drive a TV monitor. The IF signal exists at higher IF frequency (36 or 44 MHZ) and is chosen to be compatible with specific set-top box and/or cable modem devices.

2. Image Rejection and Linearity

Image rejection and linearity are two key performance parameters for RF down-conversion. Image rejection and linearity requirements depend on the specific application and the corresponding display requirements. Because of its importance, image rejection is explained further as follows.

FIG. 6Aillustrates a tuner600having a mixer and low pass filter. The tuner600down-converts an RF input602to an IF output604, using a LO input606.FIG. 6Billustrates an example frequency spectrum608that illustrates down-conversion and image rejection as performed by the tuner600. More specifically, the spectrum608illustrates an exemplary RF input602having a RF carrier610at 800 MHZ, and two RF channels612,614at 794 MHZ and 806 MHZ, respectively. For purposes of illustration, the LO input606is set to 800 MHZ so that the RF input602is frequency translated to 0 Hz by the tuner600. More specifically, the RF carrier602is translated to 0 Hz to create a DC signal618. The RF channels612,614are translated to −6 MHZ and +6 MHZ, respectively, resulting in IF signals616and620as shown. However, negative frequencies are a mathematical convenience, and fold over into the positive frequency domain. Assuming no image rejection, the signal616folds on top of the signal620as shown. Therefore, without image rejection, channels612and614interfere with each other during down-conversion, and neither channel is recoverable. Image rejection is calculated as the relative amplitude of the desired image compared to the undesired image. For example, if the channel614is the desired channel, then the image rejection of the tuner600is the ratio of the signal620amplitude compared to the signal616amplitude. If tuner600had perfect image rejection, then the amplitude of the signal616would be zero.

Before describing the invention in detail, it is useful to describe a conventional tuner for purposes of comparison.FIG. 7illustrates a conventional tuner700that is a dual conversion tuner. The tuner700performs two frequency translations (one up-conversion, one down-conversion) to meet the high image rejection requirement. The tuner700receives a RF signal701having multiple TV channels that each have a 6 MHZ bandwidth, and collectively occupy a frequency range of 57-860 MHZ. The tuner700down-converts a selected channel from the RF signal701, and outputs the selected channel as an IF signal724. In embodiments, the frequency of the IF signal724is 36 MHZ, or 44 MHZ, or some other desired IF frequency.

The detailed operation of the conventional tuner700is as follows. An RF amplifier704amplifies the RF input signal701prior to frequency translation. A first mixer706mixes the RF input signal701with a variable LO signal708. The LO710varies the frequency of the LO signal708from 1200 to 2100 MHZ. Therefore, the RF input signal701is up-converted to a frequency above the 57-860 MHZ band, resulting in an up-converted signal707. The up-converted signal707is sent off-chip to a SAW filter712, which has a narrow passband at 1200 MHz. The SAW filter712selects a desired channel713that falls within its narrow passband, and substantially rejects all of the remaining channels. Therefore, a particular channel is selected by varying the frequency of the LO signal708so that the desired channel is up-converted into the passband of the SAW filter712.

Still referring toFIG. 7, the desired channel713is sent back on-chip to a second mixer714, which is driven by a fixed local oscillator718. The mixer714down-converts the desired channel using a fixed local oscillator signal716, resulting in an IF signal715. Given that the SAW filter is centered at 1200 MHZ, the frequency of the LO signal716is appropriately selected to provide an IF at 36 MHZ, 44 MHZ, or some other desired IF frequency. The off-chip IF filter720further removes any unwanted harmonics and images from the IF signal715, resulting in the IF signal721. The IF signal721is amplified by the IF amplifier722, to produce the IF output724.

The dual conversion architecture of the conventional tuner700has several disadvantages. For instance, there are two of each component including two mixers, two high frequency local oscillators, and two off-chip filters. This results in a high DC power requirement (about 3 watts) because each of the two mixers is driven by a separate high frequency local oscillator, where each local oscillator (LO) must generate sufficient power at high frequencies to operate its corresponding mixer. A second disadvantage is that the SAW filter is an off-chip component that prevents a single chip tuner solution, which has inherent manufacturing advantages. Additionally, electrical performance is degraded by taking a high frequency signal off-chip (and back on-chip) because of the inherent parasitics that are involved.

4. Direct Conversion Tuner

FIG. 8illustrates a direct conversion tuner800according to embodiments of the invention. The tuner800receives an RF input signal801having multiple TV or cable channels, and down-converts the RF input signal801to an IF signal810. In embodiments, the IF signal810is at 36 MHZ, or 44 MHZ, which are popular frequencies for set-top box and cable modem devices. The tuner800includes a pre-select filter802, an amplifier804, an image reject mixer806, and a polyphase filter808. In embodiments, all of the mentioned components are on a common semiconductor substrate814. The detailed operation of the tuner800is described in reference to a flowchart900that is shown in FIG.9.

In step902, the pre-select filter802receives the RF input signal801. In embodiments, the RF input signal801includes multiple frequency channels from 57 MHZ to 860 MHZ, with each channel having a 6 MHZ bandwidth.

In step904, the pre-select filter802pre-selects the input frequency band of interest, and rejects out of band signals, producing a RF signal803. The pre-select filter802is a lowpass or bandpass filter having a desired passband of interest. For example, for broadcast or cable TV, the frequency band of interest is approximately 57-860 MHz. For satellite reception, the frequency band can be much higher depending on the specific system that is used.

In step906, the amplifier804amplifies the RF signal803, resulting in a RF signal805. In embodiments, the amplifier804is a low noise amplifier.

In step908, the image reject mixer806directly down-converts the RF signal805to an IF signal807having in-phase and quadrature components807aand807b, respectively. The image reject mixer806mixes the RF input signal805with a LO signal811that is generated by a variable LO812. For direct down-conversion, the LO signal811is in-band. In other words, the frequency of the LO signal811is in the same frequency band as the channels that make-up the RF input signal801.

In embodiments, the LO812is a voltage controlled oscillator (VCO), where the oscillator frequency is controlled by a voltage input. In alternate embodiments, the LO812includes one or more phase locked loops.

In step910, the polyphase filter808selects the channel of interest from the component IF signals807aand807b, resulting in an output IF signal810. The polyphase filter808has a narrow passband at is the desired IF frequency. For example, the polyphase filter808can be fabricated to have a passband centered at 36 or 44 MHZ. Channel selection is performed by changing the frequency of the LO signal811, and thereby causing the desired channel to shift into the passband of the polyphase filter808.

FIG. 10Adepicts a table1002that further illustrates channel selection using the tuner800, when down-converting to a 36 MHZ IF. More specifically, the table1002indicates two LO frequencies that can be used to down-convert specific RF input channels to an IF frequency of 36 MHz. For example, if channel4is the desired channel, then the frequency of the LO812should be set to 111 MHZ or 39 MHZ. By setting the LO812to 111 MHZ or 39 MHZ, then CH4will be down-converted to 36 MHZ and is shifted into the passband of the polyphase filter808. Continuing this example, assuming that the LO is set to 111 MHZ, an unwanted image frequency occurs at 147 MHZ, which is channel16in this example. However, this image frequency is not down-converted to the 36 MHZ IF because of the image reject features of the image reject mixer806.

FIG. 10Billustrates a table1004that is similar to the table1002except that the LO frequencies are determined for a 44 MHZ IF, instead of a 36 MHZ IF.

In embodiments, the steps904-908are performed using differential circuitry that operates on differential signals, as will be understood by those skilled in the arts. In alternate embodiments, the steps904-908are performed using single-ended circuitry that operates on single-ended (i.e. non-differential) signals.

4.1 Image Reject Mixer

As shown, the direct conversion TV tuner800includes an image reject mixer806for down-conversion. The image reject mixer806is key to performing the down-conversion operation in a single frequency conversion, instead of the conventional dual-conversion operation. In other words, the image reject feature supplants the need for a dual conversion architecture and the off-chip SAW filter.

FIG. 11illustrates an image reject mixer1100as an embodiment of the image reject mixer806in FIG.8. Image reject mixer1100includes an in-phase divider1102, component mixers1104a,1104b, and a quadrature divider1106. The in-phase divider1102receives the RF input signal805, and divides the signal805into component signals1103aand1103b, where the signals1103aand1103bare substantially equal phase and equal amplitude. The quadrature divider1106receives the LO signal811and divides the LO signal811into component LO signals1105aand1105b, where the signal1105bis phase shifted by 90 degrees relative to the signal1105a. The mixer1104amixes the I component signal1103awith the LO signal1105a, resulting in the in-phase IF component807a. The mixer1104bmixes the component signal1103bwith the LO signal1105b, resulting in the quadrature IF component807b. The in-phase IF component807aand the quadrature IF component807bare combined by the polyphase filter808(FIG.8). The image rejection occurs when the I and Q components807a,807bare combined because the phase relationship between I and Q components causes signal cancellation at the image frequency.

Theoretically, infinite image rejection is achievable if the I and Q channels of the mixer1100are perfectly balanced at the frequency of interest. However, if the phase relationship between the I and Q channels varies from 90 degrees at some frequency, then the actual image rejection deteriorates at this frequency. Additionally, if the amplitude varies between the I and Q channels, then the image rejection also deteriorates. The amplitude and phase relationship between the I and Q channels is often collectively referred to as I/Q balance. Perfect I/Q balance is achieved when the amplitude response of the I and Q channels is equal over frequency, and the phase difference between the I and Q channels is 90 degrees over frequency.

The invention is not limited to the IQ image reject mixer that is shown in FIG.11. Other image reject mixers can be utilized, as will be understood by those skilled in the arts, including but not limited to passive mixers.

As discussed above, the actual image rejection that is achieved for an image reject mixer deteriorates if there are amplitude and/or phase errors between the I and Q channels (i.e. I/Q imbalance). More specifically, a small I/Q imbalance over frequency can cause a large variation in the corresponding image rejection. In a high volume manufacturing environment, these amplitude and phase errors can originate from part-to-part variations of electrical parameters that are within stated tolerances of the electrical components. The result is that image rejection can be reduced to below a desired level.

4.2.1 Overview of Tuner Calibration for I/Q Balance

The tuner800can be calibrated for each channel frequency to correct for I/Q imbalance, and therefore improve image rejection. The calibration is done by injecting a test signal at the selected channel frequency into the tuner, and adjusting the tuner components to improve I/Q balance, and therefore image rejection. The calibration procedure is described in more detail according to a flowchart1200that is shown in FIG.12.

In step1202, a new channel selection is received. The selected channel is one of several channels carried by the RF input signal801. In embodiments, the RF input signal801includes multiple channels from 57 MHZ to 860 MHZ, with each channel having a 6 MHZ frequency bandwidth.

In step1204, the LO812generates a test signal814at the selected channel frequency. The test signal814is injected into the input of the filter802.

In step1206, the pre-select filter802is calibrated to improve I/Q balance at the frequency of the selected channel.

In step1208, the amplifier804is calibrated to improve I/Q balance at the frequency of the selected channel.

In step1210, the image reject mixer806is calibrated to improve I/Q balance at the frequency of the selected channel. More specifically, regarding phase, the image reject filter806is calibrated so that the phase difference between the I and Q channels is as close to 90 degree as possible, so as to maximize image rejection.

In step1212, the calibration procedure, defined by steps1204-1210, can be repeated after a pre-defined time delay. The calibration procedure is repeated, even though the channel has not been changed, to compensate for circuit parameter variations that are time sensitive. For instance, electrical characteristics of components may vary over time because of temperature variations, especially at initial power-up.

4.2.2 Detailed Discussion of Tuner Calibration for I/Q Balance

FIG. 13illustrates tuner1300, which is a more detailed embodiment of the tuner800. The tuner1300will be utilized to further explore tuner calibration for image rejection, according to embodiments of the present invention. The tuner1300includes a bandpass pre-select filter1304, an amplifier1306, an I/Q image reject mixer1310, a local oscillator1314, and a polyphase filter1318, which are all similar to the corresponding components in tuner800. Additionally, the tuner1300includes a synthesizer1312, a signal amplitude and phase analyzer1316(hereinafter signal analyzer1316), a signal processing module1322, a register/control module1332, and a local processor1334, which are discussed further below.

The tuner1300operates similar to the tuner800, but includes some additional features. More specifically, the tuner1300receives an RF input signal1302having multiple VHF and UHF channels. The tuner1300down-converts a selected channel from the RF input signal1302to produce an analog IF signal1320. The analog IF signal1320exists at an IF frequency of 36 MHZ, or 44 MHZ, or some other useful frequency, and is compatible with currently available set-top boxes. The down-conversion and channel selection are performed similar to that described for the tuner800. The signal processing module1322also receives the analog IF signal1320and converts it into a digital signal1330using an AID converter1324, a FIR filter1326, and a MUX1328.

Additionally, the tuner1300can be calibrated to maximize I/Q balance for each new channel selection. The calibration of the tuner1300is a three step process, where the filter1304, the amplifier1306, and the image reject mixer1310are individually calibrated to maximize overall tuner image rejection. The calibration is performed by injecting a test signal at the selected channel frequency into the filter1304, and then separately examining I/Q balance at the outputs of the filter1304, amplifier1306, and the image reject mixer1310. As described herein, the maximum image rejection is achieved when there is a balanced amplitude, but a 90 degree phase shift between the I and Q channels. The calibration of the tuner1300is described in detail with reference to the flowchart1400(FIGS. 14A-B) as follows.

In step1402, the local processor1334receives a channel selection input1336, which identifies a TV channel that is to be displayed.

In step1404, the LO1314generates a test signal1338at the selected channel frequency. The test signal1338is injected into the input of the pre-select filter1304.

In step1406, the signal analyzer1316measures the I/Q balance of the filter1304, based on the test signal input1338. More specifically, the switches1340are closed so that the filter1304output is connected to the signal analyzer1316, through the test path1308. As such, the amplifier1306and the I/Q mixer1310are by-passed so the filter characteristics can be analyzed without being influenced by the amplifier1306or the mixer1310. The signal analyzer measures the amplitude and phase for the I and Q channels at the output of the filter1304, and determines the I/Q balance between the I and Q channels. As discussed herein, the I/Q balance is the amplitude balance and phase difference between the I and Q channels. The I/Q balance information is forwarded to the processor1334for further processing. In embodiments, the signal analyzer incorporates an RF power meter.

In step1408, the processor1334calibrates the filter1304to maximize the I/Q balance at the selected channel frequency using a filter calibration signal1344. More specifically, the parameters of the filter1304are adjusted so that the amplitude of the I and Q channels are balanced, and so that the phase difference between the I and Q channels is substantially 90 degrees. In embodiments, the filter parameters that are adjusted include one or more of the following: center frequency, bandwidth, gain, and phase.

In step1410, the steps1404-1408are repeated as necessary to produce sufficient I/Q balance in the filter1304at the selected channel frequency.

In step1412, the LO1314re-generates the test signal1338at the selected channel frequency, and injects the test signal1338into the bandpass filter1304.

In step1414, the signal analyzer1316measures the I/Q balance of amplifier1306, based on the test signal input1338. More specifically, the switches1340are opened, and switches1342are closed. Therefore, the filter1304output is connected to the amplifier1306input, and the amplifier1306output is connected directly to the signal analyzer1316, bypassing the image reject mixer1310. Since the filter1304was calibrated in steps1404-1410, any I/Q imbalance that is measured by the signal analyzer1316is likely to be caused by the amplifier1306. The I/Q balance information for the amplifier1306is forwarded to the processor1334.

In step1416, the processor1334calibrates the amplifier1306to maximize the I/Q balance at the selected channel frequency using an amplifier calibration signal1346. More specifically, the parameters of the amplifier1306are adjusted so that the amplitude of the I and Q channels are balanced, and so that the phase difference between the I and Q channels is 90 degrees. In embodiments, the parameters that are adjusted include the gain and phase of the amplifier.

In step1418, the steps1412-1416are repeated as necessary to achieve sufficient I/Q balance in the amplifier1306at the selected channel frequency.

In step1420, the LO1314re-generates the test signal1338at the selected channel frequency, and injects the test signal1338into the bandpass filter1304.

In step1422, the signal analyzer1316measures the I/Q balance of the image reject mixer1310. More specifically, the switches1340and1342are opened so that the test path1308is deactivated. Additionally, the switches1315aand1315bare closed. Therefore, the filter1304, amplifier1306, and I/Q mixer1310are connected together, with the mixer1310output being connected to the signal analyzer1316. Since the filter1304and amplifier1306were calibrated in steps1404-1418, any I/Q imbalance that is measured by the signal analyzer1316is likely to be caused by the image reject mixer1310.

In step1424, the processor1334calibrates the image reject mixer1310to maximize the image rejection at the selected channel frequency. This is a two step process as described by flowchart1500in FIG.15. First, in step1502, the parameters of the mixer1310are adjusted using a mixer cal signal1348to get the best the I/Q balance for the mixer1310alone. In embodiments, the parameters that are adjusted include the gain and phase of the mixer. Next, in step1504, the relative phase-shift between the I and Q components of a LO signal1311is adjusted to produce the maximum I/Q balance at the output of image reject mixer1310. In other words, the phase difference between the I and Q components of the LO signal1311is varied from 90 degrees to compensate for any residual I/Q imbalance that is left in the mixer1310after step1502.

In step1426; the steps1420-1424are repeated as necessary to produce sufficient I/Q balance at the output of the mixer1304.

4.2.3 Automatic Frequency Control and Automatic Gain Control

In a terrestrial TV transmission system, the frequency channels of the RF input signal can vary by a few percent from their expected frequency. These frequency errors cause the NTSC signal to be shifted off of baseband, which produces unwanted distortion to appear in the TV picture that is ultimately displayed. The frequency errors stem from inaccuracies in the various TV transmitters that are utilized to transmit the individual TV channels. These inaccuracies are not typically present in cable and satellite transmissions because there is inherently more uniformity in the transmitter equipment that is used for cable and satellite systems.

To address frequency variation in terrestrial systems, conventional TV baseband processors examine the NTSC signal to determine if the NTSC signal is shifted in frequency, which indicates the corresponding RF input signal is also shifted in frequency. If a frequency shift is detected, then an automatic frequency control (AFC) signal is sent to the TV tuner to adjust the frequency of the local oscillator and thereby compensate for the frequency error in the RF input signal. In other words, the local oscillator frequency is adjusted as necessary to track the frequency error of RF input signal. Referring to the tuner1300, the local processor1334receives an AFC signal1348that is utilized to adjust the LO1314as needed, to track any frequency errors in the RF input1302.

Similar to the AFC signal1348, an automatic gain control (AGC) signal1350is received by the processor1334. The AGC signal1350contains gain control information that is used to compensate for amplitude variations of the channels in the RF input signal1302. For example, one channel in the signal1302may be stronger or weaker than the other channels in the signal1302. Therefore, the gain of amplifier1306can be adjusted appropriately to compensate for these amplitude variations.

The tuner1300outputs an analog IF signal1320at 36 MHZ, 44 MHZ, or some other IF frequency that is compatible with available set-top box or cable modem devices. However, some cable customers do not want to utilize a set-top box for cost/complexity reasoning. Therefore, it is desirable for the tuner to output a baseband NTSC signal, which can drive a TV monitor without the need for a set-top box.

FIG. 16illustrates a post processing module1600that converts the analog IF signal1320to a baseband NTSC output signal1608. The post processing module1600includes an A/D converter1602, a digital signal processor1604, and a D/A converter1606. During operation, the AID converter1602samples and down-converts the analog IF signal1320according to a clock signal1609, to generate a digital signal1603. In embodiments, the clock signal1609is a sub-harmonic clock signal so that the A/D converter1602sub-samples the analog IF signal1320. The DSP1604processes the digital signal1603to produce a NTSC digital signal1605. In other words, the NTSC signal1605contains the NTSC information at baseband, but in a digital format, instead of the desired analog format. Finally, the D/A converter1606converts the digital signal1605to an analog NTSC signal1608, which can drive a TV monitor.

5. Other Applications

The tuner invention described herein has been discussed in terms of processing broadcast television signals and/or cable signals. However, the invention is not limited to TV applications. The tuner invention is applicable to any application that requires down-conversion of an RF signal to an IF signal and/or a baseband signal, as will be understood by those skilled in the arts. These other applications are within the scope and spirit of the present invention.