Abstract:
Televisions, especially HDTV, require tremendous bandwidth, however a time interval to switch communication channels may result in the loss of several image frames or a large portion of a single image frame. Unfortunately, small glitches in television signals are not tolerable since this affects the quality of the entertainment. Thus, wireless television tuners are unacceptable to most users because of frequency of glitches occurring as a result of channel hopping. A novel wireless television tuner system is thus disclosed that switches—hops—between frequency channels for transmitting television data to the wireless television tuner. The novel tuner uses a fractional-N (FRAC-N) synthesizer with an improved settling time in order to reduce potential glitches occurring as a result of the channel hopping.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to television tuners and more particularly to a television tuner incorporating a synthesizer allowing more rapid settling time.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the world today, communication is one of the most significant luxuries. People in affluent countries have voice communication systems, data communication systems, and television systems.  
           [0003]    Unfortunately, for each communication medium, dedicated wiring is typically required. For example, television wires are usually run within a house in the form of 75 Ohm coaxial cabling. Telephone wires are twisted pair wires. Data communication often requires Ethernet cabling and so forth. For new homes, the wiring is often done before the walls are finished. This provides adequate communication ports in each room. Unfortunately, the communication ports are often in inconvenient locations. Also, for an older home, in order to support more modern forms of communication wiring is required.  
           [0004]    Another approach currently being explored for providing communication services within a house or building employs wireless transmission of data signals. Telephone signals, television signals, Internet signals and so forth can be easily transmitted at high data rates throughout a small area. This alleviates any need for wiring and, more importantly, retains a level of freedom and flexibility to home design and decoration.  
           [0005]    Clearly, with the advent of wireless communication, much of what we consider technologically proven will no longer be the case. For example, television sets will change. If television channels are available throughout a house via a wireless connection, other homes in the area and other communication signals will provide a challenge in terms of interference and signal to noise ratios. Cordless telephones experienced this problem as they became more widely accepted and, today, telephones are provided with tens of communication channels for providing the voice data signal and a best channel is preferably the one chosen.  
           [0006]    During a single telephone communication, a wireless telephone may switch between several different communication channels to ensure continuity of quality of voice communication. Because voice communication data rates are relatively slow, the time to switch communication channels is either not noticeable or is well tolerated by users.  
           [0007]    For televisions, the problem is different. Televisions, especially HDTV, require tremendous bandwidth. The interval to switch communication channels may result in the loss of several image frames or a large portion of a single image frame. Whereas people will tolerate small pauses in voice communication, they will not happily tolerate glitches in television signals since this affects the quality of the entertainment. Thus, wireless television tuners are unacceptable to most users because of frequency of glitches occurring as a result of channel hopping.  
           [0008]    Therefore, there is a pressing need for a television tuner that can support communication signals at multiple frequencies with fast switching between the signals.  
         OBJECT OF THE INVENTION  
         [0009]    It is an object of the present invention to provide a television tuner that can support communication signals at multiple frequencies with fast switching between signals at different frequencies.  
         SUMMARY OF THE INVENTION  
         [0010]    In accordance with the invention there is provided a front end tuner for receiving modulated signals and selecting therefrom modulated signals in accordance with a frequency characteristic thereof and providing an output signal including information representative of said selected signals, comprising: a frequency conversion circuit including a mixer for beating a local oscillator signal with signals within a predetermined band of frequencies to generate signals having frequencies within a predetermined channel band of frequencies and being representative of said selected signals, characterized in that the local oscillator signal is generated using a phased locked loop having a fractional-N synthesizer therein.  
           [0011]    In accordance with the invention there is provided a method of tuning a television signal comprising the steps of:  
           [0012]    providing a local oscillator for providing a first oscillating signal;  
           [0013]    providing a reference oscillator for providing a reference oscillating signal, the reference  
           [0014]    oscillator other than an integer factor of the local oscillator frequency;  
           [0015]    adjusting the local oscillator relative to the reference oscillating signal by the steps of:  
           [0016]    a. comparing an edge of a division of the first oscillating signal, the division other than an integer N division to the reference oscillating signal edge to provide a difference signal,  
           [0017]    b. adjusting the local oscillator in dependence upon the difference signal until the first oscillating signal is locked at a frequency other than an integer multiple of the reference oscillating signal;  
           [0018]    providing the first oscillating signal to a mixer; and,  
           [0019]    providing the television signal to the mixer for beating with the first oscillating signal to produce a mixed signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The invention will now be described with reference to the attached drawings in which:  
         [0021]    [0021]FIG. 1 is a simplified block diagram of a prior art home wireless communication system;  
         [0022]    [0022]FIG. 2 is a simplified block diagram of a prior art home wireless communication system shown in two adjacent homes;  
         [0023]    [0023]FIG. 3 is a simplified block diagram of a similar prior art home wireless communication system communicating with any of a plurality of frequency channels;  
         [0024]    [0024]FIG. 4 is a simplified flow diagram of a method of implementing communication using the wireless communication system of FIG. 3;  
         [0025]    [0025]FIG. 5 is a simplified block diagram of a home wireless communication system comprising a base station in communication with a wireless TV receiver;  
         [0026]    [0026]FIG. 6 is a simplified block diagram of a FRAC-N synthesizer;  
         [0027]    [0027]FIG. 7 is a simplified block diagram of a television tuner circuit according to the invention including the FRAC-N synthesizer circuit; and,  
         [0028]    [0028]FIG. 8 is a simplified flow diagram of a method of implementing a FRAC-N synthesizer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Referring to FIG. 1, shown is a simplified block diagram of a home wireless communication system  100  for supporting telephone communication. A base station  102  is wired into a telephone signal wire  104  carrying a phone signal from a central office to a home. The receiver is in communication via radio frequency (RF) signals with a telephone handset  106  carried by an individual within or about the home. Here, the base station communicates using a fixed RF frequency channel. Anywhere within or about the home within range of the RF signal of the base station, indicated by a circle, the handset is functional for receiving voice data and for transmitting same. The base station and receiver shown both support digital data communication for ensuring sound quality in either direction.  
         [0030]    Referring to FIG. 2, a similar home wireless communication system is shown in two adjacent homes. Here, circles about the base stations  202 ,  204  identify their range. As is evident, there are significant areas of interference between the two base stations and where a handset may, inadvertently, communicate via an incorrect base station. This is problematic.  
         [0031]    Referring to FIG. 3, a similar home wireless communication system to that of FIG. 1 is shown, but here the base station  302 ,  304  and the handset operate to communicate within any of a plurality of frequency channels. As is evident, even when two such wireless communication systems are adjacent one another, they can operate without interference on different communication channels.  
         [0032]    Referring to FIG. 4, a method of implementing communication using the wireless communication system of FIG. 3 is shown. A communication is initiated with a scan through the plurality of frequency channels to detect other communications. When available, a channel that is completely undetected is selected. This channel remains in use until an amount of noise above a given threshold is detected and then the communication system switches—hops—between frequency channels. Because of the nature of channel hopping, this activity is commonly detectable since a momentary silence or lack of voice data occurs. Of course, as long as the silence is short, such an event does not significantly affect communication quality or content. For voice communication 0.1 s is typically considered short.  
         [0033]    Referring to FIG. 5, a wireless communication system  50  is shown wherein a base station  51  is in communication with a wireless television receiver  52 . The base station is provided with the same features as the base station  302  but the data transmission rate is much higher. For example for HDTV, over 60 Megapixels are transmitted each second. This is much higher than the 64 Kilobits of data used to transmit voice data and is likely 4 or more orders of magnitude greater. With this increased bandwidth comes significant problems. For example, whereas the system of FIG. 3 has 0.1 s in which to channel hop—losing 6.4 KBits of data, the wireless television transmitter has only 0.00001 s or less to do the same. Also, though a small silence does not greatly affect conversation, a significantly distorted image will affect viewing and, as such, even 6.4 Kbits of data corresponding to approximately 640 pixels is noticeable and undesirable.  
         [0034]    One approach to overcoming this problem is to fix frequency channels for the receiver and the base station. Unfortunately, past experience has brought to light that such an approach is limited and therefore undesirable as discussed with reference to FIG. 2.  
         [0035]    Another approach to overcoming this problem is to design a television tuner having a rapid settling time to support very fast channel hopping. In designing a phase lock loop circuit, the settling time is a time in which the loop is not yet locked on a new reference time source. During this time, data extracted from received signals is questionably correct. Typically, until the settling time is passed, data integrity is suspect and data sampling is not performed. As such, a settling time in the order of several screen pixel data values is desired for television communication channel switching. This requires very high speed switching. For example, HDTV has approximately 1 million data points per screen times  60  screens per second. This is 60 million data points a second requiring a settling time in the order of 1/30,000,000 of a second.  
         [0036]    Conventional phase locked loop devices operating at the frequencies of television signals and supporting the bandwidth required cannot support this settling time. The settling time is inversely proportional to the bandwidth of the loop filter. Thus, a narrow band loop filter results in increased settling time and a wider band loop filter results in potentially shorter settling times. It is proposed that in order to reduce settling time within the loop, the loop filter is widened in the phase locked loop device.  
         [0037]    Unfortunately, in order to widen the loop filter for the phase locked loop device using conventional phase lock loop technology requires a difficult frequency plan selection step which may even prove impossible. Using lower frequency reference signals requires a narrow band loop filter but allows for many frequencies to be even multiples of the lower frequency reference. For example, if the reference is 250 KHz, then any frequency at one quarter, one half, three quarters or an even MHz is an even multiple of the reference frequency. Thus, a lower frequency reference is advantageous for use in tuner design. However, these advantages also prevent channel hopping from being performed rapidly in these same tuners.  
         [0038]    In order to widen the bandwidth of the loop filter, the reference frequency must be increased. In order to achieve this, either the frequency plan must support this increased reference frequency which is unlikely or a synthesizer supporting fractional divisions of clock signals is necessary. Such a synthesizer is a FRAC-N synthesizer which is used according to the invention to facilitate frequency plan design and to allow for locking of the phase locked loop to the higher frequency reference oscillating signal. This allows the television tuner to channel hop with shorter settling times and therefore permits operation of a wireless television tuner based system analogous to a wireless telephone communication system commonly available.  
         [0039]    Referring to FIG. 6, a block diagram of a FRAC-N synthesizer  600  is presented. A reference frequency f R  provided by reference oscillator  602  drives phase/frequency detector  604 . The phase/frequency detector  604  provides a voltage to voltage controlled oscillator (VCO)  606  providing an output frequency f O  in dependence thereupon. The output frequency f O  is coupled via FRAC-N divider  608  to the phase/frequency detector  604  providing a feedback signal to the phase/frequency detector  604 . The FRAC-N divider  608  comprises two counters enabling the divider to switch from divide by integer N to divide by N+1 in dependence upon a control signal provided by controller  610 . The output frequency f O  is stepped in x/m fractions of the reference frequency f R . This is realized, for example, by dividing by N+1 every m cycles and by dividing by N the other cycles resulting in an effective fractional ratio RFRAC=f O /f R =N+x/m, where x=0, 1, 2, . . . , m−1. Of course more complex control is also possible as is other forms of fractional frequency division when desired.  
         [0040]    Referring to FIG. 7, a block diagram of a wireless television tuner circuit including the FRAC-N synthesizer circuit is shown. Here, a conventional synthesizer is replaced with a FRAC-N synthesizer. The loop filter is then widened to allow for improved settling time. The wireless receiver is for receiving the entire bandwidth of a cable television signal about a known carrier channel.  
         [0041]    Television tuner  400  receives the wireless television RF signal from antenna  402  through bandpass and image reject notch filter  404 . Bandpass and image reject notch filter  404  limits the signals entering TV tuner  400  so that a minimum number of undesired signals propagate within TV tuner  400 . Filter  404  attenuates signals not in the television signal range about the known carrier frequency. Typically, known interference signals, such as FM broadcast, shortwave service signals, signals in the intermediate frequency band and Citizen Band radio signals, are specifically rejected by filter  404 .  
         [0042]    Preamplifier  406  and mixer  405  receives the output of bandpass and image reject notch filter  404  and raises the signal level (10 dB) with minimum increase in the noise level (typically 8-10 dB). The gain of preamplifier  406  is controlled by automatic gain control (AGC)  438 , so that when a very strong signal enters TV tuner  400 , overall gain is reduced, resulting in less distortion in the preamplifier than without the gain reduction.  
         [0043]    The output of preamplifier  406  is sent to bandpass and image reject notch filter  412 , with the same basic requirement of minimizing the passage of potential interference signals. Filter  412  is external to preamplifier and mixer  405  and is comprised of a plurality of discrete elements, including capacitors, inductors and varactor diodes.  
         [0044]    The output of bandpass and image reject notch filter  412  is then sent back to mixer  408  in preamplifier and mixer  405 . Mixer  408  mixes the output of filter  412  with the output of a local oscillator, FRAC-N synthesizer  415 , which has a frequency chosen to be higher than the desired receiver carrier by 43.75 MHz. Thus, the output of mixer  408  is 43.75 MHz to be compatible with present television systems. There also is an image signal due to mixer  408  at 91.5 MHz above the input frequency, which is removed by filter  404  and filter  412 .  
         [0045]    FRAC-N synthesizer  415  receives an input frequency reference signal and outputs the status signals AUTOMATIC FREQUENCY CONTROL (AFC) ERROR and FREQUENCY (FREQ) LOCK. Within the FRAC-N synthesizer, is a loop filter to ensure loop stability. Because the reference signal frequency is optionally very similar to the synthesized frequency, the loop filter supports rapid settling time. A local oscillator signal is output from FRAC-N synthesizer  415  to mixer  408 .  
         [0046]    The 43.75 MHz output signal of mixer  408  then passes through SAW (surface acoustic wave) filter  416 , which limits the bandwidth of the signal to only one (1) channel (6 MHz for NTSC standard) and applies a linear attenuation in frequency known as the Nyquist slope around the visual carrier frequency. The linear attenuation by SAW filter  416  converts the signal from a vestigial sideband signal to one which is equivalent to a single sideband with a carrier, so that the frequency response of the signal after demodulation is flat over the video bandwidth. SAW filter  416  is very “lossy” (on the order of 25 dB), so the input to SAW filter  416  is amplified by a preamplifier (not shown) by a corresponding amount to minimize noise effects.  
         [0047]    The output of SAW filter  416  is input to intermediate frequency (IF) amplifier  420  in IF and baseband signal processor  410 . IF amplifier  420  provides most of the overall gain of TV tuner  400  and receives gain control from AGC  448 .  
         [0048]    The output of IF amplifier  420  is sent to video detector  422  and is also sent to video carrier filter  424 . This is the stage at which video demodulation is performed. Video detector  422  is essentially a mixer with the local oscillator input connected to the output of video carrier filter  424  through carrier amplitude limiter  426 . The output of the carrier limiter  426  is an in-phase representation of the video carrier signal without any modulation applied to it. The output of carrier limiter  426  is received by video detector  422 , which mixes the output of carrier limiter  426  with the output of IF amplifier  420 .  
         [0049]    AFC frequency discriminator  440  is used in the prior art device to detect the difference between the carrier frequency contained in the output of carrier limiter  426  and a known valid carrier frequency reference. The output signal on the output of AFC frequency discriminator  440  is an error signal that is used to drive FRAC-N synthesizer  415  in a direction for reducing the error between the output of carrier limiter  426  and the known valid carrier frequency reference. The output of the video detector  422  is a baseband video signal combined with several high frequency mixing artifacts. These artifacts are removed by a video baseband filter  430 . The output of video baseband filter  430  is fed to synchronization pulse clamp (sync clamp)  432 , which sets the level of the sync pulses to a standard level.  
         [0050]    Next, the output of sync clamp  432  is sent to noise invertor  434 , which removes large noise spikes from the signal. The output of noise invertor  434  is sent to video buffer  436 , which is configured to drive fairly high circuit board impedances of approximately 1000 to 2000 ohms.  
         [0051]    The output of noise invertor  434  is also sent to AGC (automatic gain control)  438 , which compares the level of the synchronization pulses to the signal blanking level to measure the incoming signal strength and generates a gain control signal which is used by IF amplifier  420  and RF preamplifier  406  to dynamically adjust the gain of the TV tuner  400  for the correct level at the final output port.  
         [0052]    The audio signal is an FM signal that follows the same path as the video through video detector  422 . At the output of video detector  422 , the audio signal appears as a subcarrier at 4.5 MHz, due to the fact that the audio signal comes into prior art TV tuner 400 4.5 MHz higher in frequency than the desired video carrier. The audio subcarrier is passed on to an FM quadrature demodulator. The FM quadrature demodulator is comprised of a mixer, audio second detector  450 , and a 90-degree (at 4.5 MHz) phase shifter, audio carrier phase shifter  460 . The output of the audio second detector  450  is a baseband audio signal, which is filtered by lowpass (30 kHz) filter  452  to remove any undesired high frequency components. The output of lowpass filter  452  is finally passed on to audio buffer  454  that drives an audio amplifier that ultimately drives a speaker. Serial digital interface  444  receives SERIAL DATA and SERIAL CLOCK inputs to provide control and update status for the television receiver.  
         [0053]    Of course, other forms of RF signal tuners for use with television signals are also supported when the synthesizer therein is replaced with a FRAC-N synthesizer as disclosed herein.  
         [0054]    Referring to FIG. 8, a simplified flow diagram of a method of implementing a FRAC-N synthesizer is shown. In a typical arithmetically locked loop, a reference frequency signal and another signal are locked using a simple mathematical relationship based on a single counter. For example, if a first reference clock is at 8 KHz and a second clock is at 16 KHz, then a simple counter to count to two is gated by the 16 KHz clock allowing its output clock edge—every two—to be compared with the 8 KHz clock edge. Thus, the 16 KHz clock is locked to the 8 KHz clock. Same or opposite edge locking is achievable depending on design requirements.  
         [0055]    With a FRAC-N synthesizer, a dithering approach is used to clock synchronization. For example, if a 20 MHz clock is to be locked with an 8 MHz clock, this is achieved by alternately locking on a count of two and three. Thus a divider of 2.5 is achieved which is the ratio between the clock frequencies. Further advantageously, such a circuit allows for support of several different clock frequencies when designed accordingly. For example, a single synthesizer could be implemented to support locking of a 20 MHz, 24 MHz, and 28 MHz signal to an 8 MHz reference merely by changing the alternating count amounts between (2, 3), (3, 3), and (3, 4).  
         [0056]    Though using the above frequencies the synthesizer seems simple, it is more typically the case that frequencies vary more significantly from a simple 3:2 ratio. For example, a 1 MHz reference signal could be used to lock a 43.56 MHz signal using a counter of (43.44) alternating 56 times out of one hundred on the 44 count and 44 times out of 100 on the 43 count. Thus, very accurate and specific clock locking is achievable.  
         [0057]    In the flow diagram of FIG. 8, a reference clock is provided, its rising edge is used to lock to. A second other clock signal is provided by a VCO. The clock signal is used to strobe a counter and thereby count to a predetermined number. When the number is reached, a comparator indicates this and the rising edge of the comparator signal is compared with the rising edge of the reference clock. The VCO is provided a signal in dependence thereon to adjust a phase of the second other clock signal.  
         [0058]    While the VCO is being adjusted, a new value is provided to the comparator and the counter is reset. The entire process repeats itself.  
         [0059]    The method for providing the new value also repeats itself, here it shown repeating itself after N iterations. In the above examples N was described as 2 and N was described as 100. Of course, N could be any number and even when N is two, four iterations or six iterations are optionally employed. Of course, though the term repeats itself is used, the pattern of repetition may be complex. For example, in the above example of 43.56 MHz, locked to a one MHz reference signal, the pattern will adjust for 56/44 counts which could repeat every 100 clock adjust operations or, in this case 56:44 is reduceable to 14:11 requiring 25 counts before a repetition of the cycle.  
         [0060]    Once the FRAC-N synthesizer is implemented accordingly, the loop filter is widened to provide a desired settling time to support channel hopping in a wireless implementation of the television tuner. Of course, the FRAC-N synthesizer may also have other significant advantages when used in a television tuner circuit.  
         [0061]    Since the settling time of the loop is inversely proportional to the bandwidth of the loop filter, such an implementation allows for greatly reduced settling times and thereby supports dynamic frequency hopping for signals received at the television tuner of the invention.  
         [0062]    Accordingly, by using a Frac-N synthesizer for a phase locked loop, the invention supports faster channel hopping in, for example, a wireless communication system allowing for multichannel wireless television communication within a home that does not interfere with similar systems in adjoining homes or buildings. Of course, the invention also supports truly mobile television sets such as Sony® Watchman® televisions should they be equipped with a receiver for wireless HDTV signal broadcasts from a base station. With the mobile sets, the advantages in fast channel hopping become even more present and urgently needed. Of course, there may be numerous other applications for the television tuner device described hereinabove.  
         [0063]    Numerous other embodiments may be envisaged without departing from the spirit or scope of the invention.