MXPA97009742A - Superficial acoustic wave filter for a digi satellite receiver - Google Patents

Abstract

The present invention relates to a digital satellite television receiver for receiving and processing digital signals modulated in the respective ones of a plurality of radiofrequency signals received from an outdoor unit, including a satellite receiving antenna, and a block converter , a tuner, which comprises: a radio frequency input to receive a plurality of radio frequency signals provided by the block converter, a local oscillator to generate a signal from the local oscillator, a mixer having a first input coupled to the input of radiofrequency, a second input coupled with the local oscillator, and an output where the intermediate frequency signal is produced, and a surface acoustic wave filter coupled with the output of the mixer to provide filtering of the intermediate frequency signal, including the configuration of the symbol to reduce the interference between symbols, the surface acoustic wave filter having a lithium tantalate substrate

Description

SURFACE ACOUSTIC WAVE FILTER FOR A TUNER OF A DIGITAL SATELLITE RECEIVER The invention relates to a surface acoustic wave (SAW) filter of a tuner for a satellite receiver, especially one that can receive and process television signals transmitted in digital form. A satellite television reception system includes an "outdoor unit" that includes a dish-shaped receiving antenna, and a "block" converter, and an "indoor unit" that includes a tuner and a processing section. signal. The block converter converts the entire range of radiofrequency signals of a relatively high frequency transmitted by the satellite to a lower and more manageable range of frequencies. In a conventional satellite television reception system for receiving and processing television information transmitted in analog form, the radiofrequency signals transmitted by the satellite are in the C (3.7 to 4.2 GigaHertz) and Ku (from 11.7 to 14.2. 2 GigaHertz), and converted by the block converter to the L-band (from 900 to 2,000 GegaHertz) as a "block". A radio frequency filter section of the tuner of the indoor unit selects one of the radiofrequency signals provided by the block converter, corresponding to the selected channel, and the selected radio frequency signal is converted again by a local mixer / oscillator section of the tuner converted to a lower intermediate frequency (IF) range for filtering and demodulation. Typically, the intermediate frequency range has a center frequency of 479 MegaHertz. Analog satellite television systems typically employ F modulation, and a baseband video signal is easily obtained from the intermediate frequency signal 479 by an FM demodulator after being filtered by an intermediate frequency filter. A relatively simple surface acoustic wave (SAW) device can provide adequate filtration. In the newer satellite television systems, such as the DSSMR (Direct Satellite System) available from Thomson Consumer Electronics of Indianapolis, Indiana, television information is transmitted in digital form. The radiofrequency signals are transmitted by the satellite in the Ku band, and are converted by the block converter to the L band. The frequency range of the radiofrequency signals transmitted by the satellite is a little smaller, for example, between 12.2 and 12.7 GigaHertz, that one for the analog satellite television system, and the frequency range of the radio frequency signals produced by the block converter, in accordance with the same, is a bit smaller, for example, between 950 and 1,450 MegaHertz. As with analog satellite television reception systems, the radiofrequency signal corresponding to the selected channel must be reduced in frequency to an intermediate frequency range for filtering and demodulation. However, the type of filtering ("symbol configuration") required in the digital satellite television receiver can not be easily performed at the relatively high IF frequency (eg, 479 MegaHertz) used in a television receiver by analog satellite, especially using a SAW device. As a result, a relatively expensive digital filter will be required to filter the demodulated digital signals. Alternatively, the tuner can employ a second conversion step, to convert the first intermediate frequency signal of a relatively high frequency (e.g., 479 MegaHertz), to a second lower frequency signal (e.g. less than 100 MegaHertz) for filtration. However, the second conversion stage adds an undesirable cost to the receiver. It is also undesirable that the tuner of the digital satellite television receiver can be built using components that are already commercially available, and therefore, that are relatively inexpensive. Specifically, in this aspect, it is desirable that the tuner can be constructed using a commercially available integrated circuit (IC), incorporating an assured phase cycle (PLL) to control the frequency of the local oscillator. Since there are a large number of tuner phase locked phase integrated circuits for conventional television receivers that receive and process the available conventional and cable television transmission signals, it is particularly desirable that the tuner of the digital satellite television receiver can be built using this conventional tuner phase-locked integrated circuit. The present invention relates to a SAW filter for a tuner of a digital satellite television receiver, which comprises a single conversion stage for converting the selected radio frequency signal received from the block converter of the outdoor unit to a signal of intermediate frequency. For reasons that will be described below in detail with reference to the exemplary embodiment of the invention, it is desirable that the intermediate frequency signal has a nominal frequency in the order of 140 MegaHertz (MHz). For this tuner, the SAW must be of the type that has a lithium tantalate substrate. Different aspects of the invention will be described in detail with reference to the accompanying drawing, in which: Figure 1 is a block diagram of a digital satellite television reception system including a tuner constructed in accordance with an aspect of the invention . Figure 2 is a block diagram of an integral phase locked phase control tuning circuit used in the tuner shown in Figure 1. Figure 3 is an idealized amplitude against the frequency response of a SAW device used in the tuner shown in Figure 1. Figure 4 is a graphic representation of certain characteristics of a SAW device as a function of temperature and frequency, which are useful for understanding the selection of the particular type of SAW device that is desirably used in the tuner shown in the Figure 1. The invention will be described with reference to a digital satellite television system, wherein the television information is transmitted in a coded and compressed form according to a predetermined digital compression standard, such as MPEG. MPEG is an international standard for encoded representation of moving images and associated audio information developed by the Motion Pictures Expert Group. The television information is represented by a series or stream of digital signals organized in packets corresponding to the respective video and audio portions of the television information. The digital signals are modulated on a radiofrequency carrier signal, in what is known as QPSK (Quaternary Phase Shift Keystroke) modulation, and the radio frequency signal is transmitted to a satellite in Earth orbit, from where it returns to transmit back to earth. A satellite typically includes a number of transponders to receive and relay the respective modulated radiofrequency carriers. DirecTV's satellite television transmission system, operated by the Hughes Corporation of California, is a digital satellite television transmission system of these. In the digital satellite television reception system shown in Figure 1, the radiofrequency signals modulated with the digital signals representing video and audio information are transmitted by a satellite (not shown), and are received by an antenna parabolic 1. The radiofrequency signals received, of a relatively high frequency (for example, in the Ku frequency range between 12.2 and 12.7 GigaHertz) are converted by a block converter 3 to radiofrequency signals of a relatively lower frequency (for example example, in the L band between 950 and 1,450 MHz). The block converter 3 includes a low noise amplifier, and therefore, is often referred to by the initials "LNB". The antenna 1 and the LNB 3 are included in a so-called "outdoor unit" 5 of the reception system. The remaining portion of the receiving system is included in a so-called "indoor unit" 7. The indoor unit 7 includes a tuner 9 for selecting the particular radio frequency signal corresponding to the desired channel from the plurality of radiofrequency signals received from the outdoor unit 5, and for converting the selected radio frequency signal to a lower intermediate frequency (IF) signal. The tuner 9 is constructed in accordance with the present invention, and will be described in detail below. A QPSK demodulator 11 demodulates the output signal of the tuner 9, to produce two digital signals in analog quadrature phase (I and Q). A decoder 13 produces a stream of video and audio packets from the I and Q signals. The decoder 13 includes analog-to-digital converters, to convert the analog I and Q signals, to the respective series of digital samples, and a error corrector that corrects the transmission errors based on the error codes embedded in the transmitted digital signals. The video and audio packages of the digital current, produced by the decoder 13, are directed by a transport unit 15 to the respective sections of a digital signal processing unit (DSP) 17. The digital satellite television receiver described so far, is similar to the DSSMR satellite television receiver commercially available from Thomson Consumer Electronics, Inc. of Indianapolis, Indiana. The present invention relates to the details of the implementation of the tuner 9. The tuner 9 receives the radiofrequency signal provided by the LNB 3 at an input 901. The radio frequency input signals are filtered by a broadband filter 203, amplified by a radio frequency amplifier 905, and filtered by a tunable bandpass filter 907. The resulting radio frequency signal is coupled to a first input of a mixer 909. A local oscillator signal, produced by a local oscillator (LO 911, is coupled with a second inlet of the mixer 909. The output of the mixer 909 is amplified by an amplifier 912, and coupled with the input of an intermediate frequency filter 913 comprising a SAW device. The output of the intermediate frequency filter 913 is coupled with the output 915 of the tuner 9.

The frequency of the local oscillator 911 is controlled by an assured phase cycle (PLL) 917, which comprises an integrated circuit (IC). The frequency of the local oscillator signal is controlled by the phase-locked integrated circuit, according to the data generated by a microprocessor 919. As shown in Figure 2, the phase-locked integrated circuit includes a divider frequency "prescalar" 917-1, to divide the frequency of the local oscillator signal, follower by a programmable frequency divider (+ N) 917-3. The phase-locked integrated circuit also includes an amplifier 917-5, which in combination with an external crystal network 917-7, comprises a reference frequency oscillator. The output of the reference frequency oscillator is coupled to the output of the reference frequency divider (+ R) 917-9. The output signals of the programmable divider (+ N) 917-3, and of the reference divider (+ R) 917-9, are coupled with the respective inputs of a phase detector 917-11. The output signal of the phase detector 917-11 is coupled with an amplifier 917-13, which together with an external filter network 917-15, comprises an integrator to produce a control voltage for the local oscillator 911. When ensures the assured phase cycle, the frequency of the local oscillator signal is proportionally related to the frequency of the reference frequency signal produced by the reference frequency divider (+ R) 917-9, by the programmable division factor (N) of the programmable divider (+ N) 917-3. The programmable division factor N is controlled by the data generated by the microprocessor 919. As noted above, it is desirable that the tuner have the following three characteristics: 81) include only one conversion stage; (2) providing an intermediate frequency signal with a sufficiently low frequency to allow a SAW device to be used for the digital symbol configuration, as well as a normal intermediate frequency filtering; and (3) being able to be constructed using an integrated phase locked tuning control circuit conventionally used for the transmission and cable receivers. This is done in the present tuner by selecting the center frequency of the intermediate frequency signal at 140 MHz, and controlling the frequency of the local oscillator signal at 140 MHz below the RF signal of the frequency for the respective channel (transponder). As a result, with a frequency range for the radio frequency input signal between 950 and 1,450 MHz, the frequency range of the local oscillator signal is between 810 and 1,310 MHz. The IF frequency of 140 MHz allows a SAW device with the required characteristics to be used, as will be described later. The frequency range of 810 to 1,310 MHz of the local oscillator signal allows a conventional phase locked tuning control integrated circuit to be used conventionally for transmission and cable receivers. This integrated circuit is the TSA5515T commercially available from Philips Semiconductors and others. In this aspect, it is noted that the maximum frequency of the local oscillator available using the TSA5515T and similar integrated circuits, is of the order of 1,300 MHz, which is adequate. It will be noted that different IF frequencies are possible, and in general terms, the IF frequency can be selected to be of the order of the difference between the highest frequency of the radiofrequency signal received from the LNB, and the highest local oscillator frequency available, through the use of a phase locked integrated cycle of conventional tuner normally used in conventional transmission and cable television receivers. Desirably, the tunable bandpass filter 907 should remove the image of the desired radio frequency signal, which is at a frequency 280 MHz lower than the frequency of the desired radio frequency signal. In a digital transmission system, it is desirable to perform what is known as "symbol configuration", to provide a signal relatively free of interference between symbols. This interference can occur due to improper filtering of the high frequency energy of the pulse components of the digital signals in the transmitter, due to the limitations of the bandwidth. The desired symbol configuration function can be shared between the transmitter and the receiver. In the receiver, it is desirable that the intermediate frequency filter provide the symbol configuration, as well as the normal intermediate frequency filtering function, so that a separate digital filter is not required. By way of example, the intermediate frequency filter can provide what is known in the art of digital filters as a "high root cosine" response. This response is shown in Figure 3. A SAW device can be used to provide the configuration of the symbol, provided that its characteristics are carefully selected, in accordance with aspects of the invention that will now be described. Two properties of a SAW filter are considered important for being applied to a tuner of a digital satellite television receiver. These are: (1) the global change or phase shift of the filter characteristic (ie the change of the center frequency) with the temperature; and (2) the change in relative bandwidth (i.e., the amplitude of the bandpass divided by the center frequency). The most common type of SAW device uses lithium niobate (LiNb03) as a substrate. A lithium niobate SAW has a typical temperature coefficient of -90 ppm / degree C. The present tuner uses another type of SAW, which uses lithium tantalate (LiTa03) as a substrate. A lithium tantalate SAW has a typical temperature coefficient of -23 ppm / degree C. Assuming a temperature range of -20 to +70 degrees C, and a center frequency of 140 MHz, the following evaluation can be made with respect to to the change in frequency with temperature: the lithium niobate produces a temperature change of 140E6X-90E-6X +/- 45 = +/- 567.0 Hz; and lithium tantalate produces a temperature change of 140E6X-23E-6X +/- 45 = +/- 144.9 Hz, Assuming that a change of 500 kHz is undesirable, which can produce a degradation of the noise margin of slightly less than 0.1 dB, a lithium niobate SAW exceeds the target of 500 kHz over the range of temperature radiation. To use a lithium niobate SAW, the center frequency would have to be reduced to 123 MHz, or lower to maintain the 500 kHz target. With a lithium tantalate SAW, the center frequency only needs to be 483 MHz or lower. With respect to the relative bandwidth, the following is noted. In general, wider relative bandwidth filters are more difficult to make, and filters with a relative bandwidth greater than 15 to 18 percent require the use of a lithium niobate SAW. The lower relative bandwidth requirements allow the use of any type of SAW. An amplitude filter of 20 MHz with a center frequency of 140 MHz only has a relative bandwidth of 14 percent. If a relative bandwidth of 18 percent were required, an IF central frequency of 110 MHz would be required. Figure 4 graphically summarizes the features discussed above. Figure 4 shows the frequency regions where a lithium niobate SAW or a lithium tantalate SAW, or both, can satisfy the requirements for temperature change and relative bandwidth, and the result when both requirements are considered. As you can see: an IF frequency below 110 MHz, requires a SAW filter of lithium niobate; an IF between 110 MHz and 123 MHz can use either a lithium niobate SAW or a lithium tantalate SAW; An IF between 123 MHz and 483 MHz requires a SAW of lithium tantalate, and a SAW for an IF beyond 483 MHz can not satisfy the requirements, due to excessive temperature change. For a central frequency of 140 MHz, a SAW of lithium tantalate should be used.

Claims (15)

1. A digital satellite television receiver for receiving and processing digital signals modulated in the respective ones of a plurality of radiofrequency signals received from an outdoor unit (5), including a satellite receiving antenna (1), and a block converter (3), a tuner (9), which comprises: a radio frequency input to receive a plurality of radiofrequency signals provided by the block converter (3); a local oscillator (911) for generating a local oscillator signal; a mixer (909) having a first input coupled with the radio frequency input, a second input coupled with the local oscillator (911), and an output where the intermediate frequency signal is produced; and a surface acoustic wave (SAW) filter (913) coupled with the output of the mixer (909) to provide filtering of the intermediate frequency signal, including the symbol configuration to reduce inter-symbol interference; the surface acoustic wave filter (913) having a lithium tantalate substrate.

2. The apparatus described in claim 1, wherein: the block converter (3) provides radiofrequency signals of the order of 950 to 1450 MHz frequency range, and the local oscillator (911) has a frequency range of the order of 810 at 1310 MHz. The apparatus described in claim 2, wherein: the surface acoustic wave filter (913) has a nominal center frequency of 140 MHz. The apparatus described in claim 1, wherein: the oscillator local (911) is included in an assured phase cycle (911, 917) to control the frequency of the local oscillator (911). The apparatus described in claim 4, wherein: the phase locked cycle (911,917) includes an integrated phase locked phase control tuning circuit of the type that is normally used in terrestrial broadcast television receivers and conventional cable. The apparatus described in claim 5, wherein: the frequency of the intermediate frequency signal is selected to be of the order of the difference between the highest frequency of the radio frequency signals received from the block converter (3), and the highest local oscillator frequency available through the use of the integrated phase locked phase tuning control circuit. The apparatus described in claim 6, wherein: the block converter (3) provides radio frequency signals in a frequency range of the order of 950 to 1,450 MHz, and the local oscillator (911) has a frequency range of order of 810 to 1310 MHz. The apparatus described in claim 6, wherein: the surface acoustic wave filter (913) has a nominal center frequency of 140 MHz. 9. In a digital satellite television receiver for processing digital signals modulated in the respective ones of a plurality of radiofrequency signals received from an outdoor unit (5) including a satellite receiving antenna (1) and a block converter (3), a tuner (9) comprising: a radio frequency input to receive a plurality of radiofrequency signals provided by a block converter (3); a local oscillator (911) for generating a local oscillator signal; a mixer (909) having a first input coupled with the radio frequency input, a second input coupled with the local oscillator (911), and an output where an intermediate frequency signal is produced; And a surface acoustic wave (SAW) filter (913) coupled with the output of the mixer (909), for filtering the intermediate frequency signal, including the configuration of the symbol to reduce inter-symbol interference; having this surface acoustic wave filter (909) a center frequency that varies less than 500 kHz on a temperature scale of 45 ° C, and a relative bandwidth of less than 15 percent. The apparatus described in claim 9, wherein: the local oscillator (911) is included in the secured phase cycle (911, 917) to control the frequency of the local oscillator (911). The apparatus described in claim 10, wherein: the secured phase cycle (911, 917) includes an integrated phase locked phase control tuning circuit of the type that is normally used in terrestrial broadcast television receivers. and conventional cable. The apparatus described in claim 11, wherein: the frequency of the intermediate frequency signal is selected to be in the order of the difference between the highest frequency of the radiofrequency signals received from the block converter (3) , and the highest local oscillator frequency available, through the use of the integrated phase locked phase tuning control circuit. The apparatus described in claim 12, wherein: the block converter (3) provides radiofrequency signals in a frequency range of the order of 950 to 1450 MHz, and the local oscillator (911) has a frequency range of order of 810 to 13.10 MHz. The apparatus described in claim 12, wherein: the surface acoustic wave filter (913) has a nominal center frequency of 140 MHz. The apparatus described in claim 9, wherein: the surface acoustic wave filter (913) has a lithium tantalate substrate.