HK1015563B - Tuner for digital satellite receiver - Google Patents

Description

Tuner of digital satellite receiver

The present invention relates to a tuner for a satellite receiver, in particular a tuner capable of receiving and processing television signals transmitted in digital form.

A satellite television receiving system includes an outdoor unit having a dish and a block converter, and an indoor unit having a tuner and a signal processing section. The block converter converts the entire relatively higher frequency range RF signal transmitted by the satellite into a more manageable lower frequency range signal.

In conventional satellite television reception systems for receiving and processing television information transmitted in analog form, the RF signals transmitted by the satellite are in the C (3.7 to 4.2 gigahertz) and Ku (11.7 to 14.22 gigahertz) frequency bands, which are "block" converted by block converters to the L (900 to 2000 megahertz) frequency band. An RF filter section of the tuner of the indoor unit selects one of the RF signals provided by the block converter corresponding to the selected channel, which in turn is converted to a lower Intermediate Frequency (IF) range by a mixer/local oscillator section of the tuner for filtering and demodulation. Typically the IF frequency range has a center frequency of 479 mhz. Analog satellite television systems typically employ FM modulation and a baseband video signal is quickly obtained from the 479 IF signal by an FM demodulator after filtering by an IF filter. A relatively simple Surface Acoustic Wave (SAW) device can provide suitable filtering. Examples of conventional satellite television receiving systems are found in us patent 5325401 to haik et al entitled "L-band tuner with quadrature down-converter for PSK data applications" and uk patent GB2228383 to o.hideki entitled "series connected band conversion filter and satellite broadcast receiving system using the same".

In newer satellite television systems, such as DSS produced by Thomson consumer electronics of Indianapolis, Ind^TM(direct satellite system), television information is transmitted in digital form. The RF signal is transmitted by a satellite in the Ku band and converted to the L band by a block converter. The frequency range of the RF signals transmitted by the satellite is slightly smaller than that of an analog satellite television system, for example between 12.2 and 12.7 gigahertz, and thus the frequency range of the RF signals generated by the block converter is also slightly smaller, for example between 950 and 1450 megahertz. As in the analog satellite television receiving system, the RF signal corresponding to the selected channel is reduced in frequency to an IF frequency range for filtering and transmissionAnd (6) demodulating. However, the type of filtering required in digital satellite television receivers ("symbol shaping") cannot be immediately implemented at the relatively high IF frequencies used in analog satellite television receivers (e.g., 479 mhz), particularly when SAW devices are employed. Therefore, a relatively expensive digital filter is required to filter the demodulated digital signal. In addition, the tuner may employ a second conversion stage to convert the first IF signal at a relatively high frequency (e.g., 479 mhz) to a second signal at a lower frequency (e.g., less than 100 mhz) for filtering. However, the second conversion stage adds unnecessary expense to the receiver.

When constructing a tuner for a digital satellite television receiver, it is also required that it can be constructed using components that are readily available on the market and therefore relatively inexpensive. In this respect it is particularly desirable that the tuner is constructed using a readily available Integrated Circuit (IC) on the market, which is equipped with a Phase Locked Loop (PLL) for controlling the frequency of the local oscillator. Since a large number of tuner PLL integrated circuits of conventional television receivers for receiving and processing conventional broadcast and cable television signals are widely available, tuners for digital satellite television receivers are particularly required to be able to use such conventional tuner PLL integrated circuits when constructed.

The invention aims to provide a tuner of a digital satellite television receiver. According to one aspect of the invention, the tuner includes a single conversion stage for converting selected RF signals received from the block converters of the outdoor unit to IF signals at a frequency range that allows the use of a SAW device for "symbol shaping" of the IF signals as required for digitally transmitting information. For reasons that will be described in detail below in connection with the exemplary embodiments of the present invention, it is assumed that an IF signal having a center frequency on the order of 140mhz meets these requirements. However, different IF frequencies are possible and according to another aspect of the invention, the IF frequency may be selected to be on the order of the difference between the highest frequency of the RF signal received from the block converter (e.g. 1450MHz) and the highest local oscillation frequency (1300MHz) obtained by employing one conventional tuner PLL integrated circuit typically used in conventional broadcast and cable television receivers.

Various aspects of the invention will be described in detail with reference to the drawings, in which:

fig. 1 is a block diagram of a digital satellite television receiving system including a tuner constructed in accordance with an aspect of the invention;

FIG. 2 is a block diagram of a phase-locked loop tuning control integrated circuit for use in the tuner of FIG. 1;

FIG. 3 is an idealized amplitude-frequency response of a SAW device used in the tuner of FIG. 1;

fig. 4 is a graphical representation of certain characteristics of a SAW device as a function of temperature and frequency, useful in understanding the particular type of selection required for a SAW device used in the tuner shown in fig. 1.

The present invention will be described with reference to a digital satellite television reception system in which television information is transmitted in a form encoded and compressed in accordance with a predetermined digital compression standard, such as MPEG. MPEG is an international standard for the encoded representation of moving pictures and related audio information, as set by the moving pictures experts group. Television information is represented by a sequence or stream of digital signals arranged into packets corresponding to the video and audio portions of the television information. The digital signal is modulated on an RF carrier signal in the well-known QPSK (quadrature phase shift keying) scheme and the RF signal is transmitted to a satellite in earth orbit from which it is returned to earth. Satellites typically include some sort of transponder for receiving and retransmitting a corresponding modulated RF carrier. DirectV run by Houss corporation of Califolia^TMA satellite television transmission system is such a digital satellite television transmission system.

In the digital satellite television receiving system shown in fig. 1, an RF signal modulated with digital signals representing video and audio information is transmitted by a satellite (not shown) and received by a dish antenna 1. The relatively high frequencies of the received RF signal (for example in the Ku frequency range between 12.2 and 12.7 gigahertz) are converted by one block converter 3 into a relatively low frequency RF signal (for example in the L-band between 950 and 1450 MHz). The block converter 3 comprises a low noise amplifier and is therefore indicated by the first letter "LNB". The antenna 1 and the LNB 3 are comprised in a so-called "outdoor unit" 5 of the receiving system. The remainder of the receiving system is included in the tuned "indoor unit" 7.

The indoor unit 7 includes a tuner 9 for selecting a particular RF signal corresponding to a desired channel from a plurality of RF signals received by the outdoor unit 5 and converting the selected RF signal to an Intermediate Frequency (IF) signal of lower frequency. The tuner 9 is constructed in accordance with the present invention and will be described in detail below.

The QPSK demodulator 11 demodulates the output signal of the tuner 9 to generate two analog four-phase digital signals (I and Q). The decoder 13 generates a stream of video and audio packets from the I and Q signals. The decoder 13 comprises an analog/digital converter for converting the analog I and Q signals into corresponding digital sample sequences, and an error corrector for correcting transmission errors on the basis of an error correction code contained in the transmitted digital signal. Video and audio packets of the digital stream generated by the decoder 13 are sent by a transfer unit 15 to respective parts of a Digital Signal Processing (DSP) unit 17.

The digital satellite television receivers described so far are similar to DSSTM satellite television receivers sold by thomson consumer electronics. The invention is relevant for the details of the implementation of the tuner 9.

The tuner 9 receives at an input 901 an RF signal provided by the LNB 3. The RF input signal is filtered by a wide band filter 903, amplified by an RF amplifier 905, and filtered by a tunable bandpass filter 907. The resulting RF signal is coupled to a first input of a mixer 909. A local oscillator signal generated by Local Oscillator (LO)911 is coupled to a second input of mixer 909. Two input ends. The output of mixer 909 is amplified by amplifier 912 and coupled to the input of an IF filter 913, which comprises a SAW device. The output of the IF filter 913 is coupled to the output 915 of the tuner 9.

A Phase Locked Loop (PLL)917 formed by an Integrated Circuit (IC) controls the frequency of the local oscillator 911. The PLL ic controls the frequency of the local oscillator signal in accordance with data generated by the microprocessor 919.

As shown in FIG. 2, the PLL integrated circuit includes a "front scalar" divider 917-1 for dividing the local oscillator signal equally followed by a programmable divider (÷ N) 917-3. The PLL integrated circuit also includes an amplifier 917-5, which in combination with an external crystal network 917-7 forms a reference frequency oscillator. The output of the reference frequency oscillator is coupled to the input of a reference frequency divider (÷ R) 917-9. The output signals of the programmable divider (÷ N)917-3 and the reference divider (÷ R)917-9 are coupled to respective inputs of the phase detector 917-11. The output signal of the phase detector 917-11 is coupled to an amplifier 917-13 which, together with an external filter network 917-15, forms an integrator for generating a control voltage for the LO 911. When the phase locked loop is locked, the frequency of the local oscillator signal is proportional to the frequency of the reference frequency signal generated by the reference frequency divider (÷ R)917-9 by the programmable division factor (N) of the programmable frequency divider (÷ N) 917-3. The programmable division factor N is controlled by data generated by the microprocessor 919.

As mentioned previously, the tuner preferably has the following three features: (1) only a single conversion stage; (2) providing an IF signal having a sufficiently low frequency to allow digital symbol shaping and typically IF filtering using SAW devices; (3) it can be constructed to use a PLL tuning control integrated circuit conventionally used in broadcast and cable receivers. This is achieved in the present tuner by selecting the IF signal to have a center frequency of 140MHz and controlling the frequency of the local oscillator signal to be 140MHz lower than the frequency of the RF signal of the corresponding channel (transponder). As a result, the frequency range of the local oscillator signal is between 810 and 1310MHz when the frequency range of the RF input signal is between 950 and 1450 MHz. The IF frequency of 140MHz allows SAW devices having the required characteristics to be used, as will be described below. The 810-1310MHz frequency range of the local oscillator signal allows the PLL tuning control IC that is conventionally used for broadcast and cable receivers to be used. Such an IC is TSA5515T, which is commercially available from Philips semiconductors and others. In this respect it is noted that the maximum local oscillator frequency that can be obtained using the TSA5515T and similar ICs is in the order of 1300MHz, which is suitable.

It will be noted that different IF frequencies are possible and that in general the IF frequency may be selected to be on the order of the difference between the highest frequency of the RF signal received from the LNB and the highest local oscillator frequency obtained by employing a conventional tuned PLL integrated circuit typically used in conventional broadcast and cable television receivers.

The tunable bandpass filter 907 should desirably remove images of the desired RF signal at a frequency 280MHz lower than the frequency of the desired RF signal.

In a digital transmission system, known as "symbol shaping" is preferably performed to provide a signal with little or no intersymbol interference. Such interference may be caused by bandwidth limitations that do not properly filter out the high frequency energy of the pulse component of the digital signal in the transmitter. The required symbol shaping function may be shared between the transmitter and the receiver. In the receiver, it is preferred that the IF filter provides symbol shaping and the usual IF filtering function, so that a separate digital filter is not required. For example, the IF filter may provide a "root raised cosine" response, which is well known in the digital filter art. This response is shown in fig. 3. A SAW device may be employed to provide symbol shaping provided its characteristics are carefully selected.

Two features of SAW filters are considered important when applied in a tuner of a digital satellite television receiver, which are: (1) the overall shift or deviation of the filter characteristics with temperature (i.e., deviation from the center frequency); (2) relative bandwidth change (i.e., the width of the pass band divided equally by the center frequency).

The most common type of SAW device employs lithium niobate (LiNbO)₃) As a substrate. Lithium niobate SAW has a typical temperature coefficient of-90 ppm/degree (C). The tuner uses another type of SAW, which is lithium tantalate (LiTaO)₃) As a substrate. 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 140MHz, the frequency-related shift with temperature can be estimated as follows:

lithium niobate produces a temperature shift of 140E6x-90E-6x +/-45 +/-567.0 Hz;

lithium tantalate produces a temperature shift of 140E6x-23E-6x +/-45 +/-144.9 Hz.

Assuming a 500kHz offset, which may produce a noise margin attenuation of slightly less than 0.1dB, is undesirable, then the lithium niobate SAW exceeds the 500kHz target over a range of temperature variations. When using lithium niobate SAW, the center frequency must be reduced to 123MHz or less to maintain the 500kHz target. To use lithium tantalate SAW, the center frequency needs only 483MHz or less.

As for the relative bandwidth, the following points are noted. Generally, wider relative bandwidth filters are more difficult to manufacture, and filters with relative bandwidths in excess of 15-18% require the use of lithium niobate SAWs. The smaller relative bandwidth requirements allow the use of any type of SAW. A 20MHz wide filter centered at 140MHz has only 14% of the relative bandwidth. IF 18% of the relative bandwidth is required, then an IF center frequency of 110MHz is required.

Figure 4 provides a graphical summary of the features discussed above. Fig. 4 shows the frequency regions where either or both lithium niobate SAW or lithium tantalate SAW can meet the temperature drift and relative bandwidth requirements and the results that occur when considering both requirements. It can be seen that: an IF frequency below 110MHz requires a lithium niobate SAW filter; the IF between 110MHz and 123MHz can adopt any one of lithium niobate SAW or lithium tantalate SAW; IF at 123MHz to 483MHz requires lithium tantalate SAW; while SAWs for IF greater than 483MHz do not meet the requirements due to excessive temperature drift. For a center frequency of 140MHz, lithium tantalate SAW should be used.

Claims (4)

1. In a digital satellite television receiver for receiving and processing digital signals modulated on each of a plurality of RF signals received from an outdoor unit (5) including a satellite receiving antenna (1) and a block converter (3), a tuner (9) comprising:

an RF input for receiving a plurality of RF 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 to said RF input and a second input coupled to said local oscillator (911) and an output for producing an IF signal;

a surface acoustic wave filter (913) coupled to said output of said mixer (909) for providing filtering of said IF signal, including symbol shaping, to reduce inter-symbol interference; and

a phase locked loop tuning control integrated circuit for controlling the frequency of said local oscillator (911); the phase-locked loop tuning control integrated circuit is suitable for use in conventional terrestrial broadcast and cable television receivers.

2. The apparatus of claim 1, wherein:

the frequency of the IF signal is selected to be of the order of the difference between the highest frequency of the RF signal received from the block converter (3) and the highest local oscillation frequency obtained by tuning the control integrated circuit using the phase locked loop.

3. The apparatus of claim 2, wherein:

the block converter (3) provides an RF signal in the 950 to 1450MHz frequency range, the frequency range of the local oscillator (911) being from 810 to 1310 MHz.

4. The apparatus of claim 3, wherein:

the nominal frequency of the IF signal is 140 MHz.