GB2389257A

GB2389257A - A dual up/down frequency conversion receiver for digital signals with a tracking filter at the input and multiple gain-controlled stages

Info

Publication number: GB2389257A
Application number: GB0206244A
Authority: GB
Inventors: Stephen John Birkill
Original assignee: NOVAPAL Ltd
Current assignee: NOVAPAL Ltd
Priority date: 2002-03-15
Filing date: 2002-03-15
Publication date: 2003-12-03
Also published as: GB0206244D0

Abstract

An apparatus for receiving digital signals, said apparatus comprising: ```means for receiving digital signals (31) comprising a plurality of digital channels; ```means (73, 81)for selecting a digital channel to be outputted from said apparatus; ```first frequency conversion means (51) for converting the frequencies of the received signals to first intermediate frequencies, said first intermediate frequencies being higher than the frequencies of the received signal; ```second frequency conversion means (65) for converting the first intermediate frequencies to a second intermediate frequencies, said second intermediate frequencies being lower than said first intermediate frequencies; ```a tracking filter (43) provided to filter said received signals prior to said first frequency conversion means, said filter (43) being configured to allow transmission of a narrow band of frequencies centred about a filter frequency, said filter frequency being dependent on the selected output channel. A plurality of gain control stages are also described.

Description

1 2389257

An Apparatus for Receiving Dicital Signals The present invention relates to an apparatus for receiving digital signals. More particularly. the present invention relates to a tuner of the type which is used to receive television signals or the like.

Digital television signals are transmitted using digital modulation schemes such as quaternary phase shift keying (QPSK). quadrature amplitude modulation (QAM).

vestigial sideband (e.. 8 VSB) or coded orthogonal frequency division multiplexing (COFDM). The digital television signals are transmitted as a plurality of digital television channels. each channel occupying a number of frequencies. Typically. these frequencies are in the UHF spectrum and range from about 400 MEIz to up to 90() N1Hz.

In the UK. digital TV channels are located from 474 MHz to 858 MHz. the channels are separated by 8MHz steps.

In order to convert the digital television signal to an audio and video signal. the digital signal is demodulated. Current demodulators do not worl; at the high transmission Frequencies of digital TV signals. Therefore. the frequency of the digital signal is shifted to a lower frequency (down converted) prior to demodulation.

To output a particular digital channel. the frequency of this channel needs to be selected and down converted to the sampling frequency of the demodulator. This is difficult to achieve at the low frequencies used in a single down conversion process as the tuner needs to be carefully aligned to minimise interference of the outputted channel.

To avoid this problem, the digital signal is first up converted and then down converted.

The selected channel is isolated during the up conversion process and then the channel is down converted to the sampling frequency of the demodulator.

( Tuners which use up conversion and down conversion to receive and output digital television channels are l;noun and are explained in US 5 640 213 and US 6 343 209.

Although such tuners are proposed for use with terrestrial digital broadcasts. in practice.

problems arise due to the large frequency band which has to be up conversed and the varying signal strengths of the digital channels contained in the signal. Thus. systems which use troth up conversion and then dozen conversion have been found to he of more use with cable based digital transmission systems.

The present invention seeks to address the above problems and in a first aspect provides an apparatus for receiving digital signals. said apparatus comprising: means for receiving digital signals comprising a plurality of digital channels; means for selecting a digital channel to be outputted from said apparatus: first frequency conversion means for converting the frequencies of the received signals to first intermediate frequencies said first intermediate frequencies being higher than the frequencies of the received signal; second frequency conversion means for converting the first intermediate frequencies to a second intermediate frequencies. said second intermediate frequencies being lower than said first intermediate frequencies; a tracking filter provided to filter said received signals prior to said first frequency conversion means. said filter being configured to allow transmission of a narrow band of frequencies centred about a filter frequency. said filter frequency being dependent on the selected output channel.

Preferably. the filter frequency of said tracking filter is the frequency of the selected output channel. Preferably. the tracking filter will have a bandwidth from SO MHz to 200 MHz.

The frequencies outputted by the second frequency conversion means may be low enough to be sampled by a demodulator. Alternatively. one or more further down conversion processes may be necessary to further reduce the frequency.

According to a preferred embodiment of the invention a first amplifier is preferably provided to amplify the signal prior to the first frequency conversion means. The signal

is preferably filtered prior to, being amplified to prevent overloading of the first amplifier. The first amplifier preferably has a first gain control means. said first gain control means being controlled by feedback from the signal outputted by the first amplifier.

Positioning the tracking filter before the first gain control means is also advantageous since the gain is easier to control if only a narrow band of frequencies are amplified.

The first frequency conversion means will generally comprise first mixing means which mixes the digital signal with a first mixing signal having a first mixing frequency. The first frequency mixing means outputs a first intermediate frequency or more likel! a plurality of first intermediate frequencies. The first intermediate frequencies are preferably equal to the differences between the first mixing frequency and the frequencies of the received digital signal. However. the apparatus ma, be configured to use the sum of these frequencies.

The first mixer frequency is used to select the desired digital output channel. To achieve this the first mixer frequency is chosen so that the frequency of the desired digital channel within the digital signal is up converted to a predetermined first intermediate frequency. For example. the first mixer frequency will be F: + 1220 MHz if the first predetermined intermediate frequency is 1220 MHz and the desired digital channel frequency is Fc.

The first intermediate frequencies are preferably in the range from 95() MHz to 1400 MHz. Other frequencies may be output from the first mixer means which correspond to the sums of the first mixer frequency and the digital signal frequencies and possibly harmonics of the frequencies. Also, frequencies from channels other that the desired digital channel which were not removed by the first tracking filter may have been up converted by the first frequency conversion means. These spurious frequencies. may be removed by second filter means provided after the first frequency conversion means.

( As both the first mixing means and the tracking filter require information about the frequency of the selected digital channel. the first mixing means preferably has an output to the tracking filter to inform the tracking filter of the selected digital channel frequency Fc.

A second amplifier is preferably provided to amplify the signal having the first intermediate frequency prior to the second frequency conversion means. More preferably, this second amplifier is provided with second gain control means.

The second frequency conversion means preferably comprises second mixing means which is configured to mix a second mixing frequency with the first intermediate frequency to produce a second intermediate frequency. The second intermediate frequency being the difference of the second mixing frequency and the first intermediate frequency. The second intermediate frequency corresponds to the sampling frequency of the demodulator. As the first intermediate frequency is fixed. the second mixing frequency may also be fixed.

The second intermediate frequencies are preferably in the range from 36 MHz to 45 MHz. The second intermediate frequency is determined by the sampling capabilities of the demodulator.

Often. the digital channels may be offset from their designated frequencies by +/-

166.7 kHz. This offset may be corrected by appropriate choice of the second mixing frequency. A third amplifier is preferably provided to amplify the signal with the second intermediate frequency. This third amplifier is preferably provided with third gain control means. The third gain control means preferably uses feedback from the demodulator to control the gain of the third amplifier.

The gain of the third gain control means is also preferably used to control the second gain control means. If the gain of the third gain control means falls below a threshold value then the gain of the second amplifier is reduced by the second gain control means.

( if the gain of the third gain control means rises above a threshold then the gain of the second amplifier is increased by the second gain control means.

Alternatively. the second gain control means may also use feedback directly from the demodulator. The use of a three point control of the gain allows the tuner to cope with wide range of signal strengths. Thus. in a second aspect. the present invention provides an apparatus for receiving digital signals. the apparatus comprising: means for receiving a digital signal; first frequency conversion means for converting the frequencies of the received signal to first intermediate frequencies' said first intermediate frequencies being higher than the frequencies of the received signals; second frequency conversion means for converting the first intermediate frequencies to a second intermediate Frequencies. said second intermediate frequencies being lower than said first intermediate frequencies.

first gain control means for controlling the gain of a first amplifier provided before the first frequency conversion means: second gain control means for controlling the gain of a second amplifier. said second amplifier being provided between the first and second frequency conversion means; and third gain control means for controlling the gain of a third amplifier. said third amplifier being provided aRer the second frequency control means.

Preferably the second gain control means operates using feedback from the third gain control means. The third gain control means preferably uses feedback from a demodulator. The second gain control means may also use feedback directly from the demodulator. The first gain control means preferably uses feedback from the output of the first amplifier.

( The apparatus will generally receive both digital and analogue signals. If the apparatus needs to cope with analogue signals a loop-through is provided to divide the received signals.

The apparatus is primarily intended for use with terrestrial digital signals.

However. it may also be modified to receive both satellite or cable digital signals.

The apparatus may be used as part of a digital set top box (STB) which is used to convert digital TV signals for output in an analogue form. the tuner may also be provided within a digital television set. The present invention may also be used to demodulate digital radio signals.

The present invention will now be explained with reference to the following preferred non-limiting embodiment in which: Figure] schematically illustrates the basic components of a tuner which operates using up conversion and down conversion: Figure is a schematic graph illustrating up conversion and down conversion performed by a tuner having the features of Figure 1; and Figure 3 illustrates a radio tuner in accordance with the preferred embodiment of the present invention.

Figure I schematically illustrates the basic components which constitute a tuner which uses up conversion of the frequency of the incoming signal followed by down conversion. In figure 1, incoming digital signal comprises a frequency band having a plurality of different frequency components corresponding to a plurality of digital television channels. The whole frequency band is first up-converted using a mixer 3 having a first mixer frequency. The first mixer up-converts the frequencies of the incoming digital signal to a plurality of first intermediate frequencies 4.

The first mixer 3 outputs frequencies which are the difference between the incoming digital signal frequencies and the first mixer frequency. The first mixer frequency is thus chosen such that the difference between it and the incoming frequencies is larger than the magnitude of the incoming frequencies.

The first mixer 3 is also used in the selection of the channel which is to be output from the tuner because the first mixer frequency is selected to up-convert the frequency of a selected digital channel to a predetermined frequency. This will be explained in more detail with reference to figure 2.

The plurality of first intermediate frequencies emitted by mixer 3 is then filtered using filter S. The filter is configured to allow transmission of a narrow band of frequencies centred about the said predetermined frequency. As previously explained. the selected digital channel has been up converted to the said predetermined frequency.

The outputted signal fron1 filter 5 is then shifted to the sampling frequency of demodulator 9 using second mixer 7. Second mixer 7 mixes the filtered signal with a second mixer signal having a second mixer frequency. The second mixer frequency is selected so that the difference between the frequency of the filtered signal and the second mixer signal equals the sampling frequency of demodulator 9.

The output from second mixer 7 is demodulated by demodulator 9 which outputs digital data in the form of a transport stream to a source decoder. the source decoder then outputs an audio. video and/or data signal. l he demodulator 9 may comprise a channel decoder. In this particular example. the demodulator outputs an MPEG transport stream, which may contain several sound. vision and data channels to an MPEG decoder (not shown). The MPEG decoder then outputs an audio and video output signal. Figure 2 illustrates a schematic plot of signal strength against frequency for the frequency conversion processes of figure 1. The plot is purely schematic and is not intended to give any indication of signal strength. only a guide to relative frequencies.

The incoming digital signal (reference numeral] of figure I) is indicated by broad band UHF spectrum I 1 of figure 2.

The first mixer frequency may be selected from plurality of frequencies 13. In this particular example. the plurality of frequencies 13 are separated by2 MHz steps. In this particular example' the desired channel has a UHF frequency of Fc. the target first intermediate frequency for this channel is 120 MHz. thus a first mixer frequency 14 of Fc + 1920 MHz is selected.

The first intermediate frequency signal 15 is then filtered using a filter having a central frequency of 1,220 MHz and a filter width of plus or minus 10 MHz. Due to the operation of the frequency mixer. a higher frequency will also be produced 7. This frequency is outside the operation range of the tuner and will be ignored by the tuner and the filter.

The first intermediate frequency is then down converted by mixing the first intermediate frequency 15 with a second fixed mixer frequency 19. The second fixed mixer frequency 19 is at 1.183.875 MHz. Mixing these two frequencies results in second intermediate frequency 2 I which is centred at 36.195 MHz. This signal is then demodulated using demodulator 9 to produce a digital output signal in the form of an MPEG stream. The MPEG stream is then converted into an audio and video output signal. Figure 3 illustrates a preferred embodiment of the present invention.

Incoming signal 31 will be referred to as incoming digital signal 31 for convenience.

However, strictly. it is a digitally modulated RF signal which is thus analogue in character. The incoming digital signal 31 comprises a plurality of digital channel in a UHP band of frequencies is first amplified using bipolar linear amplifier 33. As the apparatus will receive both digital channels and analogue signals. an analogue loop-

through is provided at junction 35.

At junction 35. the signal is split. one part (the first part) of the signal is processed using the up conversion and down conversion system described with reference to Figures I and 2. the second part of the signal is then passed second linear amplifier 39 and into signal output box 41. The outputted signal is then fed into a television set or if the circuit is provided within a television set. it is directed towards the part of the television set which generates images from the analogue signal.

The first part of the signal is then filtered using tracking filter 43. For example. a tracking filter may be used which has a band width of 50 MHz at a central frequency of 470 MHz and a bandwidth of 200 MHz at a central frequency of 860 MHz. The tracking filter is centred on the desired digital channel. The control of tracking filter 43 will be described in more detail later.

The filtered signal is then directed into first amplifier 47. The gain of first amplifier 47 is controlled by automatic gain control 45. Automatic gain control 45 is a stand-alone gain control. The output of first amplifier 47 is divided at junction 49 and part of this signal is fed back into first automatic gain control 45. The automatic gain control comprises an amplitude detector which may detect the peak. mean or other level of output of first amplifier 47. The automatic gain control then compares the detected level with a reference. Depending on the variations from the reference. the automatic gain control adjusts the gain of amplifier 47 accordingly.

The outputted signal is then up converted by first frequency converter 51. First frequency converter 51 comprises a phase locked loop circuit and mixer. for example. a SL2 100 chip may be used as this component. The first frequency converter may also comprise a separate oscillator. mixer and phase locked loop synthesiser circuits.

First conversion means 51 receives a clock signal which in this particular example is 4 MHz, but other clock frequencies may be used. The first conversion means 57 is also connected to a two wire serial bus 53 and 55. Other control buses such as a parallel or 3 wire serial may be used. The serial bus 53 and 55 instructs the desired mixer frequency to be applied by said first conversion means 51. This information is then sent using connection 57 to tracking filter 43 so that both the tracking filter and the first conversion

( means operate in harmony with each other. For example. if the desired digital channel has an original frequency of 474 MHz. the up converter will provide a first mixer frequency of 1.694 MHz (474 MHz + 1.220 MHz). the tracking filter will allow a narrow band of frequencies to pass. centred around 474 MHz..

The tracking filter primary function is to reduce the number of frequencies which need to be converted by the first frequency conversion means. Thus the bandwidth of the filter may be large enough such that the filter does not need to be a precision component. The first frequency conversion means outputs a first intermediate signal having a first intermediate frequency. In this particular example. the first intermediate frequency should be centred around 1220 MHz. The output is then passed through narrow band filter 59 which is centred around 1220 MHz and with a band width of 0 MHz.

This filtered first intermediate frequency is then amplified by second amplifier 61. The gain of second amplifier 61 is set by second automatic gain control 63. The control of the second gain will be described later. The control signal may be applied to attenuator 61 as shown in Figure 3 or directly to amplifier 63.

The amplified first intermediate frequency is then directed into second frequency conversion means 65. Second frequency conversion means 65 down converts the first intermediate frequency from 1220 MHz to 36.125 MHz. It achieves this by mixing the frequency with a mixer frequency of approximately 1 183.875 MHz.

The received digital channels may often be offset by +166.7 KHz. The second mixer frequency \vill be selected to compensate for this offset to ensure that the desired digital channel data will reside at a frequency of 36.125 MHz.

The apparatus itself determines if the channels are offset. When the apparatus is first switched on, the apparatus is configured to scan the incoming digital channels by sweeping the first mixer frequency and the tracking filter together. Digital channels are identified during the process and any offsets are saved. These offsets are then used by

the second frequenc\ conversion means to select an appropriate second mixing frequency. The second intermediate frequency is then filtered by third fixed filter 67 which is centred at 36.125 MHz and has a bandwidth of 8 MHz. This signal is then amplified using static amplification means 69.

The output trom the static amplifier 69 is then directed into third automatic gain control means 71. The third automatic gain control means comprises third variable amplifier 72. The output from third variable amplifier 79 is then filtered using filter 74. The filtered signal is amplified using second static amplifier 76. The output from this amplifier 76 is then directed out of third automatic gain control means and into COFDM chip 73.

An output from COFDM chip 73 is then fed back into third automatic gain controller 71 in order to appropriately adjust the gain of third amplifier 72 using third static amplifier 78 within the third automatic gain control means. If the signal measured by the COFDM chip is too high. then the third automatic gain controller will reduce the gain of amplifier 72. However. if the signal receipted by the COFDM chip it; too low. the third automatic gain controller will increase the gain of amplifier 72.

The first static amplifier 69 may alternatively be part of automatic gain control 71 The filter 74 may be omitted.

If the gain ofthird automatic gain control 71 falls below a certain level. a signal is fed back 75 to second automatic gain controller 63. Second automatic gain controller 63 will then reduce the gain of second amplifier 61. In other words. second automatic gain controller 63 is controlled by the gain set by the third automatic gain controller and reduces the gain of the third amplifier if the third automatic gain control means falls below a certain level and increases the gain of the third amplifier if the gain of the third automatic control means raises above a certain level.

Alternatively. since the COFDM chip 73 sets the gain of the third automatic gain control means. the COFDM chip 73 may directly control the gain of the second gain control means 63.

The COFDM chip receives information (not shown) concerning the requested channel to be output he the tuner. The COFDM chip outputs information onto bus 77 and 79 which in turn supplies information concerning the first mixer frequency and the second mixer frequency to first conversion means 51 and second conversion means 65.

The output from the COFDM controller which is an MPEG stream is then outputted to MPEG decoder and controller 81. The MPEG decoder decodes the MPEG output stream from the COFDM chip and outputs audio and video signals along control lines 83 and 85 to output unit 41. Output unit 41 comprises a UHF modulator which outputs the television signal to a television or the lilies In an alternative arrangement bus 77 and 79 bypasses COFDM chip 73 and carries information directly from controller 81.

Claims

( CLAIMS:

I. An apparatus for receiving digital signals. said apparatus comprising: means for receiving digital signals comprising a plurality of digital channels: means for selecting a digital channel to be outputted from said apparatus; first frequency conversion means for converting the frequencies of the received signals to first intermediate frequencies. said first intermediate frequencies being higher than the frequencies of the received signal: second frequency conversion means for converting the first intermediate frequencies to a second intermediate frequencies. said second intermediate frequencies being lower than said first intermediate frequencies: a tracking filter provided to filter said received signals prior to said first frequency conversion means. said filter being configured to allow transmission of a narrow band of frequencies centred about a filter frequency. said filter frequency being dependent on the selected output channel.
2. An apparatus according to any preceding claim. wherein the tracking filter is controlled by an output from the first frequency conversion means.
3. An apparatus according to claim 1, further comprising first gain control means for controlling the gain of a first amplifier provided before the first frequency conversion means.
4. An apparatus according to claim 3. wherein said first gain control means and first amplifier are provided after said tracking filter.
5. An apparatus according to any preceding claim. further comprising a second amplifiers provided between the first and second frequency conversion means. a second gain control means being provided for said second amplifier.
6. An apparatus according to any preceding claim' further comprising a third amplifier. provided after said second frequency conversion means a third gain control means being provided for said third amplifier.
7. An apparatus for receiving digital signals. the apparatus comprising: means for receiving a digital signal; first frequency conversion means for converting the frequencies of the received signal to first intermediate frequencies. said first intermediate frequencies being: higher than the frequencies of the received signals; second frequency conversion means for converting the first intermediate frequencies to a second intermediate frequencies. said second intermediate frequencies being lower than said first intermediate frequencies.

first gain control means for controlling the gain of a first amplifier provided before the first frequency conversion means; second gain control means for controlling the gain of a second amplifier. said second amplifier being provided between the first and second frequency conversion means; and third gain control means for controlling the gain of a third amplifier. said third amplifier being provided after the second frequency control means.
8. An apparatus according to either of claims 3 or 7. wherein said first gain control means is controlled by feedback from the output of said first amplifier.
9 An apparatus according to either of claims 6 or 7. further comprising demodulator means for converting the second intermediate signal to an audio and/or video signal. said third gain control means being controlled by feedback from said demodulator means.
10. An apparatus according to any of claims 6 when dependent on claim 5 or claims 7 or 9. wherein the gain applied by the second gain control means is selected on the basis of the gain applied by the third gain control means.
I 1. An apparatus according to claim 10' wherein said second gain control means is configured to decrease the gain of the second amplifier if the gain of the third gain control means falls below a lower threshold value.
( 19 An apparatus according to either of claims 10 or 1 1. wherein said second gain control means is configured to increase the gain of the second amplifier if the gain of the third gain control means rises above an upper threshold value.
13 An apparatus according to any preceding claim wherein said first frequency conversion means comprises first mixing means configured to mix a signal having a first frequency with the received digital signals and to output a first intermediate signal having said first intermediate frequencies. said first intermediate frequencies being the difference between the frequencies of the received digital signal and the first mixing frequency.
14. An apparatus according to claim 13. wherein said first mixing frequency is chosen so that the frequency of the selected digital channel is shifted to a predetermined first intermediate frequency
15. An apparatus according to any preceding claim. wherein said second frequency conversion means comprises second mixing means configured to mix a signal having a second mixing frequency with the first intermediate signal and to output a second intermediate signal having the second intermediate frequencies. said second intermediate frequencies being the difference between the first intermediate frequencies and the second mixing frequency.
16. An apparatus according to claim 15. wherein said second mixing: frequency is selected to correct for any offset in the frequency of the received digital signal.
17. An apparatus according to any preceding claim. further comprising a loopthrough for an analogue signal
18. An apparatus as substantially hereinbefore described with reference to figure 3.