Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/06—Receivers
  • H04B1/16—Circuits
  • H04B1/26—Circuits for superheterodyne receivers
  • H04B1/28—Circuits for superheterodyne receivers the receiver comprising at least one semiconductor device having three or more electrodes

Definitions

• This invention relates to a receiver circuit, and more particularly a radio frequency receiver circuit
• the invention relates to a radio receiver for digital terrestrial television using the DVB-T European standard
• the known technique would cause this signal to produce an output which would interfere with the wanted output signals in an unpredictable way.
• the interferer may alias to a frequency close to that at which a wanted output will appear, and moreover may be a stronger signal than the wanted signal, such that it cannot easily be removed by filtering.
• the present invention seeks to avoid the problems associated with the known techniques by determining a relationship between the centre frequency of an analog-digital converter input signal, a frequency of an unwanted signal, and the sampling rate of the analog-digital converter, in order to avoid such aliasing problems .
• a radio receiver circuit for receiving an input signal in a received signal frequency band, the input signal including a wanted signal in a wanted frequency band between a lower wanted frequency and an upper wanted frequency, the wanted signal being centered at a wanted signal center frequency, the input signal also including an interference signal at an interference frequency within the received signal band, the radio receiver circuit comprising: an analog-digital converter for downconverting the input signal, having a sampling frequency which is less than twice the wanted signal center frequency, the sampling frequency being selected such that the degree of aliasing of the interference signal into the first wanted frequency band after downconversion is kept below a predetermined threshold.
• a method of receiving an input radio signal m a received signal band the input signal including a wanted signal m a wanted frequency band between a lower wanted frequency and an upper wanted frequency, the wanted signal being centered at a wanted signal center frequency, the input signal also including an interference signal at an interference frequency within the received signal band, the method comprising: downconverting the input signal by sampling the input signal at a sampling frequency which is less than twice the wanted signal center frequency, thus obtaining a downconverted input signal including a downconverted interference signal and a downconverted wanted signal, the sampling frequency being selected such that the degree of aliasing of the interference signal into the wanted frequency band is kept below a predetermined threshold.
• a method of processing a television signal comprising: downconverting the signal to a wanted frequency band centered on a intermediate frequency; filtering the downconverted signal using a filter which passes the wanted frequency band and which is able to pass an interference signal; further downconverting the filtered downconverted signal by subsamplmg said filtered downconverted signal at a sampling frequency, the sampling frequency being selected such that the degree of aliasing of the interference signal into the first wanted frequency band after further downconversion is kept below a predetermined threshold.
• a radio receiver circuit for receiving an input signal in a received signal band between a lower received frequency IFl - CW/2 and an upper received frequency IFl +
• the radio receiver circuit comprising: an analog-digital converter having a sampling frequency SR ⁇ 2IFl, wherein:
• a radio receiver circuit for receiving an input signal in a received signal band between a lower received frequency IFl - CW/2 and an upper received frequency IFl + CW/2, the input signal including a wanted signal in a wanted frequency band between a lower wanted frequency IFl - SB/2 and an upper wanted frequency IFl + SB/2, the wanted signal being centered at a wanted signal center frequency IFl, the input signal also including an interference signal at an interference frequency IFl + N within the received signal band, the radio receiver circuit comprising: an analog-digital converter having a sampling frequency SR ⁇ 2IFl, wherein:
• a sixth aspect of the invention there is provided a method of receiving an input radio signal in a received signal band between a lower received frequency IFl - CW/2 and an upper received frequency IFl + CW/2, the input signal including a wanted signal in a wanted frequency band between a lower wanted frequency IFl - SB/2 and an upper wanted frequency IFl + SB/2, the wanted signal being centered at a wanted signal center frequency IFl, the input signal also including an interference signal at an interference frequency IFl + N within the received signal band, the method comprising.
• a seventh aspect of the present invention there is provided a method of receiving an input radio signal m a received signal band between a lower received frequency IFl - CW/2 and an upper received frequency IFl + CW/2, the input signal including a wanted signal m a wanted frequency band between a lower wanted frequency IFl - SB/2 and an upper wanted frequency IFl + SB/2, the wanted signal being centered at a wanted signal center frequency IFl, the input signal also including an interference signal at an interference frequency IFl + N within the received signal band, the method comprising: downconverting the input signal by sampling the input signal at a sampling frequency SR ⁇ 2IFl, wherein.
• the sampling frequency for the subsampling is chosen relative to the first wanted frequency band centre frequency, which advantageously is a first intermediate frequency after initial downconversion, such that the interference signal is maintained after subsampling, allowing the interference signal to be removed at baseband.
• Figure 1 shows m schematic form a receiver circuit m accordance with one aspect of the invention.
• Figure 2 illustrates a first example of the aliasing of received signals m the circuit of Figure 1.
• Figure 3 illustrates a second example of the aliasing of received signals m the circuit of Figure 1.
• Figure 4 illustrates a third example of the aliasing of received signals m the circuit of Figure 1.
• Figure 5 illustrates a fourth example of the aliasing of received signals in the circuit of Figure 1.
• Figure 6 illustrates a fifth example of the aliasing of received signals in the circuit of Figure 1.
• Figure 1 shows a receiver circuit. The invention is described herein with reference to its application in the reception of digital terrestrial television (DTT) signals using the European DVB-T standard based on Coded Orthogonal Frequency Division Multiplexing (COFDM) , although it will be appreciated that its use is independent of the type of signals being received.
• Figure 1 shows an antenna 2, for receiving broadcast
• UHF/VHF signals containing video data modulated using Coded Orthogonal Frequency Division Multiplexing (COFDM) , which are supplied to an analog tuner 4.
• the tuner 4 includes a mixer 6 which receives a first local oscillator signal LOl for downconversion of the received signals to a first intermediate frequency, and a band-pass filter 8, which may for example be formed from a pair of SAW filters.
• the filter 8 is assumed to attenuate all signals outside a channel of width CW, at least to a level at which they cannot interfere with wanted received signals.
• a conventional downconversion process wil l typically invert the f requency sense of the received signal spectrum.
• the analog-digital converter 12 should therefore be designed to have an adequate response to signal s at the f irst intermediate frequency IFl .
• the automatic gain control circuit 11 maintains a signal level of the input signal supplied to the analog-digital converter, such that the wanted signal and interference signal can be accurately sampled by the analog- digital converter.
• the baseband output from the analog-digital converter 12 is supplied to a filtering device 13, and then to a demodulator 14 in the form of digitised samples of the signals.
• the filtering device 13 includes a mixer 16, which receives a second local oscillator signal L02.
• the second local oscillator signal L02 is at the second intermediate frequency IF2.
• the output from the mixer 16 is passed to a low-pass filter 18, for removal of unwanted components.
• the demodulator 14 then removes the COFDM modulation, and supplies output signals which can be converted into a form suitable for display.
• Figure 1 shows several discrete blocks, it will be appreciated that the different stages may be integrated as far as is desirable, for example onto a single chip, or other arrangements of functions can be used.
• the analog tuner 4 may be one component, while the analog-digital converter 12, demodulator 14, and subsequent processing circuitry are combined.
• FIG. 1 shows in schematic form the signal present at the output 10 of the tuner 4.
• the downconverted signal is centered at the first intermediate frequency IFl, as discussed above.
• the band-pass filter 8 has a channel width CW centered at IFl, thus signals in the range (IF1-CW/2) to (IFl+CW/2) appear at the output 10.
• the shaded area 20 represents the signal bandwidth SB, which contains wanted COFDM signals.
• the complication is that the channel width CW is great enough to pass not only the wanted signal bandwidth SB, but also any adjacent, potentially interfering unwanted signal, which may appear at a frequency (IF1+N) .
• a NICAM Near Instantaneous Companding Audio Multiplex
• the NICAM signal may be strong (for example +10dB) relative to the wanted COFDM signals.
• the bandpass filter 8 it would be possible to design the bandpass filter 8 such that this unwanted signal is filtered out at that point.
• the gap between the edge of the wanted signal bandwidth and the adjacent unwanted signal is relatively narrow, at least compared to the intermediate frequency IFl, and so it is relatively difficult to achieve this filtering at the intermediate frequency.
• the present invention relates to a way of avoiding this problem, thereby allowing the use of a tuner which has a single downconversion stage, without placing excessive demands on the filter or filters in the tuner.
• one aspect of this involves maintaining the interfering signal unaffected, right until it is removed.
• the analog-digital converter 12 should have sufficient headroom, that is, enough effective bits, to be able to represent both the interfering signal and the wanted signal accurately.
• the automatic gain control circuit 11 scales the tuner output so that it fits optimally into the available range of the analog-digital converter.
• Figure 3 shows a possible situation after sub-sampling, at the output of the analog-digital converter 12.
• the sampling rate SR has been chosen such that the whole of the tuner pass-band from (IF1-CW/2) to (IFl+CW/2) appears within the frequency range from (k- 1 / )-SR to k-SR, where k is an integer.
• the whole tuner pass-band appears, inverted, in the frequency range from 0 to 1 / 2 -SR.
• the pass-band from (IFl-CW/2) to (IFl+CW/2) aliases to the range from (FS1-CW/2) to (FS1+CW/2), while the potentially interfering unwanted signal aliases from (IFl+N) to (FS1-N) .
• the signal is preferably mixed in a mixer 16 with a complex carrier at FS1.
• the unwanted signal which is further from FS1 than the wanted signal, is mixed to a higher frequency, and can be removed by a low-pass filter 18, to an extent sufficient to avoid affecting further processes.
• a second automatic gain control circuit (not shown) can be used to boost the signal to an appropriate level .
• Figure 4 shows an alternative possible situation after sub-sampling, at the output of the analog-digital converter 12.
• the sampling rate SR has been chosen such that k-SR, where k is an integer, falls within the tuner pass-band from (IFl-CW/2) to (IFl+CW/2) .
• the aliasing means that the upper end of the tuner pass-band seems to reflect about the zero frequency point in the downconverted signal.
• the centre frequency of the pass-band the intermediate frequency IFl
• the part of the pass-band from (IF1- CW/2) to k-SR aliases to the range from 0 to (FS2+CW/2), while the part of the pass-band from k-SR to (IFl+CW/2) aliases from 0 to (IFl+CW/2-k-SR) , in other words from 0 to (CW/2-FS2). It is of course important that there should be no aliasing of the COFDM wanted signal into itself.
• the upper end of the wanted signal, at (IF1+SB/2) aliases to (FS2-SB/2), and it is therefore important that: (FS2-SB/2) > 0
• the potentially interfering unwanted signal at (IF1+N) aliases to (FS2-N) , if FS2>N, or to (N-FS2) , if N>FS2.
• the unwanted signal should be sufficiently far outside the signal band to be filtered therefrom, even allowing for any frequency offset which may be present.
• the sampling rate SR is chosen such that (k- 1 / 2 ) • SR falls within the pass-band
• the part of the pass- band from (IFl-CW/2) to (k- 1 / 2 )-SR aliases to the range from 0 to 1 / 2 SR without frequency inversion
• the part of the pass-band from (k- 1 / 2 )-SR to (IFl+CW/2) also aliases into the range from 0 to 1 / 2 -SR, with frequency inversion.
• the aliasing means that the lower end of the tuner pass-band seems to reflect about the 1 / 2 -SR frequency point in the downconverted signal.
• the potentially interfering unwanted signal at (IFl+N) aliases to (FS3-N) , and cannot alias into the wanted signal.
• FIG. 6 A further alternative to that shown in Figure 3 is illustrated in Figure 6.
• the sampling rate SR has been chosen such that the whole of the tuner pass-band appears within the frequency range from k-SR to (k+ 1 / 2 )-SR, where k is an integer.
• the unwanted NICAM signal remains outside the signal band centred on FS4 after this downconversion, and can be filtered out in the demodulator 14
• Figure 3 shows the sampling rate SR chosen such that the whole of the tuner pass-band appears within the frequency range from (k- 1 / 2 ) • SR to k-SR
• Figure 6 shows the sampling rate SR chosen such that the whole of the tuner pass-band appears within the frequency range from k-SR to (k+ 1 / 2 )-SR
• Figures 4 and 5 show the sampling rate SR chosen such that the tuner pass-band appears largely (but not entirely) within the frequency range from (k- 1 / )-SR to k-SR
• sampling frequency SR when the sampling frequency SR is selected such that a portion of the received signal band is within a frequency band from k-SR to (k + 1/2) -SR after sampling, with k-SR ⁇ IF1-CW/2 and IFl + SB/2 ⁇ (k + 1/2) -SR ⁇ IFl + CW/2, k being an integer, and that :
• SR sampling rate
• choosing a sampling rate of 21.0Ms/s means that the lower end of the pass-band falls below 1.5-SR, as shown in Figure 5.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Superheterodyne Receivers (AREA)
• Noise Elimination (AREA)

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• 1999
  • 1999-05-07 GB GB9910662A patent/GB2349783A/en not_active Withdrawn
• 2000
  • 2000-05-04 JP JP2000617568A patent/JP4594535B2/ja not_active Expired - Lifetime
  • 2000-05-04 EP EP00930391A patent/EP1177635A1/en not_active Withdrawn
  • 2000-05-04 WO PCT/US2000/012298 patent/WO2000069083A1/en not_active Application Discontinuation

Non-Patent Citations (1)

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