EP1177635A1 - Receiver circuit - Google Patents

Receiver circuit

Info

Publication number
EP1177635A1
EP1177635A1 EP00930391A EP00930391A EP1177635A1 EP 1177635 A1 EP1177635 A1 EP 1177635A1 EP 00930391 A EP00930391 A EP 00930391A EP 00930391 A EP00930391 A EP 00930391A EP 1177635 A1 EP1177635 A1 EP 1177635A1
Authority
EP
European Patent Office
Prior art keywords
signal
frequency
ifl
wanted
band
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP00930391A
Other languages
German (de)
French (fr)
Inventor
Jonathan Parker
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Conexant Systems LLC
Original Assignee
Conexant Systems LLC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Conexant Systems LLC filed Critical Conexant Systems LLC
Publication of EP1177635A1 publication Critical patent/EP1177635A1/en
Withdrawn legal-status Critical Current

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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.

Abstract

A radio frequency receiver circuit, for example for receiving digital terrestrial television broadcast signals. Such signals include wanted signals in wanted frequency bands, and may also include interference signals close to the wanted frequency bands. The receiver circuit receives a first intermediate frequency signal, and achieves downconversion by sub-sampling in an analog-digital converter. The sampling rate of the analog-digital converter is chosen relative to the intermediate frequency such that a known interference signal does not alias into the wanted frequency band, allowing it to be removed at baseband.

Description

RECEIVER CIRCUIT
FIELD OF THE INVENTION
This invention relates to a receiver circuit, and more particularly a radio frequency receiver circuit
Specifically, m one preferred embodiment, the invention relates to a radio receiver for digital terrestrial television using the DVB-T European standard
BACKGROUND OF THE INVENTION
Conventional radio receiver circuits are widely known, m which a received analog signal is downconverted m a first mixer stage to a first intermediate frequency, and then downconverted m a second mixer stage to a second intermediate frequency. The analog signal at the second intermediate frequency is then sampled m an analog-digital converter.
Also known is the technique of digital sub-sampling, whereby an analog-digital converter is used to achieve downconversion of a signal. This relies on the phenomenon of aliasing. An analog-digital converter having a sampling rate
(or frequency) of F can only entirely reliably reproduce signals having a frequency below F/2. However, higher frequency signals are still detected, but they appear m the output digital signal at frequencies m the range from O-F/2. Thus, input analog signals having frequencies f , (F-f ) , (F+f ) ,
(2F-f) , (2F+f) , etc, all appear m the output signal at frequency f .
It is known, for example from US Patent No. 5,630,227, that this technique can be used to achieve downconversion of a signal, m particular a signal which has components only m a relatively narrow range For example, if an analog signal only has frequency components at one or more frequencies designated (3F+f) within the range 3F-3.5F, and is sampled by an analog digital converter at a sampling frequency F, the output digital signal will have corresponding components at the frequency or frequencies f in the range 0-0.5F. In other words, the frequency range 3F-3.5F is aliased to 0-0.5F. However, this known system is unable to deal with the effects of adjacent channel interference. More specifically, in the event of a signal appearing on a frequency close to that of one of the wanted signals in the input, the known technique would cause this signal to produce an output which would interfere with the wanted output signals in an unpredictable way. In other words, 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.
SUMMARY OF THE INVENTION
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 .
According to a first aspect of the present invention, there is provided 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.
According to a second aspect of the invention, there is provided 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.
According to a third aspect of the invention, there is provided 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.
According to a fourth aspect of the invention, there is provided 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 +
C /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:
(I): (N - k-SR +IF1) < (k-SR - IFl - SB/2) when the sampling frequency SR is selected such that a portion of the received signal band is within a frequency band from (k
- 1/2) -SR to k-SR after sampling, with (k-l/2)-SR < IF1-C /2 and
IFl + SB/2 < k-SR < IFl + CW/2, k being an integer, and wherein:
(II) : (k-SR - IFl + SB/2) < 1/2-SR when the sampling frequency SR is selected such that a portion of the received signal band is within a frequency band from (k
- 1/2) -SR to k-SR after sampling, with IF1-CW/2 < (k-l/2)-SR < IFl - SB/2 and k-SR > IFl + SB/2.
According to a fifth aspect of the invention, there is provided 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:
(I): (N - (k + 1/2) -SR +IF1) < ( (k + 1/2) -SR - IFl - SB/2) 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 < IFl-CW/2 and
IFl + SB/2 < (k + 1/2) -SR < IFl + CW/2, k being an integer, and wherein: (II) : ( (k + 1/2) -SR - IFl + SB/2) < 1/2 -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 IF1-CW/2 < k-SR < IFl - SB/2 and (k + 1/2) -SR > IFl + SB/2.
According to 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. downconverting the input signal by sampling the input signal at a sampling frequency SR < 2IFl, wherein- (I) (N - k-SR +IF1) < (k-SR - IFl - SB/2) when the sampling frequency SR is selected such that a portion of the received signal band is within a frequency band from (k - 1/2) SR to k-SR after sampling, with (k-l/2)-SR < IF1-CW/2 and IFl + SB/2 < k-SR < IFl + CW/2, k being an integer, and wherein: (II) : (k-SR - IFl + SB/2) < 1/2-SR when the sampling frequency SR is selected such that a portion of the received signal band is within a frequency band from (k - 1/2) -SR to k-SR after sampling, with IF1-CW/2 < (k-l/2)-SR < IFl - SB/2 and k-SR > IFl + SB/2. According to 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. (I): (N - (k + 1/2) -SR +IF1) < ( (k + 1/2) -SR - IFl - SB '2) 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 wherein: (II) : ( (k + 1/2) -SR - IFl + SB/2) < 1/2 -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 IF1-CW/2 < k-SR < IFl - SB/2 and (k + 1/2) -SR > IFl + SB/2.
Thus, preferably, 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.
BRIEF DESCRIPTION OF DRAWINGS
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. DETAILED DESCRIPTION OF THE INVENTION
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.
It should be noted that a conventional downconversion process wil l typically invert the f requency sense of the received signal spectrum.
The output signal from the analog tuner 4 , at its output 10 , i s therefore at a first intermediate frequency IFl , and this signal is applied to an automatic gain control circuit 11 , and then to an analog-digital converter 12 , which has a sampl ing rate SR which i s les s than twice the f irs t intermediate frequency IFl , and theref ore sub-samples the signal , and ef f ectively downconverts i t by aliasing to a second intermediate frequency IF2 which is close to baseband . 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.
Although 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.
For example, the analog tuner 4 may be one component, while the analog-digital converter 12, demodulator 14, and subsequent processing circuitry are combined.
Figure 2 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) . For example, in the United Kingdom, a NICAM (Near Instantaneous Companding Audio Multiplex) sound signal can appear at this point. Moreover, the NICAM signal may be strong (for example +10dB) relative to the wanted COFDM signals.
In principle, it would be possible to design the bandpass filter 8 such that this unwanted signal is filtered out at that point. However, 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.
It is preferable to be able to filter out this unwanted signal at baseband, but, in order to be able to do this, it is necessary to avoid a situation where the unwanted signal appears within the wanted signal in the downconverted signal as a result of aliasing.
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.
Moreover, one aspect of this involves maintaining the interfering signal unaffected, right until it is removed. Thus, 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. Further, 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.
In this case, 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. After sub-sampling, the whole tuner pass-band appears, inverted, in the frequency range from 0 to 1/2-SR. In particular, if the centre frequency of the pass-band, the intermediate frequency IFl, is separated from the relevant multiple of the sampling frequency k-SR by a frequency separation FS1, that is: k-SR - IFl = FS1, then the centre frequency of the downconverted signal appears at FS1, which is in effect a second intermediate frequency at close to baseband.
It should be noted that, if the first downconversion stage inverts the frequency sense of the spectrum, this re- inversion is desirable. However, this inversion can later be removed if necessary, by inverting the sign of all Q values in the I and Q digital samples.
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) .
Since the unwanted signal remains outside the signal band
SB, which is now centered on FS1, it can relatively easily be filtered out in the demodulator 14 before the signal is further processed. Specifically, 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. If necessary, 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.
In this case, 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) .
After sub-sampling, that part of the tuner pass-band from
(IFl-CW/2) to k-SR appears, inverted, in the frequency range from 0 to 1/2-SR. Further, however, that part of the tuner pass-band from k.SR to (IFl+CW/2) also appears, uninverted, in the frequency range from 0 to 1/2-SR.
In effect, the aliasing means that the upper end of the tuner pass-band seems to reflect about the zero frequency point in the downconverted signal.
In this case, if the centre frequency of the pass-band, the intermediate frequency IFl, is separated from the relevant multiple of the sampling frequency k-SR by a frequency separation FS2 , that is: k-SR - IFl = FS2, then the centre frequency of the downconverted signal appears at FS2.
As mentioned above, 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. That is, in Figure 4, 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 Further, and in particular, the potentially interfering unwanted signal at (IF1+N) aliases to (FS2-N) , if FS2>N, or to (N-FS2) , if N>FS2. In order to allow the unwanted signal to be filtered out in the demodulator 14, it is important that it should remain outside the signal band SB, which is now centered on FS2. Moreover, 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.
If FS2>N, then, because N>SB/2 (because the unwanted signal is known to appear outside the wanted signal band in the signal at the first intermediate frequency) , the unwanted signal will be aliased outside the wanted signal band. However, if N>FS2 , it is possible that the unwanted signal will be aliased into the wanted signal band.
In order to avoid this, it is therefore desirable that: (N-FS2) < (FS2-SB/2) or, preferably, that: (N-FS2) + Δ < (FS2-SB/2), where Δ is the allowed frequency offset, possibly reflecting the fact that the unwanted signal at (IF1+N) may have a finite bandwidth and be centered at that frequency.
Conversely, if 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 1/2SR without frequency inversion, while 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. In effect, 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.
This is shown in Figure 5. In this case, if the centre frequency of the pass-band, the intermediate frequency IFl, is separated from the relevant multiple of the sampling frequency k-SR by a frequency separation FS3 , that is: k-SR - IFl = FS3, then the centre frequency of the downconverted signal appears at FS3. As mentioned above, the part of the pass-band from (IF1-
CW/2) to (k-1/2) -SR aliases to the range from 0 to 1/2-SR, while the part of the pass-band from (k-1/2) -SR to (IFl+CW/2) aliases from (FS3-CW/2) to 1/2-SR.
It is of course important that there should be no aliasing of the COFDM wanted signal into itself. That is, in
Figure 5, the lower end of the wanted signal, at (IFl-SB/2) aliases to (FS3+SB/2) , and it is therefore important that: (FS3+SB/2) < 1/2-SR, or, preferably that:
(FS3+SB/2) + Δ < 1/2-SR, where Δ again is a possible offset.
However, in this case, the potentially interfering unwanted signal at (IFl+N) aliases to (FS3-N) , and cannot alias into the wanted signal.
A further alternative to that shown in Figure 3 is illustrated in Figure 6. Here, 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. After sub-sampling, the whole tuner pass-band appears, non-inverted in this case, in the frequency range from 0 to 1/2-SR, with centre frequency FS4, where FS4 = IFl - k-SR.
As in the example of Figure 3 , the unwanted NICAM signal remains outside the signal band centred on FS4 after this downconversion, and can be filtered out in the demodulator 14 Just as 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, and 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, while 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, it is also possible to choose the sampling rate such that the tuner pass-band appears largely but not entirely within the frequency range from k-SR to (k+1/2) -SR, with the same constraints .
In this case, and in line with what already disclosed in the above figures 3 to 5 , it is desirable that:
(N (k + 1/2) -SR +IF1)) < ( (k + 1/2) -SR - IFl - SB/2)
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 :
(k + 1/2) -SR - IFl + SB/2)< 1/2-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 IF1-CW/2 < k-SR < IFl - SB/2 and (k + 1/2) -SR > IFl + SB/2. Also in this case, should the interference signal have a bandwidth Δ, the above equations can be respectively replaced by :
(N - (k + 1/2) -SR +IF1)+ Δ < ( (k + 1/2) -SR - IFl - SB/2) and
( (k + 1/2) -SR - IFl + SB/2) + Δ < 1/2-SR.
These cases will now be illustrated for the case of a received COFDM signal, which has been downconverted in a first stage to a first intermediate frequency of 36.167MHz, with a pass-bandwidth of 9.40MHz. The actual wanted signal bandwidth is 7.61MHz, centered at the intermediate frequency of 36.167MHz. The nearest adjacent interference signal is a NICAM signal at (36.167 + 4.1981) = 40.3651MHz.
Choosing a sampling rate, SR, of 20.5Ms/s means that the whole of the pass band from 31.467MHz-40.687MHz falls within the range from 1.5-SR-2-SR, and there is no aliasing of any part of the pass-band into any other. This means that the unwanted signal can be filtered out.
On the other hand, 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. In this case, the lower edge of the pass-band at 31.467MHz aliases to (1.5-SR - 31.467) = 0.033MHz below 0.5-SR, while the lower edge of the wanted signal band, at (36.167-7.61/2)MHz aliases to (36.167 - 7.61/2 - 1.5-SR) = 0.862MHz below 0.5-SR. Thus there is no interference, and the unwanted signal can be filtered out.
Choosing a sample rate of 20Ms/s means that, as in Figure 4, the upper end of the pass-band aliases into the output, and the unwanted signal can potentially interfere with the wanted signal.
In this case, the upper edge of the wanted band at
(36.167 + 7.61/2) = 39.972MHz aliases to 0.028MHz, while the unwanted signal at (36.167 + 4.1981) = 40.3651MHz aliases to 0.3651MHz, which is within the wanted band. This will mean that the requirement : (N-FS2) + Δ < (FS2-SB/2) cannot be met, for any value of Δ. There is thus disclosed a receiver which, by sub-sampling a first intermediate frequency signal in such a way that an unwanted signal is not aliased into a wanted signal, and can therefore be filtered therefrom after sub-sampling, allows the use of a relatively simple tuner, with a single downconversion stage, without imposing excessive requirements on the filtering in the tuner.

Claims

1.
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.
2.
The radio receiver circuit as claimed in claim 1, further comprising: an automatic gain control circuit, for maintaining a signal level of the input signal supplied to the analog- digital converter.
3.
The radio receiver as claimed in claim 1 or 2 , further comprising: a mixer, for mixing a local oscillator signal, having a frequency equal to the wanted band center frequency after downconversion, with the downconverted input signal; and a low-pass filter, connected to the mixer output to filter the interference signal from the mixed downconverted input signal .
4. A method of receiving an input radio signal in a received signal 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 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.
5.
The method as claimed in claim 4 , comprising maintaining a signal level of the input signal supplied to the analog- digital converter .
6.
The method as claimed in claim 4 , further comprising : mixing the downconverted input signal with a local oscillator signal having a frequency equal to the wanted signal center frequency after downconversion thus obtaining a mixed downconverted signal ; and low-pass f iltering the mixed downconverted signal to remove the downconverted interf erence s ignal f rom the downconverted wanted signal.
7.
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 subsampling 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.
8.
The method as claimed in claim 7, wherein the step of further downconverting comprises subsampling the filtered downconverted signal at a sampling frequency, the sampling frequency being selected such that the interference signal can be filtered from the wanted frequency band after further downconversion .
9.
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:
(I): (N - k-SR +IF1) < (k-SR - IFl - SB/2)
when the sampling frequency SR is selected such that a portion of the received signal band is within a frequency band from (k
- 1/2) -SR to k-SR after sampling, with
(k-l/2)-SR < IF1-CW/2 and
IFl + SB/2 < k-SR < IFl + CW/2,
k being an integer, and wherein:
(II) : (k-SR - IFl + SB/2) < 1/2-SR
when the sampling frequency SR is selected such that a portion of the received signal band is within a frequency band from (k
- 1/2) -SR to k-SR after sampling, with
IF1-CW/2 < (k-l/2)-SR < IFl - SB/2 and
k-SR > IFl + SB/2.
10. The radio receiver circui t of claim 9 , wherein the interference signal has an interference signal bandwidth Δ, and wherein equations ( I ) and ( II ) are respectively replaced by : (III) : (N - k-SR +IF1) + Δ < (k-SR - IFl - SB/2) and
(IV) : (k-SR - IFl + SB/2) + Δ < 1/2-SR.
11.
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 -t N within the received signal band, the radio receiver circuit comprising: an analog-digital converter having a sampling frequency SR < 2IFl, wherein:
(I): (N - (k + 1/2) -SR +IF1) < ( (k + 1/2) -SR - IFl - SB/2)
when the sartpling 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 wherein:
(II) : ( (k + 1/2) -SR - IFl + SB/2) < 1/2-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
IF1-CW/2 < k-SR < IFl - SB/2 and
(k + 1/2) -SR > IFl + SB/2.
12. The radio receiver circuit of claim 11, wherein the interference signal has an interference signal bandwidth Δ, and wherein equations (I) and (II) are respectively replaced by:
(III) : (N - (k + 1/2) -SR +IF1) + Δ < ( (k + 1/2) -SR - IFl - SB/2) and
(IV) : ( (k + 1/2) -SR - IFl + SB/2) + Δ < 1/2-SR.
13. 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: downconverting the input signal by sampling the input signal at a sampling frequency SR < 2IFl, wherein: (I): (N - k-SR +IF1) < (k-SR - IFl - SB/2)
when the sampling frequency SR is selected such that a portion of the received signal band is within a frequency band from (k - 1/2) -SR to k-SR after sampling, with
(k-l/2)-SR < IF1-CW/2 and
IFl + SB/2 < k-SR < IFl + CW/2,
k being an integer, and wherein:
(II) : (k-SR - IFl + SB/2) < 1/2-SR
when the sampling frequency SR is selected such that a portion of the received signal band is within a frequency band from (k - 1/2) -SR to k-SR after sampling, with
IF1-CW/2 < (k-l/2)-SR < IFl - SB/2 and
k-SR > IFl + SB/2.
14.
The method of claim 13, wherein the interference signal has an interference signal bandwidth Δ, and wherein equations (I) and (II) are respectively replaced by:
(III) : (N - k-SR +IF1) + Δ < (k-SR - IFl - SB/2) and
(IV) : (k-SR - IFl + SB/2) + Δ < 1/2-SR.
15.
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: downconverting the input signal by sampling the input signal at a sampling frequency SR < 2IF1, wherein:
(I): (N - (k + 1/2) -SR +IF1) < ( (k + 1/2) -SR - IFl - SB/2)
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 wherein:
(II) : ( (k + 1/2) -SR - IFl + SB/2) < 1/2-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
IF1-CW/2 < k-SR < IFl - SB/2 and (k + 1/2) -SR > IFl + SB/2.
16.
The method of claim 15, wherein the interference signal has an interference signal bandwidth Δ, and wherein equations (I) and (II) are respectively replaced by:
(III) : (N - (k + 1/2) -SR +IF1) + Δ < ((k + 1/2) -SR - IFl - SB/2) and
:ιv) (k + 1/2) -SR - IFl + SB/2) + Δ < 1/2-SR.
EP00930391A 1999-05-07 2000-05-04 Receiver circuit Withdrawn EP1177635A1 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
GB9910662A GB2349783A (en) 1999-05-07 1999-05-07 Receiver circuit using sub sampling analogue to digital converter to frequency shift the signal
GB9910662 1999-05-07
PCT/US2000/012298 WO2000069083A1 (en) 1999-05-07 2000-05-04 Receiver circuit

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EP1177635A1 true EP1177635A1 (en) 2002-02-06

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US8131490B2 (en) 2007-12-20 2012-03-06 Honeywell International Inc. Methods and systems for determining a received signal frequency
TWI347097B (en) 2007-12-31 2011-08-11 Ind Tech Res Inst Circuit with programmable signal bandwidth and method thereof
US8064560B2 (en) 2008-02-05 2011-11-22 Honeywell International Inc. Systems and methods for detecting a signal across multiple Nyquist bands

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JP4594535B2 (en) 2010-12-08
GB9910662D0 (en) 1999-07-07

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