FR2767985A1 - VARIABLE BANDWIDTH FILTER AND CORRESPONDING FILTERING METHOD - Google Patents

Abstract

A bandpass filter of a variable bandwidth according to the invention comprises a filter means (6, 12) of a fixed frequency response for filtering an input signal and a variable frequency converter (2, 8 ) for selectively shifting the frequency profile of the filtered signal relative to the frequency response of the filter means (6, 12), wherein the frequency shifted signal is further filtered by the filter means to produce a filtered signal bandpass with a bandwidth determined by the frequency offset.

Description

The present invention relates to a variable bandwidth filter and

  more particularly, a bandpass filter for use in radio receivers and transmission modems

by cable.

  In recent years, there has been a significant increase in the number of radio receivers designed for use in both professional equipment and consumer equipment. Radio receivers are used for the reception of digital information and analogue information in relation to various information media. Mobile telephone networks, satellites, cable networks, cable modems, microwave links, broadcast video information and broadcast audio information are just some examples of the various signals

  radio that are broadcast and received around the world.

  Virtually all the systems mentioned above use tunable frequency receivers which receive radio signals in fixed channel bandwidths. The bandwidths of particular channels are set according to the speed

  the modulation scheme used.

  More particularly, since the birth of "multimedia services", the type and quantity of information to be sent by radio has varied considerably, giving rise to the need for more flexible methods of transmitting radio signals. In some communication systems, multiple modulation schemes, multiple data rates, and multiple

  bandwidths are now specified.

  When a received radio signal is to be demodulated, it is first converted into a lower frequency frequency suitable for the demodulator. In particular, a receiver intermediate frequency (IF) stage converts the radio frequency (RF) signal to a lower intermediate frequency. This can occur in a stage or in multiple stages involving multiple intermediate frequencies. So that adjacent channels of the total available frequency spectrum for the radio receiver do not interfere with the

  demodulator or do not block it, the received signal is filtered pass-

  to pass only the desired channel to the demodulator. This filtering can be implemented on the lower intermediate frequency signals instead of on the radio frequency signal. In addition, filters can be used at multiple intermediate frequency stages so that

  improve the tape rejection output.

  With systems that use multiple bandwidths, there is a problem. In particular, the filtering implemented with the intermediate frequencies must correspond to the channel bandwidth of the desired signal so as to reject all the signals outside the desired bandwidth. Therefore, the use of multiple bandwidth signals also requires the use of bandwidth filtering at multiple bandwidths. The use of variable bandwidth filters or a plurality of different sets of filters for each required bandwidth can be considered. However, variable bandwidth filters are extremely difficult to design for use in consumer products. Moreover, the use of a plurality of sets of fixed frequency filters requires a large number of components, only some of which will one day be used at any moment. This use also requires an additional circuit to switch games in and out of the various intermediate frequency channels. These problems are multiplied as the number of intermediate frequency stages increases. In addition, the filters become more and more expensive to implement when the

  number of bandwidths to support the receiver increases.

  According to the present invention there is provided a method of filtering an input signal comprising: filtering the input signal with a fixed frequency response; selectively, shifting the frequency profile of the filtered signal and inverting the filtered signal in the frequency domain; and further filtering the frequency-shifted signal with a fixed frequency response so as to produce a band-pass filtered signal having a bandwidth determined by

  the frequency shift step.

  According to the present invention, there is also provided a bandwidth filter of variable bandwidth comprising: filtering means of a fixed frequency response for filtering an input signal; and a variable frequency converter for selectively shifting and inverting in the frequency domain the frequency profile of the filtered input signal, wherein the frequency shifted signal is further filtered by the filtering means to produce a filtered filtered signal. -band of a

  bandwidth determined by the frequency offset.

  In this way, fixed frequency filtering is used to eliminate undesired frequencies but the band rejection points of the input frequency spectrum are determined in accordance with the variable frequency converter. Frequency converters or frequency synthesizers are relatively easy and inexpensive to manufacture. It follows that the present invention allows the filtering of a signal with a variable bandwidth without the use of expensive and complex variable bandwidth bandpass filters or networks or sets of filters.

fixed frequency.

  It is possible to apply the same filtering before and after the

  frequency conversion since the frequency converter itself

  even creates the required bandwidth. Of course, the frequency converter can also reflect the frequency spectrum so that a low pass filter can be used to reject frequencies both above and below the desired bandwidth. It is also possible to apply frequency conversion before and after bandwidth filtering. In this way, the frequency rejection point of the input signal filtering can be freely positioned with respect to the entire frequency spectrum and similarly, the resulting bandpass filtered signal can be freely down converted to at the frequency

required for a demodulator.

  The filtering means may comprise a first filter and a second filter, the first filter filtering the input signal and the second filter filtering the signal shifted in frequency. In addition, the frequency response

  of the first filter may be different from that of the second filter.

  In this way, the first and second filters may have frequency responses positioned in substantially different frequency ranges such that the frequency converter downconverts the frequency of the input signal at the same time as it adjusts its position. relative to the point of rejection of the filter. In this way, the present invention is implemented very advantageously as part of

  intermediate frequency stages of a receiver.

  According to the present invention, there is provided a converter-converter radio receiver for frequency conversion of a received radio signal according to a signal for demodulation, the converter-down-converter radio receiver comprising a plurality of frequency converters and a band-pass filter as defined above. , the variable frequency converter of the bandpass filter being one of

  the plurality of frequency converters.

  According to the present invention, there is also provided a receiver

  radio including the down-converter radio receiver defined above.

  The present invention will be more fully understood at the

  light of the following description presented as an example

  only, with reference to the accompanying drawings in which: Figs. 1 (a) to 1 (d) show the principles of operation of an embodiment of the present invention; Fig. 2 shows an embodiment of the present invention; Fig. 3 shows an embodiment of the present invention; Figs. 4 (a) to 4 (d) show the operating principles of an embodiment of the present invention; Fig. 5 shows an embodiment of the present invention; and Fig. 6 shows an embodiment of the present

invention.

  In a radio receiver, it is desirable to down-convert the received radio signal to a lower frequency signal suitable for demodulation. Furthermore, in order to eliminate undesired signals and to pass only the desired channel to the demodulator, it is desirable to use filters adapted to the signal channel bandwidth in order to reject undesired signals at the desired time. above and below the desired frequencies. In general, a bandpass filter is used for each intermediate frequency although sometimes a low pass filter may be used when the intermediate frequency is at the baseband. After all intermediate frequency stages, a demodulator

  then directly samples the final intermediate frequency.

  It is now proposed to reject undesired signals which are above the desired signals at a different intermediate frequency stage from the rejection of undesired signals which are below the desired signals. In other words, a bandpass filter is implemented on two

  intermediate frequency stages.

  Referring to Figure 1, we can see how two filters pass

  may be used to reject frequencies above and below

below the desired channel ch.

  Figure 1 (a) shows received radio frequency channels ch-3 at ch + 3 in relation to two intermediate frequencies IF1 and IF2 used in the down-conversion of

radio frequency received.

  As shown in Fig. 1 (d), the received radio frequency spectrum is first passed to a first frequency converter comprising a frequency synthesizer 2 and a multiplier 4 so that the desired channel ch is positioned at the same time. IF1 intermediate frequency. It should be noted that this frequency conversion operation has the effect of reflecting the spectrum of

  frequencies received in the vicinity of the IF1 intermediate frequency.

  The frequency converted signal is then passed to a first low pass filter 6 with a filter cutoff frequency of fcl. The filter cutoff frequency fcl is fixed but the frequency synthesizer 2 is controlled to produce an intermediate frequency IF1 equal to the filter cutoff frequency fcl minus half the signal bandwidth of the desired channel ch1. In this way, as shown in FIG. 1 (b), the frequency synthesizer 2 positions the ch channel with respect to the frequency response of the low-pass filter 6 so that the low-pass filter 6 rejects all frequencies. above the channel bandwidth of the desired channel ch. In particular, with the frequency spectrum

  reflected, channels ch-1 to ch-3 are rejected.

  It will be appreciated that in special applications where one end of a channel bandwidth is still fixed, then the first frequency converter 2, 4 can be omitted and the filter

  Lowpass 6 can operate directly on the input signal.

  Referring again to Fig. 1 (d), the filtered intermediate frequency signal is passed to a second frequency converter

  comprising a frequency synthesizer 8 and a multiplier 10.

  As with the first frequency converter, the second frequency synthesizer 8 produces a signal f2 which produces an intermediate frequency IF2 of the desired frequency. As shown in Fig. 1 (c), the frequency spectrum of the IF2 intermediate frequency signal is reflected so that the unwanted channels ch + 1 to ch + 3 are now back to

  higher frequencies than the desired channel ch.

  The intermediate frequency signal IF2 has passed to a second

  low-pass filter 12 with a filter cut-off frequency of fc2.

  As with the first low-pass filter, the filter cutoff frequency fc2 is fixed. However, the second frequency synthesizer 8 is controlled to produce a second intermediate frequency IF2 equal to the second filter cutoff frequency fc2 minus half the signal bandwidth of the desired channel ch. In this way, as shown in FIG. 1 (c), all the frequencies above the desired channel ch are rejected so that only the frequencies of the desired channel ch remain in the second signal of

intermediate frequency filtered.

  Other intermediate frequency stages may use other similar frequency converters and other similar filters to produce a steeper break between the

  desired channel band and undesired frequencies.

  Therefore, the filter taken as a whole can be constituted by a variable bandwidth by positioning each intermediate frequency so that the edge of the desired signal is aligned with the cutoff frequency of the individual fixed frequency filter being used. Theoretically, any type or what

  mixture of filters could be used, that is, low-pass, pass-

  high, bandpass etc ... The easiest however is to use either

  a band-pass filter, a low-pass filter.

  As explained above, the frequency of the first synthesizer is chosen such that: IF1 = fcl - signal bandwidth / 2 Similarly, the frequency of the second synthesizer is chosen so that: IF: = fc2 - band The effective bandwidth of the apparatus is established by programming the two frequencies of the frequency synthesizers f1 and f2. In this case, the signal frequency in each stage is down converted to a lower frequency so that the frequencies satisfy RF> IF1> IF2 with frequencies f1> RF and f2> IF1. The lower mixer product is

  chosen for the two intermediate frequencies.

  The stages could be operated using a mixture of elevation conversion and down conversion and the lower mixer product would not necessarily need

to be selected.

  In the examples presented, the second frequency conversion is arranged such that the mixer products to be used and filtered are the mixer products that have been reflected under the frequency conversion. This reflection allows a low pass filter or the upper edge of a bandpass filter to remove

  channels both above and below the desired channel.

  When IF1> IF2 and IF1> f2, there are also two mixer products. These are the difference mixer or lower IF1 - f2 products and the sum mixer or higher IF1 + f2 products. As mentioned above, for the upper mixer product, the signal is not reflected. With the lower mixer product, the signal is not reflected either. The present invention can be applied by selecting frequencies using

  low-pass / high-pass / bandpass filters.

  If low-pass filters are used, then the only limitation on the maximum signal bandwidth that the intermediate-frequency stages can use is the relative shift of the radio frequency and the intermediate frequencies (RF-fl and fl-f2) and the maximum intermediate frequency that the demodulator can handle. If a bandpass filter is used, the limitation of the maximum signal bandwidth is determined by the bandwidth of the filter and the relative offsets of the RF frequency and the intermediate frequency. Again, the maximum bandwidth is also determined by the sampling frequency of the demodulator. Figures 2 and 3 show an example of an IF (intermediate frequency) stage that could be used in the receiver section of a cable modem for the DVB / DAVIC outside the downstream band specification [prETS 300 800]. 1 MHz and 2 MHz channels for data rates of 1.544 Mb / s and 3.088 Mb / s are specified. The RF downstream frequency band is 70 to 130 MHz and the modulation scheme is QPSK (four-phase phase modulation or four-state phase shift keying). Figure 2 shows a system block diagram of the receiver IF band for this application that uses two filters

low-pass with fixed bandwidth.

  As shown, the low-pass filters have respective cut-off frequencies of fc1 = 55 MHz and fc2 = 2 MHz. It follows that for 0.772 megabauds and 1 MHz channels, IF1 is chosen to be 54.6 MHz and IF2 is chosen to be 1.6 MHz. For 1, 544 megabauds and for 2 MHz channels, IF1 is chosen to be 54.2 MHz and IF2 is

1.2 MHz.

  If the cutoff frequency, fc2, for the last pass filter

  low is increased, then the IF band can also be used for higher data rates such as those used in

  different specifications of cable transmission modems.

  This application could also be implemented using other types of filters, for example Figure 3 shows the cable transmission modem receiver IF band set up.

  using a band-pass filter and a low-pass filter.

  With this example, the band pass filter has a center frequency at 55 MHz having a 3 dB bandwidth of 1.6 MHz and a 3 dB band edge higher than 55.8 MHz. The low-pass filter such as for example that of Figure 2 has a cutoff frequency of 2 MHz. It follows that for 0.772 megabauds and 1 MHz channels, IF1 is chosen to be 55.4 MHz and IF2 is 1.6 MHz. For 1.544 megabauds and 2 MHz channels, IF1 is chosen to be 55.0 MHz and IF2 is equal to 1.2 MHz. The IF band in Figure 3 combines the use of a standard dual conversion IF band for 2 MHz channel spacing

  with split filtering on both IF stages, for the 1 MHz channel.

  Referring to FIGS. 4, 5 and 6, an example of an IF band which could be used in a modem of

  cable transmission for the current provisional MCNS specification.

  This specifies baud rates of 160 kilobauds, 320 kilobauds, 640 kilobauds, 1.28 megabauds, and 2.56 megabauds. QPSK and 16-QAM modulation schemes can both be used in a

  frequency band from 5 to 42 MHz.

  When the frequency band is very low, an IF band of up / down conversion is used. In particular, the RF signal is first converted at an IF frequency which is greater than the RF input range and then is down converted.

  up to a second low IF frequency.

  Figures 4 (a) to 4 (d) are equivalent to Figures 1 (a) to 1 (d) and show how a variable bandwidth up / down conversion IF band operates in

  using the fixed filter technique.

  As can be seen from FIG. 4 (a), the first intermediate frequency IF1 is greater than the various channels CH-3 to CH + 3 intended to be received. Therefore, after frequency conversion by a frequency synthesizer 14 and a multiplier 16, the channels are upconverted as shown in FIG.

Figure 4 (b).

  The low-pass filter 18 has a cut-off frequency of fcl and as with the embodiment of FIG. 1, the frequency f 1 of the frequency synthesizer 14 is chosen so that the

  frequencies above the required channel are rejected.

  The frequency synthesizer 20 and the multiplier 22 then operate on the filtered intermediate frequency signal IF1 in order to downconvert and reflect the signal as shown.

in Figure 4 (c).

  The low-pass filter 24 has a cut-off frequency of fc 2 and the frequency f 2 of the frequency synthesizer 20 is chosen such that the low-pass filter 24 rejects all the non-tuned frequencies.

desired remaining.

  Using the architecture presented above, an IF receiver band for the MCNS specification could be implemented

as shown in Figure 5.

  As shown, the low-pass filters have respective cut-off frequencies of fc1 = 120 MHz and fc2 = 4 MHz. It follows that for channels at 160 kilobauds and at 200 kHz, IF1 and IF2 are respectively 119.92 MHz and 3.92 MHz, for channels at 230 kilobauds and 400 kHz, IF1 and IF2 are set respectively at 119.84 MHz and 3.84 MHz, for 640 kilobaud and 800 kHz channels, IF1 and IF2 are respectively 119, 68 MHz and 3.68 MHz for 1.28 megabaud and 1.6 MHz channels , IF1 and IF2 are respectively at 119, 36 MHz and 3.36 MHz, and for 2.56 megabaud and 3.2 MHz channels, IF1 and IF2 are established

  respectively at 118.72 MHz and 2.72 MHz.

  In reality, it is better to use a fixed bandpass filter for the first IF filter having a bandwidth which is of the largest channel size. This would allow rejection of the lower mixer products in the first IF frequency and

  reduce the risk of any intermodulation products.

  Fig. 6 shows an implementation for the same specification that uses a fixed bandpass filter in the first

IF frequency.

  With this example, the bandpass filter has a center frequency of 120 MHz, a 3 dB bandwidth of 2.56 MHz and a 3 dB band edge of 121.28 MHz higher. The low-pass filter as in the case of the example of Figure 5 has a cutoff frequency of 4 MHz. It follows that for channels at 160 kilobauds and kHz, IF and IF2 are respectively 121.20 MHz and 3.92 MHz, for channels at 320 kilobauds and 400 KHz, IF1 and IF2 are respectively 121, 12 MHz and 3.84 MHz, for channels at 1.28 megabaud and 1.6 MHz, IF1 and IF2 are respectively at 64 MHz and 3.36 MHz, and for channels at 2.56 megabaud and 3, 2 MHz, IF1 and IF2 are respectively 120.00 MHz and 2.72 MHz. It should be noted that all the figures represent simplified system functional diagrams. Other system blocks such as

  amplifiers and a mixer image termination etc ...

  should be added in practice. In addition, in practice, tolerances of frequency responses and filters should also be considered. For example, in a real system, the offset of the intermediate frequency should be IF = fc- ((width of

signal band / 2) + tolerances).

Claims (8)

  A variable bandwidth bandpass filter characterized in that it comprises: filtering means (6, 12; 18, 24) of a fixed frequency response (6) for filtering an input signal; and a variable frequency converter (2, 8; 14, 20) for selectively shifting and inverting in the frequency domain the frequency profile of the filtered input signal, wherein the frequency shifted signal is further filtered by the means. in order to produce a filtered bandpass signal of a   bandwidth determined by the frequency offset.   2. Bandpass filter according to claim 1, characterized in that the filtering means comprises: a first filter (6; 12) and a second filter (18; 24), the first filter filtering the input signal and the second filter filtering the offset signal in frequency.   Band pass filter according to Claim 2, characterized in that the first filter (6; 18) has a first fixed frequency response (fcl) and the second filter (12; 24) has a second fixed frequency response ( fc2) different from said first response in fixed frequency.   4. Bandpass filter according to claim 2 or 3, in   which at least one of the first and second filters is a pass filter   low. Band pass filter according to claim 2, 3 or 4, characterized in that at least one of the first and second filters is a bandpass filter.   A down-converter radio receiver for frequency converting a received radio signal according to a signal for demodulation, the down-converter radio receiver being characterized by comprising: a plurality of frequency converters (2,8,14; 20); and   a bandpass filter according to any one of the claims   previous, the variable frequency converter of the filter pass-   band being one of the plurality of frequency converters.   7. Radio receiver characterized in that it comprises a converter-step-down radio receiver according to claim 6. 8. A method of filtering an input signal characterized in that it comprises: filtering (6; 18) the input signal with a fixed frequency response; selectively, shifting the frequency profile of the filtered signal and inverting the filtered signal in the frequency domain; and further filtering (12; 24) the frequency shifted signal using a fixed frequency response to produce a band-pass filtered signal having a bandwidth   determined by the frequency offset step.