WO2010084192A1

WO2010084192A1 - Method and device for filtering desired filter frequency band from received tv signal

Info

Publication number: WO2010084192A1
Application number: PCT/EP2010/050807
Authority: WO
Inventors: Martin Skutek
Original assignee: Unitron; Jarkovsky, Ladislav
Priority date: 2009-01-23
Filing date: 2010-01-25
Publication date: 2010-07-29

Abstract

Method and for filtering a desired filter frequency band containing a desired set of one or more TV channels from a received TV signal (2) containing a plurality of TV channels, comprising: (a) shifting the received signal (2) in frequency by means of a first tunable local oscillator signal (4); (b) filtering the first frequency shifted signal (6) by means of a first high selectivity fixed frequency filtering circuit (7), thereby defining a first side of the desired filter frequency band; (c) shifting the first filtered signal (8) in frequency by means of a second tunable local oscillator signal (10); (d)filtering the second frequency shifted signal (12) by means of a second high selectivity fixed frequency filtering circuit (13), thereby defining a second side of the desired filter frequency band and generating a second filtered signal (14) containing only the desired set of TV channels; and (e) shifting the second filtered signal (14) in frequency by means of a third tunable local oscillator signal (16) to obtain an output signal (18) centered at a predetermined frequency.

Description

Method and device for filtering desired filter frequency band from received

TV signal

Field of the invention The invention relate to a method and device for filtering a desired filter frequency band containing a desired set of one or more TV channels from a received TV signal.

Background of the invention In the 1980s, a fixed filter circuit was commonly used to filter and combine different aerial signals onto 1 cable. Fixed, meaning the installer needed to replace the filter circuit by a totally new one if frequencies of the received TV channels (or simply the whole application) had changed.

In the 1990s, programmable filter circuits came on the market with the same purpose. But these circuits could be reprogrammed to serve different frequencies or new applications without replacing it by a new filter. It could be simply programmed by the installer on site. More detailed descriptions can be found in the patent GB2272341. Also the filter was the same for all possible frequency applications, so there was no need to develop, produce and store different versions. Different types of programmable filter circuits exist on the market.

A first category of programmable filter circuits is the tunable frequency and fixed bandwidth LC filter circuits, which keep the output frequency equal to the input frequency. Such circuits have been the cheapest filtering solution but their biggest disadvantage is an insufficient unwanted signal rejection and a fixed bandwidth.

A second category of programmable filter circuits is the fixed bandwidth filter circuits which use a heterodyne technique, well known from the superheterodyne radio receivers. Such filter circuits are using a fixed frequency filtering circuits with a high selectivity (high order LC, quartz, SAW, BAW or garnet filters) which enables to keep the selectivity while the input frequency can be tuned. Those circuits have a very high Q-factor, hence a very good roll-off, but they still keep the disadvantage of the fixed bandwidth. A third category of programmable filter circuits is the tunable frequency and variable bandwidth LC filter circuits. They keep the LC filtering principle of the first category but they need to cover more possibilities, which implies that the filtering is even not as sharp as the first category. The variable bandwidth feature does not allow to use the above mentioned filters of the second category with a high selectivity to improve the filter sharpness. An example of this third category is the Profiler produced by Unitron n.v. and described in EP-A- 1794883

Disclosure of the invention

It is an aim of the invention to provide a method and device for filtering a desired filter frequency band containing a desired set of one or more TV channels from a received TV signal, which achieves a high selectivity in combination with a variable bandwidth as well as a variable output centre frequency.

This aim is achieved with the method and device of the independent claims.

According to the invention, two filtering circuits with a high selectivity (high Q-factor, good roll-off) are cascaded, each time preceded by a frequency shifting block. One filtering circuit is defining one side of the desired filter frequency band at the output and the second is defining the second side. In this way, the output signal is filtered with the sharpness of the filtering circuits whereas each filter brings its selectivity to only one side of the output signal frequency band. The final bandwidth of the output signal can be varied by changing the frequency shift which is applied by the first and second frequency shifting blocks. In addition, the centre frequency of the output signal can be varied by changing the frequency shift which is applied by the third frequency shift. As a result, according to the invention, high selectivity on both sides of the desired frequency band is achieved in combination with a variable bandwidth and centre frequency of the desired frequency band.

The first and second filtering circuits are fixed frequency filtering circuits, meaning that the roll-off frequency and slope on the side which is used for defining the respective side of the output filter frequency band are fixed. Hence, according to the invention, the desired filter frequency band at the output can be fully set by only varying the frequency shift which is applied in the first and second frequency shifting blocks (the third one being used for controlling the centre frequency. As a result, the control of the device and method of the invention can be very simple. As used herein, with "normal selectivity" is intended to mean a filtering circuit having a roll-off of about 10 dB/ 10 MHz.

As used herein, with "high selectivity" is intended to mean a filtering circuit having a roll-off of an order higher than normal selectivity, e.g. at least 20 dB/10 MHz, preferably at least 30 dB/10 MHz, more preferably at least 40 dB/ MHz.

The filtering circuits are high selective on at least one side. They can be low-pass filters, high-pass filters or bandpass filters, notch filters or any other type of high selectivity filters known to the person skilled in the art. The filtering circuits are preferably surface acoustic wave filters, although other types are also possible.

The frequency shifting blocks preferably comprise a tunable oscillator with a mixer, such as for example a voltage controlled oscillator. However, any other frequency shifting blocks known to the person skilled in the art may also be used. In preferred embodiments, the first and second filtering circuits are identical, i.e. twice the same circuit is used. This is possible when the second frequency shifting block is provided for spectrally inverting the first filtered signal. This embodiment has the advantage that the design can be simplified, as twice the same circuit is used, and that cost can be reduced. In preferred embodiments, the first and second tunable local oscillator signals are generated from a common frequency source providing a mother frequency. In this way, the desired filter frequency band can be varied by changing the mother frequency of the common frequency source, thereby synchronously tuning the first and second tunable local oscillator signals. This has the advantage that any inaccuracy in the mother frequency only leads to an inaccuracy in the bandwidth at the output.

Description of the drawings The invention will be further elucidated by means of the following description and the appended figures.

Figure 1 schematically shows a first embodiment of the invention.

Figure 2 schematically shows a second embodiment of the invention.

Figure 3 shows an example of how the spectrum is filtered according to the invention.

Detailed description of preferred embodiments of the invention There is shown a possible embodiment of the invention in Figure 1.

This embodiment comprises a filter circuit input (1 ), an input signal (2), a first tunable local oscillator circuit (3), a first oscillator signal (4), a first mixer circuit (5), a first mixer output signal (6), a first filtering circuit (7), a first filter output signal (8), a second tunable local oscillator circuit (9), a second oscillator signal (10), a second mixer circuit (1 1 ), a second mixer output signal (12), a second filtering circuit (13), a second filter output signal (14) and a filter circuit output (15).

The filters (7) and (13) have a roll-off with high selectivity on at least one side. The filters (7) and (13), in case they are bandpass filters, need to have a bandwidth which is equal or wider than the highest requested bandwidth of the input signal of interest (2). Otherwise, a combination of a low-pass filter and a high-pass filter can be used. Even a combination of two low/high-pass filters is possible, provided that the second oscillator signal (10) inverts the spectrum. In the following, the example of bandpass filters is used, but it is understood that the invention is not limited thereto. An explanation will be made here for the situation that both oscillators (3) and (9) frequencies are set to provide the low-side injection, meaning that the oscillator (3) frequency is below the signal (2) frequency and oscillator (9) frequency is below the signal (8) frequency. But this is only one of the possible configurations. The principle of the invention can be applied for any combination of low-side and high-side injections of both oscillators (3) and (9).

The input signal (2) comes from the source such as an antenna to the filter circuit input (1 ). The first tunable local oscillator circuit (3) generates a first oscillator signal (4) which is mixed by a first mixer circuit (5) which gives a first mixer output signal (6) to the first filtering circuit. The signal (6) frequency is a sum of the signals (2) and (4). The spectral order of the signal is kept intact, meaning the lowest frequency component of the input signal is also the lowest frequency component of the output signal due to the low-side injection. The same is applicable for the highest frequency component. The first oscillator (3) frequency is set to shift the left (lower frequency) side of the input signal of interest (2) frequency band to the left side of the first filtering circuit frequency band. The first filter output signal (8) includes not only a frequency transferred input signal of interest (2) with sharp filtered left frequency side of the useful bandwidth but also an unwanted frequency transferred signals from input (1 ) which are filling the gap between the not yet filtered right side of the signal of interest (2) frequency band and the right side of the first filtering circuit (7) frequency band. This is especially the case if the first filtering circuit (7) has a wider bandwidth than the requested bandwidth of the input signal of interest (2). The parts (3), (5) and (7) apply a first filtering step. There is used a second filtering step with the parts (9), (1 1 ) and (13) for the sharp filtering of the right side of the signal of interest (2) frequency band. The first filter output signal (8) is mixed in a second mixer circuit (1 1 ) with a second oscillator signal (10) from a second tunable local oscillator circuit (9). The signal (12) frequency is a sum of the signals (8) and (10). Again, no spectral inversion is introduced by the low-side injection. The second oscillator (9) frequency is set to shift the right (higher frequency) side of the first filter output signal of interest (8) frequency band to the right side of the second filter (13) component frequency band. The second filter output signal (14) is leaving the filter circuit via the filter circuit output (15).

An alternative configuration of the present invention is to use the first oscillator (3) frequency to shift the right (higher frequency) side of the input signal of interest (2) frequency band to the right side of the first filtering circuit frequency band and the second oscillator (9) frequency to shift the left (lower frequency) side of the first filter output signal of interest (8) frequency band to the left side of the second filter (13) component frequency band. The results of the procedure above described are:

• The signal (14) has a frequency which can be calculated from the signal (2), (4) and (10) frequencies.

• The input (2) and output (14) signals have the same spectral order of the signal's frequency components due to the double low-side injection. • The output signal (14) is filtered with a sharpness of the filters (7) and (13) whereas each filter brings its selectivity to only one side of the output signal (14) frequency band.

• The final bandwidth of the output signal (14) can be variable. This can be achieved by changing the frequency of the first and/or second oscillator signal (10). A more detailed explanation can be found below.

Figure 3 shows another example to explain a possible embodiment of the invention, based on the circuit of figure 2. The input signal (2) comprises a plurality of channels, of which the ones desired at the output (the desired spectrum) are shown by shading. The first oscillator (3) and mixer (5) shift the input signal (2) to align the left side of the desired spectrum with the slope of the first filtering circuit (7), which is in this case a high-pass filter. The result after the first filtering step is the first filter output signal (8). The second oscillator (9) and mixer (11 ) shift the first filter output signal (8) a second time to align the right side of the desired spectrum with the slope of the second filtering circuit (13), which is in this case a low-pass filter. The result after the second filtering step is the second filter output signal (14). Finally, third oscillator (15) and mixer (17) shift this second filter output signal (14) to obtain the output signal (18) at the desired frequency. It is clear that by simply varying the first and/or second oscillator frequencies, the first and/or second frequency shifts can be varied to select a different spectrum. By varying the third oscillator frequency, the output centre frequency can be selected. So the result is a high quality output signal with a selectable bandwidth (containing one or more of the channels at the input) at a selectable centre frequency. If depending on the application, the bandwidth of the output signal

(14) must be variable (because 1 or more UHF channels could be needed), one way how this can be achieved is changing the frequency of the second oscillator signal (10). If for example the UHF channels are 8 MHz wide and the filtering circuits (7) and (13) are both 32 MHz wide, the bandwidth of the output signal (14) can be set to 1 , 2, 3 or 4 channels wide. If the first filtering circuit (7) has a centre frequency of 200 MHz and the second filtering circuit (13) has a centre frequency of 300 MHz, and the left lower frequency of the input signal of interest (2) is aligned with the left lower side of the first filtering circuit (7), in this case aligned to 184 MHz, a single UHF channel can be filtered out by setting the frequency of the second oscillator signal (10) to 124 MHz. This aligns the right upper frequency of the requested UHF channel to the right upper side of the second filtering circuit (13). If in the same application as mentioned above, not a single UHF channel is needed but 4 adjacent channels are requested, the frequency of the second oscillator signal (10) is changed to 100 MHz to let all 4 channels pass both filters. Second way of achieving the variable bandwidth of the output signal (14) is synchronously changing the frequency of both oscillator signals (4) and (10). There is valid a simple rule in such case, if we need to increase a bandwidth of the filtered signal of interest (2) by x MHz, then we need to change the frequency of the first oscillator signal (4) by x MHz together with changing the frequency of the second oscillator signal (10) by x MHz. The advantage of this second more complex way is if the oscillators (3) and (9) have a common source of the frequency then the inaccuracy of this mother frequency source does not influence an inaccuracy of the output signal (14) centre frequency but it affects only an inaccuracy of the output signal (14) bandwidth. Other solutions are possible, ranging from different bandwidths of the filtering circuits (7) and (13) over high- side injection to smaller or bigger frequency steps of the second oscillator signal (10) than the bandwidth of the received channels.

In the filter circuit of Figure 1 , the second filter output signal (14) has a fixed centre frequency, which can be further varied by adding a third frequency shifting stage. A possible embodiment of the invention which adds such a third frequency shifting stage is shown in Figure 2. This embodiment comprises a filter circuit input (1 ), an input signal (2), a first tunable local oscillator circuit (3), a first oscillator signal (4), a first mixer circuit (5), a first mixer output signal (6), a first filtering circuit (7), a first filter output signal (8), a second tunable local oscillator circuit (9), a second oscillator signal (10), a second mixer circuit (11 ), a second mixer output signal (12), a second filtering circuit (13), a second filter output signal (14), a third tunable local oscillator circuit (15), a third oscillator signal (16), a third mixer circuit (17), a third mixer output signal (18) and a filter circuit output (19). Comparing the Figure 1 with the Figure 2, the filter circuit is extended by a third frequency shifting section, comprising the parts (15) and (17). This way the second filter output signal can be transferred by mixing it in a third mixer circuit (17) with a third oscillator signal (16) from a third tunable local oscillator circuit (15) to a third mixer output signal at a predetermined centre frequency, suitable for further distribution to for example a plurality of TV sets. By the addition of the third frequency shifting stage, multiple selected frequency bands coming from multiple units like the one shown in figure 2 can be "stacked" above each other on a common distribution line, by selecting different output centre frequencies for each unit such that they are sufficiently spaced apart and can be retrieved downstream by means of suitable filters.

The description above is an example, but surely not the only possible solution. The filter circuit of the invention generally uses at least two filtering steps but it can use even more filtering steps to achieve special features, such as for example removing one of the channels in the middle of the output spectrum. As mentioned above the signal mixing and the local oscillators frequency setting can be done by any combination of the low-side and high-side injections. The oscillators can be tuned for example by any programmable electronic control unit. The electronic product which uses the invention can comprise one or more filter circuits described above. The product can also comprise any combination of previously known filter circuits and the filter circuits according to the invention. The filtering circuits (7) and/or (13) can be based on any other principle than the above mentioned if they have sufficient selectivity. The filtering circuits (7) and (13) have typically fixed bandwidth due to the high selectivity but the invention is applicable for the usage of filtering circuits (7) and/or (13) with variable bandwidth as well. The filtering circuits (7) and (13) have typically fixed frequencies due to the high selectivity but the invention is applicable for the usage of the filtering circuits (7) and/or (13) with variable frequency as well. One or two of the independent local oscillator circuits (3) and/or (9) and/or (15) can be replaced by any other sort of circuit(s) which is (are) generating one or two oscillator signal(s) (4) and/or (10) and/or (16) from the other oscillator circuit (4) or (10) or (15).

Claims

1. A method for filtering a desired filter frequency band containing a desired set of one or more TV channels from a received TV signal (2) containing a plurality of TV channels, comprising the steps of: a) shifting the received signal (2) in frequency by means of a first tunable local oscillator signal (4), thereby generating a first frequency shifted signal (6); b) filtering the first frequency shifted signal (6) by means of a first high selectivity fixed frequency filtering circuit (7), thereby defining a first side of the desired filter frequency band and generating a first filtered signal (8); c) shifting the first filtered signal (8) in frequency by means of a second tunable local oscillator signal (10), thereby generating a second frequency shifted signal (12); d) filtering the second frequency shifted signal (12) by means of a second high selectivity fixed frequency filtering circuit (13), thereby defining a second side of the desired filter frequency band and generating a second filtered signal (14) containing only the desired set of TV channels; and e) shifting the second filtered signal (14) in frequency by means of a third tunable local oscillator signal (16) to obtain an output signal (18) centered at a predetermined frequency.

2. The method according to claim 1 , wherein the first and second high selectivity filtering circuits have a roll-off of at least 20 dB/10 MHz.

3. The method according to claim 1 or 2, wherein the first and second filtering circuits (7, 13) are surface acoustic wave filters.

4. The method according to any one of the previous claims, wherein in step c) the first filtered signal (8) is spectrally inverted by the mixing with the second local oscillator signal (10), and wherein the first and second filtering circuits (7, 13) are identical.

5. The method according to any one of the previous claims, wherein the first and second tunable local oscillator signals (4, 10) are generated from a common frequency source providing a mother frequency.

6. The method according to claim 5, further comprising the step of varying the desired filter frequency band by changing the mother frequency of the common frequency source, thereby synchronously tuning the first and second tunable local oscillator signals.

7. An electronic sharp filter device for filtering a desired filter frequency band containing a desired set of one or more TV channels from a received TV signal (2) containing a plurality of TV channels, the filter circuit comprising:

- a first frequency shifting block comprising a first mixer (5) and a first tunable local oscillator (3) for shifting the received signal (2) in frequency, thereby generating a first frequency shifted signal (6);

- a first high selectivity fixed frequency filtering circuit (7) connected to the first frequency shifting block and provided for filtering the first frequency shifted signal to define a first side of the desired filter frequency band, thereby generating a first filtered signal (8); - a second frequency shifting block connected to the first high selectivity filtering circuit and comprising a second mixer (11 ) and a second tunable local oscillator (9) for shifting the first filtered signal in frequency, thereby generating a second frequency shifted signal (12);

- a second high selectivity fixed frequency filtering circuit (13) connected to the second frequency shifting block and provided for filtering the second frequency shifted signal (12) to define a second side of the desired filter frequency band, thereby generating a second filtered signal (14) containing only the desired set of TV channels; and

- a third frequency shifting block connected to the second high selectivity filtering circuit and comprising a third mixer (17) and a third tunable local oscillator (15) for shifting the second filtered signal in frequency to obtain an output signal (18) centered at a predetermined frequency.

8. The filter device of claim 7, wherein the first and second high selectivity filtering circuits have a roll-off of at least 20 dB/10 MHz.

9. The filter device of claim 7 or 8, wherein the first and second filtering circuits (7, 13) are surface acoustic wave filters.

10. The filter device of any one of the claims 7-9, wherein the first and second filtering circuits are identical, the second frequency shifting block being provided for spectrally inverting the first filtered signal (8).

1 1. The filter device of any one of the claims 7-10, wherein the first and > second tunable local oscillators (3, 9) are linked to a common frequency source providing a variable mother frequency.