Method and arrangement for filtering of signals

• US 20010019608A1
• Filed: 01/24/2001
• Published: 09/06/2001
• Est. Priority Date: 01/27/2000
• Status: Active Grant

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Lofmark, Bengt Gustav
• Granted Patent: US 7,447,305 B2
• Time in Patent Office: Days
• US Class Current: 379/398
• CPC Class Codes: H03H 11/24 Frequency-independent attenuators
• CPC Class Codes: H03H 7/38 Impedance-matching networks
• CPC Class Codes: H04L 25/03878 Line equalisers; line build-out devices

Abstract

The present invention relates to a filter (45) for filtering signals in a telecommunications system, a method of designing said filter, and a corresponding splitter filter. The inventive filter (45) is characterised in that it is passive and has a complex impedance which gives good impedance matching to the complex impedance of a transmission line. Because the filter is passive, it does not need to be powered and can thus be placed in locations that lack a power supply. The filter will also function in the event of a power failure. Because the filter has an impedance which can be well matched to the complex impedance of a transmission line, problems relating to echo and side tones can be minimised.

The design of the inventive filter has been made possible by utilising that a certain determined level of losses can often be accepted in respect of the filter. The impedance of the filter can be made similar to the impedance of a transmission line, by intentionally introducing into the filter losses (15, 17) which assist in making the impedance of the filter more complex. This can be achieved without the use of active elements.

Claims

1. A filter for filtering signals in a telecommunications system and for impedance matching to a predetermined complex impedance, wherein the filter has at least one first pass band, and wherein the filter is passive, and the characteristic impedance of the filter is complex so that it matches the predetermined complex impedance at least approximately.

2. A filter according to

claim 1

, wherein a resistance of at least one of the filter components assists in giving the characteristic impedance of the filter its complex character.

3. A filter according to

claim 2

, wherein at least one resistance is in series with at least one inductance, said resistance and said inductance assisting in giving the filter said complex characteristic impedance.

4. A filter according to

claim 3

, wherein the filter includes at least one parallel combination of a first inductance in series with a first resistance and a second inductance in series with a second, wherein said first inductance is large in relation to the second inductance, and wherein the first resistance is small in relation to the second resistance.

5. A filter according to

claim 4

, wherein the filter includes at least two circuit of which at least one circuit segment includes said parallel combination.

6. A filter according to

claim 5

, wherein the circuit segments are four structure-wise identical circuit segments.

7. A filter according to

claim 3

, wherein the filter includes at least one series combination of a first inductance in series with a first resistance and a second inductance in parallel with a second resistance, wherein the first inductance is small in relation to the second inductance, and wherein the first resistance is small in relation to the second resistance.

8. A filter according to

claim 2

, wherein the filter includes at least two cascade-coupled circuit segments of which at least one circuit segment includes at least the one resistance that assists in giving the characteristic impedance of said filter said complex character.

9. A filter according to

claim 2

, wherein the resistance that assists in giving the characteristic impedance of the filter said complex character is comprised of at least one resistor.

10. A filter according to

claim 2

, wherein the resistance that assists in giving the characteristic impedance of the filter said complex character is comprised of at least one winding resistance of an inductor.

11. A filter according to

claim 1

, wherein the predetermined complex impedance is the characteristic impedance of a transmission line.

12. A filter according to

claim 1

, wherein the predetermined complex impedance is the ETSI impedance Z_ETSI=150 nF//750 ω

+ 270 ω

.

13. A filter according to

claim 1

, wherein the filter includes at least one cable simulator section, which cable simulator section has a characteristic impedance that matches the predetermined complex impedance at least approximately;

wherein the filter also includes at least one capacitor, wherein said capacitor assists in giving the filter at least one attenuation peak in a predetermined frequency range in coaction with said cable simulator section.

14. A filter according to

claim 1

, wherein the filter includes at least one cable simulator section, which cable simulator section has a characteristic impedance that matches the predetermined complex impedance at least approximately;

and in that the filter includes at least one coupled coil, which coupled coil includes an inductance in the cable simulator section and assists in giving the filter at least one attenuation peak in a predetermined frequency range.

15. A filter according to

claim 1

, wherein the filter is a low-pass filter.

16. A filter according to

claim 1

, wherein the filter includes a further pass band in a predetermined frequency range, said further pass band differing from said at least first pass band.

17. A splitter filter which includes at least one filter according to

claim 1

.

18. A splitter filter according to

claim 17

, wherein said filter according to

claim 1

is a low-pass filter;

and wherein the low-pass filter is connected in series to a high-pass filter.

19. A method of designing a filter which is intended for filtering signals in a telecommunications system and for impedance matching to a predetermined complex impedance, said method comprising the steps of

selecting the complex impedance to which the characteristic impedance of the filter shall be matched;

and

providing the filter with at least one first pass band,

designing a cable simulator section whose characteristic impedance matches the predetermined complex impedance at least approximately, and

creating the filter from the cable simulator section by adapting said cable simulator section so that it will include at least one attenuation peak in a predetermined frequency range,

and wherein solely passive components are used in the method.

20. A method according to

claim 19

, wherein a capacitor which is connected to the cable simulator section assists in creating said at least one attenuation peak.

21. A method according to

claim 19

, wherein an inductance is implemented in the cable simulator section with a coupled coil which assists in creating said at least one attenuation peak.

22. A method according to

claim 19

, wherein the filter is provided with at least one resistance in series with at least one inductance, said resistance and inductance assisting in giving the filter said complex characteristic impedance.

23. A method for designing a filter intended for filter signals in a telecommunications system and for impedance matching to a predetermined complex impedance, in which method the filter is provided with at least one first pass band, wherein only passive components are used in the method, and wherein the method includes the further step of introducing into the filter at least one resistance which assists in giving the filter a complex characteristic impedance that matches the predetermined complex impedance at least approximately.

24. A method according to

claim 23

, wherein at least the one resistance is in series with at least one inductance, wherein said resistance and said inductance assist in giving the filter said complex characteristic impedance.

25. A method according to

claim 23

, wherein the resistance that assists in giving the filter its said complex characteristic impedance is implemented with the aid of at least one resistor.

26. A method according to

claim 23

, wherein the resistance which assists in giving the filter its said complex characteristic impedance is implemented with the aid of at least one winding resistance of an inductance.

27. A method according to

claim 23

, wherein the filter is provided with at least one parallel combination of a first inductance in series with a first resistance and a second inductance in series with a second resistance, said first inductance being large in relation to the second inductance and said first resistance being small in relation to the second resistance.

28. A method according to

claim 27

, wherein the filter is comprised of at least two circuit segments, of which at least one circuit segment includes said parallel combination.

29. A method according to

claim 28

, wherein the circuit segments are four circuit segments that are structure-wise identical.

30. A method according to

claim 23

, wherein the filter is provided with at least one series combination of a first inductance in series with a first resistance, and a second inductance in parallel with a second resistance, wherein the first inductance is small in relation to the second inductance, and wherein the first resistance is small in relation to the second resistance.

31. A method according to

claim 23

, wherein the filter is a low-pass filter.

32. A method according

claim 23

, wherein the predetermined complex impedance is the characteristic impedance of a transmission line.

33. A method according to

claim 23

, wherein the predetermined complex impedance is the ETSI impedance Z_ETSI= 150 nF//750 ω

+270 ω

.

34. A method according to

claim 23

, wherein the method also includes the step of optimising the element values of the filter components on the basis of requirements placed on the properties of said filter.

35. A method according to

claim 34

, wherein the method also includes an iteration procedure, in which the step of optimising said element values is repeated until the set requirements are fulfilled, and in which a circuit segment is added to and cascade-coupled with the earlier filter design with each iteration.

36. A method according to

claim 35

, wherein the method includes a step of establishing a least number of circuit segments with which the set requirements can be fulfilled;

and in that a filter having said least number of circuit segments is used in the optimising step at the beginning of the iteration procedure.

37. A method according to

claim 23

, wherein the filter is provided with a further pass band in a predetermined frequency range, said further pass band differing from said at least one first pass band.

38. A method of designing a splitter filter that includes a high-pass filter and a low-pass filter, wherein at least one of the filters is designed in accordance with a method according

claim 23

.

39. A method according to

claim 38

, wherein at least one of the filters designed in accordance with

claim 23

is a low-pass filter;

and wherein the low-pass filter is connected to the high-pass filter in series.