Integrated DVB filter
A filter for processing frequency signals, preferably received, particularly digital television signals is described, which filter comprises a Chebyshev filter ( 11, 12 ), a subsequent all-pass filter ( 13 ) and a control device ( 14 to 20 ) for controlling the Chebyshev filter ( 11, 12 ) and the all-pass filter ( 13 ).
1. A filter for processing frequency signals, comprising a Chebyshev filter ( 11 , 12 ), a subsequent all-pass filter ( 13 ) producing an output signal and a control device ( 14 to 20 ) comprising a frequency synthesizer ( 14 ) producing a controllable reference frequency for controlling the Chebyshev filter ( 11 , 12 ) and the all-pass filter ( 13 ), wherein the frequency synthesizer ( 14 ) generates a reference signal at a reference frequency (fref), and the control device further comprises a reference low-pass filter ( 16 ) which shifts the phase of the reference signal by 90 degrees, and a phase detector ( 18 ) which compares the phase positions of the reference signal and the output signal of the reference low-pass filter ( 16 ) and generates an output signal for controlling the frequency position of the Chebyshev filter ( 11 , 12 ), the all-pass filter ( 13 ) and the reference low-pass filter ( 16 ).
2. A filter as claimed in claim 1 , wherein the Chebyshev filter ( 11 , 12 ) is a higher-order Chebyshev filter.
3. A filter as claimed in claim 2 , wherein the Chebyshev filter ( 11 , 12 ) is a 4th-order Chebyshev filter.
4. A filter as claimed in claim 1 , wherein the Chebyshev filter comprises a plurality of filter stages ( 11 , 12 ).
5. A filter as claimed in claim 4 , wherein the Chebyshev filter comprises two filter stages ( 11 , 12 ).
6. A filter as claimed in claim 3 , wherein each filter stage ( 11 , 12 ) is a 2nd-order filter stage.
7. A filter as claimed in claim 1 , wherein the Chebyshev filter ( 11 , 12 ) has a ripple of less than 0.5 dB.
8. A filter as claimed in claim 1 , wherein the all-pass filter ( 13 ) is a higher-order all-pass filter.
9. A filter as claimed in claim 8 , wherein the all-pass filter ( 13 ) is a 2nd-order all-pass filter.
10. A filter as claimed in claim 1 , wherein the all-pass filter ( 13 ) smoothes the group delay time ripple produced in the passband to a value of less than 75 ns.
11. A filter as claimed in claim 1 , comprising a capacitor (C) connected to the output of the phase detector ( 18 ).
12. A filter as claimed in claim 1 , comprising a buffer ( 20 ) connected to an output of the phase detector ( 18 ), through which buffer the output signal of the phase detector ( 18 ) is passed.