Infinite impulse response filter architecture with idle-tone reduction
View Patent ↗A digital infinite impulse response filter has a plurality of cascaded filter elements, with each filter element defining a pole of the filter and wherein the poles lie inside a unit circle. The filter elements are configured such that the p of the last filter element is a real number. In one embodiment the poles are arranged as complex conjugate pairs. In another embodiment the real part of the output of each filter element is extracted before being passed to the next filter element. This architecture offers improved idle tone with reduced complexity.
1. A digital infinite impulse response filter, comprising:
a plurality of cascaded filter elements, each filter element defining a pole of the filter and having an input and an output, and wherein the poles lie inside a unit circle;
a string of cascaded delay lines downstream of the filter elements; and
a summer for summing the outputs of the delay lines to provide an output signal of the infinite impulse response filter,
wherein the filter elements are configured such that the output of the last filter element is a real number.
2. A digital infinite impulse response filter as claimed in claim 1 , wherein each filter element comprises a unit delay line and an adder in a feedback configuration, and wherein the output of the unit delay line multiplied by a respective pole P n is applied as one input to the adder, another input of the adder providing an input to the filter element.
3. A digital infinite impulse response filter as claimed in claim 1 , wherein the filter elements are arranged in pairs such that adjacent filter elements represent poles where one is the complex conjugate of the other.
4. A digital infinite impulse response filter as claimed in claim 1 , wherein a real part extraction module is located between adjacent upstream and downstream filter elements for passing the real part of the output of an upstream filter element to the input of a downstream filter element.
5. A digital infinite impulse response filter as claimed in claim 1 , wherein the cascaded delay lines are unit delay lines, and the output of each unit delay line is multiplied by a filter coefficient.
6. A digital infinite impulse response filter as claimed in claim 1 , which is configured to implement the transfer function:
H
(
z
)
=
∑
b
n
Z
-
n
∏
(
1
-
P
n
Z
-
1
)
where b n are the feedback coefficients and P n are the poles.
7. A sigma delta converter comprising:
a sigma delta modulator; and
a digital infinite impulse response filter having an output, comprising:
a plurality of cascaded filter elements, each filter element defining a pole of the filter and having an input and an output, and wherein the poles lie inside a unit circle;
a string of cascaded delay lines downstream of the filter elements; and
a summer for summing the outputs of the of the delay lines to provide an output signal of the infinite impulse response filter;
wherein the filter elements are configured such that the output of the last filter element is a real number.
8. A sigma delta converter as claimed in claim 7 , wherein the filter elements are arranged in pairs such that adjacent filter elements represent poles where one is the complex conjugate of the other.
9. A sigma delta converter as claimed in claim 7 , wherein each filter element comprises a unit delay line and an adder in a feedback configuration, and wherein the output of the unit delay line multiplied by a respective pole P n is applied as one input to the adder, another input of the adder provides an input to the filter element.
10. A sigma delta converter as claimed in claim 9 , wherein a real part extraction module is located between adjacent upstream and downstream filter elements for passing the real part of the output of an upstream filter element to the input of a downstream filter element.
11. A sigma delta converter as claimed in claim 7 , wherein the cascaded delay lines are unit delay lines, the output of each unit delay line being multiplied by a filter coefficient.
12. A sigma delta converter as claimed in claim 9 , wherein the infinite impulse response filter is configured to implement the transfer function:
H
(
z
)
=
∑
b
n
Z
-
n
∏
(
1
-
P
n
Z
-
1
)
where b n are the feedback coefficients and P n are the poles.
13. A method of performing a digital infinite impulse response filtering operation on an input signal, comprising:
passing the input signal through a plurality of cascaded filter elements, each filter element defining a pole of the filter and having an input and an output, and wherein the poles lie inside a unit circle; and
configuring the filter elements such that the output of the last filter element is a real number, and
wherein the output signal from the cascaded filter elements is passed to a string of cascaded delay lines downstream of the filter elements, and the outputs of the individual delay lines multiplied by filter coefficients are summed to provide an output signal of the infinite impulse response filter.
14. A method as claimed in claim 13 , wherein each filter element comprises a unit delay line and an adder in a feedback configuration, and wherein the output of the unit delay line multiplied by a respective pole P n is applied as one input to the adder, another input of the adder providing an input to the filter element.
15. A method as claimed in claim 14 , wherein the filter elements are arranged in pairs such that adjacent filter elements represent poles where one is the complex conjugate of the other.
16. A method as claimed in claim 14 , wherein the real part of the output of an upstream filter element is extracted and passed as an input signal to a following downstream filter element.