Band pass filter with improved group delay

• US 6,404,279 B2
• Filed: 01/26/2000
• Issued: 06/11/2002
• Est. Priority Date: 01/26/2000
• Status: Expired due to Term

• Current Assignee: Cirrus Logic Incorporated
• Original Assignee: Acoustic Technologies Incorporated (Cirrus Logic Incorporated)
• Inventors: Thomasson, Samuel L.
• Primary Examiner(s): LE, DINH THANH
• Application Number:
  US09/492,015
• Publication Number:
  US 20020000874A1
• Time in Patent Office: 867 Days
• Field of Search: 327/552, 327/553, 327/558, 327/557
• US Class Current: 327/557
• CPC Class Codes: H03H 11/126 using a single operational amplifier H03H11/1204 takes precedence; parallel-T filters H03H11/1295
• CPC Class Codes: H03H 11/1295 Parallel-T filters
• CPC Class Codes: H03H 17/04 Recursive filters

Abstract

An electrical signal is applied to a band pass filter, a first notch filter, and a second notch filter in any order. The center frequencies of the notch filters straddle the pass band of the band pass filter. The notch filters improve group delay and steepen the skirts of the response curve of the band pass filter.

Claims

1. A filter characterized by a band pass frequency response between an input and an output and by a short, relatively uniform group delay, said filter comprising:

a band pass filter, a first notch filter, and a second notch filter coupled in series in any sequence between said input and said output,

wherein the center frequency of the first notch filter is below the center frequency of the band pass filter and the center frequency of the second notch filter is above the center frequency of the band pass filter; and

wherein the center frequency of the first notch filter lies within the −

30 dB pass band of said band pass filter and the center frequency of the second notch filter lies within the −

30 dB pass band of said band pass filter;

whereby a relatively uniform group delay is provided across the pass band of said filter.

2. The filter as set forth in claim 1 wherein at least one of the notch filters is a twin-T active filter.

3. The filter as set forth in claim 1 and characterized by a ripple in the pass band of less than 3 dB.

4. The filter as set forth in claim 1 wherein said band pass filter includes:

a first channel coupled to said input and including a first filter having a first center frequency;

a second channel coupled to said input and including a second filter having a second center frequency;

wherein the first center frequency and the second center frequency are not equal; and

a difference amplifier coupled to said channels for subtracting the signals from the channels.

5. The filter as set forth in claim 4 wherein the frequency response of the first filter and the frequency response of the second filter intersect at approximately −

3 dB or less.

6. The filter as set forth in claim 4 wherein said first filter in said band pass filter and said second filter in said band pass filter are each a notch filter.

7. The filter as set forth in claim 4 wherein said first filter in said band pass filter and said second filter in said band pass filter are each a band pass filter.

8. A method for processing an electrical signal, said method comprising the steps of:

applying the electrical signal to a band pass filter to produce a first output signal;

applying the first output signal to a first notch filter to produce a second output signal; and

applying the second output signal to a second notch filter to produce a third output signal;

whereby the response curve of the band pass filter has steeper skirts than without the notch filters and a relatively uniform group delay is obtained across the resulting pass band.

9. The method as set forth in claim 8 wherein the center frequencies of the notch filters straddle the pass band of the band pass filter.