Method and apparatus for processing an amplitude modulated (AM) signal

• US 20040096014A1
• Filed: 11/14/2002
• Published: 05/20/2004
• Est. Priority Date: 11/14/2002
• Status: Active Grant

• Current Assignee: North Star Innovations Inc. (Wi-LAN Inc.)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Seaberg, Charles E., Hendrix, Jon, Banks, Bradley
• Granted Patent: US 7,092,465 B2
• Time in Patent Office: Days
• US Class Current: 375/320
• CPC Class Codes: H03H 17/0294 Variable filters; Programmable filters
• CPC Class Codes: H04B 1/1036 with automatic suppression of narrow band noise or interference, e.g. by using tuneable notch filters H04B1/123 takes precedence; filter circuits H03H

Abstract

In order to protect against adjacent channel interference and the effects of weak signal in AM signal processing, pre- and post-AM demodulation filters (36 and 38) may be used where the bandwidth of both is varied in the same manner by a single controlling mechanism (34). In one embodiment, two or more cascaded filters (20, 24 or 22, 26 or 30,32) may be used for the pre- and post-demodulation filters where each of the cascaded filters has a same predetermined order and uses the same set of coefficients. In one embodiment, all cascaded filters are first order IIR filters, which reduces computation complexity. The use of a same set of coefficient for all the cascaded filters results in a same bandwidth for all filters and further reduces computation complexity. In an alternate embodiment, cascaded filters may be used for only one of the pre- and post-demodulator filters.

Claims

1. A method for processing an amplitude modulated (AM) signal, comprising:

providing an input signal to a first filter having a predetermined order and having a set of coefficients;

outputting a first output signal from the first filter;

providing the first output signal to a second filter having a same predetermined order and having a same set of coefficients; and

outputting a second signal from the second filter.

2. A method as in claim 1, further comprising:

performing pre-demodulation filtering using the first filter and the second filter.

3. A method as in claim 2, further comprising:

demodulating the second signal to provide a demodulated signal;

providing the demodulated signal to a third filter having the same predetermined order and having the same set of coefficients;

outputting a third output signal from the third filter;

providing the third output signal to a fourth filter having the same predetermined order and having the same set of coefficients; and

outputting a filtered signal from the fourth filter.

4. A method as in claim 1, further comprising:

performing post-demodulation filtering using the first filter and the second filter.

5. A method as in claim 1, wherein the predetermined order is first order and the first filter and the second filter are infinite impulse response (IIR) filters.

6. A method as in claim 1, further comprising:

selectively modifying the set of coefficients.

7. A method as in claim 6, further comprising:

using a linear approximation to determine how to modify at least a portion of the set of coefficients.

8. A method as in claim 1, further comprising:

receiving a signal quality indicator.

9. A method as in claim 8, wherein receiving the signal quality indicator comprises:

receiving a signal strength indicator.

10. A method as in claim 8, wherein receiving the signal quality indicator comprises:

receiving an adjacent channel interference indicator.

11. A method as in claim 1, further comprising:

using a signal strength indicator and an adjacent channel interference indicator to selectively modify the set of coefficients.

12. A method as in claim 11, further comprising:

using a predetermined linear polynomial of the signal strength indicator to determine a first cut-off frequency; and

using a predetermined linear polynomial of the adjacent channel interference indicator to determine a second cut-off frequency.

13. A method as in claim 12, further comprising:

selecting a minimum frequency of the first cut-off frequency and the second cut-off frequency.

14. A method as in claim 13, further comprising:

using a first linear polynomial of the minimum frequency in determining a first portion of the set of coefficients; and

using a second linear polynomial of the minimum frequency in determining a second portion of the set of coefficients.

15. A method as in claim 1, further comprising:

providing gain control of the second signal.

16. A method as in claim 1, further comprising:

providing soft muting of the second signal.

17. An amplitude modulation (AM) receiver, comprising:

an input portion having a tuner for selectively receiving an input modulated signal and providing an intermediate signal;

a first filter, coupled to the input portion for receiving the intermediate signal, the first filter having a transfer function;

a second filter, coupled to the first filter, having a same transfer function;

a demodulator, coupled to the second filter;

a third filter, coupled to the demodulator, having the same transfer function;

a fourth filter, coupled to the third filter, having the same transfer function; and

a filter control unit for controlling the first, second, third, and fourth filters.

18. A receiver as in claim 17, wherein the first, second, third, and fourth filters are first order and are infinite impulse response (IIR) filters.

19. A receiver as in claim 17, wherein the filter control unit uses at least one linear approximation to approximate the same transfer function.

20. A receiver as in claim 17, wherein the at least one linear approximation is used to selectively modify coefficients of the same transfer function.

21. An amplitude modulation (AM) receiver, comprising:

an input portion;

a pre-demodulator filter, coupled to the input portion, having a transfer function;

a demodulator, coupled to the pre-demodulator filter;

a post-demodulator filter, coupled to the demodulator, having a same transfer function as the pre-demodulator filter; and

a filter control unit, coupled to the pre-demodulator filter and to the post-demodulator filter.

22. A receiver as in claim 21, wherein the pre-demodulator filter comprises a first filter having a set of coefficients and a second filter having a same set of coefficients.

23. A receiver as in claim 22, wherein the post-demodulator filter comprises a third filter having the same set of coefficients and a fourth filter having the same set of coefficients.

24. A receiver as in claim 23, wherein the first, second, third, and fourth filters are first order and are infinite impulse response (IIR) filters.

25. A receiver as in claim 21, wherein the filter control unit uses at least one linear approximation to approximate the same transfer function.