Digital FM demodulator with a reduced sampling rate

• US 5,220,583 A
• Filed: 10/03/1988
• Issued: 06/15/1993
• Est. Priority Date: 10/03/1988
• Status: Expired due to Fees

• Current Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Solomon, James D.
• Primary Examiner(s): Chin, Stephen
• Application Number:
  US07/253,174
• Time in Patent Office: 1,716 Days
• Field of Search: 375/75, 375/103, 375/122.82, 381/29-31, 358/133, 358/135, 364/723, 364/724, 455/205, 455/206, 329/315, 329/318
• US Class Current: 375/328
• CPC Class Codes: H03D 2200/005 Analog to digital conversion
• CPC Class Codes: H03D 2200/0052 Digital to analog conversion
• CPC Class Codes: H03D 2200/0056 including a digital decimation filter
• CPC Class Codes: H03D 2200/006 Signal sampling
• CPC Class Codes: H03D 3/006 by sampling the oscillations and further processing the samples, e.g. by computing techniques H03D3/007 takes precedence

Abstract

A method of recovering a frequency modulated signal wherein the frequency modulated information signal is sampled at a rate less than the Nyquist sampling rate, following which the under sampled signal can be demodulated to extract the original information signal.

Claims

1. A method of recovering an information signal that has been frequency modulated at a peak frequency deviation f_p on a radio frequency carrier and contained substantially within an allotted communication channel to provide a frequency modulated signal, said allotted communication channel having a predetermined bandwidth of approximately plus/minus f_MAX, said method comprising the steps of:

A) sampling said frequency modulated signal at a sample rate f_S, wherein said sample rate f_S is;

i) less than 2f_MAX ;

but

ii) greater than 2f_D ;

to provide a resultant sampled signal that includes aliased information;

B) demodulating said resultant sampled signal to recover said information signal without degradation that is attributable to the sample rate f_S.

2. The method of claim 1 wherein said modulated signal comprises an FM signal, wherein said allotted channel has associated therewith a peak frequency deviation f_D and a maximum frequency f_M, and wherein said sampling rate f_S is also no less than two times the greater of f_M and f_D.

3. The method of claim 1 wherein f_MAX is substantially equal to 25 kHz and f_S is substantially equal to 11 1/9 kHz.

4. The method of claim 1 wherein said modulated signal is sampled at a first sampling rate higher than said sampling rate f_S, and then further decimated down to said sampling rate f_S.

5. The method of claim 1 wherein said modulated signal comprises an angle modulated signal.

6. The method of claim 1 wherein said modulated signal comprises an FM signal.

7. A device for recovering an information signal that has been frequency modulated at a peak frequency deviation f_D on a radio frequency carrier and contained substantially within an allotted communication channel to provide a modulated signal, said allotted communication channel having a predetermined bandwidth of approximately plus/minus f_MAX, said device comprising:

A) sampling means for sampling said frequency modulated signal at a sample rate f_S, wherein said sampling rate f_S is;

i) less than 2f_MAX ;

but

ii) greater than 2f_D ;

to provide a resultant sampled signal that includes aliased information;

B) demodulating means for demodulating said resultant samples signal to recover said information signal without degradation that is attributable to the sample rate f_S.

8. The device of claim 7 wherein said modulated signal comprises a frequency modulated signal.

9. A radio for use with an information signal that has been frequency modulated at a peak frequency deviation f_D on a radio frequency carrier and contained substantially within an allotted communication channel to provide a modulated signal, said allotted communication channel having a predetermined bandwidth of approximately plus/minus f_MAX, said radio comprising:

A) receiving means for receiving said modulated signal;

B) sampling means for sampling said modulated signal at a sample rate f_S,

wherein said sample rate f_S is;

i) less than 2f_MAX ;

but

ii) greater than 2f_D ;

to provide a resultant sampled signal that includes aliased information;

C) demodulating means or demodulating said resultant sampled signal to recover said information signal without degradation that is attributable to the sample rate f_S.

10. The radio of claim 9 wherein said modulated signal comprises a frequency modulated signal.