Direct conversion receiver with dithering local carrier frequency for ...

[54] DIRECT CONVERSION RECEIVER WITH DITHERING LOCAL CARRIER FREQUENCY FOR DETECTING TRANSMITTED CARRIER FREQUENCY

[75] Inventor: Yoichiro Minami, Tokyo, Japan

[73] Assignee: NEC Corporation, Tokyo, Japan

[21] Appl. No.: 511,696

[22] Filed: Apr. 20,1990

[30] Foreign Application Priority Data

Apr. 20, 1989 [JP] Japan 1-100617

May 18, 1989 [JP] Japan 1-57855[U]

Oct. 25, 1989 [JP] Japan 1-277711

[51] Int. CI.5 H04L 27/14

[52] U.S. CI 375/88; 375/97;

329/302

[58] Field of Search 375/45, 80, 82, 83,

375/84, 85, 88, 104, 97; 329/300, 302, 303;

328/134, 141; 333/215

[56] References Cited

U.S. PATENT DOCUMENTS

4,193,033 3/1980 Voorman 333/215

4,322,851 3/1982 Vance 375/88

4,475,219 10/1984 Puckette 375/82

4,517,519 5/1985 Mukaiyama 375/88

4,521,892 6/1985 Vance et al 329/302

4,571,738 2/1986 Vance 329/303

4,672,636 6/1987 Marshall et al 375/97

4,716,579 12/1987 Masterton 375/88

4,752,742 6/1988 Akaiwa 329/303

Primary Examiner—Stephen Chin

Attorney, Agent, or Firm—Sughrue, Mion, Zinn

Macpeak & Seas

[57] ABSTRACT

An FSK receiver includes first and second channel paths to which the received FSK signals are applied. Each channel path includes a mixer circuit, a iow-pass filter connected to the mixer circuit, and a limiting amplifier connected to the low-pass filter. A voltagecontrolled oscillator generates a local- carrier and a phase shifter applies its output to the mixer circuits with a 90° phase difference therebetween. A frequency detector is coupled to one of the limiting amplifiers for generating a signal representative of the deviation frequency minus the offset frequency of the received FSK signal. An average circuit detects an average value of the signal from the frequency detector. A comparator circuit detects whether the frequency representative signal lies in or outside of the neighborhood of the average value. A ramp voltage of constant amplitude is periodically generated if the signal is detected as lying outside of the neighborhood and maintained at an instantaneous value if the signal is detected as lying in the neighborhood. The voltage-controlled oscillator is responsive to the ramp generator to control the frequency of the local carrier in search of the transmitted carrier frequency.

26 Claims, 5 Drawing Sheets