Direct conversion FM receiver with offset

• US 4,944,025 A
• Filed: 08/09/1988
• Issued: 07/24/1990
• Est. Priority Date: 08/09/1988
• Status: Expired due to Term

• Current Assignee: Protocol-ip.com, LLC, Seiko Instruments Incorporated (Seiko Group)
• Original Assignee: AT&T, Inc.
• Inventors: Gehring, Mark R., Suter, Richard R., Ragan, Lawrence H.
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): KUNTZ, CURTIS A
• Application Number:
  US07/229,976
• Time in Patent Office: 714 Days
• Field of Search: 455/207-209, 455/214, 455/47, 455/258, 455/61, 455/264, 455/316, 455/318, 455/319, 455/141-143, 455/324, 455/303-306, 455/192, 455/234, 329/50, 329/122, 329/124
• US Class Current: 455/207
• CPC Class Codes: H03B 21/02 by plural beating, i.e. for frequency synthesis ; Beating in combination with multiplication or division of frequency digital frequency synthesis using a ROM G06F1/02; digital frequency synthesis in general H03K; indirect frequency synthesis using a PLL H03L7/16
• CPC Class Codes: H03B 2200/008 making use of a reference frequency
• CPC Class Codes: H03D 2200/0047 Offset of DC voltage or frequency
• CPC Class Codes: H03D 7/161 all the frequency changers being connected in cascade
• CPC Class Codes: H03D 7/166 using two or more quadrature frequency translation stages
• CPC Class Codes: H03J 7/04 where the frequency control is accomplished by varying the electrical characteristics of a non-mechanically adjustable element or where the nature of the frequency controlling element is not significant
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22

Abstract

A direct conversion FM receiver that includes AC coupling and automatic gain control employs an offset frequency at the first local oscillator. The offset frequency prevents the frequency spectrum occupied by the signal modulation from being affected by AC coupling. The offset frequency is chosen so that it translates the frequency spectrum of the received signal outside the DC notch created in the spectrum by the AC coupling. To conserve battery supplied power, an error amplifier coupled between the output of the receiver and the first local oscillator maintains the offset frequency after it has been established by a frequency synthesizer, which is then turned off.

Claims

1. A direct conversion receiver for processing frequency modulated radio signals that are centered about a carrier frequency, the frequency modulation extending a first sideband bandwidth above and below the carrier frequency, the receiver comprising:

means for receiving the radio signals, said receiving means having an output;

a pair of signal paths each having an input coupled to the output of the receiving means;

a local oscillator tuned to a local oscillator frequency and having an output;

each of said signal paths including a first mixer serially coupled therein, each of said first mixers having a local oscillator input coupled to the local oscillator output;

each of said signal paths further including an AC coupling means serially coupled therein, each of said AC coupling means producing a notch centered at DC in a frequency response of each of said signal paths, said notch extending a second sideband bandwidth above and below DC;

wherein the local oscillator frequency is set equal to the carrier frequency plus an offset frequency, said offset frequency being greater than or equal to the sum of the first and second sideband bandwidths.

2. The receiver of claim 1 including a frequency synthesizer for generating the local oscillator frequency and AFC means for maintaining the frequency when the frequency synthesizer is switched off to conserve power.

3. The receiver of claim 1 including RF amplifying means for amplifying the radio signals and automatic gain control means for controlling the gain of the RF amplifying means and thereby preserve linearity of the signals in each path.

4. The receiver of claim 1 including low-pass filter means within each path and coupled to the output of the first local oscillator for filtering the radio signals to extract signals modulated on the offset frequency.

5. The receiver of claim 1 including a second local oscillator coupled to the output of each low-pass filter means for mixing in each signal path a second oscillator frequency to convert the occupied frequency spectrum rom the offset frequency to a predetermined output frequency.

6. The receiver of claim 1 including means for shifting the first local oscillator frequency in quadrature for one of the signal paths.

7. A direct conversion receiver for processing frequency modulated radio signals that are centered about a carrier frequency, the frequency modulation extending a first sideband bandwidth above and below the carrier frequency, the receiver comprising:

means for receiving the radio signals, said receiving means having an output;

a pair of signal paths each having an input coupled to the output of the receiving means;

a first local oscillator tuned to a first local oscillator frequency and having an output;

each of said signal paths including a down conversion mixer serially coupled therein, each of said down conversion mixers having a local oscillator input;

means for coupling local oscillator signals in phase quadrature from the output of the first local oscillator to the local oscillator inputs of the down conversion mixers;

each of said signal paths further including AC coupling means serially coupled therein, each of said AC coupling means producing a notch centered at DC in a frequency response of each of said signal paths, said notch extending a second sideband bandwidth above and below DC;

said first local oscillator frequency being equal to the carrier frequency plus an offset frequency, said offset frequency being greater than or equal to the sum of the first and second sideband bandwidths;

RF amplifying means coupled to the receiving means for amplifying the radio signals and providing amplified radio signals to the pair of signal paths, said RF amplifying means thereby coupling the receiving means to the pair of signal paths;

automatic gain control means responsive to the amplified radio signals for preserving the linearity of the signals in each of said signal paths;

each of the signal paths further including a low pass filter serially coupled therein, each of said filters having an input coupled to the output of the down conversion mixer in said path, and each of said filter means having an output;

each of said signal paths further including an up conversion mixer serially coupled therein, each of said up conversion mixers having an input coupled to the output of the low pass filter in said path, and each of said up conversion mixers having an output and a second local oscillator input;

a second local oscillator tuned to a second local oscillator frequency and having an output;

means for coupling local oscillator signals in phase quadrature from the output of the second local oscillator to the second local oscillator inputs of the up conversion mixers; and

means for recombining signals from each of the two signal paths to produce a frequency modulated output signal.

8. The receiver of claim 7 wherein the offset frequency is at least 100 kHz.

9. The receiver of claim 7 including a frequency synthesizer for generating the first local oscillator frequency and AFC means for maintaining the frequency when the frequency synthesizer is switched off to conserve power.

10. A direct conversion receiver for processing frequency modulated radio signals that are centered about a carrier frequency, the frequency modulation extending a first sideband bandwidth above and below the carrier frequency, the receiver comprising:

means for receiving the radio signals, said receiving means having an output;

a pair of signal paths each having an input coupled to the output of the receiving means;

a voltage controlled local oscillator responsive to a voltage applied to an input thereof for producing a local oscillator frequency, said oscillator having an output;

each of said signal paths including a first mixer serially coupled therein, each of said first mixers having a local oscillator input coupled to the local oscillator output;

each of said signal paths further including an AC coupling means serially coupled therein, each of said AC coupling means producing a notch centered at DC in a frequency response of each of said signal paths, said notch extending a second sideband bandwidth above and below DC;

a synthesizer for generating a voltage corresponding to a desired frequency;

switch means operable in a first position for applying the voltage generated by the synthesizer to the input of the voltage controlled oscillator;

said switch means operable in a second position for applying a voltage from an AFC circuit the input of the voltage controlled oscillator;

wherein the synthesizer can be switched off to conserve power and control of the local oscillator frequency can be assumed by switching the input of the voltage controlled oscillator to the AFC circuit.