Zero intermediate frequency receiver having an automatic gain control circuit

• US 5,483,691 A
• Filed: 07/14/1994
• Issued: 01/09/1996
• Est. Priority Date: 06/08/1992
• Status: Expired due to Term

• Current Assignee: Motorola Solutions, Inc.
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Heck, Joseph P., Ferrer, Enrique
• Primary Examiner(s): Urban, Edward F.
• Application Number:
  US08/275,088
• Time in Patent Office: 544 Days
• Field of Search: 455/234.1, 455/234.2, 455/236.1, 455/239.1, 455/241.1, 455/251.1, 455/253.2, 455/245.1, 455/245.2, 455/315, 455/324, 330/278, 375/345
• US Class Current: 455/234.2
• CPC Class Codes: H03G 3/3068 Circuits generating control signals for both R.F. and I.F. stages
• CPC Class Codes: H04B 1/109 by improving strong signal performance of the receiver when strong unwanted signals are present at the receiver input

Abstract

A receiver automatic gain control (AGC) circuit includes a first adjustable gain control amplifier (158) which is responsive to a gain control signal (156). The AGC circuit further includes a second adjustable gain control amplifier (114) and a control circuit (116) which receives the gain control signal (156) and provides a modified gain control signal or VCNTRL (152) to the second adjustable control amplifier (114). The control circuit (116) also limits the amount of gain control applied to adjustable gain control amplifiers (114 and 118) when the gain control signal (156) reaches a certain predetermined level. This provides for all further gain reduction to occur at the first adjustable gain control amplifier (158) and thereby reduce the chances for distortion under high input signal conditions.

Claims

1. A zero intermediate frequency (zero IF) receiver, comprising:

a first adjustable gain stage having an input terminal for receiving an input signal, the first adjustable gain stage being responsive to a gain control signal for adjusting the amount of gain provided to the input signal and providing an amplified output signal at an output terminal;

first and second down mixers coupled to the output terminal of the first adjustable gain stage and providing first and second phase-related baseband signals;

a second adjustable gain stage for receiving the first and second phase-related baseband signals and providing first and second amplified baseband signals in response to a modified gain control signal;

a control means coupled to the first and second adjustable gain stages and responsive to the gain control signal for providing the modified gain control signal to the second adjustable gain stage for adjusting the gain of the second adjustable gain stage depending on the level of the modified gain control signal, the control means including a limiting means for limiting the maximum amount of gain reduction applied to the second adjustable gain stage upon the gain control signal reaching a predetermined gain reduction limit threshold level, wherein the gain of the first adjustable gain stage is reduced prior to the gain of the second adjustable gain stage;

first and second up mixers for converting the first and second amplified baseband signals into first and second amplified phase-related signals; and

an adder for summing the first and second amplified phase-related signals to produce the gain control signal.

2. A receiver as defined in claim 1, wherein the first adjustable gain stage begins to have its gain reduced upon the gain control signal reaching a first threshold level and the second adjustable gain stage has its gain reduced upon the gain control signal reaching a second threshold level.

3. A receiver as defined in claim 2, wherein the second threshold level is greater than the first threshold level causing the first adjustable gain stage to begin gain reduction at a lower gain control signal level than the second adjustable gain stage.

4. A receiver as defined in claim 2, wherein the first adjustable gain stage comprises an intermediate frequency (IF) preamplifier.

5. A zero intermediate frequency receiver as defined in claim 1, wherein the limiting means comprises:

an operational amplifier having an input terminal for receiving the gain control signal and an output terminal for providing the modified gain control signal;

a current reference; and

a set of current mirrors coupled to the current reference and responsive to the modified gain control signal; and

switching means coupled to the current mirrors and modified gain control signal for limiting the signal level of the modified gain control signal.

6. A zero intermediate frequency (zero IF) receiver, comprising:

a preamplifier having an input terminal for receiving an input signal, the preamplifier being responsive to a gain control signal for adjusting the amount of gain provided to the input signal and providing an amplified output signal at a preamplifier output terminal;

first and second down mixers coupled to the preamplifier output terminal and providing first and second phase-related baseband signals;

a baseband amplifier for receiving the first and second phase-related baseband signals and providing first and second amplified baseband signals in response to a modified gain control signal;

a control means responsive to the gain control signal for providing the modified gain control signal to the baseband amplifier for adjusting the gain of the baseband amplifier depending on the level of the modified gain control signal, the control means including a limiting means for limiting the maximum amount of gain reduction applied to the baseband amplifier, said gain control signal reducing the gain of the preamplifier prior to the baseband amplifier'"'"'s gain being reduced by the modified gain control signal;

first and second up mixers for converting the first and second amplified baseband signals into first and second amplified phase-related signals; and

an adder for summing the first and second amplified phase-related signals to produce the gain control signal.

7. A receiver as defined in claim 6, wherein the limiting means limits the maximum amount of gain reduction applied to the baseband amplifier upon the gain control signal reaching a predetermined gain reduction limit threshold level thereby causing all further receiver gain reduction to take place at the preamplifier.

8. A zero intermediate frequency (zero IF) receiver as defined in claim 6, further comprising a limiter for normalizing the gain of the zero IF receiver.