Method and apparatus for automatic gain control and DC offset cancellation in quadrature receiver

• US 5,617,060 A
• Filed: 04/13/1995
• Issued: 04/01/1997
• Est. Priority Date: 04/28/1994
• Status: Expired due to Term

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Black, Peter J., Peterzell, Paul E., Wilson, Nathaniel B.
• Primary Examiner(s): MOTTOLA, STEVEN J
• Application Number:
  US08/423,332
• Time in Patent Office: 719 Days
• Field of Search: 330/129, 330/141, 330/279, 330/281, 332/123, 332/124, 332/125, 455/239.1, 455/240.1, 455/245.1, 455/250.1
• US Class Current: 330/129
• CPC Class Codes: H03D 2200/0025 Gain control circuits
• CPC Class Codes: H03D 2200/0047 Offset of DC voltage or frequency
• CPC Class Codes: H03D 2200/005 Analog to digital conversion
• CPC Class Codes: H03D 2200/0052 Digital to analog conversion
• CPC Class Codes: H03D 3/002 Modifications of demodulators to reduce interference by undesired signals H03D3/248 takes precedence
• CPC Class Codes: H03D 3/008 Compensating DC offsets
• CPC Class Codes: H03D 7/165 at least two frequency changers being located in different paths, e.g. in two paths with carriers in quadrature combined with amplitude demodulation H03D1/2245, combined with angle demodulation H03D3/007; N-path filters H03H19/002
• CPC Class Codes: H03G 3/3052 in bandpass amplifiers (H.F. or I.F.) or in frequency-changers used in a (super)heterodyne receiver H03G3/32, H03G3/34 take precedence
• CPC Class Codes: H03G 3/3089 Control of digital or coded signals

Abstract

An automatic gain control (AGC) and D.C. offset correction method and apparatus for controlling signal power of a received RF signal within a dual mode quadrature receiver is disclosed herein. In a preferred implementation the automatic gain control apparatus may be adjusted to provide a desired control response to various fading characteristics of a received FM, FSK, GMSK, QPSK, or BPSK signal. The AGC apparatus includes an adjustable gain amplifier having an input port for receiving an input signal, a control port for receiving a gain control signal, and an output port for providing an output signal. A quadrature downconverter coupled to the output port serves to translate the frequency of the output signal to a baseband frequency, thereby producing baseband signals. In a preferred implementation the downconverter is operative to map the carrier frequency of the output signal to a baseband frequency offset by a predetermined margin from D.C. Two high gain active lowpass filters provide out-of-band signal rejection for the baseband signals. A D.C. feedthrough suppression loop, disposed to receive said baseband signal, suppresses D.C. offsets produced by the downconverter and lowpass filters, hence providing a compensated baseband signal. The AGC apparatus is further disposed to generate a received power signal based on the power of the output signal. A saturating integrator compares the received power signal to a reference signal and produces the gain control signal by integrating or by refraining from integration based on values of the reference, received power signal, and gain control signals, thereby extending the usable dynamic range of the receiver for FM mode.

Claims

1. An automatic gain control apparatus including an adjustable gain amplifier, the adjustable gain amplifier having an input port for receiving an input signal, a control port for receiving a gain control signal, and an output port for providing an output signal, the automatic gain control apparatus comprising:

a downconverter coupled to said output port for downconverting frequency of said output signal to a baseband frequency so as to produce a baseband signal, said downconverter being operative to map a carrier frequency of said output signal to a baseband frequency offset by a predetermined margin from D.C.;

a D.C. feedthrough suppression loop, disposed to receive said baseband signal, for suppressing D.C. feedthrough signals produced by said frequency downconverter and for providing a compensated baseband signal;

means for generating a received power signal based on power of said compensated baseband signal; and

saturating integrator means for comparing said received power signal to a reference signal and for generating an error signal in response to a result of the comparison, said saturating integrator means including means for providing said gain control signal by selectively integrating said error signal based on values of said error and gain control signals.

2. The automatic gain control apparatus of claim 1 wherein said D.C. feedthrough suppression loop further includes:

a subtractor having a first input for receiving said baseband signal, and an output port operatively coupled to an input of a low-pass filter; and

an integrator having an integrator input port operatively coupled to an output port of said low-pass filter, and having an integrator output port operatively coupled to a second input of said subtractor.

3. The automatic gain control apparatus of claim 2 wherein said D.C. feedthrough suppression loop further includes:

an analog to digital converter coupled to said output port of said low-pass filter; and

a digital to analog converter interposed between said integrator output port and said second input of said subtractor.

4. The automatic gain control apparatus of claim 1 wherein said saturating integrator means includes first means for selectively enabling said error signal to be integrated only while magnitude of said gain control signal is less than a first predefined threshold, and second means for selectively enabling said error signal to be integrated only while magnitude of said gain control signal exceeds a second predefined threshold.

5. The automatic gain control apparatus of claim 1 wherein said downconverter includes:

a mixer having a first input port for receiving said output signal; and

a local oscillator connected to a second input port of said mixer wherein frequency of said local oscillator is selected such that said center frequency of said output signal mapped to said baseband frequency offset by said predetermined margin from D.C.

6. The automatic gain control apparatus of claim 5 wherein said downconverter includes an offset modulator circuit for varying said frequency of said oscillator circuit so as to vary said predetermined margin by which said center frequency of said output signal is mapped relative to D.C.

7. The automatic gain control apparatus of claim 6 wherein said downconverter includes a mixer coupled to said output port of said adjustable gain amplifier, said mixer being operative to downconvert said output signal to I and Q baseband signal components of said baseband signal.

8. The automatic gain control apparatus of claim 7 wherein said D.C. feedthrough suppression loop includes first and second low-pass filters for filtering said I and Q baseband signal components, respectively.

9. A method for automatic gain control using an adjustable gain amplifier, the adjustable gain amplifier having an input port for receiving an input signal, a control port for receiving a gain control signal, and an output port for providing an output signal, the method comprising the steps of:

downconverting frequency of said output signal to a baseband frequency so as to produce a baseband signal wherein a carrier frequency of said output signal is mapped to a baseband frequency offset by a predetermined margin from D.C.;

suppressing D.C. feedthrough signals accompanying said baseband signal so as to provide a compensated baseband signal;

generating a received power signal based on power of said compensated baseband signal; and

integrating selectively a difference between said received power signal and a reference signal based on values of said error and gain control signals.

10. An automatic gain control apparatus for compensating for variations in received signal power, the automatic gain control apparatus comprising:

an adjustable gain amplifier having an input port coupled to a received signal, an output port for generating an output signal having a frequency, and a control port for receiving a gain control signal;

a downconverter coupled to the output port for downconverting the frequency of the output signal to produce a baseband signal having a baseband frequency, the downconverter mapping a carrier frequency of the output signal to a baseband frequency offset by a predetermined margin from D.C.;

a filter, coupled to the downconverter, for removing D.C. offset errors and signal in the baseband signal to generate a filtered signal;

a power detector, coupled to the filter, for generating a power level signal in response to a power of the filtered signal; and

an integrator having a first input coupled to the power detector and a second input coupled to a reference signal, the integrator generating the gain control signal by selectively integrating a difference between the reference signal and the power level signal.

11. The automatic gain control apparatus of claim 10 and further including control logic for selectively enabling the difference to be integrated while a magnitude of the gain control signal is less than a first predetermined threshold and greater than a second predetermined threshold.

12. The automatic gain control apparatus of claim 10 wherein the downconverter is comprised of:

an intermediate frequency filter coupled to the output port of the adjustable gain amplifier;

an oscillator for generating a frequency reference signal;

a mixer, coupled to the oscillator and the intermediate frequency filter, for generating at least one baseband component in response to the frequency reference signal and the output signal; and

at least one low pass filter, coupled to the mixer, for generating at least one lowpass transfer function from the at least one baseband component.

13. The automatic gain control apparatus of claim 12 wherein the apparatus operates in either a code division multiple access mode (CDMA) or a frequency modulated (FM) mode and the at least one low pass filter is comprised of a first filter for operation in the CDMA mode and a second filter for operation in the FM mode.

14. The automatic gain control apparatus of claim 11 wherein the integrator includes a switch controlled by the control logic, in a closed position the switch coupling the power level signal to an input of the integrator and, in an open position, a capacitor holding the integrator input at one of a plurality of predetermined voltage levels.

15. An automatic gain control apparatus for compensating for variations in received signal power, the automatic gain control apparatus comprising:

an adjustable gain amplifier having an input port coupled to a received signal, an output port for generating an output signal having a frequency, and a control port for receiving an analog gain control signal;

a downconverter, coupled to the output port, for downconverting the frequency of the output signal to produce at least one baseband signal having a baseband frequency, the downconverter mapping a carrier frequency of the output signal to a baseband frequency offset by a predetermined margin from D.C.;

at least one analog to digital converter, each coupled to a different baseband signal of the at least one baseband signal, each analog to digital converter generating a digital representation of the respective baseband signal;

a filter, coupled to the at least one analog to digital converter, for generating at least one filtered signal;

a power detector coupled to the filter, the power detector generating a power level signal in response to the at least one filtered signal;

an integrator, coupled to the power detector, for comparing the power level signal to a predetermined reference signal to generate an error signal, the integrator generating a digital gain control signal by selectively integrating the error signal in response to values of the error signal and the digital gain control signal; and

a digital to analog converter coupled between the integrator and the adjustable gain amplifier, the digital to analog converter generating the analog gain control signal from the digital gain control signal.

16. The apparatus of claim 15 wherein the integrator comprises:

a subtractor coupled to the power detector, the subtractor generating the error signal in response to a difference between the power level signal and the predetermined reference signal;

a scaling multiplier, coupled to the subtractor, that generates a scaled error signal by multiplying the error signal by a first constant when the power level signal is decreasing in value and multiplying by a second constant when the power level signal is increasing in value; and

an accumulator, coupled to the scaling multiplier, that generates the digital gain control signal by accumulating the scaled error signal, the accumulator holding the digital gain control signal at a minimum predetermined threshold when the accumulated scaled error signal decreases to the minimum predetermined threshold and the accumulator holding the digital gain control signal at a maximum predetermined threshold when the accumulated scaled error signal increases to the maximum predetermined threshold.

17. The apparatus of claim 15 and further including a lowpass filter coupling the digital to analog converter to the adjustable gain amplifier.

18. A method for compensating for variations in received signal power in an automatic gain control apparatus having an adjustable gain amplifier, the adjustable gain amplifier having an input port coupled to a received signal, an output port for generating an output signal having a frequency, and a control port for receiving a gain control signal, the method comprising the steps of:

downconverting the frequency of the output signal to produce a baseband signal having a baseband frequency;

generating a filtered signal by removing D.C. offset errors and signal in the baseband signal;

generating a power level signal in response to a power of the filtered signal; and

generating a gain control signal by selectively integrating a difference between the power level signal and a reference signal.

19. The method of claim 18 wherein the step of generating a gain control signal further includes integrating the difference only when the value of the gain control signal is greater than a minimum predetermined threshold and less than a maximum predetermined threshold.

20. A method for compensating for variations in received signal power in an automatic gain control apparatus having an adjustable gain amplifier, the adjustable gain amplifier having an input port coupled to a received signal, an output port for generating an output signal having a frequency, and a control port for receiving an analog gain control signal, the method comprising the steps of:

downconverting the frequency of the output signal to produce at least one baseband signal having a baseband frequency;

generating a digital representation of each respective baseband signal;

generating at least one filtered signal by filtering the digital representations of the at least one baseband signal;

generating a power level signal in response to the at least one filtered signal;

comparing the power level signal to a reference signal to generate an error signal;

generating a digital gain control signal by selectively integrating the error signal in response to values of the error signal and the digital gain control signal; and

converting the digital gain control signal to the analog gain control signal.

21. The method of claim 20 wherein the step of generating a digital gain control signal further includes integrating the error signal only when the value of the digital gain control signal is greater than a minimum predetermined threshold and less than a maximum predetermine threshold.