Automatic gain control system that responds to baseband signal distortion

• US 6,154,503 A
• Filed: 06/05/1997
• Issued: 11/28/2000
• Est. Priority Date: 06/07/1996
• Status: Expired due to Term

• Current Assignee: Sharp KK Corporation (Hon Hai Precision Industry Co., Ltd.), Sarnoff Corporation (SRI International, Inc.)
• Original Assignee: Sharp KK Corporation (Hon Hai Precision Industry Co., Ltd.), Sarnoff Corporation (SRI International, Inc.)
• Inventors: Strolle, Christopher Hugh
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Ha, Dac V.
• Application Number:
  US08/869,590
• Time in Patent Office: 1,272 Days
• Field of Search: 375/345, 375/264, 375/261, 375/346, 455/232.1, 455/234.1
• US Class Current: 375/264
• CPC Class Codes: H03G 3/3068 Circuits generating control signals for both R.F. and I.F. stages
• CPC Class Codes: H04L 27/3809 Amplitude regulation arrangements

Abstract

An automatic gain control (AGC) system that monitors the distortion of the outer constellation points of a complex modulation format such as quadrature amplitude modulation (QAM) and adjusts both the RF and IF AGC gain to minimize such distortion. In particular, the receiver contains at least two stages of AGC gain. Typically, the RF AGC amplifier is contained in the tuner that is followed by an IF AGC amplifier. The IF amplifier is coupled to a mixer and an associated low pass filter which, in combination and when driven by a particular frequency, produces a baseband (or near baseband) signal, The baseband signal is digitized and processed by a gain control circuit. The gain control circuit produces IF and RF AGC signals that control the gain of the respective AGC stages.

Claims

1. Apparatus for automatically adjusting gain of an input signal comprising:

a common mode signal generator for generating a common mode signal from the input signal, where the common mode signal represents the difference between the magnitude of the input signal and a target signal level;

a demodulator for demodulating the input signal to form symbol values and determining an error value for each symbol value;

a signal analyzer, coupled to the demodulator, for identifying at least two sets of error values having common characteristics and calculating an average error value for each set of error values; and

a control signal generator, coupled to said signal analyzer and said common mode generator, for manipulating the average error values and common mode signal to produce a gain control signal.

2. The apparatus of claim 1 wherein the common mode signal generator further comprises:

a rectifier/integrator for determining an average value of the input signal,

a target signal level generator for generating a nominal average value of the input signal,

a subtractor, coupled to the rectifier/integrator and target signal level generator, for calculating a difference value between the nominal average value and the average value, and

a loop filter, coupled to the subtractor, for filtering the difference value to form the common mode signal.

3. The apparatus of claim 2 wherein the demodulator is a quadrature amplitude modulation (QAM) demodulator.

4. The apparatus of claim 3 wherein one set of average error values is derived from inner constellation points of a QAM demodulated input signal and another set of average error values is derived from outer constellation points of a QAM demodulated input signal, where the signal analyzer integrates the inner constellation points to establish an inner point average error value and integrates the outer constellation points to establish an outer point average error value.

5. The apparatus of claim 4 wherein the control signal generator comprises:

first and second amplifiers having first (α

) and second (β

) amplification factors for amplifying the outer point average error value, respectively,

a first subtractor, coupled to said first amplifier and said signal analyzer, for subtracting the first amplified outer point average error value from the inner point average error value,

a second subtractor, coupled to said second amplifier and said signal analyzer, for subtracting the second amplified outer point average error value from the inner point average error value,

first and second low pass filters, connected to the first and second subtractors, for forming first and second biasing signals, respectively, and

first and second adders, coupled to the first and second low pass filters, respectively, for adding the first and second biasing signals to the common mode signal to produce a first and a second control signal.

6. The apparatus of claim 5 wherein the first control signal forms a radio frequency tuner automatic gain control signal.

7. The apparatus of claim 5 wherein the second control signal forms an intermediate frequency amplifier automatic gain control signal.

8. The apparatus of claim 5 wherein α

≦

1.

9. The apparatus of claim 5 wherein β

≧

1.

10. The apparatus of claim 8 wherein α

=0.5.

11. The apparatus of claim 9 wherein β

=1.5.

12. Method of determining automatic gain control signals comprising the steps of:

creating a common mode signal from an input signal, where the common mode signal represents the difference between the magnitude of the input signal and a target signal level;

determining a plurality of error values, where each of the error values corresponds to a symbol value in an input signal;

determining, in response to the plurality of error values, at least two sets of average error values; and

generating the automatic gain control signals in response to the sets of average error values.

13. The method of claim 12 wherein the average error value determining step further comprises the steps of:

deriving one set of average error values from quadrature amplitude modulation (QAM) demodulated inner constellation points, and

deriving another set of the average error values from QAM demodulated outer constellation points.

14. The method of claim 13 wherein the generating step further comprises the steps of:

amplifying the outer points average error value with first (α

) and second (β

) amplification factors, respectively,

subtracting the amplified outer points average error value from the inner points average error value, respectively, to produce a first and second difference signal,

filtering the first and second difference signals to form first and second biasing signals, respectively, and

adding the first and second biasing signals to the common mode signal, respectively, to form first and second automatic gain control signals.

15. The method of claim 14 wherein the first automatic gain control signal is a radio frequency automatic gain control signal.

16. The method of claim 14 wherein the second automatic gain control signal is an intermediate frequency automatic gain control signal.

17. The method of claim 14 wherein α

≦

1.

18. The method of claim 14 wherein β

≧

1.

19. The method of claim 17 wherein α

=0.5.

20. The method of claim 18 wherein β

=1.5.