Method and apparatus for adaptive automatic gain control
Systems and methods for dynamically adjusting the gain in a receiver front end to have a desired amount of headroom, based upon a measurement of the signal to noise ratio (SNR) of the output of a digital to analog converter and the amount of degradation to the SNR due to previous adjustments to the gain.
1. A receiver front end comprising:
an automatic gain control (AGC) module having an input and an output;
an analog to digital converter (ADC) having an input and an output, the input coupled to the output of the AGC module; and
an AGC control module having an input for receiving information regarding the signal output from the ADC and an output for controlling adjustments to the gain of the AGC module;
wherein the AGC control module determines a signal to noise ratio (SNR) for the signal output from the ADC, controls the gain of the AGC module to maintain the SNR at a desired value, and determines a delta SNR level that is based on a change of the SNR resulting from a change in the gain of the AGC module.
2. The receiver front end of claim 1 , wherein:
when the SNR is above a minimum level, then the AGC control module determines whether a delta SNR is above a maximum level, and when the SNR is not above a minimum level, the AGC control module adjusts the gain up; and
when the SNR is above a minimum level and the delta SNR is above a maximum level, the AGC control module adjusts the gain up, otherwise, the AGC module adjusts the gain down.
3. The receiver front end of claim 1 , wherein:
when the SNR is above a minimum level, then the AGC control module determines whether a delta SNR is above a maximum level, and if the SNR is not above a minimum level, the AGC control module determines whether there is sufficient headroom to adjust the gain up, and if so, then adjusts the gain up;
when the SNR is above a minimum level and the delta SNR is above a maximum level, the AGC control module determines whether there is sufficient headroom to adjust the gain up and if so, then adjusts the gain up, otherwise, the AGC control module adjusts the gain down.
4. The receiver front end of claim 1 , wherein the receiver front end comprises a low noise amplifier (LNA), the output of the LNA being coupled to the input of the AGC module, and wherein the SNR is the ratio of the desired signal level to the level of the noise that is due to distortion that occurs in the ADC, noise that is received at the input of the LNA together with a signal coupled to the input of the LNA and any distortion that occurs to the incoming signal prior to reception at the input of the LNA and any noise generated within the LNA.
5. The receiver front end of claim 1 , comprising an analog filter module coupled between the output of the AGC module and the input of the ADC.
6. The receiver front end of claim 2 , wherein the determining whether a delta SNR is above a maximum and adjusting the gain of the AGC are repeated.
7. The receiver front end of claim 2 , wherein determining whether a delta SNR is above a maximum level includes:
storing a first SNR value;
determining whether a previous SNR value has been stored; and
when a previous SNR value has been stored, then determining the delta SNR by calculating the absolute value of the difference between the previous SNR value and the current SNR value.
8. The receiver front end of claim 3 , wherein:
when the SNR is not above a minimum level and there is sufficient headroom, the AGC control module adjusts the gain up, and if either the SNR is above the minimum or there is insufficient headroom, the AGC control module adjusts the gain down.
9. The receiver front end of claim 3 , wherein the determining whether a delta SNR is above a maximum level, determining whether there is sufficient headroom to adjust the gain up, and adjusting the gain up or down are repeated.
10. The receiver front end of claim 3 , wherein adjustments to increase the gain are equal in size to adjustments to reduce the gain.
11. The receiver front end of claim 3 , wherein adjustments to increase the gain are larger than adjustments to reduce the gain.
12. The receiver front end of claim 3 , wherein adjustments to increase the gain are smaller than adjustments to reduce the gain.
13. The receiver front end of claim 3 , wherein gain adjustments during an acquisition mode are greater than gain adjustments during a maintenance mode.
14. The receiver front end of claim 3 , wherein the size of the gain adjustment is based on the number of consecutive times the gain has been adjusted in the same direction.
15. A method for controlling the gain of a receiver front end, comprising:
receiving an input signal;
measuring the signal to noise ratio (SNR) of the output signal
determining a delta SNR level; and
when the SNR is not below a minimum desired level and the delta SNR level is above a minimum, adjusting the gain of an AGC module down using an AGC gain control module; and
repeating the gain adjusting until either the SNR is below a minimum desired level or the delta SNR is above a minimum.
16. The method of claim 15 , wherein determining the delta SNR comprises making a first measurement of an SNR before making an adjustment to the gain of the AGC module, making a second measurement of an SNR after making an adjustment to the gain of the AGC module and determining the difference in the SNR between the first and second measurement.
17. The method of claim 15 , comprising increasing the gain of the AGC module when either the SNR is below the minimum desired level or the delta SNR level is above a maximum and there is sufficient headroom, and then repeating the method.
18. The method of claim 15 , wherein:
when the SNR is above a minimum level, then the AGC control module determines whether a delta SNR is above a maximum level, and when the SNR is not above a minimum level, the AGC control module adjusts the gain up; and
when the SNR is above a minimum level and the delta SNR is above a maximum level, then the AGC control module adjusts the gain up, otherwise, the AGC module adjusts the gain down.
19. The method of claim 15 , wherein:
when the SNR is above a minimum level, then the AGC control module determines whether a delta SNR is above a maximum level, and when the SNR is not above a minimum level, the AGC control module determines whether there is sufficient headroom to adjust the gain up, and when so, then adjusts the gain up;
when the SNR is above a minimum level and the delta SNR is above a maximum level, the AGC control module determines whether there is sufficient headroom to adjust the gain up and when so, then adjusts the gain up, otherwise, the AGC control module adjusts the gain down.
20. A method for controlling the gain of a receiver front end, comprising:
receiving an input signal;
measuring a signal to noise ratio (SNR) of the input signal;
when the SNR is not below a minimum desired level, adjusting the gain of an AGC module down, otherwise determining whether there is sufficient headroom and when so, adjusting the gain up;
determining a delta SNR level that is based on a change of the SNR resulting from a change in the gain of the AGC module; and
repeating the gain adjusting and delta SNR determining.