LNA gain adjustment in an RF receiver to compensate for intermodulation interference

A method for adjusting the gain of a Low Noise Amplifier (LNA) in an RF unit that includes the LNA, a mixer coupled to the output of the LNA, a Band Pass Filter (BPF) coupled to the output of the mixer, a first received signal strength indicator coupled to measure the wideband signal strength at the output of the mixer, and a second received signal strength indicator coupled to measure the narrowband signal strength after the BPF, the method comprising:

• determining that a signal of interest is not present in a received RF signal present at an input of the LNA;
• measuring wideband signal strength indication (RSSI_A) by use of the first received signal strength indicator at the output of the mixer;
• determining if mixer non-linearity is present based upon RSSI_A;
• reducing gain of the LNA if mixer non-linearity is present based upon RSSI_A, until the mixer is no longer driven into non-linearity;
• determining if RSSI A exceeds a predefined threshold (THUR_B) that will cause the mixer to operate in an upper range of mixer operation;
• measuring narrowband signal strength indication (RSSI_B) by use of the second received signal strength indicator after the BPF if RSSI_A exceeds the predefined threshold;
• determining if intermodulation interference is present by using RSSI_B;
• optimizing a signal-to-interference ratio of the mixer by setting the gain of the LNA to have the mixer operate significantly within its linear region of operation, if intermodulation interference is detected;
• and
• setting the gain of the LNA so that RSSI_B exceeds a predetermined minimum, in which a combination of RSSI_A and RSSI_B based gain adjustments of the LNA is employed to compensate for intermodulation interference.