What is the relationship between IP2 and IP3 in a direct conversion receiver?

In a direct conversion (zero-IF) receiver, both IP2 and IP3 are critical, but IP2 is uniquely important because the second-order intermodulation products fall directly at baseband (DC), interfering with the desired signal: (1) IP2: the second-order intercept point is the theoretical power where the second-order intermodulation product (IM2 = f1 ± f2) equals the fundamental. IM2 products: f1 + f2 (sum) and f1 - f2 (difference). In a direct-conversion receiver: the difference frequency (f1 - f2) falls at baseband (near DC). This is catastrophic: two interferers at, say, f1 = f_LO + 1 MHz and f2 = f_LO + 1.5 MHz generate an IM2 product at f1 - f2 = 0.5 MHz. This 0.5 MHz product falls directly in the desired signal band. (2) IP3 vs IP2 in direct conversion: IP3 creates IM3 at 2f1 - f2, which typically falls within the RF band (near the desired channel). IP3 behaves the same as in any superheterodyne receiver. IP2 creates IM2 at f1 - f2, which falls at baseband. In a superheterodyne receiver: the IM2 at f1 - f2 is far from the IF frequency and is easily filtered. In a direct-conversion receiver: the IM2 at f1 - f2 falls directly at IF = 0 (baseband). (3) IP2 requirements: for a direct-conversion LTE receiver: IIP2 > +40 to +70 dBm (extremely high). For comparison: IIP3 typically needs only -15 to +5 dBm. The IP2 requirement is 50-80 dB higher than the IP3 requirement. (4) Achieving high IP2: balanced mixer topology: the symmetry of a balanced or double-balanced mixer cancels even-order products (including IM2). A well-balanced mixer: IIP2 > +50 dBm. IP2 is very sensitive to balance: 0.1 dB gain mismatch or 1° phase mismatch between the I and Q paths can degrade IIP2 by 20-30 dB. On-chip calibration: digital calibration trims the I/Q balance to maximize IP2. Production testing: each device is calibrated individually (the IP2 is highly sensitive to process variations).