Linearity and Distortion: What Do ACPR, IMD, and EVM Really Mean?
Time：2026-04-05 Views：4
In the digital communication era, Engineers commonly use ACPR, IMD, and EVM to quantify PA linearity.
1. ACPR: Adjacent Channel Power Ratio
ACPR measures how much energy from the main channel leaks into adjacent channels. In cellular communications, leaked signals interfere with users in neighboring frequency bands. A lower (more negative) ACPR value means less leakage. For example, 5G base stations typically require ACPR < -45dBc. The main causes of poor ACPR are AM-AM and AM-PM distortion in the PA.
2. IMD and IP3: Intermodulation Distortion
When two or more signals enter a PA simultaneously, they generate new frequency components called intermodulation distortion (IMD). Third-order intermodulation (IMD3) is particularly harmful because it falls close to the main channel and is hard to filter. The third-order intercept point (IP3) is a key linearity figure – higher IP3 means better linearity. For a highly linear PA, the output IP3 (OIP3) can be 10-15dB above P1dB.
3. EVM: Error Vector Magnitude
EVM is the most intuitive linearity metric in digital communications. It represents the vector error between an ideal symbol point and the actual transmitted symbol point. Lower EVM means higher signal quality. For 64QAM, EVM must be below 8% for reliable demodulation. PA nonlinearity, noise, and phase noise all contribute to EVM.
4. How to Improve Linearity?
Besides optimizing bias and circuit topology, modern systems widely use digital pre-distortion (DPD). DPD adds a “correction signal” in the digital domain that is opposite to the PA’s nonlinearity, making the overall link linear. Ampbuc’s DPD solutions improve ACPR by 15-20dB while maintaining high efficiency, and reduce EVM to below 2%.
1. ACPR: Adjacent Channel Power Ratio
ACPR measures how much energy from the main channel leaks into adjacent channels. In cellular communications, leaked signals interfere with users in neighboring frequency bands. A lower (more negative) ACPR value means less leakage. For example, 5G base stations typically require ACPR < -45dBc. The main causes of poor ACPR are AM-AM and AM-PM distortion in the PA.
2. IMD and IP3: Intermodulation Distortion
When two or more signals enter a PA simultaneously, they generate new frequency components called intermodulation distortion (IMD). Third-order intermodulation (IMD3) is particularly harmful because it falls close to the main channel and is hard to filter. The third-order intercept point (IP3) is a key linearity figure – higher IP3 means better linearity. For a highly linear PA, the output IP3 (OIP3) can be 10-15dB above P1dB.
3. EVM: Error Vector Magnitude
EVM is the most intuitive linearity metric in digital communications. It represents the vector error between an ideal symbol point and the actual transmitted symbol point. Lower EVM means higher signal quality. For 64QAM, EVM must be below 8% for reliable demodulation. PA nonlinearity, noise, and phase noise all contribute to EVM.
4. How to Improve Linearity?
Besides optimizing bias and circuit topology, modern systems widely use digital pre-distortion (DPD). DPD adds a “correction signal” in the digital domain that is opposite to the PA’s nonlinearity, making the overall link linear. Ampbuc’s DPD solutions improve ACPR by 15-20dB while maintaining high efficiency, and reduce EVM to below 2%.
