EVM &ACLR

time：

2026-02-06

category：

basic

News

Considerations for ACLR and EVM Indicators

EVM and ACLR are two distinct dimensional characterizations of transmitter nonlinearity: the former focuses on the time domain/modulation domain, while the latter focuses on the frequency domain/spectrum domain. Digital Pre-Distortion (DPD) and power back-off are the only technical means that can optimize both indicators simultaneously; IQ calibration and low-phase-noise local oscillators only optimize EVM; high-performance filters only optimize ACLR. In scenarios such as cellular base stations and high-order modulation terminals, it is necessary to prioritize ensuring that EVM meets demodulation requirements, and then optimize ACLR through filters or DPD to control adjacent channel interference.

What does EVM measure? EVM is a "quality indicator within the main channel". In essence, EVM quantifies how far the constellation points deviate within the main channel bandwidth: it only focuses on the main bandwidth and cares solely about demodulation correctness, with complete insensitivity to out-of-band performance. The calculation of EVM is essentially confined to the main bandwidth.

In conclusion: No matter how poor the out-of-band performance is, as long as the main channel can still achieve correct demodulation, the EVM will remain excellent.

What does ACLR measure? ACLR, or 'Clean Channel Outside the Main Band,' indicates how much 'noise' you're introducing into your neighbor's spectrum. It specifically tracks out-of-band leakage, which is highly sensitive to power amplifier (PA) nonlinearity. PA nonlinearity often hides in areas invisible to EVM, such as third-order intermodulation (IMD3) and fifth-order intermodulation (IMD5), which are predominantly distributed outside the main bandwidth.

Sometimes the constellation diagram is very round, and the EVM is good, but the ACLR is very poor. This is because: the PA produces significant spectrum regrowth in the medium and high power regions, and the regenerated energy mainly goes to the adjacent channels, while the main channel "seems to be normal".

EVM and ACLR measure two "almost orthogonal" things. A good EVM only indicates that your "signal is correct"; a good ACLR indicates that your "signal is clean".

DPD prioritizes optimizing EVM first, then considers ACLR. Because demodulation errors are "immediate death", while a slightly poor adjacent channel is a "chronic disease". However, DPD has an upper limit on effective bandwidth, the modeling accuracy of edge frequency points decreases, and the out-of-band intermodulation "cannot be compensated".

Correlation Analysis Table between EVM and ACLR

Comparison dimension	（EVM）Error Vector Magnitude	（ACLR）Adjacent Channel Leakage Ratio
Core definition	The ratio of the actual modulation vector to the ideal reference vector in magnitude characterizes the signal modulation quality.	The ratio of the main channel power to the adjacent channel leakage power, which characterizes spectral purity.
Physical essence	Caused by the amplitude/phase distortion of the signal, the constellation diagram shows scatter offset.	Caused by nonlinear distortion of the signal (intermodulation, harmonics), the frequency spectrum expands to adjacent
Core source	AM-AM/AM-PM distortion in power amplifier (PA) 2. Baseband IQ modulation imbalance 3. Local oscillator phase noise/frequency offset 4. Filter phase distortion	1Third/fifth-order intermodulation products generated by PA nonlinearity 2. Intermodulation of multi-carrier signals 3. Excessively high peak-to-average power ratio (PAPR) of modulated signals 4. Insufficient out-of-band rejection of filters
Association mechanism	Both have the same origin, dominated by the nonlinearity of the transmission link (especially the PA):PA nonlinearity increases → EVM deteriorates + ACLR worsens → adjacent channel interference increases
Interrelationship	When EVM deteriorates, signal distortion is accompanied by spectrum expansion, which is likely to cause ACLR degradation.2Insufficient out-of-band rejection of a simple filter will cause ACLR to deteriorate, but it does not necessarily affect EVM	1.ACLR directly determines the upper limit of adjacent channel interference. 2. Adjacent channel interference can be derived from the formula to calculate the equivalent ACLR: main channel interference. 3. ACLR and the receiver's ACS jointly determine the adjacent channel interference ratio (ACIR): ACIR= (ACLR1 + ACS1) − 1.
The standard requires relevance.	The higher the modulation order, the stricter the requirements (e.g., 5G 1024QAM requires EVM ≤ 2%).	It is strongly related to the bandwidth of the communication system and the channel spacing. 3GPP uniformly requires that the adjacent channel ACLR of cellular systems is ≤ -45dBc.
Relevance of optimization strategies	1. Digital Pre-Distortion (DPD): Simultaneously improving PA nonlinearity and synchronously optimizing EVM and ACLR (core solution) 2. Power back-off: Reduce the output power of the PA to the linear region, while improving EVM and ACLR, but sacrificing efficiency. 3.IQ Calibration: Only improves EVM, has no impact on ACLR 4. Filter optimization: only improves ACLR with minimal impact on EVM
Typical test scenarios	Single-carrier/multi-carrier modulated signals, different PA output power points (saturation point, 1dB compression point, linear region)