What is the difference between ACPR and ACLR?

They measure the same thing with slightly different conventions. ACPR (Adjacent Channel Power Ratio) is the general term used across the industry: the ratio of main channel power to adjacent channel power, expressed as a negative dBc number (e.g., -45 dBc). ACLR (Adjacent Channel Leakage Ratio) is the 3GPP term used in LTE and 5G NR specifications, defined identically but sometimes expressed as a positive number (45 dB). Some sources define ACPR as the inverse ratio (adjacent to main), making it a positive dB number. Always check the sign convention when comparing datasheets.

How does digital predistortion improve ACPR?

DPD applies a nonlinear pre-correction to the digital signal before it reaches the PA, such that the PA's compression and AM-PM distortion are cancelled. A well-designed DPD system improves ACPR by 15 to 25 dB, allowing the PA to operate 3 to 5 dB closer to compression. Without DPD, a typical GaN Doherty PA achieves -35 to -38 dBc ACLR. With DPD, the same PA achieves -50 to -60 dBc, well within the 3GPP limit of -45 dBc for LTE and 5G NR base stations.

What measurement bandwidth and offset should I use?

The measurement bandwidth and offset are defined by the standard. For LTE 20 MHz: integrate over 18 MHz bandwidth, with the adjacent channel center offset by 20 MHz and the alternate channel center offset by 40 MHz. For 5G NR 100 MHz: integrate over 98 MHz, adjacent offset 100 MHz. The spectrum analyzer resolution bandwidth (RBW) should be narrow enough to resolve the spectral shape (typically 30 kHz for LTE, 100 kHz for NR), and the measurement uses channel power integration, not peak power.

Test & Measurement

ACPR

Adjacent Channel Power Ratio

When a power amplifier compresses, the signal spectrum grows wider. Energy that was confined to the assigned channel spills into neighboring channels, interfering with other users on adjacent frequencies. ACPR (or ACLR in 3GPP standards) measures exactly how much power leaks: it is the ratio of the power integrated across the main channel to the power integrated across the adjacent channel, expressed in dBc. For a 5G NR base station, the limit is −45 dBc. Without digital predistortion, most PAs fail this limit by 10 dB. With DPD, they pass with margin to spare.

Category: Test & Measurement

3GPP Term: ACLR (E-UTRA)

Limit: −45 dBc (LTE/NR BS)

Measuring the Spectral Regrowth Your PA Creates

ACPR definition:
ACPR (dBc) = 10·log₁₀(P_adj / P_main)
where P_adj is the integrated power in the adjacent channel and P_main is the integrated power in the main channel.

Relationship to two-tone IMD3:
For a memoryless PA, ACPR ≈ IMD3 − 3 to 5 dB (the ACPR is slightly better than two-tone IMD3 because the modulated signal's peak statistics differ from a two-tone test).

This approximation breaks down for PAs with significant memory effects (thermal, bias network, or envelope-dependent), where ACPR can be 5 to 10 dB worse than the two-tone prediction.

Channel Offset and Integration BW by Standard

Standard	Channel BW	Integration BW	Adjacent Offset	Alternate Offset	ACLR Limit (BS)
WCDMA	5 MHz	3.84 MHz	5 MHz	10 MHz	−45 dBc
LTE 10 MHz	10 MHz	9 MHz	10 MHz	20 MHz	−45 dBc
LTE 20 MHz	20 MHz	18 MHz	20 MHz	40 MHz	−45 dBc
5G NR 50 MHz	50 MHz	48 MHz	50 MHz	100 MHz	−45 dBc
5G NR 100 MHz	100 MHz	98 MHz	100 MHz	200 MHz	−45 dBc

DPD Impact on ACPR

Without DPD: GaN Doherty PA at 3 dB back-off typically achieves −33 to −38 dBc ACLR. Fails the −45 dBc spec.
With memoryless DPD: Improves to −48 to −52 dBc. Passes spec with 3 to 7 dB margin.
With memory DPD (GMP model): Achieves −55 to −60 dBc. Provides 10 to 15 dB margin and allows the PA to operate 2 to 3 dB closer to compression, improving efficiency by 5 to 10 percentage points.

Common Questions

Frequently Asked Questions

ACPR vs. ACLR: what is the difference?

Same measurement, different names. ACPR is the general industry term. ACLR is the 3GPP term for LTE/NR specs. Watch the sign convention: ACPR is usually negative dBc (−45 dBc), while some datasheets express ACLR as a positive number (45 dB). Always verify the reference direction.

How does DPD improve ACPR?

DPD pre-corrects the digital signal to cancel PA compression and AM-PM distortion. A GaN Doherty goes from −35 dBc (fail) to −55 dBc (pass with margin) with memory DPD. This allows operating 3 to 5 dB closer to compression, boosting average efficiency.

What measurement settings should I use?

Integration bandwidth and offset are standard-defined. For LTE 20 MHz: 18 MHz integration BW, 20 MHz adjacent offset. RBW should be 30 kHz (LTE) or 100 kHz (NR). Use channel power integration mode, not peak power.

Related Terms

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Test Equipment

PA Test Bench Setup Guide

Step-by-step procedures for ACPR, EVM, and harmonics measurement with proper attenuator placement, calibration, and signal generator configuration.

Read the Guide