For adjacent-channel leakage measurements, the external path must support the main channel and all measured offset bands. Its loss must keep the analyzer in a useful operating range.
Why this matters in the industry
Small adjacent-channel signals are measured alongside a much stronger carrier. Excessive attenuation raises the importance of the analyzer noise floor, while too little can introduce instrument distortion.
The technical reasoning
ACLR requires defined main-channel and adjacent-channel integration regions. A route correction must cover all of those regions, and the instrument must resolve unwanted power without generating it. The relevant question is not simply whether the carrier is visible, but whether the adjacent measurement has adequate dynamic range and an appropriate reference.
Interpreting modulation and adjacent-channel measurements
Error vector magnitude compares measured symbols with a defined ideal reference, while adjacent-channel measurements compare integrated power in specified frequency regions. Their definitions depend on measurement bandwidths, waveform settings and processing conventions. Too little input level makes analyzer noise influential; too much can create instrument distortion. External paths can introduce amplitude ripple, delay variation or nonlinearity, so measured impairment can include both the device and the measurement system.
How to structure the investigation
Check the test definition's integration bandwidths and offsets. Verify the accessory response across that entire span. Compare results at more than one safe analyzer input level, using the same transmitter settings, to identify measurement-system contributions.
Sweep input level over a useful range while holding waveform settings constant. A stable result over an appropriate region is more credible than a single convenient reading. Check the exact integration regions and reference normalization, then compare a suitable reference signal through the same path. Include route response across the whole measured span, not just the central carrier frequency. Retain instrument settings so the experiment can be reproduced.
Worked example or engineering scenario
Suppose a carrier is reported at 0 dBm and the selected adjacent region at minus 45 dBm after valid corrections. Their difference is 45 dB under that convention; instrument residuals can still limit interpretation.
Evidence to collect
| Record | Purpose |
|---|---|
| List integration bands | Defines the tested state and scope of the comparison. |
| Check broadband response | Makes the stimulus or route condition reproducible. |
| Verify analyzer noise floor | Supports interpretation of variation and possible confounding effects. |
| Compare safe input settings | Connects the observation with the stated engineering decision. |
Trade-offs and common interpretation errors
Do not subtract an analyzer's advertised residual error from a DUT result without a valid measurement model. Likewise, one external loss correction cannot recover distortion already introduced by a nonlinear stage. Report which impairments are resolved by the setup and which remain combined with its residual behavior.
What the result can support
Report integration bandwidths and conventions together with the ratio rather than presenting an unlabeled adjacent-channel number.
Flat nominal attenuation at one frequency is not evidence of broadband measurement accuracy.
Further technical reading
- NIST: Modulated-Signal Measurement and Traceability
- Keysight: Cellular Base Station Performance Testing
Related industry knowledge
- TDD Burst Power: What a Cellular Test Load Must Handle
- Using a Power Divider for Cellular Receiver Comparisons
Numerical scenarios are illustrative assumptions, not reported measurements of a supplied product or installation.