Demystifying 5G NR Transmitter Testing: Power Spectrum, Spectral Flatness & EVM(Constellation Error)

Advantest Spectrum Analyzer for Constellation Error, Spectral Flatness & Power Spectrum

This Error Vector Magnitude test evaluates how clean and accurate the radio’s transmitted signal is. When the signal becomes distorted, devices have a harder time decoding it. Lab Engineers run this test to confirm that the radio unit(5G RU) is performing as desired and meets required 5G quality standards.

1. Validate 5G NR Modulation Quality

5G uses advanced modulation schemes (QPSK, 16‑QAM, 64‑QAM, 256QAM). If the radio distorts the signal, the phone cannot decode it correctly.

EVM tells professionals whether the RU is good enough to support high‑speed data.

2. Ensure the Power Amplifier Is Not Distorting

When the PA is driven too hard, it introduces noise and distortion. This validation shows whether the PA is operating within safe limits.

3. Confirm Compliance With 3GPP Standards

Every radio must meet strict limits for:

• EVM

• Frequency error

• Phase error

• Power accuracy

This test verifies that the RU passes certification requirements.

4. Detect Hardware Problems Early

Undesired EVM can indicate:

• Faulty PA

• Damaged RF chain

• Calibration issues

• LO phase noise

• IQ imbalance

This test empowers technical team to trace issues before deployment.

5. Validate Manufacturing Quality

Every RU manufactured undergoes this test to ensure:

• No defective components

• No assembly errors

• No calibration drift

6. Constellation Diagram (top right)

This is a picture of the radio signal’s modulation quality. Each dot represents a symbol the radio is transmitting.

If the dots are tight and clustered → good signal If the dots are everywhere or spread out → poor/worse signal

7. Power Spectrum (top right)

This shows how the signal’s power is distributed across frequencies. It helps testers visualize whether the radio is transmitting in the correct channel and whether unwanted noise is leaking outside the band.

8. Constellation Error / EVM (left side)

This is the most important part of the test.

The red‑boxed value “Constellation Error” (also known as EVM – Error Vector Magnitude) tells you how distorted the signal is.

Lower EVM → cleaner signal Higher EVM → worse/Poor signal