Modulation Error Ratio Calculator

1) What modulation error ratio tells you

Modulation Error Ratio (MER) is a power ratio that shows how tightly measured constellation points cluster around their ideal locations. It compares average reference signal power to average error vector power. In practice, MER is a fast health check for a digital transmitter, receiver, or link.

2) MER and EVM describe the same impairment

RMS EVM is the square root of the error-to-signal power ratio. In dB form, MER(dB) = 10·log₁₀(P_signal/P_error) and MER(dB) = −20·log₁₀(EVM_rms). Higher MER therefore means lower EVM percent.

3) Typical operational targets by modulation

Higher-order constellations demand better modulation accuracy. Common operational minimums used in cable and lab work are about 18 dB for QPSK, 24 dB for 16‑QAM, 27 dB for 64‑QAM, and 31 dB for 256‑QAM. Many teams keep an added 3–6 dB safety margin.

4) Measurement bandwidth changes the number

MER depends on the measurement bandwidth and receiver processing. Wider filters include more noise and spurs inside P_error, which reduces MER. Equalization can raise MER by removing linear distortion, so note whether EQ is enabled. To compare runs, keep bandwidth, equalizer state, averaging window, and capture length consistent.

5) Impairments you can infer from MER trends

Compression and clipping raise error power as level increases, so MER falls with output drive. Phase noise often produces a circular smear. IQ imbalance stretches the constellation into an ellipse. Reflections, group delay, and frequency offset add systematic vector errors that also drop MER.

6) IQ samples mode for DSP validation

Paste lines as Iref,Qref,Imeas,Qmeas. The calculator averages reference power and error power across all valid rows, then reports MER and EVM. This is useful for checking demod logs, comparing algorithm versions, or verifying a captured burst. Use enough samples to represent the full constellation.

7) Turning MER into a pass or fail limit

Set limits based on modulation order and receiver sensitivity. Some receivers may struggle near 24 dB for 64‑QAM and near 30 dB for 256‑QAM. If you operate 256‑QAM, aiming around 35 dB provides room for drift and plant changes.

8) Build repeatable reports with exports

Snapshot MER before and after adjustments, then export the results. CSV works well for trending, and PDF works well for job records. Save your measurement settings with each report so later comparisons stay fair. Tracking MER with power level and error rates helps separate low-level issues from distortion or interference.

1) Is higher MER always better?

Yes. Higher MER means less error-vector power relative to signal power, so demodulation is usually more reliable for the same modulation order.

2) Why does MER change when bandwidth changes?

Filters control how much noise and distortion energy is counted as error. Wider bandwidth typically increases P_error, which lowers MER.

3) How do I convert EVM percent to MER?

Divide EVM% by 100 to get RMS, then MER(dB) = −20·log₁₀(EVM_rms). Example: 10% → 0.10 → 20 dB.

4) What should I enter for signal and error power?

Use average reference signal power and average error-vector power from the same capture, bandwidth, and processing. Many analyzers report both metrics directly.

5) Why can two analyzers show different MER?

Equalization, timing recovery, averaging, and correction methods vary. Standardize instrument settings and procedures when you compare results or set acceptance limits.

6) What does “infinite MER” imply in IQ mode?

It means computed error power is essentially zero for your pasted samples. That can happen with idealized data, too few samples, or heavy rounding.

7) How much MER margin should I keep?

A 3–6 dB headroom over the minimum target is a common practice. Margin helps tolerate drift, noise bursts, temperature changes, and component aging.