Signal-to-Noise Ratio

The Eye mode Signal-to-Noise Ratio is the ratio of the signal difference between one level and zero level relative to the noise present at both levels. Four annotation labels are placed on the eye diagram. Two labels annotate the mean values of the one and zero levels, and two labels annotate the one level (minus RMS noise) and the zero level (plus RMS noise). The one and zero level measurements are made in a section of the eye referred to as the eye window boundaries. The default value for the NRZ eye window is the central 20% of the symbol period. Signal-to-Noise Ratio is similar in construction to a Q-factor measurement. However, because noise levels contributed by the instrument cannot be removed, a slightly pessimistic Q-factor measurement may result.

To define the signal, histograms are constructed using the sampled portions of the eye diagram within the eye window boundaries. One histogram is comprised of data points from only the upper half of the eye diagram (one level). The second histogram is comprised of data points from the lower half of the eye (zero level). The histogram means and standard deviations are determined.

The noise is defined as 1 σ (standard deviation) of the histogram at the one and zero levels.

Signal-to-noise ratio is computed as:

This measurement does not remove the effect of noise generated by the instrument. If the noise on the signal being tested is of a magnitude similar to that of the instrument or smaller, the signal-to-noise measurement error will be significant.

Configurable Measurement Parameters

This measurement is affected by the following setting (click Measure > Configure Base Measurements):

• Eye Boundary tab

To measure

1. Select Eye/Mask Mode.
2. Click the toolbar's Eye Meas tab.
3. Click Auto Scale in the menu bar.
4. Click the More buttons to locate the Signal-to-Noise Ratio button.
5. Click the button.

SCPI Command

:MEASure:EYE:ESN