Also refer to Digital Television Signal Analyzer Technology.

This submenu key opens the Select Mode, GI list box. Guard Intervals are used to ensure that distinct transmissions do not interfere with one another. The use of Guard Intervals introduces immunity to propagation delays, echoes, and reflection, to which digital data is normally very sensitive. Protection is inversely proportional to data rate efficiency. For example, a 1/4th guard interval (the largest choice) provides the best protection but also provides the lowest data rate efficiency.

The instrument uses the FFT Start position to obtain the data that is used for measurement. The Select FFT Start list box allows selecting: 0/8, 1/8, 2/8, 3/8, 4/8, 5/8, 6/8, 7/8, 8/8 and 0/8 Fixed, 1/8 Fixed, 2/8 Fixed, 3/8 Fixed, 4/8 Fixed, 5/8 Fixed, 6/8 Fixed, 7/8 Fixed, 8/8 Fixed.

The relationship between FFT Start Position and GI is described in Digital Television Signal Analyzer Technology and in Figure: FFT Start and Guard Interval.

Using a 0/8 FFT Start position means that the instrument is measuring the Effective symbol, but is not including any portion of the Guard Interval.

Using a 1/8 FFT Start position means that the instrument is measuring the Effective symbol plus 1/8 of the Guard Interval. Stated in another way, an FFT Start of 1/8 means that the measurement starts with 1/8 of the Guard Interval and includes the complete Effective symbol.

Using a 2/8 FFT Start position means that the instrument is measuring the Effective symbol plus 2/8 of the Guard Interval.

Using an 8/8 FFT Start position means that the instrument is measuring the Effective symbol plus 8/8 (all) of the Guard Interval.

When only a segment of the Guard Interval is included in the measurement, the instrument is truncating the beginning of the guard interval before interpreting the effective symbol.

0/8 Fixed to 8/8 Fixed:

When a fixed FFT Start position is selected, the FFT Start position is fixed to one of the Guard Intervals (0/8, 1/8, 2/8, to 8/8).

Signals without interference can be cut out and measured by adjusting the FFT Start position. This allows separating the effective symbol from a delayed wave or a pre‑ghost signal.

Spectrum Reverse is used to measure a signal in which the sub carrier location is inverted on the frequency axis, such as the IF signal of the transmitter.

Layer Segment setting: The total sum of segments for all three layers (A, B, and C) must add up to 13. Layer Modulation is 64QAM, 16QAM, or QPSK.