Custom APSK Tab (Digital Demod)

Menu Path: MeasSetup > Digital Demod Properties > Custom APSK tab

The Custom APSK tab configures constellations composed of up to 8 rings and 256 states.

must be selected for the Format in the tab of the dialog box to make a custom APSK measurement.

The following are the parameters used to configure a custom APSK constellation:

Ring ID specifies the individual ring to be defined. The number in Ring ID corresponds directly to the ID number shown for the Mag and Phase results in the APSK Cust Syms/Errs table.
Enabled check box enables or disables the individual ring definitions. The check box is greyed for invalid entries. See below.
# States specifies the number of phase states on the ring. These states are uniformly distributed around the ring . The minimum number of states is 1. The maximum number of states summed across all active rings is 256.
Relative Magnitude specifies the magnitude or radius of the ring. This must be 0 or a positive number. The magnitudes are relative to each other so arbitrary scaling is possible. This allows a variety of ways to normalize entries, ie. they can be normalized to the inner ring, outer ring or any other ring.
Phase specifies the angle of the first state on the ring relative to the x-axis. Entries are in degrees and limited to -180 deg to 180 deg. Positive phase is counter clockwise from the x-axis and negative phase is clockwise.

The rings do not need to be sorted in any particular order. Any rings in the list can be disabled. This allows the Ring ID to be based on any ring, for example zero based (Ring ID "0" enabled) or one based (Ring ID "0" disabled and Ring ID "1" enabled).

The state numbering is sequential, starting at 0 on the first (lowest ID) enabled ring and proceeding down the list. The states on an individual ring start at the state denoted by the Phase for that ring and counting up counter-clockwise around the ring. This means that the state numbering on an individual ring can be rotated around the ring through the Phase entry. For example, on a 4-state ring even though the constellation would look the same for Phase entries of 45, -45, 135, -135, the first state number for that ring would be in 4 different locations.

Since the state numbering starts on the lowest Ring ID, the numbering can be started on the inner ring, the outer ring, or any other ring depending on which is the lowest Ring ID.

"Enable non-uniform phase shift definition" Check Box

The Enable non-uniform phase shift definition check box allows a different interpretation of the phase parameter under certain circumstances. The check box is available only when Ring 0 is enabled with 4 or 6 states and phase of greater than 3 degrees. When selected, the Phase parameter determines the phase rotation away from the x-axis of the 4 states that are closest to the x-axis, as shown in the diagram below. When cleared, the Phase parameter affects the constellation as described previously in this topic (rotation of a ring with evenly-spaced states).

Invalid Entries

Invalid entries will cause the Enabled checkbox to be greyed and the ring to be disabled.

A Relative Magnitude of 0 but # States greater than 1.
A # States defined that causes the constellation to go over the 256 state limit when the ring is Enabled.
A Relative Magnitude that is close to the Relative Magnitude of a lower ID ring. The minimum linear distance is 4% of the Relative Magnitude of the largest Enabled ring. For example, if the outer ring Relative Magnitude is 5, then no rings can be closer than .2 to each other. This includes a ring at the origin (Relative Magnitude = 0 and #States = 1) when a lower ID ring at the origin is already Enabled.