BNC 630 - Manual (Page 72)

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More on Phase Offsets
Phase Offsets are accomplished using a reset signal which is shared by all units. This signal tells all units to jump to a particular
waveform point at the same time. By changing which point in the waveform is jumped to, the starting phase of the waveform may be
adjusted.
The 630 has a phase offset field which allows the user to specify a starting phase from 0 degrees to 359.99 degrees. This phase is
relative to the Master Unit’s waveform phase, which is always 0 degrees.
The 630 translates the phase value entered on the LCD (from 0 to 359.99) to a starting address as follows:
Starting Address = Number of waveform points * Phase Offset
360
which is illustrated in the following diagram:
Master Unit
waveform
Slave Unit
waveform
The reset pulse always forces
the Master Unit to the first point.
This gives 0 deg of phase shift.
The reset pulse can force
the Slave Unit to any point.
A 90 deg phase shift is shown here.
Phase Resolution
Because the phase offset is done by advancing by whole samples, the phase resolution depends on the number of waveform points.
The more points you have, the better the resolution. The resolution is given by:
Phase Resolution = 360
.
(in degrees) # of waveform points
The phase value on the LCD display may be adjusted with .01 degree phase resolution. If the Phase resolution given by the above
formula is greater, the 630 will generate the nearest phase offset to the specified value.
Waveform Size Restrictions
When the arbitrary waveform generator skips points to produce a phase offset, the generator will reach the end of the waveform
earlier. To prevent a discontinuity in the waveform, the 630 must make a duplicate copy of the waveform and tack it onto the end of
the original downloaded waveform. For this to be possible, enough arbitrary waveform memory must be reserved for this duplicate
copy. One exception is where the waveform fills every memory location (32,768 points). In this case, the 630 will not make a
duplicate copy of the waveform since the generator will “wrap” around from the last point to the first point and thus avoid any
waveform discontinuity. Another exception is when the desired phase offset is limited to less than 359.99 degrees. In this case, the
630 does not need to copy the entire waveform since the starting address is bounded.
It is for these reasons that the length of the waveforms used for Multiple Unit Locking must be:
1. 32,768 points
or
2.
≤
16,384 points
or, if the desired phase offset is limited
3.
≤
32,768
.
points
1 + max phase
360
Example:
Suppose you need phase offsets only up to 95 degrees. Then your waveforms may be up to 25,926 points in length. At 25,926 points,
the phase resolution will be .014 degrees. (To obtain the best phase resolution, it is desirable to use as many points as possible.)