User Manual LW120
Adjusting Phase
Offset Between
Channels
The LW120 has two output channels that can generate various and
numerous waveforms. Although the control over waveform
parameters is separate for each channel, the sample clock is
derived from a single source. Having a single source for both
channels is of great advantage because of two main reasons: 1)
There is no jitter between the two channels and 2) If we ignore the
initial skew, both waveforms start at exactly the same phase.
Understanding the initial skew term is very important. If you set both
channels to output square waveforms and then connect these
signals to an oscilloscope. If you then set the oscilloscope to its
fastest time base setting, you’ll see the two rising edges of the
LW120 signals. They do not overlap because the instrument has a
skew spec of
±
2 ns. Skew is caused as a result of many factors.
Although the two channels were designed exactly the same, small
variations in printed circuit board layout or component values are
enough to cause skew. These factors were known during the
design phase and were minimized as practical. On the other hand,
skew can also be generated from external factors that are
controlled by the user alone. Examples for these factors are
variation in cable length and quality, as well as, non-symmetrical
end termination. Therefore, if you want to eliminate skew between
channels, you have to use exactly the same cable type, the same
cable length and the same termination for the two channels.
There are times, however, that you do need to offset phase
between channels. In that case, the LW120 lets you adjust phase-
offset variations to a resolution of one point. When you do, just
keep in mind that the initial skew will escort your programmed
phase offset throughout the entire phase offset range.
Notice that phase offset resolution is given in points, not degrees.
This is because the LW120 can generate arbitrary waveforms that
you download from an external application. After you download
waveforms, there is no way for the instrument to determine phase
because all it has is just number of points and no further knowledge
of the shape of your waveform. If you insist, you can convert points
to degrees on your waveform from the following equation:
Phase
(in
°
) = Phase point x 360 / number of waveform points
If you are using standard waveforms, you can determine the
number of points that the LW120 is using from the FREQUENCY
menu. Note the Freq and the Sclk values. The number of points is
derived from the following relationship:
Number of Waveform Points = Sclk / Freq
After you know the number of points you can easily calculate the
Using The Instrument 3-40
