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Understanding the
Basics of Phase
Offset between
Channels
Adjusting Phase
Offset for Standard
Waveforms
Figure 3-42 and access the Start Phase field in the
Synchronization Properties group. The start phase defines the
delay time that the instrument will hold off before it will start
generating the output waveform. Setting resolution is 20 ns and
the delay can be programmed from 200 ns to 20 s.
10. There is always some skew between adjacent instruments,
which is due to circuit delays and cable length. Always consider
the initial skew in your inter-instrument delay calculations. The
initial waveform skew on the slave unit is roughly 25 ns.
The 2074 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
2074 signals. They do not overlap exactly because the instrument
has a skew spec of ±1 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 on both channels.
There are times, however, that you do need to offset phase
between channels. In that case, the 2074 lets you adjust phase-
offset variations with 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.
The 2074 can generate an array of standard waveforms however,
one should bear in mind that the 2074 is a digital instrument and
that standard waveforms are created from lookup tables or
computed from equations; The lookup tables or equations are
converted to waveform coordinates, placed in the arbitrary
waveform memory and then clocked to the DAC with the sample
Using the Instrument
Using the Half Cycle Waveforms
3
3-69
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