8
TIMEBASE DRIFT
The drift of the timebase over several hours is
substantially less (x10 to x100 less) than the
absolute timebase error. The major factor in the
timebase drift is the instrument's temperature:
after the instrument has warmed up, the
timebase drift is about 0.5 ppm/°C for the
standard timebase, and about 0.05 ppm/°C for
the optional timebase. The drift between several
DG535's which are used in the same experiment
may be eliminated by daisychaining the
reference output from one DG535 to the
reference input on the other DG535's.
JITTER
Various noise sources in the DG535 modulate
the time delay for the outputs causing "jitter".
Some of these noise sources are common to all
of the channels, others are independent. The
distribution of the pulses around the desired
time can be approximated by a Gaussian (or
"normal") distribution:
where,
p(t)= probability of pulse occurring at time t
T
= set time for the output (mean value)
σ
= standard deviation of the distribution
Figure 4 shows the shape of the Gaussian
distribution and the relations between the rms jitter
and the peak-to-peak jitter.
The rms jitter,
σ
, is a function of the delay setting.
The jitter is about 50 ps rms for delays less than100
µs. For short delays, the peak-to-peak jitter
measured on an oscilloscope is about four times the
rms jitter. For long delays, the observed jitter
measured on an oscilloscope is
primarily due to the jitter of the oscilloscope's
horizontal (timebase) deflection circuits. For a good
300 MHz oscilloscope, the rms jitter is typically (25
ps + 10 ppm of the time base).
The rms jitterm, as a function of time delay, is
shown in Figure 5 for a Tektronix 2465
oscilloscope, the DG535 using an internal timebase,
and for a DG535 with an ideal external source. The
jitter for time delays longer than 10 seconds, using
the ideal external source, is due to the drift of the
analog jitter compensation circuits in the DG535. It
can be seen from this figure that the Tek 2465 scope
would show an apparent jitter of 1 ns rms at 100 µs
even though the DG535's jitter is only 50 ps rms.
CHANNEL to CHANNEL INTERACTION
There is a small interaction (pulling) between
channels A and B and between channels C and D
when these channels are set within 10 ns of each
