Adding Waveforms - Tektronix 11A32 User's Reference Manual

Operating the 11A32 in 11300-series mainframes
Adding Waveforms
NOTE
Before adding waveforms, check that each channel's display is
independently on screen.
The 11300-series oscilloscopes allow the addition of any two channels. That is,
one channel may be added to a channel from another plug-in unit. A simple
restriction applies. Each channel must be in its linear operating range. This is
assured if each channel separately is within the screen area before addition.
Those portions of a trace which are off screen will not be valid when brought
back on screen using another channel's input signal or Offset control. This
general restriction applies to any dual-channel oscilloscope.
Overdrive occurs when any 11A32 channel is driven out of its linear range of
approximately ±15 divisions.
The 11A32 has extraordinarily good overdrive recovery, and this feature may be
used to greatly extend measurement resolution. For example, suppose a signal
changes from -1.7 V to +0.8 V in 1 ns. The 11A32 could be used to determine if
the signal stabilized immediately at +0.8 V or if perhaps had some small
aberration following the transition. By setting the 11A32 offset to +0.8 V and the
sensitivity to 1 mV/division, aberrations of just 0.1% of the original transition
will be 2.5 divisions in amplitude (0.1% of 2.5 V is 2.5 mV or 2.5 divisions at 1
mV/division).
Any amplifier will ultimately reach an equilibrium value after an input step
(although its accuracy will determine how far that equilibrium value is from the
correct value). The HA32/s ability to settle quickly to within a veiy small
fraction of its equilibrium value is exceptional. The time it takes the 11A32 to
settle to within a stated fraction of the equilibrium value is its overdrive
recovery time.
Measuring the overdrive recovery time of an 11A32 takes some care and can
lead to some surprising results. An interesting experiment is to use a very flat
pulse generator, such as the Tektronix PG 506, to pulse the 11A32. Connect the
generator's fast-rise output to the 11A32 input through a short (one foot or less)
coaxial cable and select 50 Q, input impedance. Adjust the pulse amplitude to 1
volt. Set the 11A32 sensitivity to 1 mV/division. Each division now represents
0.1% of the input signal. Trigger the oscilloscope and observe the recovery of the
11A32 using 50 ns/division sweep rate. Now increase the cable length by about
three feet (for example, use a 42-inch length of RG-58 cable, Tektronix part 012-
0057-01) and observe the new waveform. See Figure 2-2.
11A32 User Reference Supplement
2-5