Tektronix 7B51 Instruction Manual page 17

Operating Instructions-Type 7B51
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(A) (B)
Fig. 2-5_ LEVEL/SLOPE control operation for (P-P) AUTO mode triggering. (A) LEVEL/SLOPE control set so displayed waveform starts at
about 25% point of a one-division waveform. (B) displayed waveform still triggered at about 25% of leading edge, when signal amplitude is
increased, to produce a four-division display (LEVEL/SLOPE control at same setting for both waveforms).
NORM. When the NORM push-button is pressed, a trig-
gered display is presented with the correct setting of the
LEVEL/SLOPE control whenever an adequate trigger signal
is applied. The range of the LEVEL/SLOPE control in this
mode is ±8 vertical divisions. The TRIG'D light indicates
when the display is triggered.
The NORM trigger mode must be used to produce trig-
gered displays with trigger repetition rates below about 30
hertz. When the LEVEL/SLOPE control is at a setting out-
side the amplitude range of the trigger signal, when the
trigger repetition rate is outside the frequency range selec-
ted by the COUPLING switch or when the trigger signal is
inadequate, there is no trace (TRIG'D light off).
SINGLE SWP. When the signal to be displayed is not
repetitive or varies in amplitude, waveshape or repetition
rate, a conventional repetitive type display may produce an
unstable presentation. A stable display can often be obtain-
ed under these circumstances by using the single-sweep
feature of this unit. The SINGLE SWP MODE is also useful
to photograph non-repetitive or unstable displays.
To obtain a single-sweep display of a repetitive signal,
first obtain the best possible display in the NORM MODE.
Then without changing the other TRIGGERING controls,
press the SINGLE SWP pushbutton. When ready to view
the single-sweep display, press the RESET-READY push-
button_ A single trace is presented each time the RESET-
READY pushbutton is pressed (as long as the repetitive
signal remains connected to the system and TRIGGERING
controls are correctly set) and further sweeps cannot be
presented until the RESET-READY pushbutton is pressed
again_ If the displayed signal is a complex waveform com-
2-8
posed of pulses of varying amplitude, successive single-
sweep displays may not start at the same point of the wave-
~orm_
To avoid confusion due to the CRT persistence, allow
the display to decay before pressing the RESET-READY
pushbutton again_ At fast sweep rates, it may be difficult to
view the single-sweep display _ The apparent trace intensity
can be increased by reducing the ambient light level or
using a viewing hood as recommended in the indicator
instruction manual.
Non-repetitive, random signals can be displayed in the
single-sweep mode by first obtaining the best possible dis-
play in the NORM MODE with a signal which is about the
same amplitude and frequency as the random signal. Then
without changing the other TRIGGERING controls, press
the SINGLE SWP pushbutton_ When ready for the random
signal, press the RESET-READY pushbutton_ The RESET-
READY pushbutton remains lighted to indicate that the
unit has been reset and is ready to produce a sweep_ The
light goes out after the random signal triggers the unit. To
prepare the unit for another single-sweep display, press the
RESET-READY pushbutton again.
When using the single-sweep mode to photograph wave-
forms, the graticule must be photographed separately in the
normal manner to prevent over-exposing the film. Be sure
the camera system is well protected against stray light, or
operate the system in a darkened room. For repetitive
waveforms, press the RESET-READY pushbutton only
once for each waveform unless the signal is completely
symmetrical. Otherwise, multiple waveforms may appear on
the film. For random signals, the camera lens can be left
open until the random signal triggers the unit (RESET-
READY pushbutton lighted). Further information on
photographic techniques is given in the camera instruction
manual.