Tektronix 7B51 Instruction Manual page 16

Operating Instructions--Type 7851
2- 8
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-
Fig . 2-5 . LEVEL/SLOP E 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) .
posed of pulses of varying amplitude, successive single-
sweep displays may not start at the same point of the wave-
form . 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 .