Tektronix 7B70 Time Base Instruction Manual page 17

Operating Instructions-7B70 (SN B205000 and Up)
this mode, the P-P AUTO trigger mode can be used for
most applications and the remaining modes used only for
special applications or when stable triggering is not obtain-
able in this mode.
AUTO. When the AUTO push-button is illuminated, a
triggered display is presented with the correct setting of the
LEVELISLOPE control (see Trigger Level discussion)
whenever an adequate trigger signal is applied. The range of
the LEVELISLOPE control in this mode is k8 vertical divi-
sions. The TRIG'D light indicates when the display is trig-
gered.
When the trigger repetition rate is below about 30 hertz
(or outside the frequency range selected by the COUPLl lVG
switch) or when the trigger signal i s inadequate, the sweep
free runs at the sweep rate indicated by the TIMEIDIV
switch t o produce a reference trace (TRIG'D light off).
When an adequate trigger signal is again applied, the free-
running condition ends and a triggered display is presented.
When the LEVELISLOPE control is at a setting outside the
amplitude range of the trigger signal, the sweep also free
runs at the sweep rate indicated by the TIMEIDIV switch.
This type of free-running display can be useful when it is
desired t o measure only the maximum peak-to-peak ampli-
tude of a signal without observing the waveshape (such as in
bandwidth measurements).
NORM. When the NORM push-button is pressed, a trig-
gered display is presented with the correct setting of the
LEVELISLOPE control whenever an adequate trigger signal
is applied. The range of the LEVELISLOPE control in this
mode is +8 vertical divisions. The TRIG'D light indicates
when the display i s triggered.
The NORM trigger mode must be used to produce trig-
gered displays with trigger repetition rates below about 30
hertz. When the LEVELISLOPE control is at a setting out-
side the amplitude range of the trigger signal, when the
trigger repetition rate i s outside the frequency range
selected by the COLlPLlNG switch or when the trigger sig-
nal is inadequate, there is no trace (TRIG'D light off).
SINGLE SWP. When the signal t o 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 ob-
tained under these circumstances by using the single-sweep
feature of this unit. The SINGLE SWP MODE i s 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 R ESET-R EADY pushbutton is pressed
again. I f the displayed signal is a complex waveform
composed of pulses of varying amp1 itude, successive single-
sweep displays may not start at the same point of the wave-
form. To avoid confusion due t o the CRT persistence, allow
the display to decay before pressing the RESET-READY
pushbutton again. A t 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
R ESET-READY pushbutton again.
When using the single-sweep mode t o 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.
Trigger
Coupling
The pushbuttons located below the COUPLING title
(see Fig. 2-4) select the method in which the trigger signal is
connected t o the trigger circuits. The selected coupling is
indicated by a lighted pushbutton. Each position permits
selection or rejection of the frequency components of the
trigger signal which trigger the sweep. Fig. 2-6 graphically
illustrates the band of frequencies covered by each position
of the COUPLllVG switch.
Scans by ArtekMedia O 2006