Scientech 801C
Scientech Technologies Pvt. Ltd.
18
Trigger and Time Base:
Time Base operation is important for obtaining a satisfactory stable display. If the
"Auto" push button is not pressed the sweep generator will be triggered automatically.
The time axis (baseline) is then also visible without applying a signal voltage. In this
position practically all uncomplicated, periodically recurring signals above 30Hz
repetition frequency can be displayed in a stable locked-in position. Operation of the
time base is then restricted mainly to adjusting the time setting.
To obtain a stable display at all frequencies the time base must be triggered
synchronously with the applied signal. Triggering can be initiated by this signal itself or
by a different external fed-in voltage, which must also be in synchronism. For this
purpose, press the Ext button. The trigger signal (at least 0.6Vpp) is applied to the
socket marked "Trigger Input". On single channel operation a trigger signal may also be
applied to the input of channel 2 (in this case trigger selector button "Trig 1/2" must be
pressed). This method is recommended if the amplitude of the trigger signal does not
fall in the range 0.6 - 6Vpp, or if it is of unknown value. Using this method the signal
can be adapted to the trigger input of the time base within a range of 2mVpp to
approximately 160Vpp by means of the CH2 attenuator switch. Initially the unknown
external trigger signal should be displayed and then adjusted to peak-to-peak amplitude
of 3-6cm. The trigger signal can be taken either from CH1 or CH2. Selection is made
by means of the button marked "Trig 1/2" on the front panel. If possible, it is always
better to trigger with the less complicated signal. To select the trigger edge, use the"+/-"
button. When it is not pressed, all displays start with a positive going rise.
If the pulse duty factor of a square signal changes drastically, and a part of this square
wave deforms to a needle pulse, the operation of the "Level" control may become
necessary, after pressing "Auto/Level" switch. With composite signals the trigger
facility is dependent on the occurrence of certain periodically recurring levels. The
"Level" adjustment of these signals will require care.
If no triggering point can be found on complex signals even after repeated and careful
adjustment of the "Level" control, it may be possible to obtain one by adjusting the
"Var" control.
All coefficient settings on the "Time Base" switch are calibrated when the "Var"
control is set at the "Cal" position. With "Var" control in the clockwise position the
sweep rate is made faster by a factor of 2.5. This factor in not precisely calibrated. With
the x10 expansion of the sweep a maximum sweep speed of approximately 20ns/cm is
obtained. The choice of the optimum time coefficient depends on the repetition rate of
the signal being measured.
