Philips PM3320 Operating Manual page 118

4-98E
LEVEL and NEGATIVE SLOPE controis
The position of the LEVEL control determines the starting point of the signal acquisition.
The circuit behind the LEVEL control functions as follows:
The trigger signal is fed to the input of a comparator and the voltage on the other input of this
comparator is determined by the position of the LEVEL control.
li the trigger signal reaches the voltage level of the LEVEL conirol, a trigger pulse is generated by
the comparator and the signal acquisition starts.
In this way, signal acquisition is started at a fixed point of the trigger signai, which means that by
using the LEVEL control, it is possible to scan the shape of the waveform.
The NEGATIVE SLOPE pushbutton permits selection of the trigger slope.
If NEGATIVE SLOPE is selected, the signal acquisition starts on the negative-going slope of the
trigger signal, otherwise on the positive-going slope.
lf the LEVEL control is turned higher or lower than the maximum resp. minimum level of the
selected input signai, the internal comparator does not generate a trigger pulse. The led "NOT
TRIG'D" on the frontpanel will then light. In AUTO level triggering, the trigger level can not be
turned "out of range".
The LEVEL and NEGATIVE SLOPE controls enable the trigger level to be set at a predetermined
value without the need of an input signal. This is important when the signal to be measured is not
available in advance as when single phenomena are to be observed. The selected trigger level
point is indicated on the left side of the C.R.T. screen by one of the marksf, or}. When input
signals which exceed a known trigger level have to be displayed, the trigger level can be set in
advance and an input signal of sufficient amplitude will initiate the recording of the signal.
When external triggering is selected, the | is replaced by an X. For line triggering an Lis used as
trigger level indication. Both the X and the L do not indicate the exact trigger level, related to the
signal, but give an indication where the triggerlevel is set.
AUTO-triggering: The acquisition system becomes free running 100 ms after the last
irigger pulse. This means that even during the absence of trigger
pulses the RO/ contents is refreshed, although the pilot lamp NOT
TRIG'D lights. lf a trigger is again applied, normal triggering is
obtained. The lamp NOT TRIG'D will extinguish.
An additional feature of the AUTO mode is that the LEVEL range lies
within the peak-to-peak value of the signal amplitude.
Due to the fact that the system becomes free-running 100 ms after the
last trigger pulse, the AUTO-mode can not be used for signals with a
repetition rate of 100 ms or more; for this the triggered mode must be
selected.
DUAL triggering
By selecting DUAL triggering, the oscilloscope is able to trigger on either positive or negative—
going edges of the signal. This feature can very well be used in the SINGLE-shot mode.