Differential Mode; Selection Of Trigger Mode - Philips PM 3214 Anleitung

22
2.3.6 Differential mode
The A — B mode can be selected by depressing ADD and pulling the channel B POSITION control.
In measurements where signal lines carry substantial common mode signals (e.g. hum) the differential mode
will cancel out these signals and leave the remainder of interest (A — B►. The capability of the oscilloscope to
suppress common mode signals is given by the CMR factor (see Fig. 2.4).
To obtain the degree of common mode rejection as specified, channel A and B gains must first be equalised.
This can be done by connecting both channels to the CAL output connector, and adjusting one of the con-
tinuous controls with the AMPL/DIV switch for minimum deflection on the screen.
When passive 10:1 probes are used a similar equalisation process is recommended by adjusting their compen-
sating controls for minimum deflection.
A
e
A-B
~~ a ,
!~~d
1 ~ ~~
CMRlcommon mode
re~ectionl_~cm
ud
As drawn here CMR ~ 10
MA 9397
Fig. 2.4. Suppression of common mode signals
2.3.7 Selection of trigger mode
(AC
AUTO DC)
L TV L —a ~- TV F —~
The AUTO mode is most useful because it provides traces) on the screen even in the absence of trigger signals.
Furthermore, for a signal amplitude larger than 1 division, this mode provides stable triggering independently
of the position of the LEVEL control; its range is automatically adjusted to the peak -to -peak value of the
signal selected for triggering.
In this way the setting of the LEVEL control is facilitated at small amplitudes of the trigger signal.
The AUTO mode cannot be employed for signals with low repetition rates (10 Hz or lower) because the sweeps
would be able to free run in between triggers. Therefore, for low repetition signals normal triggering must be
used (AC or DC depressed►.
In normal triggering, sweeps are only initiated with a trigger signal applied and the LEVEL control set
appropriately.
With AC or DC depressed the range of the LEVEL control is fixed (+ or —8 divisions or more at the extremes
of the LEVEL control with respect to mid screen).
The DC component in the trigger signal can be blocked by depressing AC. This is useful when triggering must
take place on a.c. signals superimposed on a large d.c. level.
With AC coupling, the level at which the display starts will change with alterations in the average value of the
trigger signal. The trigger level is thus no longer referenced to signal ground. This may cause instability with
waveforms that vary in time interval from cycle to cycle. Normally it is preferable to use the DC position.
Slope selection is made with pushbutton +/—. In TV mode —must be selected for negative video signals and
+for positive video signals. The LEVEL control is inoperable in the TV mode.
No buttons depressed offers an extra mode of use, a trace is on screen in the absence of a trigger signal, but the
LEVEL range is fixed.
~.
~.
i
...
r
Y