Philips PM3219/01 Notice D'emploi page 38

3.4.2.5.
3.4.3.
3.4.3.1.
3.4.3.2.
43
Single shot mode
To write a zero line in single sweep mode proceed as follows:
— Depress 0" of the input coupling switches of the channels.
— Depress AUTO (AC+DC) of the trigger mode switch.
After 100 ms a zero line is written on the CRT screen.
To obtain a multi-trace display in single sweep mode the following should be born in mind:
— SINGLE + ALT depressed: both traces of channel A and B are written after each other on the CRT screen.
For each trace one trigger pulse derived from the depressed trigger source is required.
— SINGLE + Aor B+ ALT TB depressed: both traces of the MTB and the DTB are written after each other
on the CRT screen. For each trace a trigger pulse derived from the depressed trigger source is required.
— SINGLE + ALT + ALT TB depressed: both traces of channel A and B + both traces of the MTB and the
DTB are written arbitrary on the CRT screen. Now four trigger pulses are necessary.
Input A and B facilities
The oscilloscope has two identical vertical channels, each of which is suitable for either YT measurements in
combination with one or both time-base generators, or XY measurements in combination with the external
horizontal channel.
YT measurements
To display one signal, one of the two vertical channels can be selected by operating either pushbutton A or
pushbutton B of the vertical display-mode controls.
When pushbutton ALT or CHOP is depressed, two different signals can be displayed simultaneously. The Y
deflection coefficient (and the polarity for channel B only) can be selected individually for each channel.
When the ALT button is selected, the display switches over from one channel to the other during the flyback
of the time-base signal. Although the ALTERNATE mode can be used at all sweep speeds of the time-base
generator, the CHOPPED mode will give a better display quality for long sweep times, because in the ALT
mode, these slow sweep alternations become clearly visible.
In the CHOP mode, the display is switched over from one channel to the other at a fixed frequency. If the
ADD pushbutton of the display mode switch is selected, the signal voltages of both vertical channels are
added. Depending on the position of the polarity switch, either the sum or the difference of the input signal is
displayed. The ADD mode also enables differential measurements to be made. With these measurements,
advantage is taken from the common mode rejection in the ADD position. When the channel B polarity switch
is set to PULL TO INVERT, the common mode portions of the signals on sockets A and B will only be
subjected to very slight amplification compared with the differential mode portions.
Differential mode
As stated, the A-B mode is 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 largely cancel out these signals and leave the remainder of interest (A-B). The capability of the oscillo-
scope 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 conti-
nuous 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
compensating controls for minimum deflection.