Philips PM 3214 Anleitung Seite 24

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-- 2.3.8 Trigger sources
The main time base trigger sources can be selected by the front-panel TRIG or X DEFL pushbuttons.
A B EXT LINE
"` I—COMP-~
— /nterna/ triggering will be most commonly used because it requires only one signal (the input signal) to
obtain stable triggering.
"' — External triggering. When tracing many signals it is advantageous to use an external signal for triggering.
There is no need to set and reset the trigger controls (LEVEL, SLOPE and SOURCE) on changing the input
signal. Furthermore the two A and B inputs remain free for examining waveforms.
— Selection of trigger source. In comparing waveforms that are a multiple of each other's frequency, always
select the signal, that has the lowest repetition rate as the trigger source. Not doing so may lead to double
pictures (chopped mode► or untrue time-shifts (alternate mode).
— Composite triggering. With internal triggering signals are obtained from either the A channel, the B channel
preamplifier stages or, when COMP is selected by depressing both A and B pushbuttons, from the delay
line driver stage that follows the electronic channel switch.
Composite triggering offers three advantages:
1. In differential mode (A — B measurements) triggering is derived from the differential signal. Triggering is
not disturbed by common mode signals.
2. For one channel operation it is not necessary to switch trigger sources from A to B or vice versa.
3. In the alternate mode, it is possible to compare signals that are not related in time.
Note:
When composite triggering is employed in dual channel operation (chopped or alternate), and
1
there is only one signal applied (to A or B input), stable triggering cannot be obtained. This is only
normal since the trigger source is also switched from A to B (see Fig. 2.5J.
— Line (mains) triggering is useful when the signal input is mains (line) frequency related.
.,.
--•
_.
~.
...
A
PREAMP.
1
Channel A triggering
B
PREAMP
ELECTRONIC
SWITCH
i
Channel B triggering
Fig. 2.5. Block diagram of composite trigger circuit
DELAY
LINE
DRIVER
MA9365
Composi te triggering