Philips PM32671 Service Manual page 19

2-4
The trigger signals derived from the AMPLIFIER circuits of channel A and B, and from the DELAY LINE
DRIVER are routed t o the TRIGGER INPUT STAGE. These signals are in current form, which makes them
less sensitive to interference; often a problem with long signal wires. I n the TRIGGER INPUT STAGE, these
current signals are converted into voltage form, and fed t o the TRIGGER SELECTION CIRCUIT.
The EXT trigger signal from the EXT input socket, and the LINE signal from the LINE TRIGGER PICK-OFF
are fed t o the TRIGGER SELECTION CIRCUIT via the EXT-LINE TRIGGER INPUT circuit.
In this stage, the EXT and LINE trigger signals are converted t o symmetrical current signals and adapted to the
A, B and COMP signals.
The EXT-LINE TRIGGER INPUT
EXT depressed simultaneously).
I n addition, the A and B trigger signals are fed to t h e TRIGGER SELECTION CIRCUIT of the DTB.
2. 1 . 2 . 1. Main time-base
The trigger signals are selected by the MTB trigger source switches A, B, EXT, COMP, LINE, which control the
TRIGGER SELECTION CIRCUIT.
Common-mode interference is reduced by using a symmetrical configuration for the TRIGGER SELECTION
CIRCUIT output signal. This output current signal is fed t o the TRIGGER AMPLIFIER INPUT STAGE.
This converts the symmetrical current signal t o an asymmetrical voltage signal, which is fed to the
stage
INTERFACE via the TRIGGER COUPLING
switches DC, LF, HF.
Several signals are produced by the INTERFACE, e.g. X DEFL, TRIG VIEW and AUTO/TV.
The X-DEFL signal is an asymmetrical signal that i s fed to the X DEFLECTION AMPLIFIER.
The symmetrical TRIG VIEW signal is routed to the VERTICAL CHANNEL SWITCH; the asymmetrical
The FINAL TRIGGER AMPLIFIER comprises the SLOPE circuit under the control of the SLOPE switch
incorporated in the LEVEL control. It permits positive and negative triggering.
The output of the FINAL TRIGGER AMPLIFIER is fed to the CURRENT-VOLTAGE CONVERTER.
In the AUTO CIRCUIT, the TOP DETECTOR detects the amplitude of the AUTO SIGNAL. When in the
AUTO Mode, the LEVEL range is determined by this detected amplitude. The MULTIPLEXER is an electronic
switch which, depending on the selected mode, selects the different ranges for the LEVEL control.
Each mode has i t s own specific LEVEL range, for example:
T V : fixed level
AUTO : determined by TOP DETECTOR
TRIG 8 divisions
:
X DEFL : 0 divisions (LEVEL inoperative)
If the instrument is provided with the TTL/ECL option, the TV pushbutton will function as the TTL or ECL
mode switch.
The TV trigger signal is fed to the T V CIRCUIT.
When the T V pushbutton i s selected, the T V CIRCUIT is inserted between the INTERFACE and the CURRENT-
VOLTAGE CONVERTER of the MTB.
I n the T V mode, the FINAL TRIGGER AMPLIFIER is switched off and also the LEVEL control is inoperative,
fixed trigger level being
a
For FRAME and LINE synchronisation,
the CURRENT-VOLTAGE CONVERTER, the trigger signal is routed to the SWEEP-GATING LOGIC.
The SWEEP-GATING LOGIC determines the start of the MAIN TIME-BASE GENERATOR sweep.
The SWEEP-GATING LOGIC is controlled by signals derived from the TRIGGER MODE SELECTION, the
HOLD-OFF CIRCUIT and the CURRENT-VOLTAGE CONVERTER.
The TRIGGER MODE SELECTION is controlled by the MTB trigger mode selection pushbuttons AUTO,
TRIG, SINGLE. In the AUTO mode, the MAIN TIME-BASE GENERATOR runs automatically when no
trigger pulses are available.
In the TRIG mode, the MAIN TIME-BASE GENERATOR must be normally triggered by trigger signals derived
fromthe CUR RENT-VOLTAGE CONVERTER.
If the SINGLE pushbutton is selected, the SWEEP-GATING LOGIC will start the MAIN TIME-BASE GENERATOR
for one sweep.
The MAIN GATE OUT signal (optional) i s taken from the SWEEP-GATING LOGIC
This MAIN GATE OUT signal output is
The NOT TRIG LED lights up when the MTB is not triggered.
stage is controlled by the EXT pushbutton and LINE (pushbuttons B and
stage. The coupling is controlled by the MTB trigger coupling
set.
frame or line f i l t e r is selected with the MTB TIME/DIV switch. Via
a
logic H during the MTB sweep and L for other conditions.
a t
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