Triggering And Time Bases - Fluke Autoranging CombiScope PM3370B Service Manual

DESCRIPTIONS
3.2.3

Triggering and time bases

The sections for Main Time Base (MTB) and Delayed Time Base (DTB) are for the greater part
identical. Therefore the MTB part is extensively described and then the DTB part briefly. The EXTernal
TRIGger input of the 2 channel versions is only different in the attenuator part, after that it is equal to
and described as CH4.
Main Time Base Description.
The possible trigger sources are applied to the block MTB TRIGGER FILTERS + AMPLIFIER. The
selection for triggering on the vertical channels 1, 2, 3 or 4 is done in the Y-FUNCTIONS blocks in
the vertical section. Triggering is also possible on the LINE trigger signal that is derived from the
mains. The MTB TRIGGER FILTERS + AMPLIFIER incorporates filters for HF- reject, LF-reject and
ac or dc signal coupling. The block also incorporates the +/- slope selection. The dc control signal
LEVEL MTB originating from the CONTROL CIRCUITS is routed via the PEAK-PEAK DETECTOR.
This block limits in peak-peak mode the LEVEL MTB range just within the peak-peak value of the
signal. For this purpose the trigger signal is applied to an input of the detector. The LEVEL MTB
control signal is not limited if the peak-peak mode is inactive.
For triggering on TV synchronization pulses the blocks TV CLAMP CIRCUIT and TV SYNC
SEPARATOR are used. The CLAMP CIRCUIT separates the synchronization pulses from the
composite video signal. The video information is not necessary for triggering. The TV SYNC
SEPARATOR filters out line, frame and field pulses. This is done for various TV systems including
HDTV. A VSYNC output is connected to digitizer unit A8.
The block MTB TRIGGER FILTERS + AMPLIFIER sends trigger pulses to the MTB LOGIC. This logic
is combined with the DTB LOGIC. The three output signals are used to start MTB (applied to MTB
TIMING CONTROL), to switch intensity on and off (applied to Z CONTROL) and for ALTernate display
switching (applied to DISPLAY MODE + TRIGGER SOURCE CONTROL). TV HOLD-LD is received
from digitizer A8. TB40TXT and MTB trigger DSOM are applied to A8.
The MTB is based on the principle that selectable capacitors (inside block MTB TIMING) are charged
with a selectable constant current (from MTB CURRENT SOURCE). This results in a sawtooth
voltage across the capacitor(s) that rises linearly with the time. As a consequence a time-linear
horizontal deflection is obtained. The sawtooth voltage is routed to the horizontal deflection part via
the MTB SAWTOOTH PICK OFF. This block serves as a high to low impedance converter so that the
load to the timing capacitor(s) is minimal. Various time base sweep speeds are obtained by selecting
different combinations of current values and timing capacitors. The function MTB VARiable works via
the MTB CURRENT SOURCE.
The MTB TIMING CONTROL discharges the capacitor(s) if the maximum level of the sawtooth is
reached. The MTB TIMING CONTROL allows the charging process to start again if a trigger occurs.
The functions X POSition, HOLD OFF and 10x MAGN are applied to and realized in the MTB TIMING
CONTROL. HOLD OFF determines the time between discharge of the timing capacitors and the
moment that a trigger pulse is allowed to start the MTB again.
The MTB TIMING CONTROL accomplishes the DELAY time function. The sawtooth voltage is
compared with an adjustable dc voltage. The DTB is started where both voltages have the same
level: this condition is signalled to the DTB LOGIC via signal START.
The MTB TIMING CONTROL also plays a role in the automatic MTB calibration. A reference voltage
MTBCALREF is compared with the sawtooth voltage. The time necessary for the sawtooth to reach
the MTBCALREF level is monitored by the microprocessor via signal MTBCAL. If necessary the MTB
is readjusted. Automatic calibration of the X-path (including output circuit of MTB TIMING CONTROL)
is done with accurate voltages from the block X DEFLECTION CALIBRATION.
Delayed Time Base Description.
The function of DTB TRIGGER FILTERS + AMPLIFIER is identical to the corresponding block in the
MTB part. A "peak-peak detector" for LEVEL DTB is not present. The range of this control is always
fixed and not related to the peak-peak level of the signal. Blocks for TV triggering are not present in
the DTB section. TV triggering of the DTB occurs via output signals from the TV SYNC SEPARATOR
in the MTB section.
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