Fluke PM3082, Service Manual

Page: 67 / 496

5.1 -12
UNIT DESCRIPTIONS
Diagram 16
This diagram indicates the timing circuits for the MTB. The principle of the time base is that a
capacitor is charged with a constant current. This gives a time-linear voltage across the capacitor;
the so- called sawtooth or sweep signal. The timing capacitance consists of C6011 that is always in
circuit and C6012, C6013 that are activated via the switching transistors V6016 and V6019. The MTB
control is done in IC D6011.
The constant current is supplied via transistor V6003 and V6005. The current source consists of
voltage divider R6002 through R6007 with precision resistors. The voltage across this divider can be
influenced by DAC output voltage MTBVAR via N6007/5,6,7 and V6001. This is necessary for
continuous time/div control (VAR) and calibration. The voltage on a tap of the voltage divider can be
selected via multiplexer D6001. This voltage (MCSCONTROL) is applied to the base of V6003 and
V6004. This occurs via operational amplifier N6006. Via multiplexer D6002 two different emitter
resistance values can be selected for V6003 and another two for V6004. The resistance values differ
by a factor of 10: the emitter resistance for V6003 is switchable between R6014 (time base magnifier
x10) and R6013 (magnifier x1). The emitter resistance for V6004 is switchable between R6016 (time
base magnifier x10) and R6015 (magnifier x1).
The current from V6004 is applied to pin 16 of D6011 and is used as a reference. The sawtooth that
is generated across the timing capacitor(s) is applied to pin 18 of D6011. Inside this IC is the time
base switching transistor. The start of the sawtooth is initiated by signal MTBGATE that is applied to
pin 20 of D6011. The sawtooth is also applied to pin 17 and converted into a balanced output signal
that is available pin 12 and 13. The MTB sweep pickoff circuit consists of Source follower V6013 (and
matching V6014) and emitter follower V6012. Horizontal position control is achieved via DAC output
signal TBXPOS at pin 14.
Input signal TBCALREF at D6011/28 and output signal MTBCALTST at D6011/24 provide
autocalibration of the time base. The sweep is switched on via STRCALM-HT and MTBGATE and
compared with reference voltage TBCALREF. Via switching of output signal MTBCALTST, the
microprocessor knows if the current source delivers the correct current. In case of inaccuracies the
microprocessor makes corrections.
The start of the DTB is initiated by a low level of signal TBSTD-LX at pin 1 of D6011. The signal is
generated via comparison of the MTB sawtooth signal and the adjustable DC voltage DTBM (delay
time multiplier). The DTBM signal is coming from DAC output N6014/6 via operational amplifier
N6008/5,6,7.
The signal TBEOM-HX at pin 4 of D6011 becomes high at the end of the MTB sweep. The signal
TBEOHO-LX at pin 5 of D6011 becomes low at the end of the hold off period. The length of the hold
off period is determined by the DAC output voltage HOLDOFF that is applied to D6011/7.
The MTB is switched on by making MTBONOFF (D6011/11) high. This signal is derived from control
signal TBSEL. TBSEL is high during X deflection by the MTB and low during DTB. The MTB is also
off in external X- deflection mode.
V6006 is on during the fast time base positions. It activates a stabilizing circuit that is connected with
D6011/21.
The circuit with V6042 and C6033 assures that the hold off time in the fastest time base positions will
never become shorter than 3 us. The circuit with C6035, V6002, C6017, V6008 and C6415 is
responsable for the hold off timing. C6035 is always in circuit and C6017 and C6415 are activated by
switching transistors. The capacitors are charged in parallel with the MTB timing capacitors. During
the hold-off time that follows they are discharged by a current source inside D6011. The hold- off time
can be varied by varying the discharging current.