Trigger Pulse Shaper - Philips GM 5605 Manual

Circuit description 25
valve B501 via C501. The anode of this amplifier valve is d.c. coupled to
the input of the Schmitt trigger (B502-B502'). A Schmitt trigger is a
multivibrator with two stable conditions, viz.
B502 conductive, B502' cut off, and
B502 cut off, B502' conductive.
The condition is changed by applying a positive pulse to the control grid
of the non-conductive valve or a negative pulse to the grid of the conduc-
tive valve. By means of R513 the Schmitt trigger B502-B502' is adjusted
in such a way that in the waiting position (i.e. if no trigger voltage is
applied) valve B502 is cut off and B502' conductive. During the positive-
going part of the cycle the amplified trigger signal applied to the control
grid of B502 will drive the valve into conduction at a certain level (the
upper threshold voltage). Due to the common cathode resistor and the
d.c. coupling between the anode of valve B502 and the control grid of
valve B502' a cumulative switching phenomenon takes place, as a result
of which valve B502' is cut-off. This results in a positive voltage at the
anode of B502'.
During the negative half of the cycle of the amplified trigger signal, valve
B502 will be cut-off at a certain level (lower "threshold voltage"). Due to
the repetitive switching action, valve B502' becomes conductive, so that
a negative voltage arises at the anode of this valve. In the case of a repeti-
tive trigger signal it is necessary for the correct functioning of the trigger
pulse shaper that both threshold voltages are exceeded. The trigger
sensitivity of the trigger pulse shaper is thus determined by the difference
in level between the two threshold voltages. This difference is adjusted by
means of potentiometer RSI3, so that the position of this potentiometer
is decisive for the trigger sensitivity.
The positive and negative voltage transients at the anode of valve B502'
are differentiated by capacitor C503 and resistor R521 into small positive
and negative pulses, which are applied to the Schmitt trigger B503-
B503' in the time base generator.
The control range of R3 is adjusted by selection of resistor R522. The
capacitors C502 and C517 in the anode leads of valves B502 and B501
respectively serve to improve the rise time of the pulse shaped voltages.