Tektronix 516 series Instruction Manual Download Page 24

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Circuit Description — Type 5 1 6

FROM

TIME-BASE

GENERATOR

Fig. 4 -4 . Time-Base Trigger block diagram .

Trigger Input Amplifier

The

input to the

Trigger Input Amplifier, V24,

may

be se

lected from any one of three sources by means of the black

TRIGGER  SELECTOR  control,  SW10A.  When  SW10A  is  in
one  of  the  INT.  positions,  the  signal  is  obtained  from  the
Trigger  Pickoff circuit  in  the Vertical  Amplifier.  When  SW10A

is in one of the EXT. positions, the signal may be obtained
from an external source through the TRIGGER INPUT con

nector on the front panel. When SW10A is in one of the

LINE positions, the signal is obtained from one of the 6.3-
volt secondary windings of the line transformer.

The + and — positions of SW10A provide a means of in

verting  or  not  inverting,  as  desired,  the  triggering  signal  in
the  Trigger  Input  Amplifier.  This  is  done  so  that  the  negative
spike  at  the  output  of  the  Time-Base  Trigger  can  be  made
to  occur  during  either  a  positive-going  or  a  negative-going

portion of the triggering signal. (The negative spike occurs

only when there is a negative-going signal at the input to
the Trigger Multivibrator.)

When SW10A is in any of the —positions, the incoming

triggering  signal  is  applied  to  the  grid  of  V24A,  and  V24  is
a  cathode-coupled  amplifier.  With  this  configuration,  the
signal  at  the  plate  of  V24B  (output  of  the  Trigger  Input  Am

plifier) is in phase with the incoming triggering signal. When

SW10A is in any of the + positions, the incoming triggering
signal is applied to the grid of V24B, and V24B is a plate-

loaded amplifier. In this case, the signal at the plate of
V24B is 180° out of phase with the incoming triggering

signal.

The TRIGGERING LEVEL control, R17, varies the average

dc  level  at  the  plate  of  V24B  from  about  + 60  volts  to  +100
volts.  This  is  true  whether  SW10A  is  in  a  +   position  or  a
—  position.  The  minimum  level  of  + 60  volts  represents  the
point  where  V24B  is  taken  into  saturation  by  a  sufficiently
positive  setting  of  R17  when  SW10A  is  in  a  —  position  or

by a sufficiently negative setting of R17 when SW10A is in

a + position. The maximum level of +100 volts represents

the  point  where  V24B  is  taken  into  cutoff  by  a  sufficiently
negative  setting  of  R17  when  SW10A  is  in  a  —  position  or
by  a  sufficiently  positive  setting  of  R17  when  SW10A  is  in
a  +   position.  As  will  be  seen  later,  the  voltage  at  the  plate
of  V24B  must  pass  through  the  approximate  center  of  this
range  (about  + 80  volts)  in  order  to  cause  the  Trigger  Multi
vibrator to  change states.

For small triggering signals, R17 is set such that the aver

age dc level at the plate of V24B is close to the center of

its range so that the small signal, as amplified by V24, is

sufficient to carry the plate voltage through the +80-volt

point. When a large triggering signal is applied and it is

desired  to  trigger  on  an  extreme  positive  or  negative  point
of  it,  R17  is  set  such  that  V24B  is  well  into  saturation,  or  cut
off  (depending  upon  whether  triggering  is  desired  at  a  more
negative  or  more  positive  point  on  the  signal  and  on  a  nega
tive-going  or  positive-going  slope).  In  this  case,  the  trig
gering  signal  must  be  sufficient  to  overcome  the  saturation
or  cutoff  of  V24B  and  produce  an  additional  20  volts  of
swing  at  the  plate  of  V24B  in  order  to  cause  the  Trigger
Multivibrator  to  switch  states.

It should be noted that the voltages given in the foregoing

discussion are typical nominals only and will vary somewhat
from instrument to instrument and with time.

Trigger Multivibrator

The Trigger Multivibrator is a two-state Schmitt circuit.

When  the  voltage  at  its  input  grid  (grid  of  V45A)  is  above
a  certain  critical  level  (neglecting  hysteresis),  the  Trigger
Multivibrator  is  in  one  state,  with  V45A  conducting  and
V45B  cut  off.  When  the  Trigger  Multivibrator  is  in  this  state,
the  voltage  as  its  output  (plate  of  V45B)  is  at  300  volts.
When  the  voltage  at  the  input  grid  is  below  the  critical  level
(still  neglecting  hysteresis),  the  Trigger  Multivibrator  is  in  the

other state, with V45A cut off and V45B conducting. When
the Trigger Multivibrator is in this second state, the voltage

at its output is about +290 volts. The transition from one

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