Circuit Description—Type 422 AC-DC
the Sweep Generator circuit. These trigger pulses are
derived either from the internal trigger signal from the verti
cal deflection system or an external signal connected to the
TRIG IN connector. Controls are provided in this circuit to
select trigger source, coupling, slope, level and mode. Fig.
3-6 shows a detailed block diagram of the Sweep Trigger
circuit. A schematic of this circuit is shown on diagram 5 at
the back of the manual.
Trigger Source
The TRIGGERING Source switch SW305 selects the
source of the trigger signal. Three trigger sources are avail
able; internal from both channel 1 and 2, internal from
channel 1 only, and external. The internal trigger signal is
obtained from the vertical deflection system. In the CH 1 &
2 position of the TRIGGERING Source switch, the trigger
signal is obtained from the CH 1 & 2 Trigger Pickoff Net
work in the Vertical Switching and Output Amplifier cir
cuit. This signal is a sample of the displayed channel (or
channels for dual-trace operation). Since this trigger signal
source follows the Diode Gate stage, the trigger signal in
this position also includes the chopped switching transients
when operating in the CHOPPED vertical mode. The trigger
pickoff for CH 1 & 2 triggering also follows the POSITION
control. Therefore, the DC level of the trigger signal in this
Source switch position changes as the vertical position of
the display is changed.
When the TRIGGERING Source switch is in the CH 1
position, the internal trigger signal is obtained from the
Channel 1 Trigger Pickoff stage in the Channel 1 Input
Amplifier circuit. This signal is a sample of only the signal
applied to the INPUT 1 connector. Since this trigger pick
off precedes the POSITION control, the trigger signal is not
affected by changes in vertical position of the display.
External trigger signals applied to the TRIG IN connec
tor can be used to trigger the sweep in the EXT position of
the TRIGGERING Source switch. Input resistance at DC is
about 100 kilohms paralleled by about 35 picofarads. Vari
able capacitor C302 provides high-frequency compensation
for the external triggering circuit.
Trigger Coupling
The TRIGGERING Coupling switch SW310 offers a
means of accepting or rejecting certain frequency compo
nents of the trigger signal. In the AC position of the
Coupling switch, the DC component of the trigger signal is
blocked by coupling capacitors C306 and C309. Frequency
components below about 50 hertz are attenuated. In the
AC LF REJ position, additional capacitors C305 and C308
are connected in series with C306 and C309 to block DC
and attenuate frequency components below about 50 kilo
hertz. The DC position passes all signals from DC to at least
15 megahertz.
Nonlinear Feedback Amplifier
The trigger signal at the output of the TRIGGERING
Coupling switch is connected to the Nonlinear Feedback
Amplifier stage through R321. Diodes D320 and D321
limit the input voltage swing to about ±0.5 volts to protect
this stage if a high-amplitude signal is inadvertently con
nected to the TRIG IN connector. Transistors Q323 and
Q324 are connected as a feedback amplifier with two paral
lel feedback paths. When the output voltage swing of this
stage is less than about ±0.3 volts, the feedback path is
through C353-R353. For higher output voltages, the feed-
back is through the limiting network
C332-C333-D331-D332-D333-D334-R331-R332-R333-
R334. The effect of this configuration is a higher gain for
low-amplitude signals than for high-amplitude signals. To
Channel 1 & 2
Trigger pulse
to Sweep
Generator cir
cuit
Auto gate to
’ Sweep Gener
ator circuit
3-10
Fig. 3-6. Sweep Trigger detailed block diagram.
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