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SECTION
3
CIRCUIT
DESCRIPTION
Change
information,
if any, affecting
this section will be found at the
rear
of
the
manual.
General
Information
This
section
of
the
manual
contains
a
block
diagram
analysis
of the
Type
S-2
Sampling
Head
followed
by
a
detailed circuit description.
The Type S-2 Sampling Head is
designed
to be
the
signal
input
section
of the
sampling
system,
and
determines
the
input
characteristics
of
the
sampling system.
The reader may find it helpful to refer to
the
associated
sampling
unit
manual
for
information
on
sampling
principles if the purpose of a particular circuit is
not clear.
For example Type 3S2 manual, Section 4, 'Basic
Sampling
Principles".
The sampling
unit manual also shows
interconnections
and
circuits
referred
to
in
this
section.
Schematic and block diagrams of the Type S-2 are located
at the rear of this manual.
BLOCK
DIAGRAM
Strobe
Generator
The
Strobe
Generator
develops
fast-rise
short-duration
push-pull
pulses
that drive
the Sampling
Bridge
into bal-
anced conduction.
Output occurs at the time of each sample
when
a command
pulse arrives from the Delay and Strobe
Driver
circuit of the
associated
ampling
unit.
Shape
and
amplitude of the output strobe pulses is set by the Avalanche
Volts and Snap-off Current controls.
Strobe pulse duration
is fixed by the two shorted clipping lines.
Sampling
Bridge
The Sampling
Bridge allows no connection other than the
normal
stray capacitance
of the bridge between
the input
connector
and
the
Preamplifier
input except
when
driven
into conduction by the Strobe Generator.
When
the Strobe
Generator
drives
the Sampling
Bridge
into conduction,
a
portion of the signal across the Sampling Bridge is applied
to the Preamplifier input.
Reverse bias is applied to the Sampling Bridge diodes by
the Bridge Volts and Bridge Bal circuit. The sampling system
feedback
signal
and
DC
Offset voltage
is applied
to the
Sampling
Bridge
output
side
and
the
Preamplifier
input
through the biasing network.
Blow-by
and
Trigger Pickoff
The primary function of the Blow-by and Trigger Pickkoff
circuit
is
to
cancel
capacitively-coupled
unwanted
signals
that normally
bypass
the Sampling
Bridge.
Very high fre-
quencies pass to the Preamplifier input by the normal stray
®
capacitances of the Sampling Bridge. These unwanted signals
are called "blow-by".
The Blow-by circuit receives an attenuated portion of the
input signal, amplifies and inverts the signal, and applies tt,
as a blow-by correction signal, through a capacitor to the
output terminal of the Sampling
Bridge.
Magnitude of the
blow-by
correction
signal
is adjusted
by the Transient Re-
sponse control to effectively cancel the capacitively-coupled
blow-by signal.
The
trigger
pickoff function of the Blow-by
and Trigger
circuitry
provides
a signal
source for externally triggering
the sampling
sweep
unit.
The trigger pickoff circuit output
signal
drives an additional
amplifier and
channel
selector
circuit in the sampling
unit.
Preamplifier
The
Preamplifier circuit both amplifies and time-stretches
the signal it receives from the Sampling Bridge.
The signal
received
is a portion of the difference between
the Feed-
back with the DC Offset voltage and the input signal.
This
"error signal" is amplified and AC coupled to the Post Ampli-
fier in the sampling unit. The Preamplifier gain is adjustable
to aid in setting the overall sampling
head
and sampling
unit "loop"
gain for proper
unity dot response.
CIRCUIT
DESCRIPTION
The Type S-2 Sampling
Head
uses the power supplies of
the
indicator
oscilloscope
and
associated
sampling
unit.
Interconnections to some of the circuits in the sampling unit
are by two connectors
at the rear of the sampling
head.
This Circuit Description
covers the circuits in the Type S-2,
and refers to circuits within the sampling unit.
Reference to
the sampling
unit instruction
manual
diagrams
and
circuit
description
may
be
useful
to fully
understand
the circuit
relationships.
Strobe
Generator
The Strobe Generator
circuits are located on the Strobe
board.
The
generator
contains
two
basic
circuits, the Ava-
lanche circuit and the Snap-off diode circuit.
Both circuits
work
together to produce
the push-pull
strobe pulses that
drive the Sampling
Bridge through
two equal
transmission
lines.
See Fig. 3-1.
Avalanche
circuit.
The
Avalanche
circuit converts
the
Strobe
Drive
pulse
from
the
sampling
unit
to very
fast
push-pull pulses to drive the Snap-off diode to non-conduc-
tion.
3-1
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