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Section 3—S-4 Sampling Head
CIRCUIT DESCRIPTION
General
This section of the manual contains a block diagram de-
scription of the Type S-4 Sampling Head followed by a de-
tailed circuit description. Both the block and schematic
diagrams are located at the rear of this manual.
The Type S-4 is the signal input section of a sampling
system,
and
determines
the
vertical
channel
input
characteristics.
BLOCK DIAGRAM
Both the Block Diagram and the circuit schematic show
the Type S-4 circuits outlined by dashes, indicating theindi-
vidual circuit boards within the unit. One circuit board is de-
voted to the generation of the sampling gate strobe pulses,
one to the sampling gate, one to the blow-by and trigger
pickoff amplifier and one to the preamplifier.
Stobe Generator
The Strobe Generator develops heavy-current (several
mA), short-duration, push-pull pulses that drive the Sam-
pling Gate into balanced conduction. Output occurs at the
time of each sample when a command pulse arrives from
the Stobe Driver circuit of the associated sampling unit. The
pulse lasts for a period of about 125 ps, turning on the Sam-
pling Gate diodes. At the end of the pulse, the diodes turn
off very quickly, retaining a portion of the input signal.
Strobe pulse duration andfall time are fixed by the Snap-Off
diode D61, and the twoclipping lines. The Strobe Generator
parts layout andcircuit board construction are carefully con-
trolled during manufacture to ensure proper strobe pulses
being sent to the Sampling Gate.
Sampling Gate
The Sampling Gate (called the Sampling Bridge in other
Tektronix sampling circuitry) connects the input signal to the
Preamplifier only during the short time when each sample is
taken. The six diodes, D2A through D2F, form whatis tech-
nically termed a traveling wavegate. It is the way the end of
the Strobe Drive pulse travels through the gate that deter-
mines the step response of the sampler. In other Tektronix
sampling bridges, the step responserisetime is controlled
by the strobe pulse duration. In the Type S-4, the step re-
sponserisetime is controlled instead by the length of timeit
takes the end of the strobe pulse to travel through part of
the Sampling Gate.
At the end of each Strobe pulse, part of the input signal
is stored temporarily between Sampling Gate diodes andis
then fed to the Preamplifier input at a rate much slower than
the step responserisetime. It is only the special traveling
wavegate that hasits electrical environment controlled for
minimum reflections of fast pulse signals. All circuit parts
that pass the sampled error signal (see the sampling unit
manualsection on Basic Sampling Principles for definition of
error signal) to the Preamplifier handle only moderate rate-of
rise signals. A special 10X (20 dB) 50 attenuator helps
serve as a high-quality, high-frequency termination to the
input connector and the 50 2 environment of the traveling
wave gate.
Part of the input signal is continuously passed to the
Trigger Amplifier Board and the Blow-by compensating and
Trigger Pickoff circuit. The signal passes through the 10X
attenuatordirectly to the Trigger Amplifier Board andis ter-
minated in 50 © there.
Gate Bias
Reverse bias is applied to the Sampling Gate diodes by
the Gate Biascircuit. The average voltage of the gate biasis
controlled by the Gate Balancecircuit and the associated
sampling unit DC Offset and Feedback signals. The Sam-
pling Gate output is DC coupled to the Preamplifier input
through a portion of the Gate Biascircuit.
Blow-by and Trigger Pickoff
The primary function of the Blow-by and Trigger Pickoff
circuit is to cancel capacitively-coupled unwanted signals
that normally bypass the Sampling Gate. These unwanted
signals are called "blow-by"signals.
The Type S-4 special traveling wave Sampling Gate con-
tains two blow-by reducing diodes that minimize the un-
wanted capacitively-coupled signals. The special Sampling
Gate construction and the blow-bycircuit essentially cancel
all of the unwanted signals that would otherwise distort the
oscilloscope sampling display.
The Blow-by and Trigger Pickoff circuit receives an atten-
uated portion of the input signal and terminates the 10X
attenuator that is located between the Sampling Gate and
the Trigger Amplifier Board. The circuit amplifies and inverts
the signal and applies it, as a blow-by correction signal,
3-1
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