Tektronix S-4 User manual

TEK aaa
070-0896-01
Product Group 42
S-4
SAMPLING HEAD
Please Check for
CHANGE INFORMATION
at the Rear of This Manual
First Printing JAN 1969
—,_ Revised OCT 1986

Copyright © 1969, 1983 Tektronix, Inc. All rights reserved.
Contents of this publication may not be reproduced in any
form without the written permission of Tektronix, Inc.
Products of Tektronix, Inc. and its subsidiaries are covered
by U.S. and foreign patents and/or pending patents.
TEKTRONIX, TEK, SCOPE-MOBILE, and oi are
registered trademarks of Tektronix, Inc. TELEQUIPMENT
is a registered trademark of Tektronix U.K. Limited.
Printed in U.S.A. Specification and price change privileges
are reserved.
INSTRUMENT SERIAL NUMBERS
Each instrument has a serial number on a panel insert, tag,
or stamped on the chassis. The first number or letter
designates the country of manufacture. The last five digits
of the serial number are assigned sequentially and are
unique to each instrument. Those manufactured in the
United States have six unique digits. The country of
manufacture is identified as follows:
BO00000 Tektronix, Inc., Beaverton, Oregon, USA
100000 Tektronix Guernsey, Ltd., Channel Islands
200000 Tektronix United Kingdom, Ltd., London
300000 Sony/Tektronix, Japan
700000 Tektronix Holland, NV, Heerenveen,
The Netherlands

S-4 Sampling Head
CONTENTS
Section 1 Specification
Section 2 Operating Instructions
THE FOLLOWING SERVICING INSTRUCTIONS
ARE FOR USE BY QUALIFIED PERSONNEL ONLY.
TO AVOID PERSONAL INJURY, DO NOT PERFORM
ANY SERVICING OTHER THAN THAT CONTAINED
IN OPERATING INSTRUCTIONS UNLESS YOU ARE
QUALIFIED TO DO SO.
Section 3 Circuit Description
Section 4 Performance Check/Calibration
Section 5 Maintenance
Section 6 Options
Section 7 Electrical Parts List
Section 8 Diagrams
Section 9 Mechanical Parts List Information
Mechanical Parts List Illustrations
Accessories
Abbreviations and symbols used in this manual
are based on or taken directly from IEEE Stan-
dard 260 “Standard Symbols for Units,” MIL-
STD-12B and other standards of the electronics
industry. Change information, if any, is located
at the rear of the manual.

S-4 Sampling Head
mVOLTS/DIV
TYPE S-4
SAMPLING HEAD
fe anton +5v MAX
TEKTRONIX® 4
Fig. 1-1. Type S-4 Sampling Head.
0896-03
~
—j}
—
J)
~~
J
__)
a

Section 1—S-4 Sampling Head
TYPE S-4 SPECIFICATION
GENERAL INFORMATION
The type S-4 Sampling head is an input signal processing
unit for Tektronix sampling units. Input characteristics of the
sampling system are determined by the Type S-4. The Type
S-4 has an input impedance of 50 Q and rise time of 25 ps
or less.
The Type S-4 can be installed directly into a sampling
unit or used remotely on optional 3 foot or 6 foot extender
cable. A portion of the signal delivered to the sampling head
input is coupled to the sampling unit for use as an internal
trigger signal. Use of this trigger pickoff signal is dependent
upon the type of vertical sampling unit and sampling sweep
unit used in the system. See the instruction manual for your
vertical sampling unit and sampling sweep unit for further
information.
The vertical deflection factor of the sampling system is
labeled at the top of the Type S-4 as mVOLTS/DIV. The
label refers to the sampling unit Units/Div switch.
NOTE
The electrical characteristics in this section are appli-
cable for an instrument calibrated with ambient tem-
peratures between +20°C and +30°C, and after a 5
minute warm-up. Unless otherwise stated, electrical
characteristics apply over an operating temperature
range from 0°C to +50°C and to an altitude of
15,000 feet. Supplemental information items are for
information only and are not specifications.
Table 1-1
SPECIFICATIONS
Characteristic
Performance Requirements
Supplemental Information
Step Response
Displayed Noise
Risetime 25 ps or less
Observed with
S-50 or S-52 35 ps or less
Aberrations Time From Step
With S-50 or With 284
S-52
0 - 400 ps 400 ps - 25ns After 25 ns
—10%, +10% 0%, +10% or +2%, —2% Ol
or less; total less; total less less; total less
less than 20% than 10% p-p than 4% p-p
p-p
5 mV or less, measured tangentially
1-1

Type S-4 Specification—S-4 Sampling Head
Table 1-1 (cont)
Characteristic
Performance Requirements
Supplemental Information
Dot Transient Response
Within 5% for input signals up to 500 mV P-P
Plug-in unit may require adjustment of
DOT RESPONSE control when switch-
ing Units Div. Avalanche voltage may
have to be readjusted with extreme
ambient temperature.
Signal Voltage
Maximum Operating
1V P-P
Maximum DC limits which may be dis-
played depend on DC OFFSET range
and Deflection Factor
Safe Overload
Do not exceed + or —5 V limits
Input R
50 2 within 5%
Baseline Shift With
Repetition Rate Change
10 mV or less from 30 Hz to 50 kHz
Temperature
Nonoperating —40°C to +65°C
Operating 0°C to +50°C
Altitude
Nonoperating To 50,000 feet
Operating To 15,000 feet
Vibration
Operating 15 minutes along each axis at 0.015”. Vary
the frequency from 10 to 50 to 10 c/s in
1-minute cycles. Three minutes at any
resonant point or at 50 c/s
Shock
Nonoperating 1000 g’s, 1/2 sine, 1/2 ms duration,
2 shocks per axis. Total of 6 shocks
Transportation Qualifies under National Safe Transit
Committee Test Procedure Category IV
(48 inch drop)
—,

Section 2—S-4 Sampling Head
OPERATING
INSTRUCTIONS
General Information
This section provides the basic information required for
operation of the Type S-4 Sampling Head.
The Type S-4 can be used on an extender cable without
compromising the response of the measurement system.
Signals are applied to the 50 Q input of the samping head
through a 3 mm, 50 2 coaxial connector.
Unity loop gain can be obtained with input signals of up
to 0.5 volt (peak-to-peak). Unity loop gain ensures the most
accurate displays during fast changes in input signal ampli-
tude. See the Basic Sampling Principals Section of your
sampling unit manual for further information.
The Type S-4 may be used with input signals up to 1 volt
(peak-to-peak). Specifications for the Type S-4 Sampling
Head are given in Section 1 of this manual.
NOTE
Attenuators, with threaded 3mm connectors, are
available as optional accessories. These attenuators
are useful in reducing the amplitude of large signals.
Other optional accessories with 3mm connectors in-
clude coaxial cables, a 50 2 termination, and adapters
for interconnecting various types of connectors.
Installation instructions are located in the vertical sam-
pling unit instruction manual.
Effect of Excessive Input Voltage
A maximum operating signal voltage of 1 volt (peak-to-
peak) is specified for the Type S-4 in Section 1 of this man-
ual. Signals having an amplitude of more than 1 volt (P-P) at
the Type S-4 input may be distorted by the samping head.
Large signals must be reduced in amplitude to not more
than 1 volt (P-P) at the Type S-4 input.
Signals in excess of plus 5 volts or minus 5 volts may
destroy the sampling diode bridge.
A 15 volt (P-P) signal can be reduced to a suitable ampli-
tude at the Type S-4 input by inserting a series combination
of 10X and 2X attenuators in the coaxial line delivering the
signal. A series combination of 10X and 5X attenuators
would also be satisfactory.
See the discussion on Sine Wave Signal measurements
later in this section for information on signal amplitude affect
upon display amplitude accuracy.
Risetime Measurements
Accurately determining risetime from an observed display
requires that the position of the 10% and 90% points be
accurately known and that the horizontal distance between
these points be accurately measured. The timing accuracy
of the sampling sweep unit must also be known. Switching
the sampling sweep unit from one oscilloscope to another
requires readjustment of the horizontal gain control on the
sampling sweep front panel. See your sampling sweep unit
instruction manual for timing accuracy specifications and
complete instructions for checking timing.
Displayed Risetime and Actual Risetime
Differences in displayed and actual signal risetime will
result unless the risetime of the sampling system is about
four times faster than the risetime of the signal to be mea-
sured. As used in this discussion, the sampling system is
considered to be made up of the sampling head and any
coaxial line, connectors or accessories between the signal
source and the sampling head. Displayed risetime is depen-
dent upon the risetime of the sampling head, the risetime of
the pulse generator, and the risetime of the cable and con-
necting components between the generator and sampling
head. With signals as fast as about 100 ps arriving at the
Type S-4 input, the displayed risetime will be very close to
the actual signal risetime.
Since the risetime of the Type S-52 Pulse Generator out-
put signal is much faster than 100 ps, the displayed risetime
(with this pulse applied to the Type S-4 input) will not be the
actual signal risetime.
The risetime of signals that are faster than 100 ps can
still be determined using the Tektronix Type S-4 Sampling
head. However, the signal risetime is calculated using for-
mula (2-2). Signal risetime calculated using formula (2-2) will
provide a close approximation of the actual signal risetime.
2-1

Operating Instructions—S-4 Sampling Head
| Reference te | uzee kid
|_| | point 2 a 35 a)
100% Level wn nnn nga N t
po |) dt 6g ioe
| | pad \ N
| | | + 35 psy Zone
t yf center __|
<< Tr
Jt fe eas dad, pS pg 4 j poppy pp ty
Zone | cc
center | oi
t 1 x ie
| \ +35 peo} a
Soo
| «ans 2 oo -—0% Level
100 %—+ — Reference
zor r point 1
$ t |
pet 0896-04
Fig. 2-1. Special S-4/S-52 system risetime measurement
technique.
The relationship between displayed risetime, signal
risetime, signal path risetime, and sampling head risetime is
shown by formula (2-1). T, of signal path, and T, of Type S-4
in formula (2-1). T, of signal path includes the combined
risetime of all coaxial cables, connectors, or accessories be-
tween the signal source and the Type S-4 input. If the signal
path consists of a coaxial cable having a risetime of 8 ps
and connectors and accessories having a risetime of 6 ps,
the combined signal path risetime is
(8)? + (6)? = 10 ps
T, (displayed) ~
V(T, signal)? + (T, of sig. path)? + (T, of Type S-4)? (2-1)
For example, assuming an input signal having a risetime
of 25 ps; connecting cable, accessories, and connectors
having a risetime of 10 ps; and a sampling head with a
risetime of 25 ps gives:
T,
(displayed)
~
V(25)
+ (10
+
(25)
T,
(displayed)
~
y625
+
100
+
625
T,
(displayed)
~
37
ps
Transposing formula 2-1 to permit calculation of signal
risetime when the displayed risetime, signal path risetime,
and sampling head risetime is known gives:
T, (signal) ~
(T, displayed)? + (T, of Type S-4)* — (T, of sig. path)?
(2-2)
Additional information concerning the effect of connect-
ing cables on the displayed signal risetime may be found
under Cable and Accessory Considerations later in this
section.
2-2
Sine Wave Signal measurements
The parameters of the Type S-4 are specified using pulse
or time domain parameters and nomenclature. The fre-
quency response characteristics of the unit, while not speci-
fied, are such that it may be used for relative sine wave
measurements through its calculated upper 3 dB rolloff
point of 14 GHz.
Because of the traveling wave gate used in the Type S-4,
it is capable of handling very large signals as compared to
other microwave samplers. Even so, the peak to peak signal
amplitude should be held to as low level as possible con-
sistent with good signal to noise ratio when making checks
in the 5 to 15 GHz region to avoid compression effects.
Cable and Accessory Considerations
When connecting a signal to the Type S-4 it is important
that the signal coupling cable, connectors, attenuators and
any other devices between the signal source and sampling
head have the correct characteristic impedance, be capable
of handling the signal frequency without causing excessive
loss or distortion, and be of high quality. Care must be taken
to firmly tighten all connectors, or unwanted reflections may
result.
Accurate displays of signals containing very fast transi-
tions or very high frequencies are possible only if the cables
carrying the signal to the sampling head do not distort the
signal. The Type S-4 input resistance is 50 2 and requires
the use of high quality, low loss, 50 2 coaxial cables and
attenuators between the signal source and the sampling
head input cohnector. Using an adapater to change from
3 mm to a larger diameter coaxial cable will result in a loss
of
system bandwidth.
The Type S-4 sampling head is useful from DC up to at
least 14 GHz. Standard and optional 3mm accessories
available from Tektronix are intended for use at frequencies
to 18GHz and are generally useful at frequencies to
25 GHz. Since any accessories in the path between the sig-
nal source and the Type S-4 input will increase the displayed
signal risetime, use only high quality accessories of ade-
quate high frequency capability. Accessories (3 mm) sup-
plied with the Type S-4, or available as optional equipment,
include flexible and rigid coaxial cable, attenuators, adapters
to change from 3mm to larger size coaxial cable, and a
power divider.
The physical and electrical characteristics of the cable
determine the characteristic impedance, velocity of propa-
gation and nature of signal losses.
It is important to use cables or airlines, in the signal-
handling side of the system, that are as short as possible.

Use of the optional 3 foot or 6 foot head extender cable
permits locating the Type S-4 near the signal source and
thereby keeps the input coaxial cable short and signal
losses to a minimum. Signal losses along a coaxial line, due
to skin effect or dielectric loss, result in a loss of high-fre-
quency signal amplitude as the signal travels along the line.
Dielectric loss along the coaxial line also results in an in-
crease in the displayed risetime of fast step pulses. Dielec-
tric and skin effect losses increase with signal frequency
and the length of the coaxial cable. Fig. 2-2A shows the
displayed waveform when the 25 ps output pulse of the
Type S-52 is fed to the Type S-4 input through a rigid 3 mm
coaxial line. In Fig. 2-2B the 25 ps pulse is fed to the S-4
input through a 5 ns length of flexible 3 mm coaxial cable.
Fig. 2-2C shows the risetime of the displayed signal when
the same 25 ps pulse is sent through a 5 ns length of good
quality coaxial cable similar to Type RG-58/U. The impor-
tance of using a short length of the proper type coaxial be-
tween the signal source and the Type S-4 input is apparent
from comparison of the waveforms of Fig. 2-2.
Another factor that can result in loss of signal in a coaxial
cable is use of coaxial cable having a diameter that is large
compared to the wavelength of the highest frequency com-
ponents of the signal. The Type S-4 input connector is in-
tended for use with Teflon dielectric coaxial cable having an
outer conductor with an outside diameter of 0.141 inches.
Resistive losses in large diameter coaxial cable is less
than in smaller diameter cable for frequencies below the crit-
ical value.
The two coaxial lines listed under part numbers 015-
1015-00 and 015-1005-00 must be treated with care
to obtain maximum life and least signal distortion. The
rigid line may not have its shape changed and still
have the propagation delay and Z, guaranteed. In
cases where the line is not used at its maximum per-
formance limits, small amounts of bending can be
made providing mating connectors are on each end
before bending. If bent to a curve radius of 1 1/2
inches, the line may be made totally inoperative. The
flexible cable must not be given a bend radius less
than 10 times its OD, or approximately 2 1/4 inches.
Flexible cables may suffer outer conductor breakage
by tighter bending. Few flexures assure longer life.
Operating Instructions—S-4 Sampling Head
a
eae ror
eee ee
20 ps/div
100 mV/div
20 ps/div
100 mV/div
bi.
aes
Me
LL
(C) 5ns length of cable similar to RG 58/U.
0896-05
Fig. 2-2. Effect of cable length, diameter, and quality on dis-
played risetime.
2-3

ii
4]
)
__]
4]
a
|

WARNING
THE FOLLOWING SERVICING INSTRUCTIONS
ARE FOR USE BY QUALIFIED PERSONNEL
ONLY. TO AVOID PERSONAL INJURY, DO NOT
PERFORM ANY SERVICING OTHER THAN THAT
CONTAINED IN OPERATING INSTRUCTIONS
UNLESS YOU ARE QUALIFIED TO DO SO.
REFER TO OPERATORS SAFETY SUMMARY
AND SERVICE SAFETY SUMMARY PRIOR TO
PERFORMING ANY SERVICE.
Be
pQ
BA
AR
WmWA
DBA

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 the indi-
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 and fall time are fixed by the Snap-Off
diode D61, and the two clipping lines. The Strobe Generator
parts layout and circuit 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 what is tech-
nically termed a traveling wave gate. 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 response risetime is controlled
by the strobe pulse duration. In the Type S-4, the step re-
sponse risetime is controlled instead by the length of time it
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 and is
then fed to the Preamplifier input at a rate much slower than
the step response risetime. It is only the special traveling
wave gate that has its electrical environment controlled for
minimum reflections of fast pulse signals. All circuit parts
that pass the sampled error signal (see the sampling unit
manual section 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
attenuator directly to the Trigger Amplifier Board and is ter-
minated in 50 © there.
Gate Bias
Reverse bias is applied to the Sampling Gate diodes by
the Gate Bias circuit. The average voltage of the gate bias is
controlled by the Gate Balance circuit 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 Bias circuit.
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-by circuit 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

Circuit Description—S-4 Sampling Head
through a small capacitor to the output side of the Sampling
Gate blow-by limiting diodes. magnitude of the blow-by cor-
rection signal is adjusted during calibration by the Transient
Response control.
The trigger pickoff function of the Blow-by and Trigger
circuit provides an in-phase signal source for internally trig-
gering the sampling sweep unit. Not all channels of all sam-
pling units (in which the S-4 operates) couple the trigger
pickoff signal to the sampling sweep unit. If the signal is not
used, the sampling unit terminates the output in 50 Q, allow-
ing the circuit to remain functional for blow-by correction.
Preamplifier
The Preamplifier circuit both amplifies and time-stretches
the signal it receives from the Sampling Gate. The signal
received is a portion of the difference between the Feedback
combined with the DC Offset voltage and the input signal.
This “error signal” is amplified and AC coupled to the Post
Amplifier in the sampling unit. The Preamplifier gain is ad-
justable to aid in setting the overall sampling head and sam-
pling unit “loop” gain to unity for proper dot response.
CIRCUIT DESCRIPTION
Refer to the main schematic diagram during the following
descriptions. The Type S-4 Sampling Head uses the power
supplies of the indicator oscilloscope and associated sam-
pling
unit.
Interconnections
to
the
sampling
unit
circuits
are
by two rear connectors. The following description includes
references to circuits in the sampling vertical unit and sam-
pling sweep unit, all units forming one sampling system.
Reference to diagrams and circuit descriptions in the in-
struction manuals for the other sampling units will help in
gaining a full understanding of the circuit relationships.
Strobe Generator
The Strobe Generator circuits are all located on the right
side Strobe circuit board. The generator contains two basic
circuits,
an
Avalanche
circuit
that
delivers
fast
push-pull
pulses and the Snap-off diode and clipping lines circuit. Both
circuits work together to produce the push-pull strobe
pulses that drive the special traveling-wave Sampling Gate
through two equal transmission lines. Both sets of circuitry
are physically arranged over separate ground planes that
are identified on the main schematic diagram. The two
ground planes are connected together by R52 and R54, to
damp out ringing between the two circuit areas at sample
time.
The Avalanche Circuit converts the Strobe Drive pulses
from the sampling unit to very fast push-pull pulses that
drive the Snap-off diode to non-conduction.
3-2
The Strobe Drive pulse is transformer-coupled by T75 to
the base and emitter of Avalanche transistor Q69. Two out-
puts are ac-coupled from Q69, one from the collector and
the other from the emitter. The Avalanche Volts control ad-
justs
the
collector
voltage
of
avalanche
transistor
Q69.
The
typical quiescent voltage at Q69 collector is about
+15 volts. This voltage sets the amplitude of the signals
that drive the Snap-off diode circuit and assures the normal
avalanche action of Q69 when driven by the Strobe Drive
signal. Before Avalanche conduction, there is a potential of
about 60 volts between Q69 collector and emitter.
The negative Strobe Drive pulse is transformed into a
hard forward bias signal to Q69. Normal avalanche action
follows with the collector going negative and the emitter go-
ing
positive.
This
fast-rise,
push
pull
signal
is
capacitively
coupled to the Snap-off circuit.
The Snap-off Circuit operates as a current switching
circuit to apply some of the push-pull Avalanche current sig-
nal at snap-off time to the Sampling Gate. The circuit con-
sists of a Snap-off Current control R57, Q55, Snap-off diode
D61, two clipping lines and associated components. Be-
tween drive pulses from the Avalanche circuit, the Snap-off
diode D61 is forward-biased by the current in Q55. The cur-
rent value is set by the Snap-off Current control R57. The
current in D61 is typically 20 mA. This forward current as-
sures that D61 has many carriers within its junction region.
The push-pull signals from the Avalanche circuit cause
D61 junction carriers to reverse direction as a heavy reverse
current. This heavy reverse current stops suddenly as all the
carriers clean out of the junction. As the reverse current
“snaps” to a stop, the push-pull avalanche signals are sud-
denly coupled into the clipping lines by C52 and C54 and
toward the Sampling Gate by R51-C51 and R53-C53. The
fast-rise step which appears at each clipping line input is
propagated down the line. About 50 ps later the steps reach
the short circuited ends of each clipping line. The step is
then reflected, equal in amplitude and opposite in polarity,
back to the input end of each clipping line. This cancels the
signals moving toward the Sampling Gate. This action re-
sults in a positive Strobe pulse being delivered to P53, and a
negative Strobe pulse being delivered to P51.
The clipping lines have a surge impedance of approxi-
mately 10 2 with a very good short circuit at the shorted
ends. The high quality of the short, and the line low imped-
ance causes the reflected signal to stop the Strobe pulses
very rapidly. Such rapid ending of the Strobe pulse and the
characteristics of the traveling wave Sampling Gate causes
each sampling diode to turn off in approximately 10 ps.
Ringing of the Snap-off diode and the clipping lines is
mainly prevented by R50 which is directly across the lines

ends. Ringing is also limited to some degree by the two
equal resistors R60 and R62 that are in DC series connec-
tion with the Snap-off diode D61. The primary function of
R60-R62, however, is to provide a DC path for D61 forward
conduction while at the same time allowing the Avalanche
signals to pass on into the clipping lines and Sampling Gate.
As the Avalanche pulses end and as both the Avalanche
and Snap-off circuits recover for another sample, R60-R62
re-establish the correct quiescent charges on C63 and C64.
C59 assures a very low impedance AC path for the ground
side of R60 so that R60 and R62 will both have the same
affect upon the circuit.
Another technique for minimizing Strobe ringing is that of
R51 and R53 being in series with the output lines to the
Sampling Gate. The output connectors that connect the
Strobe board to the Sampler board have a 50 © characteris-
tic impedance. R51 and R53 raise the Clipping lines 10
impedance drive to 50 2 to properly match the connections.
Thus, at the end of the Strobe pulse, reflections back from
the Sampling Gate are reverse terminated. These resistors
and the resistors discussed in the preceeding paragraph all
assure that there is no possibility of double strobing of the
Sampling Gate due to ringing of the Strobe Generator
circuits.
Two separate ground planes are identified on the main
schematic diagram between the Avalanche and Snap-off cir-
cuits. Two resistors connect the two ground plane ends to-
gether to damp any natural ringing when the case is off.
Ringing can occur without R52 and R54 because the two
ground planes are connected together at the instrument
front and rear through the Sampler board. Component lay-
out is organized so the strobe signal ground currents bal-
ance to zero at the center of the Sampling Gate, and thus
do not contribute unwanted currents to the input signal. Q69
circuit is capacitively isolated from both ground planes as
much as possible, with its primary signal output path being
through C63-C64 and C52-C54. There is a small third termi-
nal capacitance to ground between the two windings of T75,
but it is not large enough to disturb the balanced drive from
Q69 to D61.
Sampling Gate and Gate Bias Circuits
The Sampling Gate consists of six series diodes, D2 A
through F, and four resistors all mounted on a ceramic sub-
strate inside the input circuit holder. See Fig. 3-1 during the
following discussion.
The signal that arrives at J1 travels right on through the
Sampling Gate area to a 3 mm 50 © 10X (20 db) attenuator.
Output of the attenuator feeds the Trigger and Blow-by
Compensation circuit where it is terminated in 50 2. When
not being strobed, D2C and D2D present a very small value
of capacitance to the input signal. When the diodes conduct
Circuit Description—S-4 Sampling Head
due to Strobe current, the input line is coupled to the travel-
ing wave gate.
Gate Biasing
Quiescent condition of the Sampling Gate diodes is con-
trolled by the Gate Bias circuit. D2A, D2B, D2E and D2F are
each reverse biased about 0.5 volts by D20, D23, D25 and
D28. D2C and D2D are reverse biased about 1.2 volts each,
a total of about 2.45 V is developed across the parallel com-
bination of R24-C25 of the Gate Bias circuit. The Gate Bias
circuit receives its current from the two 50 V supplies and
R30-R31-R32-R33 and R34 of the Gate Balance control cir-
cuit. The diodes of the Gate Bias circuit are used as 0.5-volt
Zener diodes. R2A-R2F with C24 and R2B-R2E with C25
isolate the traveling wave gate segments from the Preamp
and Bias circuits R21-R26 conduct DC bias potentials to the
gate but allow a sampled error signal to drive C24 and the
Preamplifier input.
Except for the very short strobed conduction time (about
125 ps), the Sampling Gate diodes do not conduct. A por-
tion of the input signal is coupled through stray capaci-
tances to the Preamplifier input, and disturbs the normal
sampling display. This effect is known as “blow-by”.
Blow-by Correction
The basic risetime of the traveling wave Sampling Gate is
25 ps. In this extremely short interval of time, only a very
small amount of charge can be collected in one sample.
What small charge is collected, is called the error signal and
is amplified by the Preamplifier and the sampling unit cir-
cuits. The Preamplifier has a shaped frequency response |
best suited to amplifying each sample’s small charge and
delivering a proper error signal to the sampling unit.
It is impossible to build a sampling gate without a path
for blow-by displacement currents. These currents cause
the Preamplifier to respond to a part of some input signals
and distort the display. The Type S-4 Preamplifier responds
to signals in the 0.5 to 3 MHz bandpass region. Signals
much higher or lower in frequency are not amplified. Those
blow-by signals in the Preamplifier bandpass region that do
get through the Sampling Gate are virtually eliminated by
the Blow-By correction amplifier Q10.
Blow-by rejection in the Sampling Gate is assured by the
double differentiation networks that have very fast RC
times. They are located between the signal input and the
Preamplifier. The first differentiation network is formed by
reverse biased diodes D2C and D2D and their respective
resistors to ground (see Fig. 3-1), R2B and R2E. The sec-
ond network is formed by diodes and resistors D2B, D2E
and R21, R26, respectively. All the Sampling Gate diodes
3-3

Circuit Description—S-4 Sampling Head
+50V
SAMPLING GATE
GATE
BIAS
INPUT
‘
és
SC
IRCUIT SAMPLER BOARD
eae 2 PREAMP BOARD R30
HOLDER
M
\o 11
Ny | | ™
@->> K
i
[ 1,
I C5
—
nN I
-—— S PREAMP
f
-
~5
pF
me
oo Strays
R24
7 C6 rh
et} hy. Feedback
R32
| J b25 GATE
\
BAL
4
R26
D28 R33
EERE
\H
?
| R34
A
Se
502
STROBE
—50V
GENERATOR
0896-06
Fig. 3-1. Traveling wave Sampling Gate and Gate Bias circuit components, with input signal blow-by differentiating paths shown by
dotted lines.
are normally non-conducting and are therefore very small
capacitors. the differentiating networks eliminate all but
about 5% of the possible blow-by effects. The Blow-By cor-
rection amplifier feeds an inverted replica of the input signal
into the Sampling Gate output, and a small quantity of
charge passes through C19 to cancel most of the remaining
blow-by signal.
Traveling Wave Sampling Gate
Push-pull strobe pulses from the Strobe Generator cause
the six Sampling Gate diodes to conduct for about 125 ps.
While the diodes are conducting, the signal at the input con-
nector propagates down the diode transmission paths as
well as toward the 10X attenuator, R3. As the fast falling
strobe pulse end begins to propagate into the diode trans-

mission paths, the diodes are quickly switched off (into re-
verse bias), one set after another. First, diodes D2A and
D2F turn off, then D2B and D2E, and finally D2C and D2D.
Turn off time of the diode sets is controlled by the mechani-
cal spacing and thereby the propagation time between sets.
The Sampling Gate risetime is determined by these propa-
gation times, and is thus not heavily influenced by the strobe
pulse amplitude or duration. The sampling process is thus
one of trapping signal charge between sets of diodes, in
particular, between set D2A-D2F and the set D2B and D2E.
The risetime is very close to the double transit time between
sets.
Once the signal charge trapping process has taken place
at the strobe pulse end, the Preamplifier begins to receive
the trapped charge by conduction through resistors R2A
and R2F (See Fig. 3-2). The Preamplifier input capacitance
is much larger than the capacitance of the transmission line
segments between the diode sets, causing practically all of
the trapped charge to finally transfer to the Preamplifier in-
put. The transfer time constant is approximately 10
nanoseconds.
Because of the carefully adjusted balance of the plus and
minus strobe signals (set by the physical position of R51
and R53 in the Strobe Generator), the Preamplifier receives
only the trapped signal charge and nothing from the Strobe
Generator.
Preamplifier
The Preamplifier circuit amplifies and time-stretches the
error signal pulse from the Sampling Gate, and AC couples
it to the Post Amplifier in the associated plug-in unit.
Input transistor Q36 operates as a very high input imped-
ance, high-gain inverting amplifier. Temperature compensa-
tion for Q36 is accomplished by thermistor R38. A total of
4.4 mA channel current passes from the +50 volt supply
through R36, Q36, R38 and R40 to the —50 volt supply.
C39 assures that Q36 AC gain is high, while its DC gain is
less than 1.
Q43 and Q46 are connected as an operational amplifier
with a very low output impedance at Q46 emitter. The out-
put is coupled by C50 to the 90 2 input resistance of the
Post Amplifier in the sampling unit. R37 and R39 provide
temperature compensation to stabilize loop gain. D41 pro-
tects Q43 base from high negative voltage if Q46 is re-
moved from its socket.
Current in R42 with DC negative feedback by R48 sets
the output DC voltage level of the amplifier at about
7.3 volts. Negative AC feedback from the emitter of Q46
Circuit Description—S-4 Sampling Head
502
— Strobe
source
Le
502
+ Strobe
source
0896-07
Fig. 3-2. Error-signal source and path to Preamp shown by
dotted lines.
through C49 and Gain control R49 to Q43 base controls the
AC gain of the Preamplifier. Gain control R49 allows the AC
feedback to be adjusted, thereby adjusting the gain. The AC
gain is adjusted during calibration so that the sampling head
has the correct amplitude output signal to the sampling unit
Post Amplifier.
The Type S-4 sampling efficiency is less than 2%. With a
low sampling efficiency, the Preamplifier positive and nega-
tive signal output impedance must both be low. Q46 as-
sures a low output impedance for positive-going output
pulses, and D43 assures a low output impedance for nega-
tive-going output pulses. D43 conducts only for large nega-
tive output signals that fall at a rate faster than Q46 emitter
can follow. Q46 emitter remains a low output impedance for
small negative output signals.
Components not already mentioned above include: C47
and R46 that both prevent parasitic oscillations of Q46, and
C45-C46 that decouple the +15-volt supply for both low
and high frequency current pulses of Q46.
Blow-by and Trigger Pickoff
The Blow-by and Trigger Pickoff circuits consist of Q10,
Q14 and associated components connected as a common-

Circuit Description—S-4 Sampling Head
emitter paraphase amplifier. The primary purpose of the
circuit is to cancel unwanted high frequency capacitively-
coupled signals that bypass the Sampling Gate. The sec-
ondary purpose is to provide a trigger pickoff signal to the
sampling unit for internal triggering of the sampling sweep
unit.
Input signal to the Blow-by and Trigger Pickoff circuit is a
portion of the Type S-4 input signal. The signal is fed to the
base of Q10 through R3. R11 terminates the 50 Q imped-
ance of the input and assures no reflections back onto the
input circuit. The output from Q10 collector feeds an AC
coupled signal to the output side of the Sampling Gate, can-
celling the blow-by signal. The output from Q14 collector
feeds a DC coupled trigger pickoff signal to the sampling
unit for use by the sampling sweep unit.
Feedback Limiting
The Preamplifier input is the input terminal of an integrat-
ing pulse amplifier that includes the sampling unit Post Am-
3-6
plifier, AC Amplifier, Memory Gate and Memory Amplifier.
Feedback from the Memory Amplifier output is DC coupled
to the Type S-4 Preamplifier input through the sampling unit
Feedback Attenuator and the Type S-4 Gate Bal control.
As a sample is taken, the error signal is amplified and
converted to a DC signal for both the CRT and the feedback
that arrives through pins 2 and B of the sampler board. The
maximum feedback is limited to approximately + 1.2 volts
by two resistive dividers and D80 and D83. The limiting is to
prevent excessive feedback voltge from reaching the Sam-
pling
Gate
when
the
sampling
unit
Units/Div
switch
is
changed between positions. In certain conditions of opera-
tion,
the
excessive
feedback
can
cause
the
Sampling
Gate
to go into a mode of conduction that holds the CRT beam
off screen. By limiting the feedback amplitude, overdriven
displays will leave the CRT screen but will return just as
soon as the overdrive is removed. Without the limiting, an
overdrive signal could cause the display to disappear.
—

Section 4—S-4 Sampling Head
PERFORMANCE
CHECK
AND
CALIBRATION
This performance check and calibration procedure for the
Type S-4 Sampling Head requires the use of the test equip-
ment listed below. The test equipment specifications given
correctly calibrated.
are
the
minimum
acceptable
for
the
particular
use
of
each
Table 4-1
TEST EQUIPMENT REQUIRED
item. If other test equipment is substituted, it must meet or
exceed the stated requirements. All test equipment must be
Description
Requirements
Example
Oscilloscope
Mainframe
for
sampling
Plug-ins
7000
Series
Mainframe
Sampling
plug-in
Vertical
for
S-4
sampling
Head
7811
Sampling
Horizontal
Plug-in
for
use
with
sampling
Vertical
7711
Test
Oscilloscope
DC
to
30
MHz,
0.5
V/div
vertical
sensitivity
2213A
10X
probe
for
use
with
test
oscilloscope
P6120
Signal/Pulse Generator
Pulse Generator
Rigid Coax line, 3 mm connectors
GR874 to 3mm adapter plug
GR874 to 3 mm adapter jack
50 2 2X coaxial attenuator, GR874
connectors
Special variable attenuator with GR
connectors
bne 50 2 cable approximately 40
inches long.
bnc to bsm cable approximately 18
inches long
Special 3 foot extender cable for
sampling head
Pulse rise time 70 ps or or less, amplitude
approximately 20 mV into 50 Q. Trigger 180 mV
positive going at least 75 ns in advance of pulse
with risetime of <400 ps. Square wave outputs
of 1 us and 1 ns with 100 mV and 1.0 V
amplitude into 50 Q.
rise time 25 ps or less, amplitude =>200 mV into
50 2 with 180 mV positive going trigger at least
75 ns in advance of the fast pulse.
Test Procedures
Response not guaranteed
(2 required)
Tektronix Type 284
Tektronix Type S52
Tektronix Part No. 015-1015-00
015-1007-00
015-1008-00
017-0080-00
Tektronix Part No.
Tektronix Part No.
Tektronix Part No.
Tektronix Part No. 067-0511-00
Tektronix Part No. 012-0057-01
Tektronix Part No. 012-0127-00
Tektronix Part No. 012-0124-00

Performance
Check
and
Calibration—S-4
Sampling
Head
Table 4-1 (cont)
Description
Requirements
Example
Clip lead to bne adapter
DC Bridge
RMS line voltage meter
Test sampling oscilloscope
For measuring 50 2 +5 Volts maximum across
50 © resistor accurate within + 0.5%
+3% at input line voltage range
For checking strobe kickout.
Tektronix Part No. 013-0076-00
7000 series with 7T11, 7S11, anda
type S-2 sampling head.
PRELIMINARY PROCEDURE
4. Check the 50 Q DC Input Resistance. With the sam-
pling head separated from the sampling unit, use a DC Re-
sistance Bridge and measure the DC input resistance.
Connect one lead to the input connector outer conductor,
and the other lead to the center conductor. Be sure the
bridge does not apply more than +5 volts to the input
terminals.
The Type S-4 input resistance must be 50 Q, +5%, for a
resistance reading of 47.5 2 to 52.5 Q.
2. Setting up the equipment.
a. Assembly the indicator oscilloscope system. Place the
type 7S11 into the right vertical compartment of the indi-
cator oscilloscope, and the Type 7T11 into the horizontal
compartment. Install an operating sampling head into the
Type 7S11 compartment.
b. Connect the RMS line-voltage meter to the power
mains. Determine that the oscilloscope (and other equip-
ment) input voltage selector is set for the correct value of
line voltage. Connect all the equipment to the proper power
outlet and turn on the power.
c. Obtain a free-running trace on the indicator oscillo-
scope and let the equipment warm up for five minutes. After
the warm-up period, adjust the Trace Alignment control so
the free-run trace is parallel to the graticule lines.
d. Connect the Type 284 Square Wave Output connec-
tor to the sampling head input. Use a 5 ns signal delay 50 2
coaxial cable with GR 874 connectors. Connect the Type
284
Trigger Output connector to the sampling sweep unit
External Trigger input (50 2) connector, using a BNC 50 Q
coaxial cable.
4.2
Set the Type 284 Period at 100 ns Square Wave and the
Amplitude at 1 volt. Using the Type 284 as both a time and
amplitude reference adjust both the vertical and the horizon-
tal unit screwdriver adjustable Gain controls for proper de-
flection factor of each unit. Remove sampling head from the
7$11.
e. Install the Type S-4 onto the special three foot ex-
tender cable. Connect the other end of the cable into the
7S11 sampling head compartment. Allow a five minute
warm up of the system.
NOTE
The case should be in place unless access to internal
controls is necessary.
f. Set the controls as listed following Fig. 4-1.
Sampling Unit (Both channels)
Dot Response Midrange
Unit/Div 100
Variable Cal
Up Pushed in
Pasition Midrange
DC Offset 0 volts at Offset Out
Sampling Sweep Unit
Scan Rep
Sequential Pushed in
Time/Div 50 ns
Swp Range 0.5 us
Variable Cal
Time Position controls
Trig Sensitivity
Fully clockwise
Stable Display
Trigger Polarity cf
Trigger Source Ext
Scan Mid-range