Tektronix S-2 User manual

INSTRUCTION
MIAN UAL Serial Number
Tektronix, Inc.
S.W. Millikan Way @ P. O. Box 500 @ Beaverton, Oregon 97005 @ Phone [phone]
070-0764-00
@ Cables: Tektronix
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mVOLTS/DIV
TYPE S-2
SAMPLING HEAD
Fig. 1-1. Type 5-2 Sampling Head.

Type S-2
SECTION 1
TYPE S-2 SPECIFICATION
Change information, if any, affecting this section will be found at the
rear of the manual.
General Information
The Type S-2 Sampling Head is a 50Q input, 50 ps rise-
time, plug-in preamplifier for use in some Tektronix 3S-series
sampling units, usch as the Type 352 Sampling Unit. Input
characteristics of the sampling system are controlled by the
type of sampling head in use.
The Type S-2 can be installed directly into the Type 352,
or used remotely on an optional 3 foot or 6 foot extender
cable. Vertical deflection factor of the sampling system is
labeled at the top of the Type S-2 as mVOLTS/DIV; the label
refers to the sampling unit Units/Div switch that is directly
above the head.
A portion of the input signal is provided to the Type 382
Sampling Unit for externally triggering a Type 372 Random
Sampling Sweep. The Type 3S2 allows selection of the trigger
pickoff signal from the Channel A or Channel B sampling
head. The selected trigger pickoff signal is returned to the
Type 352 front panel at the Trig Out connector. The trigger
pickoff signal is useful with any sampling sweep unit for
repetitive signals where the triggering event does not have
to be displayed. (Only the Type 312 random process sampling
allows the triggering event to be displayed when using the
sampling head trigger pickoff signal.
Digital Unit Program Connections
The Type S-2 contains connections at ifs rear that program
both the decimal and the units lamps of a Type 230 Digital
Unit. The S-1 digital control connections pass through the
Units/Div switch of either the Type 355 or Type 386 Program-
mable Sampling Unit, and through the Type 568 Oscilloscope
to the Type 230 Digital Unit.
ELECTRICAL CHARACTERISTICS
The following characteristics apply over an ambient tem-
perature range of 0°C to +50°C. These characteristics
apply only after the Type S-2 has been properly mated to the
sampling unit and indicator oscilloscope, and after suf-
ficient warmup time. For the particular system warmup re-
quirements, refer to the related amplifier and indicator oscillo-
scope instruction manual. (A 5 minute warmup time is required
for Type 382 — 3T2). A procedure for mating the Type S-2 to
the vertical amplifier can be found in the Operating Instruc-
tions section of this manual.
ELECTRICAL CHARACTERISTICS
Characteristics
Performance Requirement
Supplemental Information
Input Resistance 50 within 1%
Measured at DC
Dot Response
Adjustable in the sampling unit to unity
for signals up to 200 mV P-P; when the plus
dot response is adjusted to unity, the minus
dot response will be within 5% of unity
and vice versa.
Operating Input Voltage Range
+1V to —1V with <1 V P-P signals. Step
signals greater than 200 mV require more
than one dot to display 100% of the step
transition.
Maximum Input Voltage
+5 VDC or sine wave of 10 V P-P up to
10 MHz.
Transient Response
Risetime
50 ps or less, 10% to 90%.
Pulse Flatness Deviation
< +5% and —5% in the first 2.5 ns after
the step pulse reaches 50%.
< + and —2% after the first 2.5 ns.
Applies when the step pulse is Tektronix
Type 284 Pulse Output signal through a
20 cm airline.
Displayed Noise 10 mV or less, tangential noise.
Trace Baseline Vertical Shift with
Trigger Repetition Rate Change.
10 mV or less for trigger rate change from
30 Hz to 50 kHz.
1-1

NOTES

Type S-2
SECTION 2
OPERATING INSTRUCTIONS
Change information, if any, affecting this section will be found at the
rear of the manual.
General Information
This section provides the basic information required for
operation of the Type S-2 Sampling Head, including installa-
tion and First Time Operation.
The Type S-2 completes the circuitry of some Tektronix
sampling units, and controls the input characteristics of the
vertical channel by providing the Strobe Generator, Sampling
Bridge, and Preamplifier circuits. Such a sampling unit is
the Type 352, using one or two S-series sampling heads. A
Type 382 and a sampling sweep unit plugged into an indi-
cator oscilloscope completes the sampling system. A choice
of real or equivalent time sampling is provided by the type
of the sweep unit used,
A 502 input resistance allows the use of 509 attenuators
and probes for operation over a wide range of signal volt-
ages.
A portion of the input signal is provided to the sampling
unit. In the Type 382, this signal is selected from Channel A
or B, and returned to the front panel Trig Out connector
for external triggering or sampling sweep units.
Installing the Type S-2 Sampling Head
Fig. 2-1 shows the Type S-2 partially installed into a Type
352 Sampling Unit with Type 561A Oscilloscope and Type
3T2 Random Sampling Sweep. The sampling head {or heads)
can be plugged into the sampling unit as shown, or used
remotely on a special extender cable. Three and six foot
extender cables are available, Order the three foot extender
cable by Tektronix Part No. 012-0124-00, or the six foot
extender cable by Tektronix Part No, 012-0125-00. Contact
your local Tektronix Field Engineer or Representative for
price and availability of these optional accessories.
To insert the Type S-2 into the right or left hand compart-
ments of the sampling unit, proceed as follows:
1. Pull the latch knob (Fig. 2-1} outward from the front
panel {the latch knob will push out normally when the unit
is inserted if the knob is left free to move).
2. Insert the Type S-2 slowly into the compartment, so the
two plastic guides in the compartment engage the S-2,
3. Push the Type S-2 completely into the compartment.
4. Push the latch knob to lock the S-2 in place.
To remove the Type S-2 from the compartment, pull the
latch knob away from the front panel, then pull the unit
from the compartment.
Type $-2
Sampling Head
Latch knob
Fig. 2-1. Installation information.
To use the Type S-2 on an extender cable, install as
follows:
1. Pull the latch knob located on the head end of the
extender cable outward from the panel (the latch knob
will push out normally when the extender is inserted if the
knob is free to move).
2. Insert the extender cable head end slowly into the
desired compartment in the sampling unit so the two plastic
guides in the compartment engage the unit.
3. Push the head completely into the compartment.
4, Push the latch knob to lock the extender cable head
end in place.
5. Connect the Type S-2 to the other end of the extender
cable in a similar manner, and set the latch knob to hold
it in place.
6. To remove the Type S-2 from the extender cable, pull
the latch knob on the front panel of the Type S-2, and remove
the unit from the extender cable,
7. To remove the extender cable head from the sampling
unit compartment, pull the latch knob outward from the
front of the panel, then pull the unit free.
2-1

Operating Instructions—Type S-2
Mating
The vertical deflection factor labeled by the Type S-2, and
the sampling system loop balance (adjusted by the Bridge
Bal control) must be considered when mating a Type S-2
with a sampling unit.
The mVOLTS/DIV label on the Type S-2 names the deflec-
tion factor units of the sampling unit Units/Div switch
located directly above the label. For example, with the Type
S-1 installed in a Type 382 and the Units/Div switch set at
100, each major division of CRT deflection corresponds to
100 millivolts of input signal (when the Variable control is
in the Cal position). The deflection factor is mated to the
indicator oscilloscope CRT by adjusting the sampling unit
Gain control while the Type S-2 is in Channel B. Refer to the
sampling unit manual for its Gain adjustment. Power supplies
and circuit interconnections are made through the two con-
nectors at the rear of the Type S-2.
Bridge Balance
The Bridge Balance control of the Type S-2 should be
adjusted whenever the sampling head is changed from one
sampling unit to another, or is operated on an extender
cable. Location of the control and details of its adjustment
are described in First Time Operation which follows.
FIRST TIME OPERATION
This First Time Operation procedure utilizes, in addition to
the Type S-2, a Type 382, Type 561A, and Type 284. The
Type 3T2 Random Sampling Sweep unit is recommended for
use with the Type S-2 because random sampling permits
viewing the signal in advance of the triggering event. Such
trigger lead-time is required by the Type S-2 with the Type
382 to view fast pulses without a pre-trigger. The Type 284
is used as a signal source.
Setup Information
1. With the Type 561A Power switch off, insert a Tektronix
Type 382 Sampling Unit into the vertical compartment (left)
and a Tektronix Type 3T2 Random Sampling Sweep into the
horizontal plug-in compartment (right).
2. Insert the Type S-2 Sampling Head info Channel A
compartment (left) in the Type 382 leaving the latch knob
free to move. Once the S-1 is seated, push the latch to lock
it in place.
3. Set the Intensity control on the Type 561A fully counter-
clockwise.
4, Connect the Type 561A to a power source which meets
its voltage and frequency requirements.
5. Set the Power switch to on. Allow about 5 minutes
warm-up so the units reach operating temperature before
proceeding.
6. For single-trace operation, set the controls as follows:
2-2
Type 352
Display Mode CH A
Normal-Smooth Normal
A and B Position Midrange
DC Offset Midrange (5 turns
(both Channels} from one end}
Units /Div 100
(both Channels}
Variable (both channels} Cal
Invert (both Channels} Push in
Dot Response Midrange
(both Channels)
B Delay Midrange
Type 312
Horiz Position Midrange
Samples/Div 9 o'clock position
Display Mode Normal
Start Point With Trigger
Sweep Rate 200 ns/Div
Range 10 ps
Display Mag XI
Time Magnifier x5
Variable Cal
Time Position Both fully clockwise
Trig Sensitivity Fully clockwise
Recovery Time Optional
Trigger Polarity +
Trigger Source Ext
Type 284
Square Wave Amplitude 1.0 Vt
Period 1 ps ‘
Mode Square Wave Output
Lead Time Optional
7. Connect the square wave output signal from the Type
284 to the input connector of the Type S-2 through a 50Q
coaxial cable and a X5 attenuator. Connect the trigger
output signal from the Type 284 through a coaxial cable to
the External Input 50Q connector on the Type 372.
8. Advance the Type 561A Intensity control until the free
running trace brilliance is at the desired viewing level.
9. Center the trace on the graticule with the Type 352 A
Position control and/or the DC Offset control. Adjust the
Type 312 Trig Sensitivity control for a stable triggered dis-
play of a two-cycle square wave with an amplitude of
about 2 divisions.
‘Used through <5 attenuator for 0.2 V into the Type §-2.

Adjusting Dot Response
Sequential Sampling
One method of adjusting the Dot Response to unity is to
use a double or multiple-triggered sweep. This causes each
sample to respond te the full signal amplitude and produce
a display similar to Fig. 2-2A or B.
10. To obtain a display similar to that of Fig. 2-2A or B,
turn the Type 312 Trig Sensitivity control clockwise into the
free run region. Then adjust the Recovery Time control until
the desired display appears.
11, Turn the Channel A Dot Response control to obtain
a display showing unity dot response (see Fig. 2-2B). Fig,
2-2A shows greater than unity dot response. This requires
counter-clockwise rotation of the Dot Response control to
obtain unity dot response. The double triggering shown in
Fig. 2-2A and B is useful to adjust the dot response to unity,
but should not be used in making measurements,
12, After the dot response is adjusted to unity, turn the
Trig Sensitivity control counterclockwise into the triggered
region for a stable trace of a properly triggered display
similar to Fig, 2-2C,
Random Sampling
13, Another convenient method of adjusting the dot re
sponse to unity can be shown with random process sampling.
Change the following controls:
Type 3172
Start Point Before Trigger
Time Magnifier < 50
Type 284
Period 100 ns
14. A dot response greater than unity will produce a dis
play similar to Fig. 2-3A. Dot response deviations from unity
are best observed when the Type 372 Time Magnifier is set
at X20 or 50. Adjust the Type 382 Dot Response control
for a unity dot response display similar to Fig. 2-38.
Bridge Balance Adjustment
15. Connect a bench multimeter set for = 30 V full scale
between ground and the Type 382 Channel A Offset jack
Adjust the Type 382 DC Offset contro! until the multimeter
reads 0 Volts. Increase the meter sensitvity and repeat the
adjustment for greater accuracy,
16. Disconnect the coaxial cable from the Type 284, leaving
the other end of the cable connected to the Type S-2 input
connector. Turn the Type 352 Trig Sensitivity control clock-
wise to free run the trace
NOTE
Operating the sampling head without the input
connector terminated by a 501 resistor or coaxial
cable will cause a vertical shift of the zero signal
baseline by a few millivolts. This*is caused by the
Operating Instructions—Type $-2
(Al Double-triggered,
{C) Properly triggered
Fig. 2-2. Typical display for unity dot response adjustment in normal
sequential sampling
2-3

Operating Instructions—Type 5S-2
100 mV/Div
Bridge Bal
Adjustment
Fig, 2-3. Typical display for unity dot response adjustment in random
tampling operation,
strobe kickout signal being reflected from the open
input connector, and arriving back at the sampling
bridge while the bridge is conducting, Because of
this phenomena, set the display zero reference
point with the input circuit terminated, Also use
at least 20 cm airline between the Type S-2 input
and any tunnel-diode pulse generator or circuit
that is sensitive to signals entering its output
circuit.
17. Switch the Type 382 Units/Div control throughout its
range, adjusting the Type S-2 Bridge Bal control for a trace
shift of not over one division as the Units/Div control is
switched from one end to the other, See Fig. 2-4 for the loca-
tion of the Bridge Bal control. This adjustment must be made
whenever the Type S-2 is shifted from one sampling unit to
another, or operated on a sampling head extender cable.
GENERAL OPERATING INFORMATION
Input Signal Connection Precautions
Accurate displays of signals containing very fast transi-
tions or very high frequencies are possible only when the
2-4
Fig, 2-4, Bridge Bal Location.
coupling circuit does not distort the signal. The Type S-2
nput resistance is 502 requiring the use of high quality
509 coaxial cables and attenuators between the signal
source and the sampling head input connector, Tektronix,
Inc, offers several special probes that permit the signal source
oading to be less than that caused by the 502 input resis
tance. See your Tektronix Field Engineer or Representative
about such special coupling devices
Input Voltage Considerations
Proper displays of various signal amplitudes require special
attention to the type of time base generation in use. There
are also input voltage limits that must be observed to prevent
either display distortion or damage to the Type S-2,
For greater details on coupling of high frequency input
signals, and on precautions to be observed, refer to the
A
sampling unit Instruction Manual supplied with your
sampling instruments
Maximum input signal voltage is an important consideraion
in any sampling system. Signals greater than +1 or —1 volt
generally will not permit a valid display. Thus any signals
of greater amplitude should be attenuated in some manner
before being applied to the Type S-2 input connector, Maxi-
mum input voltage without damage to the input termination
resistor is +5 or volts DC, or 10 volts peak to peak
sine wave signals in the frequency domain of 10 MHz or
below
Where unity dot response 1s required, accurate displays
are possible to achieve only when the input amplitude and
the time base generation mode are both considered. The two
time b generation modes are: (1) normal process sampling,
; the CRT, and
; sampling where the dots are not neces-
where each dot is displayed in sequence acro
(2) random process
sarily displayed in a@ sequence that progresses uniformly
across the CRT. [The Type 3T2 Random Sampling Sweep

unit presents random process sampling displays in a con-
trolled sequence very similar to normal process sampling
when the Time Magnifier switch is at 1 or X2 and the
trigger signal repetition rate is uniform.) The random process
dot sequence is definitely not sequential when the Time
Magnifier switch is anywhere between 5 and X50, with
the dot presentation completely random at 50. The Type
3T2 can be operated in a normal process mode, and all
other Tektronix sampling sweep units generate only a
normal process sequenial dot display.
Unity dot response displays are not usually required when
the signal rate of rise and the sweep rate selected provide a
large number of dots for any vertical display change. Unity
dot response is required for some random process ampling
displays, and for normal process displays where there are
either no samples, or less than about 6 samples in a step
dot response is required for some random process sampling
transition. (Many dots in any vertical change permits valid
random process displays without unity dot response when
the Type 3T2 Time Magnifier is at X1 or X2)}
In summary, unity dot response exists when the vertical
channel {sampling head and sampling unit) will accurately
shift a sample dot 100% of the signal amplitude. The Speci-
fication Section lists the dot response linearity limits as =5%
for certain limited signal amplitudes discussed below. A 5%
dot response linearity means that the vertical channel will
shift a dot in the positive direction 100% of the full signal
amplitude, but minus transitions may shift a dot between
—95% and —105% of the full signal amplitude, and vice
versa. (The Dot Response control thus allows either the dis-
play top or bottom to be adjusted to unity dot response for
close examination of signa! detail.) Unity dot response is
obtained by proper adjustment of the sampling unit Dot
Response control as described in First Time Operation.
Input signal amplitude limits for unity dot response within
the 5% linearity tolerance are listed in Table 2-1. The input
voltage columns are for signal amplitudes of 0.2 V peak to
peak and 1.0V peak to peak anywhere between +1 V and
—1V DC.
TABLE 2-1
Input Signal Amplitude Limits Allowing Unity Dot
Response Vs Sampling Unit mVOLTS/DIV Deflection
Factor.
Sampling Unit Input Voltage
Units/Div Switch 0.2V P-P [| 1.0 V P-P
All positions 200 to yes | no
2 Either Normal or Random Process
Limited to positions yes yes
of 20 to 2 Either Normal or Random Process
All positions 200 to yes | yes
2 Normal Process only with >6 dots
ina 100% vertical transition.
Triggering from the Signal
The Type S-2 provides a Trigger pickoff signal to the
sampling unit. This signal is a portion of the input signal.
Operating Instructions—Type S-2
\
|
In the Type 352, the signal is selected from the Channel
A or Channel B sampling head and returned to the front
panel Trig Out connector by the trigger circuit. The ampli-
tude is about 10% of the input signal into 509. A small
amount of the sampling bridge strobe signal is present with
the trigger signal.
A Type 382 standard accessory trigger coupling cable
can be used between the Type 382 and Type 3T2 for trigger-
ing the sampling sweep unit from the sampling head pickoff
signal. Connect the cable between the Type 382 Trig Out
connector and the Type 3T2 500 Trigger Input connector.
Operate the Type 3T2 as if externally triggered. Repetitive
sine or square wave displays are best viewed when oepra-
ting the Type 3T2 Start Point switch at With Trigger (normal
process sequential dot sampling).
Fast pulse signals require random process sampling with
the Type 3T2 Start Point switch at Before Trigger. This mode
of operation will permit the sampling bridge strobe signal
portion of the trigger pickoff signal to sometimes trigger
the timing unit. When this occurs, it is impossible to obtain
a meaningful display. Signal amplitude to the Type S-2
must be greater than about 300 mV peak to peak in order
to obtain valid displays of fast pulses during random process
sampling. It is usually more satisfactory to obtain some
other form of pretriggering for the sampling sweep unit
when it is necessary to view fast signals.
Measuring Risetime
The Type S-2 is useful to show 10% to 90% risetime of
signals in circuits that are properly coupled to the input.
The risetime of both the generator (of source}, and the Type
S-2 must be taken into consideration.
Signals with risetimes (at the input connector} as fast as
about 150 picoseconds can be measured using the Type S-2
without any special considerations. Read the 10% to-90%
risetime directly from the CRT (or digital unit) to know the
risetime of the signal.
Signals with risetimes faster than about 150 ps can still be
measured using the Tektronix Type S-2 Sampling Head. How-
ever, only approximations of risetime can be made by cal-
culating the signal risetime using formula (1} below. Formula
(1) is based upon both the signal and the sampler having
Gaussian risetime characteristics. The Type S-2 risetime is
not Gaussian. Signals with risetimes in the order of 45 ps will
cause the Type S-2 display to show a composit risetime in
the order of 73 ps. Thus, it is recommended that displays
with risetimes faster than about 150 ps not be used to accu-
rately measure the signal 10% to 90% rate of rise. The
Performance Check and Recalibration Procedure gives equip-
ment, procedure and displays of Type S-1 and Type S-2
risetime when driven by a 50 ps risetime tunnel diode pulser
signal.
Formula (1) T, (Displayed) =\isignal T,.}? + Type S-2 T,}?
transposing:
Signal T, —\{Signal T,)? + Type S-2 T,)?
2-5

NOTES

Pa
Type
S-2
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

Circuit Description—Type S-2
—¢ Transmissi Lines
7
2
To Sampling
Bridge
nih t- N53
A A
Snap-off
Current
Strobe
Clipping Lines
Avalanche
Volts
175
From
Strobe
_ Driver
2T
Fig. 3-1. Strobe Generator circuits.
The Strobe Drive pulse is transformer-coupled by T75 to
the base and emitter of the Avanianche transistor Q69. Two
outputs are AC-coupled from Qé69, one from the collector and
the other from the emitter. The Avalanche Volts control
adjusts the collector voltage of the 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. Q69
current path is shown by a dashed line in Fig. 3-1. Before
Avalanche conduction, there is a potential of about 60 volts
between collector and emitter.
The negative Strobe Drive pulse is transformer-coupled
by T75 to the emitter and the base of Qé69, forward biasing
Q69. Normal avalanche action follows with the collector
going negative and the emitter going positive. This fast-rise
push-pull signal is capacitively coupled to the Snap-off Diode
circuit.
Snap-off circuit. The Snap-off circuit operates as a cur-
rent switching circuit to apply some of the push-pull Ava-
lanche signals at snap-off time to the Sampling Bridge.
3-2
The circuit consists of a Snap-off Current contro! R57, Q55,
Snap-off diode Dé1, two clipping lines and associated com-
ponents. Between drive pulses from the Avalanche circuit,
the Snap-off diode Dé61 is forward-biased by the current in
Q@55. The current value is set by the Snap-off Current control
R57. The current in D6] is typically 20mA, and the current
path is shown as a dashed line in Fig. 3-1. This heavy for-
ward current assures that Dé! has many carriers within its
junction region.
The push-pull signals from the Avalanche circuit cause
Dé} junction carriers to reverse direction as a heavy reverse
current. This heavy reverse current stops suddenly as all the
carriers clear out of the junction. As the reverse current
“snaps” to a stop, the push-pull avalanche signals are sud-
denly coupled toward the clipping lines and the Sampling
Bridge by C53 and C51. The fast-rise step which appears at
each clipping line input is propagated down the line. A
finite period of time later the steps reach the short circuit ends
of each clipping line. The step is then reflected, equal in
amplitude and opposite in polarity, back fo the transmission
line. This cancels the signals moving toward the Sampling
®

Circuit Description—Type S-2
To Blow-by &
Trigger Takeoff
+50V
’
t
R20!
From
Blow-by
OT
'
circuit
From DC
BRIDGE R21 Offset &
— C14
!
I
'
t
! Feedback
! @ Attenuator
I
t
|
{
!
{
VOLTS \
I
|
{
1
|
|
|
1
!
i
|
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4
R7A
R7B aN
NS
Strobe Pulses
ad @——— To Q31 in
Preamplifier
Fig. 3-2. Sampling Bridge circuit and Bridge Volts and Bridge Bal circuits.
Bridge. This action results in a positive Strobe pulse being
delivered to P51, and a negative Strobe pulse being deliv-
ered to P53.
Sampling Bridge
The Sampling Bridge consists of D5, D6, R5, R6, R7A, R8A,
C5, C6, C7, C8, and C28. See Fig. 3-2. The Sampling diodes
allow no connection between the input connector and the
Preamplifier input (other than the normal stray capacitance
of the bridge) except when driven into conduction by the
Strobe pulses from the Strobe Generator. During D5 and
Dé conduction time of about 45 to 50 ps the bridge delivers
a portion of the voltage difference between the input signal
and the bridge output signal to the Preamplifier input.
The bridge input circuit is terminated in 509. The 502
consists of R4 with the series value of R3 and R10 in parallel.
(R10 is shown in Fig. 3-3). R3 delivers a small portion of the
input signal to the Blow-by and Trigger Pickoff circuit.
The bridge output drives the high input impedance Q31
gate in the Preamplifier.
Reverse-bias voltage for the sampling bridge diodes is
developed across R21-R22-R23 in parallel with R26, the Bridge
Volts control. R26 allows the reverse-bias voltage adjustment.
A higher reverse voltage allows D5 and Dé to be turned on
@
for a shorter period of time. A lower voltage gives a longer
conduction time. During calibration the voltage is usually
set nearly to maximum.
The Bridge Bal potentiometer is adjusted to compensate
for diode, strobe, and other system unbalance signals. The
combined memory feedback and DC Offset voltage from the
sampling unit is fed through the bridge volts circuit to the
output of the Sampling Bridge.
During the sampling time, the strobe pulses forward bias
D5 and Dé. By normal bridge function, the conduction of
D5 and Dé charges or discharges C5, C6, C7 and C8. The
voltage charge on these capacitors changes about 21/,% of
the difference between the Feedback and DC offset voltage
and the incoming signal voltage. This voltage change, called
the error signal, is amplified in the Preamplifier.
Preamplifier
The Preamplifier circuit (see Fig. 3-3 amplifies and time-
stretches the error signal pulse from the Sampling Bridge,
and AC couples it to the Post Amplifier in the associated
plug-in unit.
The input transistor Q31 operates as a very high input
impedance high-gain inverting amplifier. Temperature com-
pensation for Q31 is accomplished by thermistor R33 in
parallel with R34. The 4mA current path is from the +50
3-3

Circuit Description—Type S-2
Y
a
a
k— FET Input
> ant
From Sampling NX
Operational Amplifier >
To Post
Amplifier
(Vertical
> Sampling
Plug-in Unit)
(90.9 2)
Bridge ! b4]:
C34
R33 [ T 1 R34
I
|
'
{
1 R35
'
l
—50V
Fig. 3-3. Preamplifier circuit.
volt supply through R31, Q31, the parallel combination of
R33-R34
and
R35
to
the
—50
volt
supply.
C34
assures
that
Q31 AC gain is high, while its DC gain is less than 1.
Q41 and Q45 are connected as an operational amplifier
with a very low output impedance at @45 emitter. The out-
put
is
coupled
by
C49
to
the
90Q
input
resistance
Post
Amplifier in the sampling unit. @45 current paths are shown
by
dashed
lines
in
Fig.
3-3.
D41
protects
Q41
base
from
high
negative
voltage
if
Q45
is
removed
from
its
socket.
Current
in
R40
with
DC
negative
feedback
by
R45
sets
the
output
DC
voltage
level
of
the
amplifier
at
about
7.3
volts.
Negative
AC
feedback
from
the
emitter
of
@45
through
C46
and
Gain
control
R46
to
Q41
base
controls
the
AC
gain
of
the
Preamplifier.
The
Gain
control
R46
allows
the
AC
feedback
to
be
adjusted,
thereby
adjusting
the
gain.
The
AC
gain
of
the
Preamplifier
is
usually
adjusted
to
about
16
de-
pending
upon
the
sampling
efficiency.
For
example:
For
an
input
signal
into
the
Type
S-2
input
connector
of
200
mV,
if
the
display
dot
change
is
100%
of
the
signal,
Q45
output
signal
will
be
about
80
mV.
A
positive
200
mV
input
signal
will
produce
a
+5mV
signal
at
Q31
gate
(or
a
2.5%
sampling
efficiency),
and
a
positive
80mV
output
signal.
Likewise
a
negative
input
signal
will
produce
a
negative
out-
put
signal,
3-4
Blow-by and Trigger Pickoff
The
Blow-by
and Trigger
Pickoff
circuits
consist
of
Q13,
Q17
and
associated
components
connected
as
a
common-
emitter
paraphase
amplifier.
See
Fig.
3-4.
The
primary
pur-
pose
of
the
circuit
is
to
cancel
unwanted
high
frequency
capacitively-coupled
signals
that
bypass
the
Sampling
Bridge.
The
secondary
purpose
is
to
provide
a
trigger
pickoff
signal
to
the
sampling
unit for
external
triggering
of
the
sampling
sweep
unit.
Input
signal
to
the
Blow-by
and
Trigger
Pickoff
circuit
is
a
portion
of
the
Type
S-2
input
signal.
The
signal
is
fed
to
the
base
of
Q13
through
R3.
R10
terminates
the
50.0
impedance
of
J3-P3
coaxial
connector
and
assures
no
reflections
back
into
the
input
circuit.
The
output
from
Q13
collector
feeds
an
AC
coupled
signal
to
the
output
side
of
the
Sampling
Bridge,
cancelling
the
blow-by
signal.
The
output
from
Q17
collector
feeds
a
DC
coupled
trigger
pickoff
signal
to
the
sampling
unit
trigger
amplifier.
To
trace
the
Blow-by
signal
path,
a
portion
of
the
Type
S-2
input
signal
is
coupled
to
the
base
of
Q13
and
inverted
at
the
collector
of
Q13.
The
inverted
signal
amplitude
is
adjusted
by
R13,
the
Transient
Response
control,
and
fed
through
R14
and
C14
to
the
output
side
of
the
Sampling
Bridge.
This
signal
is
out
of
phase
with
the
input
signal,
cancelling
the
®

_ Circuit Description—Type S-2
4
+15V
| P17
C-<— Trigger Take-off
.
Ferrite
Transient
>
to
Sampling
Unit
—
Bead
\
Response
|
b
il
_/
L17
WwW"
“+
Ferrite
iT
Bead
Qi3 WN
1h
R12
R10
>= C14
NAN
y- |
From —12.2V .
SIGNAL To Sampling
INPUT Gate ;
Circuit {output side)
Fig. 3-4. Blow-by and Trigger Take-off circuits.
induced displacement current of the capacitance shunting the _ the input signal coupled from Q13 emitter to Q17 emitter to
Sampling Bridge. provide an in-phase signal at QI7 collector to the Trigger
Amplifier and selector circuits in the sampling unit. Q17
The trigger pickoff signal path starts with a portion of collector load is provided in the sampling unit.
® 3-5

NOTES

y . Type S-2
SECTION 4
MAINTENANCE
Change information, if any, affecting this section will be found at the
rear of the manual.
Introduction
This section of the manual contains information for use in
corrective maintenance or troubleshooting of the Type S-2.
To remove the Type S-2 from its case, loosen the four
retaining screws on the back. Then slide the back off, and
remove the body by sliding it to the rear. Directions for
replacing the case on the sampling head will be found at the
end of this section.
Parts Replacement
All parts used in the Type S-2 can be purchased directly
through your Tektronix Field Office or Representative. How-
ever, replacements for standard electronic items can generally
be obtained locally in less time than is required to obtain
them from Tektronix. Replacements for the special parts used
in the assembly of the Type S-2 should be ordered from Tek-
tronix since these parts are either manufactured or selected
by Tektronix to satisfy a particular requirement. Before pur-
chasing or ordering, consult the Electrical or Mechanical Parts
List to determine the value, tolerance and rating required.
NOTE
When selecting the replacement parts, if is import-
ant to remember that the physical size and shape
of a component may affect its performance at high
frequencies. Parts orientation and lead dress
should duplicate those of the original part since
many of the components are mounted in a parti-
cular way to reduce or control stray capacitance
and inductance. After repair, the sampling head
may require recalibration.
Transistor Replacement. Cut the leads of a replacement
transistor to the same length as the transistor removed and
bend the leads as necessary. The dead configuration of the
transistors used in the Type S-2 are shown in Fig. 4-1. Two
transistors, Q13 and QI7, have soldered leads and are
soldered to the circuit board. The field-effect transistor
mounting has an arrow at the top position,
Leadless Capacitors. There are leadless ceramic capaci-
tors soldered directly to the circuit boards. Care must be taken
when replacing these capacitors as they are easy to crack.
The Type of solder used must be quality, with good cold-
flow characteristics. Thus, do not use 50/50 solder, but
60/40 or 62/38 solder when replacing the leadless capacitors.
Best results will be obained by applying heat from the
soldering iron directly under the leadless capacitor on the
opposite side of the board. Usually a plated-through hole
is under the leadless capacitor, allowing solder to conduct
heat through the board. Without plated-through holes, the
capacitor may be soldered into place by positioning the part,
then applying heat to the adjacent plated area.
Emitt Emitter
mitter
Collector
Collector
Base
Flat
side
Plastic-Case Transistors
Emitter Source
Base Gate Drain
Case
Tab
Collector
Field-effect (Q31)
151-1012-00
Metal-Case
Fig. 4-1. Lead configuration for board-mounted transistors,
@®

Maintenance—Type $-2
Use only enough solder to obtain a good full-flow joint.
Excess solder on either side of the capacitor can lead to a
shorted circuit.
Removal and Replacement of Snap-off Diodes, The
Snap-off diode is mounted in small metal clips, as shown on
the circuit board illustration. The diode is best removed or
replaced with a pair of shaped plastic tweezers, such as
Tektronix Part No, 006-0765-00, or equivalent.
Replacement of Sampling Bridge Diodes. The Sampling
Bridge diodes may be removed or replaced on the Sampler
Board through the top of the unit. Fig. 4-2 shows a method
for the removal or replacement of the white plastic holder in
which the diodes are mounted,
CAUTION
The diodes are very fragile and it is possible to
break one or both if the white plastic holder is
removed or replaced too rapidly. Damage is
caused when the holder ends snap together as they
are moved from the circuit board,
Fig, 4-2, Removal or replacement of sampling bridge diodes.
The position of the plastic diode holder assembly is shown
in Fig. 4-4 with the shields removed; it is not necessary to
remove these shields to replace the sampling diode assembly.
For removal or replacement use Xcelite Forceps 42H, Tek-
tronix Part No, 003-0347-00, or similar tool.
The hook-like ends of the diode holaer normally rest in a
circuit board hole and firmly hold the diodes to the pleted
area. Cautious removal, especially just as the holder clears
the circuit board, is necessary to minimize the physical shock
and possible damage to the diodes, as the hook-like ends
may snap or spring together. Replacement of the holder
should also be by slow, cautious movement; first as the hook-
4-2
like ends reach the plated area which increases the thickness
and second as hey reach the hole where he possibility of
physical
shock
exists
as
in
the
removal.
A
metal
shield
pre-
vents the holder being inserted too far.
Circuit Board Replacement. |f a circuit board is damaged
and cannot be repaired, the entire assembly including all
soldered-on-components
should
be
replaced.
The
part
number
given
in
the
Mechanical
Parts
List
is
for
the
completely
wired
board.
The
Preamp
Board
and
the
Strobe
Board
are
removed
by
gently
pulling
outward
from
the
Sampler
Board.
To
replace
the
boards,
align
the
connectors
and
pin
contacts
and
ease
the
boards
into
position.
Pin
connectors
should
not
protrude
beyond
the
clamps,
The Sampler Board is removed as follows:
1.
Remove
both
the
Preamp
and
the
Strobe
Board,
2,
Remove
the
12
sided
nut
(nutdriver,
special,
Tektronix
Part
No,
003-0459-00)
which
secures
the
GR
connector
to
the
front
panel,
3,
Slide
the
Sampler
Board
and
the
GR
connector
away
from
the
front
panel,
To remove the Shields and Input Connector:
1,
Loosen
the
threaded
nut
by
turning
it
counterclockwise
with
the
special
Tektronix
tool,
Part
No,
003-0607-00,
Do
not
remove
the
threaded
nut,
but
back
it
off
just
enough
to
free
the
notched
plate
(see
Fig,
4-3),
2.
Rotate
the
notched
plate
90°
and
slide
the
Input
Con-
jector
assembly
away
from
th»
circuit
board.
3.
Loosen,
but
do
not
remove,
the
seven
nuts
holding
the
shields.
4, Remove the 7 nuts and bolts, freeing the shields.
TROUBLESHOOTING
The
design
of
this
sampling
head
is
such
that
printed
cir-
cuit
boards
are
interchangeable
from
one
unit
to
another.
Since
the
outer
boards
are
easily
removed
and
replaced,
it
is
possible
to
quickly
troubleshoot
a
faulty
sampling
head
by
interchanging
boards
with
an
operating
one.
Thus,
removal
of
the
preamp
board
from
the
operating
unit
and
replacement
with
one
from
the
defective
unit
quickly
checks
the
faulty
units
preamp
circuits.
The
sampler
board
may
be
checked
for
defective
parts
without
disassembly
by
the
following
procedure:
1.
Remove
the
sampling
diode
plastic
holder.
2.
Remove
the
Preamp
board.
Leave
the
Strobe
board
in
place,
3,
With
an
ohmmeter,
check
the
DC
resistance
to
ground
from
the
preamp
input
pins
H
and
I.
If
a
direct
short
is
found,
C5
or
Cé
is
shorted;
if
o
3509
connection
to
ground
is
found,
then
C7
or
C8
is
shorted.
4.
To
check
C5,
Cé,
C7
and
C8
for
opens
{with
Preamp
board
removed),
use
a
Tektronix
Type
130
Direct-Reading
L-C
Meter
for
equivalent}
and
refer
to
Table
4-1
for
the
values
of
capacitance
from
either
pin
H
or
|
to
ground.
@