Tektronix S-1 User manual

INSTRUCTION
MAN UAL Serial Number
Tektronix, Inc.
S.W.
Millikan
Way
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P.
©.
Box
500
@
Beaverton,
Oregon
97005
@
Phone
644-0161
@
Cables:
Tektronix
070-0763-00
468

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SAMPLING HEAD
Fig. 1-1. Type S-1 Sampling Head.

Type S-1
SECTION 1
TYPE S-1 SPECIFICATION
Change information, if any, affecting this section will be found at the
- vear of the manual.
General Information
The Type S-1 Sampling Head is a 50 input, 350 ps rise-
time, plug-in preamplifier for use in some Tektronix 3S-
series sampling units, such as the Type 382 Sampling Unit.
Input characteristics of the sampling system are controlled
by the type of sampling head in use.
The Type S-1 can be installed directly into the Type 382,
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-1 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 352
Sampling Unit for externally triggering a Type 3T2 Random
Sampling Sweep. The Type 3S2 allows selection of the trig-
ger pickoff signal from the Channel A or Channel B sam-
pling head. The selected trigger pickoff signal is returned
to the Type 32 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 using
the sampling head trigger pickoff signal.)
Digital Unit Program Connections
The Type S-1 contains connections at its rear that pro-
gram 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 385 or Type 3586
Programmable 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-1 has been properly mated to
the sampling unit and indicator oscilloscope, and after
sufficient warmup time. For the particular system warmup
requirements, refer to the related amplifier and indicator
oscilloscope instruction manual. {A 5 minute warmup time is
required for Type 3S2-3T2). A procedure for mating the Type
S-1 to the vertical amplifier can be found in the Operating
Instructions section of this manual.
ELECTRICAL CHARACTERISTICS
Characteristics
Performance Requirement
Supplemental Information
Input Resistance 509 within 1%
Measured at DC
Dot Response
Adjustable in the sampling unit to unity
for signals up to 500 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 < 1V P-P signals.
Step signals greater than 500 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
Pulse Flatness Deviation
Displayed Noise
350 ps or less, 10% to 90%.
< + 05 and — 3% in the first 5 ns after
the step pulse reaches 50%.
< + and — 0.5% after the first 5 ns.
2 mV or less, tangential noise.
Applies when the step pulse is Tektronix
Type 284 Pulse Output signal through a
20 cm airline.
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-1
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-1 Sampling Head, including instal-
lation and First Time Operation.
The Type S-] completes the circuitry of some Tektronix
sampling units, and controls the input characteristics of
the vertical channel by providing the Strobe Generator, Sam-
pling Bridge, and Preamplifier circuits. Such a sampling unit
is the Type 382, using one or two S-series sampling heads.
A Type 382 and a sampling sweep unit plugged into an
indicator oscilloscope completes the sampling system. A
choice of real or equivalent time sampling is provided by the
type of sweep unit used.
A 502 input resistance allows the use of 50 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 of sampling sweep units,
Installing the Type S-1 Sampling Head
Fig. 2-1 shows the Type S-1 partially installed into a
Type 382 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-1 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-1 slowly into the compartment, so
the two plastic guides in the compartment engage the S-1.
3. Push the Type S-1 completely into the compartment.
4. Push the latch knob to lock the S-1 in place.
To remove the Type S-1 from the compartment, pull the
latch knob away from the front panel, then pull the unit
from the compartment.
Sampling Head
Loteh knob
Fig. 2-1. Installation information,
To use the Type S-1 on an extender cable, install as fol-
lows:
1. Pull the latch knob located on the head end of the ex-
ender 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-1 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-1 from the extender cable, pull
the latch knob on the front panel of the Type S-1, and re-
move the unit from the extender cable.
7. To remove the extender cable head from the sampling
unit compartment, pul! the latch knob outward from the front
panel, then pull the unit free.
2-1

Operating Instructions—Type S-1
Mating
The vertical deflection factor labeled by the Type S-1, and
the sampling system loop balance (adjusted by the Bridge
Bal control) must be considered when mating a Type S-1 with
a sampling unit.
The mVOLTS/DIV label on the Type S-1 names the de-
flection 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-1 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-1.
Bridge Balance
The Bridge Balance control of the Type S-1 should be ad-
justed whenever the sampling head is changed from one sam-
pling unit to another, or is operated on an extender cable.
Location of the control and details of its adjustment are de-
scribed in First Time Operation which follows.
FIRST TIME OPERATION
The First Time Operation procedure utilizes, in addition
to the Type S-1, a Type 382, Type 561A, and Type 284. The
Type 3T2 Random Sampling Sweep unit is recommended
for use with the Type S-1 because random sampling permits
viewing the signal in advance of the triggering event. Such
trigger lead-time is required by the Type S-1 with the Type
382 to view fast pulses without a pretrigger. The Type 284
is used as a signal source.
Setup Information
1. With the Type 561A Power switch off, insert a Tek-
tronix Type 382 Sampling Unit into the vertical compart-
ment {leff) and a Tektronix Type 3T2 Random Sampling
Sweep into the horizontal plug-in compartment (right).
2. Insert the Type S-1 Sampling Head into Channel A
compartment (left) in the Type 3S2 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:
Type 352
Display Mode CH A
Normal-Smooth Normal
2-2
A and B Position
DC offset
(both channels)
Units /Div
(both Channels)
Variable (both Channels)
Invert
(both Channels}
Dot Response
(both Channels)
B Delay
Horiz Position
Samples/Div
Display Mode
Start Point
Sweep Rate
Range
Display Mag
Time Magnifier
Variable
Midrange
Midrange (5 turns
from one end)
200
Cal
Push in
Midrange
Midrange
Type 312
Midrange
9 o'clock position
Normal
With Trigger
200 ns/Div
10 ps
x1
x5
Cal
Both fully clockwise
Fully clockwise
Time Position
Trig Sensitivity
Recovery Time ~ Optional
Trigger Polarity +
Trigger Source Ext
Type 284
Square Wave Amplitude 1.0 ¥*
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-1 through a 500
coaxial cable and a X2 attenuator. Connect the trigger out-
put signal from the Type 284 through a coaxial cable to the
External Input 50 connector on the Type 3712.
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 382 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
2.5 divisions.
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 to the full signal amplitude and produce
a display similar fo 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
‘Used through X2 attenuator for 0.5 V into the Type S-1.

> unity dot response
wy ns/ Div
et ae OAAae BAAD t+ ‘
(B} Double-Triggered, unity dot response
200 mV/Div 200 ns/Div
(C) Properly triggered
Fig. 2-2. Typical display for wnity dot response adjustment in
normal sequential sampling.
>)
Operating Instructions—Type S-1
free run region. Then adjust the Recovery Time control un-
til 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 372
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 20 or X50. Adjust the Type 382 Dot Response con-
trol for a unity dot response display similar to Fig. 2-38.
Bridge Balance Adjustment
15. Connect a bench multimeter set for = 30V full scale
between ground and the Type 352 Channel A Offset jack.
Adjust the Type 382 DC Offset contro! until the multimeter
reads 0 Volts. Increase the meter sensitivity and repeat the
adjustment for greater accuracy.
16, Disconnect the coaxial cable from the Type 284, leav-
ing the other end of the cable connected to the Type 5-1
input connector. Turn the Type 3S2 Trig Sensitivity con-
trol clockwise to free run the trace.
NOTE
Operating the sampling head without the input
connector terminated by a 50 © resistor or coaxial
cable will cause a vertical shift of the zero signal
baseline by a few millivolts. This is caused by the
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 between the Type S-1 input
between the Type S-1 input and any tunnel-diode
pulse generator or circuit that is sensitive to sig-
nals centering its output circuit.
2-3

Operating Instructions—Type S-1
st
200 mV/ Div
Fig. 2-3. Typical display for unity dot response adjustment in
random sampling operation,
17. Switch the Type 382 Units/Div control throughout
its range, adjusting the Type S-1 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
location of the Bridge Bal control. This adjustment must be
made whenever the Type S-1 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 tran-
sitions or very high frequencies are possible only when the
coupling circuit does not distort the signal. The Type S-1
input resistance is 509 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 loading to be less than caused by the 501 input
2-4
Bridge Bal
Adjustment
Fig. 2-4, Bridge Bal Location,
resistance. See your Tektronix Field Engineer or Represent:
ative about such special input coupling devices,
Input Voltage Considerations
Proper displays of various signal amplitudes require spec:
ial 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-1.
For greater details on coupling of high frequency input
signals, and on precautions to be observed, refer to the sam
pling unit Instruction Manual supplied with your sampling
instruments,
Maximum input signal voltage is an important consider-
ation 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-1 input con-
nector, Maximum input voltage without damage to the in-
put termination resistor is -+-5 or —5 volts DC, or 10 volts
peak to peak sine wave signals in the frequency domain of
10 MHz or below.
Where unity dot response is required, accurate displays
are possible to achieve only when the input amplitude and
the time base generator mode are both considered, The two
time base generation modes are (1) normal process sampling,
where each dot is displayed in sequence across the CRT,
and (2) random process sampling where the dots are not
necessarily displayed in a sequence that progresses uniformly
across the CRT. (The Type 3T2 Random Sampling Sweep unit
presents random process sampling displays in a controlled se
quence very similar to normal process sampling when the
Time Magnifier switch is at X1 or X2 and the trigger sig-
nal repetitfon rate is uniform.) The random process dot se-

quence is definitely not sequential when the Time Magnifier
switch is anywhere between <5 and X50, with the dot
presentation completely random at X50. The Type 3T2 can
be operated in a normal process mode, and all other Tek-
tronix sampling sweep units generate only a normal process
sequential 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 sampling
displays, and for normal process displays where there are
either no samples, or less than about 6 samples in a step
transition. Many dots in any verical change permits valid
random process displays without unity dot response
when the Type 3T2 Time Magnifier is at <1 or 2).
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
Specification Section lists the dot response linearity limits
as 5% for certain limited signal amplitudes discussed be-
low. 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 sampling unit Dot Response control thus
allows either the display top or bottom to be adjusted to
unity dot response for close examination of signal 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 0.5V peak to
peak and 1.0V peak to peak anywhere between +1V
and —1V DC.
TABLE 2-1
Input Signal Amplitude Limits Allowing Unity Dot
Response Vs Sampling Units mVOLTS/DIV Deflec-
tion Factor.
Sampling Unit Input Voltage
Units/Div Switch 0.5V P-P | 1.0V P-P
All positions yes | no
200 to 2 Either Normal or Random Process
Limited fo positions yes | yes
of 20 to 2 Either Normal or Random Process
All positions yes | yes
200 to 2 Normal Process only with >6 dots
in a 100% vertical transition.
Triggering from the Signal
The Type S-1 provides a trigger pickoff signal to the sam-
pling unit. This signal is a portion of the input signal.
In the Type 352, the signal is selected from the Channel A
or Channel B sampling head and returned to the front panel
Operating Instructions—Type S-1
Trig Out connector by the trigger circuit. The amplitude is
about 10% of the input signal into 500, A small amount
of the sampling bridge strobe signal is present with the trig-
ger signal.
A Type 382 standard accessory trigger coupling cable can
be used between the Type 382 and Type 312 for triggering
the sampling sweep unit from the sampling head pickoff sig-
nal. Connect the cable between the Type 382 Trig Out con-
nector and the Type 3T2 50. Q Trigger Input connector. Oper-
ate the Type 3T2 as if externally triggered. Repetitive sine
or square wave displays are best viewed when operating the
Type 372 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 sometime trigger
the timing unit. When this occurs, it is impossible to ob-
tain a meaningful display. Signal amplitude to the Type S-1
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-1 is useful to show risetime and detect aber-
rations of signals in circuits that are properly coupled to the
Type S-1. The risetime of both the generator (or source),
and the Type S-1 must be taken into consideration.
Signals with risetimes as fast as about 0.8 ns (800 pico-
seconds} can be measured using the Type S-1 without any
special considerations. Read the 10% to 90% risetime
directly from the CRT (or digifal unit) to know the rise-
time of the signal.
Signals with risetime faster than about 0.8 ns can be best
be measured using a Tektronix Type S-2 Sampling Head.
However, fair approximations of risetime can be made
using the Type S-1 and calculating the signal risetime by
formula (1} below. Formula (1} is based upon both the sig-
nal and the sampler having Gaussian risetime character-
istics. The Type S-1 risetime is actually trape-zoidal, with a
very linear rise and sharp corners. Signals with risetimes in
the order of 50 ps will cause the sharp corners on the Type
S-1 display to be quite obvious and signals with risetimes
in the order of 80 to 100ps will only modify the risetime
display corners. Thus, it is recommended that displays with
risetimes faster than 0.8 ns not be used to accurately measure
the signal 10% to 90% rate of rise. The Performance Check
and Recalibration Procedure gives equipment, procedure and
displays of Type S-1 and Type S-2 risetime when driven by
a 450 ps risetime tunnel diode pulser signal.
Formula (1)
T. (Displayed) = Visignal T.’ + (Type $-1 T,?
transposing:
Signal T. = ViDisplay T.? + (Type $-1 T.?

NOTES

Type S-1
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-1 Sampling Head followed by a de-
tailed circuit description. The Type S-1 Sampling Head is
designed to be the signal input section of the sampling
system. The reader may find it helpful to refer to the asso-
cated sampling unit manual for information on sampling prin-
ciples if the purpose of a particular circuit is not clear. For
example, Type 382 manual, Section 4, “Basic Sampling
Principles’. The sampling unit manual also shows intercon-
nections and circuits referred to in this section. Schematic
and block diagrams of the Type S-1 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 sam-
ple when a command pulse arrives from the Delay and
Strobe Driver circuit of the associated sampling 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 Pick-
off circuit is to cancel capacitively-coupled unwanted sig-
nals that normally bypass the Sampling Bridge. Very high
frequencies 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
it, 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
Response 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 Feedback
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 adjustable to
aid in setting the overall sampling head and sampling unit
“loop” gain for proper unity dot response.
CIRCUIT DESCRIPTION
The Type S-1 Sampling Head uses the power supplies of
the indicator oscilloscope and associated sampling unit. Inter-
connections 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 of the Type S-1, 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-conduction.
The Strobe Drive pulse is transformer-coupled by T75 to
the base and emitter of the Avalanche transistor Q69. Two
outputs are AC-coupled from @Q69, one from the collector
3-1

Circuit Description—Type S$-1
Le Transmission Lines
To Sampling
Bridge
pt ay
P51 P53
A q
Avalanche +
Snap-off
Current
Strobe
Clipping Lines
Avalanche
Volts
175
From
Strobe
_. Driver
27
Fig. 3-1. Strobe Generator circuits.
and the other from the emitter. The Avalanche Volts con-
trol adjusts the collector voltage of the avalanche transistor
Q69. The typical quiescent voltage at Qé69 collector is
about +15 volts. This voltage sets the amplitude of the sig-
nals that drive the Snap-off diode circuit, and assures the nor-
mal 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 Q69, forward bias-
ing Q69. Normal avalanche action follows with the col-
lector 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.
The circuit consists of a Snap-off Current control R57,
emitter follower Q55, Snap-off diode Dé1, two clipping lines,
3-2
and associated components. Between drive pulses from the
Avalanche circuit, the Snap-off diode D61 is forward-biased
by the current in Q55. The current value is set by the Snap-
off Current control R57. The current in Dé1 is typically 20
mA, and the current path is shown as a dashed line in Fig.
3-1. This heavy forward current assures that Dé1 has many
carriers within its junction region.
The push-pull signals from the Avalanche circuit cause
Déi junction carriers to reverse direction as a heavy re-
verse 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 suddenly 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 reflect-
ed, equal in amplitude and opposite in polarity, back to
the transmission line. This cancels the signals moving to-
ward the Sampling Bridge. This action results in a positive
Strobe pulse being delivered to P51, and a negative Strobe
pulse being delivered to P53.

Circuit Description—Type S-1
+50V
i
R20!
From
Blow-by
|
ckeatadkantackentbedeed
circuit !
l
From
DC
BRIDGE
!
R21
Offset
&
To Blow-by &
Trigger Takeoff
3 Y- Y
3
— C14
Feedback
@ Attenuator
R28
WV
=~ C28
C7 tp) ss
R7A
R7B aN
AS
“i
S4\ |
7~
Strobe
eo @—> To Q31 in
Preamplifier
Pulses
Fig. 3-2. Sampling Bridge circuit with Bridge Volts and Bridge Bal circuits.
Sampling Bridge
The Sampling Bridge consists of D5, Dé, R5, Ré, RZA,
R8A, C5, Cé, 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 conduc-
tion by the strobe pulses from the Strobe Generator. Dur-
ing D5 and Dé conduction time of about 350 ps, the bridge
delivers a portion of the voltage difference between the
input signal and the bridge output signal to the Preampli-
fier input.
The bridge input circuit is terminated in 500, The 500
consists of R4 with the series value of R3 and R10 in par-
allel. (R10 is shown in Fig. 3-3). R3 delivers a small por-
tion of the input signal to the Blow-by and Trigger Pick-
off circuit. The bridge output drives the high input imped-
ance of 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 volt-
age 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 volt-
age 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 sampling unit.
The input transistor Q@31 operates as a very high input
impedance high-gain inverting amplifier. Temperature com-
pensation
for @31 is accomplished by thermister R33 in
parallel with R34. The 4mA current path is from the +50
volt supply through R31, Q31, the parallel combination of
R33-R34 and R35 to the —50 volt supply. C34 assures that
Q@31 AC gain is high, while its DC gain is less than 1.
Q@41 and Q45 are connected as an operational ampli-
fier with a very low output impedance at Q45 emitter. The
output is coupled by C49 and L49 to the 90Q input re-
sistance Post Amplifier in the sampling unit. Q45 current
3-3

Circuit Description—Type S-1
v
oa
k— FET Input
+50V =
| D41
R31 ' +8V
WA \l
: H
' c40 +0.6V
\
1
}.Q31
|
From Sampling NN
Operational Amplifier
oe
R42
mAAA Q45
c49 ars To Post
Q41 -—~WHT Amplifier
L49 Sampling Unit
(+7.3V AT (90.9 Q)
1
}
I
R45 |
Bridge E41: ¢- NW
' c34 an 1
R33 (T 1 R34 I
|
I
| | 12 \ !
\
ras
"
'
1
R35
C46
\ R40 & Gain 1 Ra7
| 1
]
—50V
(
—12.2
say 12.2V
Fig. 3-3. Preamplifier circuit.
paths are shown by dashed lines in Fig. 3-3. D41 protects
@4]1 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 Q45
through C46 and Gain control R46 to Q41 base controls the
AC gain of the Preamplifier. The Gain contro! R46 allows the
AC feedback to be adjusted, thereby adjusting the gain.
The AC gain of the Preamplifier is usually adjusted to be
about 16 depending upon the sampling efficiency. For
example: For an input signal into the Type S-1 input con-
nector of 500 mV, if the display dot change is 100% of the
signal, Q45 output signal will be about 200mV. A posi-
tive 500 mV input signal will produce a +12.5mV signal
at Q31 gate (for a 2.5% sampling efficiency), and a posi-
tive 200 mV output signal. Likewise a negative input signal
will produce a negative output signal.
Blow-by and Trigger Pickoff
The Blow-by and Trigger Pickoff circuits consists of Q13,
QI17 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
3-4
Bridge. The secondary purpose is to provide a trigger pick-
off 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-1 input signal. The signal is fed
to the base of Q13 through R3. R10 terminates the 500
impedance of J3-P3 coaxial connector and assures no reflec-
tions back into the input circuit. The output from Q13 col-
lector 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 take-off sig-
nal to the sampling unit trigger amplifier.
To trace the Blow-by signal path, a portion of the Type
S-1 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 induced displacement current of the capaci-
fance shunting the Sampling Bridge.
The trigger pickoff signal path starts with a portion of
the input signal coupled from Q13 emitter to Q17 emitter
to provide an in-phase signal at Q17 collector to the Trigger
Amplifier and selector circuits in the sampling unit. Q17 col-
lector load is provided in the sampling unit.

Circuit Description—Type S-1
R10
Ferrite
Bead \
Qi13
+15V
R12
® C-— Trigger Take-off
Transient to Sampling Unit
Response | 4
i}
Li7 4
~—— Ferrite
PANY
From
SIGNAL
INPUT
Circuit
“ IT Bead
. I ,
$ R14
Qi7
R17
=~ C14
—12.2V
To Sampling
Gate
(output side)
Fig. 3-4. Bloweby and Trigger Pickoff circuits.

NOTES

Type S-1
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-1.
To remove the Type S-1 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-1 can be purchased directly
through your Tektronix Field Office or Representative. How-
ever, replacements for standard electronic items can gen-
erally 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-1 should be or-
dered from Tektronix since these parts are either manu-
factured or selected by Tektronix to satisfy a particular
requirement. Before purchasing or ordering, consult the
Electrical or Mechanical Parts List to determine the value,
tolerance and ratings required.
NOTE
When
selecting the replacement parts, it is impor-
tant 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 partic-
ular way to reduce or control stray capacitance
and inductance. After repair, the sampling head
may require recalibration.
Transistor Replacement. Cut the leads of a replace-
ment transistor to the same length as the transistor removed
and bend the leads as necessary. The lead configurations
of the transistors used in the Type S-1 are shown in Fig.
4-1. Two transistors, Q13 and Q17, 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 high quality, with
good cold-flow characteristics. Thus, do not use 50/50 sol-
der, but 60/40 or 62/38 solder when replacing the leadless
capacitors.
Best results will be obtained 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
Emitter
Base Collector Collector
/
Flat side
or dot
Plastic case
Emitter
Metal-Case
Gate \ Drain
Field-effect (Q31)
151-1012-00
Fig. 4-1. Lead configuration for board-mounted transistors.

Maintenance—Type S-1
capacitor may be soldered into place by positioning the part,
then applying heat to the adjacent plated area.
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 Diode. 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 Sam-
pling 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. 4-3 with the shields removed; it is not necessary to
remove these shields to replace the sampling diode as-
sembly. For removal or replacement use Xcelite Forceps
42H, Tektronix Part No. 003-0347-00, or similar tool.
The hook-like ends of the diode holder normally rest in a
circuit board hole and firmly hold the diodes to the plated
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
4-2
ends may snap or spring together. Replacement of the
holder should also be by slow, cautious movement; first
as the hook-like ends reach the plated area which increases
the thickness and second as they reach the hole where
the possibility of physical shock exists as in the removal.
A metal shield prevents the holder being inserted too far.
Circuit Board Replacement. If a circuit board is dam-
aged and cannot be repaired, the entire assembly includ-
ing all soldered-on components should be replaced. The
part number given in the Mechancial 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 Board 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. 4-4).
2. Rotate the notched plate 90° and slide the Input
Connector assembly away from the 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
circuit boards are interchangeable from one unit to an-
other. Since the outer boards are easily removed and re-
placed, it is possible to quickly troubleshoot a faulty sam-
pling 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 |. If a direct short is
found, C5 or Cé is shorted; if a 350 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
®