Tektronix 7A18 User manual

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INSTRUCTION
IAi8/tA"%H
DUAL TRACE
AMPLIFIER
7A18
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TABLE OF CONTENTS
SECTION 1 SPECIFICATION Page OPTION INFORMATION
lntroduction
Electrical Characteristics
I- I
1-1 SECTION 6 ELECTRICAL PARTS LIST
7A18 And Mainframe Frequency
Response 1-3
Environmental Characteristics 1-3
Abbreviations and Symbols
PartsOrdering lnformation
Physical Characteristics 1-3 SECTION 7 DIAGRAMSAND CIRCUIT BOARD
SECTION
2 OPERATING INSTRUCTIONS ILLUSTRATIONS
Installation 2- 1
Front Panel Controls and Connectors 2-1
General Operating Information
2-2
Basic Applications 2-4
Symbols and ReferenceDesignators
Voltage and Waveform Conditions
SECTION
3 CIRCUIT DESCRiPTlON SEC'TION
8 MECHANICAL PAR1'S LIST
Introduction 3-1
Block Diagram Description 3-1 Mechanical Parts List Information
Detailed Circuit Description 3-1 Index of Mechanical Parts Illustrations
Mechanical Parts List
SECTION 4 MAINTENANCE Accessories
PreventiveMaintenance
Troubleshooting
Replacement Parts
Component Replacement
4-3 CHANGE INFORMATION
4-4
SECTION 5 CALIBRATION Abbreviations and symbols used inthis manual are based on
Recalibration Interval 5-1 or taken directly from IEEE Standard
260 "Standard
'Test Equipment Required 5-1 Symbols for Units", MIL-STD-12Band other standards of.
Part
I -.- Performance Check 5-3 the electronics industry. Change information, if any, is
Part I I - Adjustment 5-7 located at the rear of this manual.
REV. JUNE 1974
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Changeinformation, ifany, affecting thissection willbe foundat therear of the manual.
t ntroduction
The 7A18 and 7A18N Dual Trace Amplifier plug-in
units are designed for use with l-ektronix 7000-Series
Oscilloscopes. The 7A18 and 7A18N are electrically iden-
tical except that readout encoding capabilities and an
"IDENTIFY" function are provided only in the 7A18. All
references made to the 7A18 apply equally to the 7A18N
unless othewise noted. The 7A18 is a dual-channel,
medium-bandwidth amplifier. Internal gain and compensa-
tion circuits are automatically switched to correspond to
the setting of the
VOL-I"S/DIV switch. Channel 2 can be
inverted for differential measurements. The 7A18 can be
operated in any plug-in compartment of the 7000-Series
Oscilloscopes.
The following electrical characteristics are valid over the
stated environmental range for instruments calibrated at an
ambient temperature of i-20"~to +30°c, and after a five
minutewarmup unlessotherwise noted.
TABLE 1-1
ELECTRICAL
Deflectran Factor Accuracy
Uncalibrated
(VARIABLE)
GAlN
Frequency Respar7se
Syslorn Dependent
(8 dtv refc~ence stgnal)
Upper Bandwrritlr
DC {Direct) CuuplecJ
ower ljandwiclth
-
i wrViDiv to 5 VIDIV; tcrt step5 111 6 I,
Vrrhrri 296 with GAlN adjusted at
0 mV/Div,
:ontinuously vat iablc between cal~brart
teps; extends deflection factor tn at
East 12.5 ViDiv
Pern-rits adjustrrrcnt of tic:flect~nn factor
for cdlrbrdted or>erdtiot~ wit17 all 7000-
set rcs uscrltuzcopes,
- --
7A18
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Specification-7A1817AI 8N
TABLE 1-1 (contj
Characteristic
""
Maximum InputVoltage
DC Coupled
AC Coupled
Channel Isolation
Input Rand C
--- --- -- -----
Resistance
Capacitance
RC Product
DisplayedNo~se
(TangentiallyMeasured)
". "- ," ...... .,"-'"--
Overdrive Recovery Time
Common Mode Rejection
Ratio
DC Drift
Dr~ftwith Time (ambient
temperatureand line voltage
constant)
Dr~frw~thTemperature
(linevoltage constant)
- -- --- -
Time Delay between Channels
--- ---- -* ---- - -- -----
Display Modes
PerformanceRequiremen_t-.
- -
- --- --
50: 1 display ratio up to50
megahertz.
Approximately 20.0 pf
Atleast 10:
1 up to59 megahertz.
Zhannel 1 only.
Dual-trace,alternatebetweenchannels.
Added algebraically.
Dual-tracechapped between channels.
Shannel 2 only.
Supplemental
-- Information
!50 volts, (DC + Peak AC); AC com-
Ionent 500 volts peak-to-peakmaxi-
num. one kilohertz or less.
j00 volts, (DC + Peak AC); AC compo-
lent 500 volts peak-to-peakmaxi-
num, one kilohertz or
less.
---*-----~"." .."."-"".----,-....-*-
Nithin :k 1% between all deflection
$00microvolts or less at 5 mV/Div in
7000-SeriesOscilloscope.
1.1 ms or
less to recover to within one
livision after the removal of an over-
jrive signal of up to -1-75divisions or
-75 divisions regardless of overdrive
;ignal duration.
" .,..----..-..-.. ~ ..-"
1.02 divisron or less in any one minute,
fter one hour warmup.
- -- - - -- -- -- "- ---
do more than 0.01 d~vis~onper degree C.
'00 picosecondsor less.
7A18
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TABLE 2.1 (cant)
TABLE A
7A78 AND MAINFRAME
With 1500 Scr res
60 MHz
Wutl7 7400 Sas its$
50 blE4r
Refer $0 st~c Specifaca!~un fw tbw il"iocidt~C1 OSCI~~OSCQ~~~
7 ABLE $3
7A18
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SECTlON 2
TING INSTRUCTIONS
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FRONT PAMEL CONTROLS
AND CONNECTORS
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7A18
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Operating Instructions--7A18/7A18N
VOLTSIDIV Selects calibrated deflection factors
from
5 mV/Div to 5 VIDiv; ten
steps ina 1-2-5sequence.
VARIABLE Provides continuously variable un-
(VOLTSiDIV) calibrated settings between cali-
brated steps. Extends thedeflection
factor range to 12.5 voltsidivision
or more.
GAlNAdjustment When the VARIABLE control is
pushed in, it becomes afront-panel
screw-driver adjustment for cali-
bration of deflection factor.
DISPLAY MODE Selects one of the following modes
of operation:
CH I.--A single-tracedisplay of the
signal applied toChannel 1.
ALT--A dual-trace display of the
signal applied to both channels.
'The channels are alternately dis-
played, and switching occurs at
the end of each time-base
sweep.
ADD-Algebraically adds the signals
applied to the CH 1 and CH 2
input connectors, and the
algebraic sum is displayed on
the CRT. The CH 2 POLARITY
switch allows the display to be
CH 1
+ CH 2 or CH 1 - CH 2.
Position of the trace in this dis-
play mode is controlled by CH 1
POSITION control only.
CHOP---Adual-trace display of the
signals applied to bothchannels.
The two channels time-sharethe
sweep as determined by the in-
dicator oscilloscope.
CH single-tracedisplay of the
signal applied toCH 2.
TRIGGER SOURCE Selects source of the trigger signal.
The trigger signals provide internal
triggering for the oscilloscope time-
base units.
CH 1
- internal triggering signal
obtained from signal applied to
CH
1.
MODE-.-,Internal trigger signal auto-
matically follows DISPLAY
MODE selection. In ADD or
CHOP display modes, the trigger
signal is the algebraic sum of CH
1 and CH 2 trigger.
CH 2---Internal trigger signal
obtained from signal applied to
CH 2.
CH 2 POLARITY Provides means of inverting the CH
2 display.
-WP--.A positive-going signal at the
CH
2 input connector deflects
the
CRI display upward.
INVERT--.A positive-going signal at
the CH
2 input connector de-
flects the CRT display down-
ward.
GENERAL OPERATING INFORMATION
Introduction
For single-trace operation, either of the two identical
amplifier channels can be used independently by setting the
DISPLAY MODE and TRIGGER SOURCE switches toCH
1 or CH 2 and connecting the signal to be observed tothe
appropriate input. In the discussions to follow, single-trace
operations using CH
1 only apply equally to CH 2 only.
Signal Connections
In general, probes offer the most convenient means of
connecting a signal to the input of the 7A18. A 10X atten-
uator probe offers a high input impedance and allows the
circuit under test to perform very close to normal operating
conditions.
The Tektronix P6053A probe, with its readout coding
ring, was designed specifically for use with 'Tektronix 7A-
series amplifier units equipped with readout. The readout
coding ring on the probe connects to
a circuit inthe ampli-
fier unitwhich automatically corrects the readout displayed
on the CRT to the actual deflection factor at the tip of the
probe being used. For probes to be used with amplifier
units without readout, see the Tektronix, Inc. catalog.
Vertical Gain Check and Adjustment
TOcheck the gain of either channel, set the VOLTSlDfV
switch to 10mVand connect 40 mV, 1kHz signal from the
oscilloscope calibrator to the input connector of the chan-
nel being checked. Thevertical deflection should be exactly
four divisions. If not, adjust the front-panel GAIN for
exactly four divisions of deflection. The GAlN adjustment
is engaged by pressing in the GAIN control knoband turn-
ing the knob with a narrow-blade screwdriver (see Front
7A18
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Operating Instructions--~7A'18/7A18N
Panel Controls and Connectors), Turn the knob clockwise,
CH 2 POLARITY Switch
then counterclockwise, until the GAIN control is engaged.
When the GAIN control is engaged, the vertical deflection
will change as the knob is turned. Turn the GAIN control
knob with the screwdriver until the deflection is set to
exactly four divisions, then remove the screwdriver.
Input Coupling
The Channel 1 and Channel 2 coupling (AC-GND-DC)
switches allow a choice of input coupling methods. The
type of display desiredand the applied signal will determine
the couplingto use.
The DC coupling position must be used to display the
DC component of the signal,
It mustalso be used todisplay
AC signals below about
30 hertz (ten hertz with a 10X
probe) and square waves with low-frequency components as
these signals are attenuated inthe AC position.
In the AC coupling position, the DC component of the
signal
is blocked byacapacitor inthe inputcircuit. IheAC
coupling position provides the best display of signals with a
DC component much larger than the AC components. The
precharge feature should be used with large DC inputs. 70
use this feature, first set the coupling toGND. Connect the
probe to the circuit and wait about two seconds for the
coupling capacitor to charge. Then set the coupling toAC.
The GND position provides a ground reference at the
input of the amplifier without externally grounding the
input connectors. However, the signals connected to the
inputs are notgrounded, and the same DC load is presented
to the signa! source.
VOLTSIDIV and VARIABLE Controls
The amount of vertical deflection produced bya signal is
determined by the signal amplitude, the attenuation factor
of the probe, the setting of the VOLTSlDlV switch, and
the setting of the VARlABLE control. Calibration deflec-
tion factors indicated by the settings of the VOLTS/DIV
witch apply only when the VARIABLE control is in the
calibrated (CAL IN)position.
The VARIABLE control provides variable, uncalibrated
settings between the calibrated steps of the VOLTS/DIV
switch. With the VARIABLE control fully counterclock-
wise and the VOtTSlDlV set to
5 voltsldiv the uncali-
brated vertical deflection factor
is extended to at least 12.5
volts/division. By applying a calibrated voltage source to
the input connector, any specific deflection factor can be
set within the range ofthe VARIABLE control,
The CH
2 POLARITY switch may be used to invert the
displayed waveform ofthesignal applied totheCH
2 input.
This is particularly useful in added operation of the 7A18
when differential measurements are to be made. The CH 2
POLARITY switch has two positions, +UP and INVER1. In
the +UP position, the displayed waveform will have the
same polarity as theapplied signal and a positive DC voltage
will move the CRT trace up. In the INVERT position, a
positive-goingwaveform at the CH 2 inputwill be displayed
on the CRT in inverted form and apositive DCvoltage will
move the trace down.
DISPLAY MODE Switch
For single-trace operation, apply tire signal either tothe
CH
1 inpttt or the CH 2 inputand set the DISPLAY MODE
switch tothecorresponding position: CH
f or CH 2.
To display a signal inone channel independently when a
signal is also applied to the other channel, simply select the
desired channel by setting the DISPLAY MODE switch to
the appropriate CH
1 or CH 2 position.
Alternate Mode. The
ALT position of the DISPLAY
MODE switch produces a display which alternates between
channel 1 and channel
2 with each sweep on the CRT.
Although the ALT mode can be used at all sweep rates, the
CHOP mode provides a more satisfactory display at sweep
rates below about
0.2 millisecond/division. At slow sweep
rates alternate mode switching becomes visually per-
ceptible,
Add Mode. "The ADD position of the DISPLAY MODE
switch can be used to display the sum or difference of two
signals, for common-mode rejectionto removean undesired
signal, or for DC offset (applying a DC voltage to one
channel to offset the DC component of a signal on the
other channel). The overall deflection factor in the ADD
mode with both VOLTSlDIV switches set to the same
position is the deflection factor indicated by either
VOLTS/DIV switch. However, if the CH 1 and CH 2
VOLTSlDIV switchesare set to different deflection factors,
the resultant amplitude is difficult to determine from the
CRT display. In this case, the voltage amplitude of the
resultant display can be determined accurately only if the
amplitude of the signal applied toone channel is known. In
the ADD mode, positioningof thetrace is contro:led by the
channel
1 POSITION controlonly.
Chop Mode. The CHOP position of the DISPLAY
MODE switch produces a display which
is electronically
switched between channels at approximately a
500 kilo-
hertr rate (controlled by mainframe). Ingeneral the CHOP
mode provides the best display at sweep rates slower than
REV. APR 1974 2-3
7A18
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Operating instructions~--7Al8/7A18N
about 0.2 miilisecondIdivision or whenever dual-trace, non-
repetitive phenomena is to be displayed.
TRIGGER SOURCE Switcl~
CH 1. 'The CH 1 position of the 1-RIGGER SOURCE
switch provides a trigger signal obtained from the signal
applied to the CH
1 inp~~tconnector. This provides astable
display of the signal applied to the CH
1 input connector.
CH
2. The CH 2 position of the TRIGGER SOURCE
switch provides a trigger signal obtained from the signal
applied to the CH
2 input connector. This provides astable
display of the signal applied to the CH
2 input connector.
MODE. In this position of the TRIGGER SOURCE
switch, the trigger signal for the time-baseunit is dependent
on the setting of the DISPLAY MODE switch. The trigger
source for each position of the DISPLAY MODE switch is
as follows:
MODE TRIGGER
SIGNAL SOURCE
CH
1 Channel 1
CH 2 Channel 2
ADD Algebraic sum of channel 1 and channel 2
CHOP Algebraic sum of channel 1 and channel 2
ALT Alternates between channel 1 and channel 2
'Trace Identification (7A18only)
When the IDENTIFY button is pressed, the trace is de-
flected about
0.2 division to identify the 7A18 trace. This
feature is particularly useful when multiple traces are dis-
played. In instruments with readout, also replaces deflec-
tion factor readoutwith the word "IDFNTIFY".
BASIC APPLICA'TIONS
General
The following information describes the proceduresand
techniquesfor making basic meastlrerrlents with a 7A18 and
the associated Tektronix oscilloscope and time-base.These
applications are not described in detail since each applica-
tion must be adapted to the requirements of the individual
measurements. This instrument can also be used for many
applications not described in this manual. Contact your
local 'l-ektronix Field Office or representativefor assistance
in making specific measurementswith this instrument.
Peak-to-PeakVoltage Measurements (AC)
To make peak-to-peak voltage measurements, use the
following procedure:
I. Apply the signal to either inputconnector
2. Set the DISPLAY MODE and TRIGGER SOURCE
switchestodisplay the channel used.
3. Set the co~lplingswitch to AC.
For low-
frequenc y signals helo w about 30 hertz use
the DC position to prevent attenuation of the signal.
4. Set the VOL'T'SIDIV switch to display about five
divisions of the waveform vertically.
5. Set the time-baseTriggering controls for astabledis-
play. Set the time-baseunit to a sweep rate which displays
several cyclesof thewaveform.
6. Turn the 7Af8POSITION control so the lower por-
tion of the waveform coincides with one of the graticuie
lines below the center hori~ontalline, and the top of the
waveform is within the viewing area. With the time-base
Position control, move the display so one of the upper
peakslies near the center vertical line (see Fig.
2-2).
7. Measure the divisions of vertical deflection peak-to-
peak. Check that the VARIABLE (VOLTSIDIV)control is
inthe CAL INposition.
NOTE
This techrtique can also be used to make measore-
ments between two points on the wavefom, rather
titanpeak topeak.
Ftg. 2-2. Measurtng the peak-to-peak voltage of a waveform.
7A18
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Operating Instrtictions--7Al8/7A18N
8. Multiply the deflection measured in step 7 by the
VOLTSiDlV switch setting. Include the attenl~ationfactor
of the probe if used.
EXAMPLE: Assume that the peak to peak vertical de-
flection
is 4.5 divisions (see Fig. 2-2) using a 10X atten-
uator probe, and the VOLTSIDIV switch is set to 1 V.
vertical probe
Volts
- deflection X
Peak to Peak 'OL
rS'DIV x attenuation
setting
(divisions) factor
Substitutingthe given values:
Volts Peak-to-Peak
= 4.5 X 1 X 10
The peak-to-peakvoltage is 45 volts.
InstantaneousVoltage Measurements (DC)
Po measure the
DC level at agiven point on awaveform,
proceed as follows:
1. Connect the signal to either-input connector.
7. Measure the distance in divisions between the
reference line anrf the point on the waveform at which the
DC level is to be measured. For example, in Fig. 2-3 the
measurement is between the reference line and point A.
8. Establish the polarity of the waveform. With the CH 2
POLARITY switch in the +UP position, any point above
the referenceline is ~ositive.
9. Multiply the distance measured in step 7 by the
VOLTS/DIV setting. Include the attentlation factor of the
probe,
if used.
EXAMPLE: Assume the vertical distance measured is
3.6
divisions (see Fig. 2-3) and the waveform is above the
referenceline usinq a 10X probe wit11aVOLTSIDIV setting
of 0.5 V.
Using the formula:
Instan- vertical VOLTS! probe
taneous
= distance X polarity X DIV X attenuation
Voltage (divisions) setting factor
2. Set the DISPLAY MODE and TRIGGER SOURCE
Substitllting given values:
switches todisplay the channel used.
3. Set the VOLTSIDIV switch to display about five
divisions of the waveform.
4. Set the couplinq switch to GND and pos~tionthe
trace to thc lnottom graticule line or other referenceline. If
the voltage
is ncqative with respect to ground, position the
trace to the top graticitle line. Do not move the POSl PION
control after this reference line has been establ~shed.
NOPE
Po n~easrire
a voltage Icvel with respect to a voltage
orher ti~afigrout?d, make the followirig c/)at?ges to
step 4. Set tl?~COLI/?IIII~switch to DC ar~d apply tlte
reference voltage to the inpiit connector. Then
position the trace to the refere17celtne.
5. Set the coupling switch to DC. The ground reference
line can be checked at any time by sw~tclr~nqto the GND
position.
6. Set the time-base 'I r~ggeringcontrols for a stable
display. Set the time-base sweep rate for an optinium
display of thewaveform.
Instantaneous
3.6 X i 1 X 0.5 V X 10
Voltage
The instantaneousvoltage
1s 18 volts.
Fig. 2-3. Measuring instantaneous voltage with respect to some refer-
ence.
7A18
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Operating Instructions-7Al8/7A18N
Comparison Measurements
In some applications it may be desirable to establish
arbitrary units of measurement other than those indicated
by the VOL-ISiDIVswitch. This is particularly useful when
comparing unknown signals to
a reference arnpiitcide. One
use for the comparison-measurement technitlue is to
facilitate calibration of equipment where the desired
arnplitude does not produce an exact number of tlivisions
of deflection. The adjustment will be easier and more
accurate if arbitrary units of measurement are established
so that the correct adjiistment is indicated by an exact
number of divisions of deflection. The following procedure
describes how to establish arbitrary units of measure for
conlparison measurcmc?nts.
To estatrtlish an arbitrary vertical deflection factor- based
upon aspecific refert?nccamplitude, proceed as follows:
1. Connect the reference signal to the input connector.
Set the rime-baseunit sweep rate to display several cycles
of tile signal.
2. Set tile VOLTS'DIV switch and the VARIABLE
control to produce a display which
is an exact number of
vertical div~sions in atnplitude. Do not cf.tange the
VARIABLE control after obta~nrngthe desiretl cfeflect~on.
3. To estat,lish an arbitrary vertical deflection factor so
the amplitude of an unknown signal can he measured
accurately at any setting of the VOLT-S/DIV switch, the
ai-nl,litude of the t-eierence signal must be icnown. If it is
not known, it can
be measured before the VARIABLE
VOLTS'DIV control is set instep 2.
Signal VOLT'SIDIV vertical vertical
Amplitude
- setting X conversion X deflection
factor (divisions)
EXAMPLE: Assume
a reference signal atnplitude of 30
volts, a VOLTSIDIV setting of
5 V and the VARIABLE
control adjusted to provide a vertical deflection of four
divisions. Substituting these values in tlie vertical
conversion factor formula (step
4) :
30V
Vertical
Conversion - 4 X 5 V = 1.5
Factor
]-hen with a VOLTS/DIV setting of 2 V, the
peak to-peak amplitude of an unknown signal which
produces a vertical deflection of five divisions can be
determined by using tlie signal amplitut.le formilla (step
6):
Signal
= 2 V X 1.5 X 5 = 15 volts
Amnlitude
Dual-Trace Phase Difference Measurements
Phase conlparison between two signals of the same fre-
quency can be made using the dual-trace feature of the
7A18. 'Th~smethod of phase dilference measurement can
be used up to the frequency limit ot the oscilloscope
system. To make the comparison, use the following pro-
cedure:
1. Set the CH 1 and CH 2 coi~plingswitches to the same
position, clependingon the type of cot~plir~gdesired.
4. Divide the atnplitude of the referencesignal (volts) by 2. Set the DISPLAY MODE to ALT or CHOP. In
the product of tho vertical deflection (divisions) established general, CHOP is more suitable for low frequencies and
in step
2 and the settirig of the VOLTS/DIV switch. 1-hisis ALT is more suitable for high frequencies. Set the
the vertical conversion factor. TRIGGER SOURCE to CH 1.
Vertical referencesignal
Conversion amplitude (volts)
-- - .- -- -- - - --
Factor vert~cal VOLTSIDIV
deflection
X switch
(divisions) setting
5. To measure the amplitude of an unknown signal,
disconnect the reference signal and connect the unknown
signal to the input connector. Set the VOLTSI'DIV switch
to a setting that provides sufficient vertical cleflection to
make an accurate measurement. Do not readjust the
VARIABLE control.
3. Connect the reference signal to the CH 1 input and
the comparison signal to the CH
2 input. Use coaxiat cables
or probes which have similar time delay characteristics to
connect the signals to the input connectors.
4. If the signals are of opposite polarity, set the CH 2
POLARITY switch to invert the channel 2 display.
(Signals may be of opposite polarity due to 180' phase
difference; if so, take this into account in
the final cal-
culation.)
6. Measure the vertical deflection in divisions and 5. Set the VOLTS/DIV switches and the VARIABLE
calculate the amplitude of the unknown signal using the controls of the two channels so the displays are equal and
following formula. about five divisions inamplitude.
7A18
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Operating Instructions--7A18/7A18N
6. Set the time-base unit to a sweep rate which displays Substitiitinq the given values:
about one cycle of the waveforms. Set the Triggering
controls for a stable display. Phase Difference
- 0.3 X 45"
The phase difference is 13.5".
7. Center the waveforms on
thegraticule with the 7A18
POSFTION controls.
High Resolution Phase Measure~nents
8. Adjust the time-base Variable TitneiDiv control until More accurate dual-trace phase measuremenis can be
one cycle of the reference signal occupies exactly eight made by increasing the sweep rate (witho~ltchanging the
horizontal divisions between the second and tenth vertical Variable TirneIDiv control). One of the easiest ways to in-
lines of the graticule (see Fig. 2-4). Each division of the crease the sweep rate is withthe time-baseMagnifier switch.
graticuie represents 45O
of the cycle (360° + 8 divisions =
Set the Magnifier to XI0 and determine the magnified
45"idivision). The sweep rate can now be stated interms of sweep rate t~ydividing the sweep rate obtained previously
degrees as 45'/division. by the amount of sweep magnification.
9. Measure the horizontal difference behveen corre-
sponding points onthe waveform. EXAMPLE. If the sweep rate is increased 10times by
the Magnifier, the magnified sweep rate is 45Oidivision
: 10
- 4.5°/division. Fig. 2-5 shows the same signals as used in
10. Multiply the measured distance (in divisions) by Fig. 2-4 but with the Magnifier set to X10. With
a iiori-
45Oidivision to obtain the exact amount of phase dif- zontal difference of3 divisions, tile phase difference is.
ference.
horizontal magnified
EXAMPLE: Assume a horizontal difference of 0.3 divi- Phase Difference
- difference X sweep rate
sion with a sweep rate
of 45°/division as shown in Fig. 2-4. (divisions) (cfegreesi'division)
Using the formula: Substiti~tintjthe given values:
hori7ontal sweep rate Phase Difference - 3 X 4.5"
Phase Difference
- difference X (degrees/division)
(divisions) The phase difference is 13.5".
Fig. 2-4. Measuring phase difference between two signals. Fig.
2-5. High resolution phase measurement using time-base magrii-
f ier.
2-7
7A18
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Operating Instructions--7A18/7A18N
Common Mode Rejection
The ADD feature of the 7A18 can be used to display
signals which contain undesirable components. These un-
desirable components can be eliminated through common-
mode rejection. The procedure is as follows:
1. Set the DISPLAY MODE s~zlitclito ALT or CHOP
and the TRIGGER SOURCE switch to MODE,
2. Connect the signal containing both the desired and
undesired informationto the CH
1 inputconnector.
3. Connect a signal similar to the unwanted portion of
tlie CH 1 signal to the CH 2 input connector. For example,
in Fig.
2-6 a line-freclcrency signal is connectetl to Channel 2
to cancel out the line-frequencycomponent of the Channel
1 signal.
4. Set botii coupling switches to the same setting, DC or
AC, depericlingon the applied signal.
5. Set the VOLTS/DIV switches so the signalsare about
equal in amplit~~de.
6. Set tlie DISPLAY MODE switch to ADD. Set the CH
2 POLAR1rY switch to INVERT so the common-mode
sign;ils are of opposite polarity.
7, Adjust the Channel 2 VOLTSiDlV switch and
VAR
l ABLE control for maxiinurn cancellation of the
cor~7mon-i~lodesignal. The signal which remains shocrld be
only
tlie desired portionof the Channel 1 signal.
EXAMPLE: An example of this mode of operation is
shown in Fig. 2-6. The signal applied to Charinel
1 contains
unwanted line frequency components (Fig. 2-6A).A corre-
sponding line frequency signal is connected to Channel
2
(Fig. 2-GB). Fig. 2-6C shows the desired portion of tlie
signal as displayed when common-mode rejectioli is used.
The above procedure can also be used for examining a
signal siiperimposed on some DC level when DC coupling is
Fig. 2-6. Using the ADD mode for common-mode rejection. (A)
usecf. A DC voltage of the Proper polarity
Channel 3 contains desired information along with line-
Channel 2 can be used to cancel out the DC portion of the
frequency component. (B) Channel 2 contains line frequency only.
signal applied toChannel 1.
(Cf ~estrltant CRT display using common-mode rejection.
7A18
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Change information, if any, affecting thissection will be found at the rear of thismanual.
l ntroduction
This section of the manual contains adescription of the
circuitry used inthe 7A18 dual-trace amplifier. The descrip-
tion begins with
a discussion of the instrument using the
block diagram shown in the Diagrams section. 'Then, each
circuit is described in detail using block diagrams to show
the interconnections between stages in each major circuit
and the relationship of the front-panel controls to the indi-
vidual stages.
Complete schematics of each circuit are given in the
Diagrams section. Refer to these schematics throughout the
following circuit description for electrical values and
relationship.
BLOCK DIAGRAM
The following discussion is provided to aid in nnder-
standing the overall concept of the 7A18 before the indivi-
dual circuits are discussed in detail. Only the basic inter-
connections between the individual blocks are shown on
the block diagram (see Diagrams section). Each block
represents a major circuit within the instrument. The
number on each block refers to the schematic onwhich the
complete circuit
is found.
The signal to be displayed on the CRT is applied to the
input connector. The signal passes through the input
coupling switch, where the appropriate coupling
is selected,
to the attenuators. The VOLTSIDIV switch selects the
correct amount of attenuation and the signal
is passed to
the input amplifier.
The Channel
1 Input Amplifier circuit provides gain
setting, variable gain control, and trace positioning. The
Channel
2 InputAmplifier provides signal polarity inversion
in addition to gain setting, variable gain control, and trace
positioning. The outputs of these circuits are applied push-
pull to the Signal and Trigger Channel Switches.
The ChannelSwitches select the proper signal and trigger
as determined by the DISPLAY MODE and TRIGGER
SOURCE switches. The signal and trigger outputs are
provided to the oscilloscopevia the Interface Connector.
The Readout Encoding circuit (7A18 only) provides
readout logic for the oscilloscope readout system. Data
IS
supplied to the mainframe readout system identifying the
polarity, deflection factor, the uncalibrated symbol (when
the VARIABLE control is inthe outward position), and the
plug-in mode. When the IDEN-SIFY button
IS pressed, the
trace is deflected about
0.3 division and the deflection
factor readout is replaced by the word "IDEN1-IFY".
DETAILEDCIRCUIT' DESCRIP'TION
ATTENUATOR
General
The Attenuator circuit determines the input coupling
and the 7A18deflection factor.
NOTE
The Cff I and Cff 2 Attentlator circuitsare identicai.
To minimize duplication, only CH 7 is described in
detail throi~ghouttt~isdiscussion.
AC-GND-DC Switch
Input signals connected to the input connector can be
AC-coupled, DC-coupled, or internally disconnected.
SlOOA is a cam-type switch; a contact-closure chart
showing the operation
is given on Diagram 1. The dots on
this chart indicate when the associated contacts are
III the
position shown (open or closed). When the AC-GND-DC
switch
is in the DC position, the input signal is cot~pled
directly to the input Attenuator stage. In the AC position,
the input signal passes through capacitor C10. This cap-
acitor prevents the DC component of the signal from
passing to the amplifier. The GND position opens the signal
path and connects the input circuit of the amplifier to
ground. This provides a ground reference without the need
to disconnect the applied signal from the input connector.
Resistor
R 102, connected across the AC-GND-DCswitch,
allows C10 to be precharged in the GND position so the
trace remains on screen when switching to the AC position
if the applied signal has a high DC level.
7A18
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Circuit Description--7A18/7Al8N
lnput Attenuator
The ef.fective overall deflection factor of the 7A18 is
determined by the setting of the VOLTSIDIV switch,
SIOOB. The basic deflection factor is five millivoltsper divi-
sion of CRT deflection. To increase the basic deflection
factor to the values indicated on the front panel, precision
attenuators are switched into the circuit. These attenuators
are hybrid devices which contain the necessary resistances
and capacitors. Each attenuator is replaceahie as a unit.
SIOOB is a cam-type switch and the dots on the contact-
closure chart (see Diagram 1) indicate when the associated
contacts are in the position shown (open or closed). Inthe
5 mV/Div position, inputattenuation
is not used; the input
signal is connected directly to the input amplifier.
For switch positions above five millivolts, the atten-
uators are switched into the circuit singly or in pairs to
produce the deflection factor indicated on the front panel.
These attenuators are frequency-compensated voltage
dividers. For DC and low-frequency signals, the attenuators
are primarily resistancedividers and the voltage attenuation
is determined by the resistance ratio in the circuit. The
reactance of the capacitors in the circuit is so high at low
frequencies that their effect
is negligible. However, at
higher frequencies, the reactance of the capacitors decreases
and the attenuator becomesprimarily acapacitancedivider.
In addition to providing constant atLenuation at all
frequencies within the bandwidth of the instrument, the
input attenuators are designed to maintain the same input
RC characteristics (one megohm
X 20 pF) for each setting
of the VOLTS/DIV switch. Each attenuator contains an
adjustable series capacitor to provide correct attenuation at
high frequencies and an adjustable shunt capacitor to pro-
vide correct inputcapacitance.
CHANNEL 1
INPUTAMPLIFIER
General
The Channel 1 lnput Amplifier converts the single-ended
signal applied to the Channel 1 input connector to a dif-
ferential (push-pull)output. Fig. 3-1shows a detailed block
diagram of the Channel 1 lnput Amplifier. A schematic of
this circuit is shown on Diagram 2 inthe Diagrams section.
lnput Source Follower
The InputSource Follower Q210A provides a high input
impedance with a low impedance drive for the following
stage. R210 limits the current drive to the gate of Q210A.
Dual-diode CR210 provides circuit protection by limiting
the voltage swing at the gate of 0210A to about
4 (positive
or negative) 15 volts. Q210B provides a constant current
source for 0210A. 0210A and Q210B are encapsulated in
the same case so that 0210B temperature-compensatesthe
circuit.
Paraphase Cascode Amplifier
Paraphase amplifier 0220-0320, in conjunction with
0225-0325, forms acascode amplifier. Q220-Q320convert
the single-ended input signal to adifferential output signal.
Diodes CR220-CR221 hold the voltage level at the base of
0220 close to ground to limit the voltage swing to about
40.6 volt. Common-base connected 0225-Q325 provide
isolation between the paraphase amplifier and the GAIN-
VARIABLE controls. 'Phe gain of the Channel 1 lnput
Amplifier is set in this stage by front-panel GAlN control
R237A with the CAL IN switch pressed in. When the CAL
IN switch is in the outward (uncalibrated) position and
turned fully co~~nterclockwiseto minimum resistance, the
gain of the amplifier is reduced by a factor of at least 2.5.
Adjustment 1 R321varies the base level of Q320 to provide
the same voltage levelsat the collectors of (2225 and 0325.
This prevents a rerouolt reference trace from changing
position when varying the GAlN or VARIABLE controls.
Second Cascode Amplifier
The Second Cascode Amptifier stage provides a signal
gain of approximateiy two. This stage includes the POSI-
TION control and, in the 7A18 only, trace IDENTIFY
circuitry. The emitters of common-base connected
(2250-0350 provide a low-impedance point for injection of
the POSITION control and IDENrlFY switch currents.
Position of the trace is determined by the setting of the
POSITION control, R11. This control changes the current
drive to Q250-0350. Since the emitters are a very low-
impedance point in the circuit, there is negligible voltage
change at these points. However, the change in current
from the POSI1"ION control produces a resultant DC
voltage difference at the collectors to change the position
of the trace. Trace identification is accomplished by
inserting resistor R357 from ground through CR357 tothe
junction of R11-R256. This results in a slight increase in
the emitter current of Q250 to cause the trace to move.
This aids in identifying the channel 1 trace when multiple
traces are displayed.
The network C246-C345-C245-R246-R345-R245 pro-
vides high frequency compensation. R245-C245in this net-
work provide high-frequency response adjustment for this
stage.
Emitter Follower
Emitter Follower stage 0260-(2360 provides a low out-
put impedance to drive the Signal and Trigger Channel
Switches, U270-U470. This stage also provides isolation
betweenthe Second Cascode Amplifier and U270-U470.
7A18
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Circuit Description-7Al8/7A18N
CHANNEL SWITCH
-+ U470
Fig. 3-1. Channel 1 lnputAmplifier detailed block diagram.
CHANNEL 2 INPOUAMPLIFIER
General
The Channel 2 lnput Amplifier circuit is basically the
same as the Channel
1 lnput Amplifier circuit. Only the
differences between the two circuits are described here.
Portions of this circuit not described in the following des-
cription operate in the same manner as for the Channel
1
lnput Amplifier circuit (corresponding circuit numbers
assigned in the
400 599 range).Fig. 3-2 showsadetailed
block diagram of the Channel
2 lnput Amplifier circuit. A
schematic of this circuit
is shown on Diagram 3 in the
Diagramssection.
Paraphase Cascode Amplifier
The Paraphase Cascode Amplifier for Channel 2 consists
of
Q420, Q520, (3425, Q525,0426, and Q526. Inaddition
to the functions described under Channel
1 lnput Ampli-
fier, the Channel
2 Paraphase Cascode Amplifier stage
provides a means of invertingthe displayedsignal. With the
CH
2 POLARI1-Y switch set to +UP, 0425 and 0525 are
biased on and the signal
is passed to the Second Cascode
Amplifier stage as for the Channel
1 lnputAmplifier. With
the
CH 2 POLARITY switch set to INVERT, Q425 and
Q525 are biased off and Q426-Q526 are turned on to
providesignal inversion.
FROM
INPUT
ATTEN
PARAPHASE
FROM
S23
17A18 ontv)
TO
SIGNAL
GMANNEL
SWITCH
U270
CHANNEL
SWiTCW
U470
Fig. 3-2. Channel 2 lnputAmplifier detailed block diagram.
REV. D, DEC. 1976
7A18
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Circuit Description-7Al8/7A18N
Second Cascode Amplifier
Ttie Second Cascode Amplifier for Channel 2 consistsof
0440, 0540, Q450, and Q550. Position of the trace
is set
by the POSITION control, R21 or by network R455-H555
as determined by the DISPLAY MODE switch. Inany DIS-
PLAY MODE switch position other than ADD,
i 50volts is
applied tothe center arm of the POSl I^IONcontrol through
R32. The POSITION control varies the current drive tothe
emitters of Q450-0550. Since the emitters are a very low-
impedance point in the circuit, there is negligible voltage
change at these points. However, the change in current
from the POSITION control produces a resultant DC
voltagt-:difference at the collectors to change the position
of the trace. When the DISPLAY MODE switch is in the
ADD position, +50 volts
is applied to the junction of resis-
tors R455-R555through R32 to balance the current drive
to the emitters of (2450-0550. This results in a fixed zero
volts (approximately) difference between the collectors.
Since +50 volts is not applied to the POSII-ION control in
the ADD position of the DISPLAY MODE switch, the
control setting has no effect on the circuit operation.
CHANNEL SWITCHES
General
Ttie ChannelSwitchescircuit provides Signal and Trigger
outputs to the oscilloscope via the Interface Connector as
determined by the DISPLAY MODE and THIC<GER
SOURCE switches. A schematic of this circuit is given on
Diagram 4 inthe Diagramssection.
Signal Channel Switch
The Signal Channel Switch stage consists of integrated
circuit U270and itsexternal components. This staqe selects
DISPLAY MODE
SELECTED
mm ---
*Level is switched between the HI-level and LO-level at an approxi-
mate
0.5 megahertz rate.
I
**Level is switched between the Hl-level and LO-level at a rate
determined by the setting
of the time-baseunit sweep rate.
I
Fig. 3-3. U270 input combinaliotis for DISPLAY MODE selection
'Trigger Channel Switching
The Trigger Channel Switch U470 is identical to the
Signal Channel Switch. This stage determines which input
(CH 1 or CH 2) provides the trigger stgnal for internal
triggering of the time-base unit.
7 he selection of the trigger
signal
is controlled by inputs from the TRIGGER SOURCE
switch. Resistors R476-R477 and R576-R577 set the cur-
rent gain for each channel. Networks
C474-R474-C475-R475 and C574-R574-C575-R575 pro-
vide high-frequencycompensation for each channel.
An inputloutput table for this stage is shown in Fig. 3-4.
Wlien the level at pin 14 is LO, the output of U470 is
determined
by the level at pin4. With the level at pin 14 HI
and the level at pin 4 LO, the channel
1 and channel 2
triggers are added algebraically.
one, or mixes two input analog signals in response to inputs
from tlie DISPLAY MODE switch. The Signal Cliannel
Signal and
Output
Switch stage determines which input (CH 1 or CH 2) pro-
vides the signal to the oscilloscope as controlled by the 'Phe Signal Output stage, Q280-Q380, and the Trigger
DISPLAY MODE switch setting. Resistors R276-R277and Output stage, 0480-0580, are similar. Each stage consists
R376-R377set the current gain for each channel. Networks of a pair of common-baseconnected transistors which pro-
C274-R274-C215-R275 and C374-R374-C375-R375 pro- vide the DC level shifting necessary to drive the mainframe
vide high-frequency compensation for each channel. C275 circuits.
and C315 in these networks are high-frequency compen.
sation adjustments.
DISPLAY MODE AND
Fig. 3-3 shows the U270 input combinations for each
position of the DISPLAY MODE switch. When the level at
pin 14 is LO the output of U270is determined by the level
at pin
4. With the level at pin 14 HI and the level at pin4
LO, the signals from both channel 1 and channel 2 are
passed to the Signal Output stage. This condition occurs
only when the DISPLAY MODE switch is set to ADD. In
this operating mode the signal output
is the algebraic sum
of channel 1 and channel
2 signals and the resultant signal
determines the mainframedeflection.
TRIGGER SWI"T'CHING
General
The Display Mode and Trigger Switching circuit deter-
mines which input signal (Channel
1 or Channel 2) providcs
the Signal and Trigger outputs to the mainframe as selected
by the DISPLAY MODE and TRIGGER SOURCE switches.
This circuit also provides plug-in mode information to the
mainframe chop blanking circuit, and readout control infor-
mation for proper CRT display.
7A18
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