Tektronix 2A61 User manual

INSTRUCTION
MI ANI WAL Serial Number 27.3 2.3
Tektronix, Inc.
S.W. Millikan Way @ P. ©. Box 500 @ Beaverton, Oregon 97005 @ Phone [phone] @ Cables: Tektronix
070-0328-01 367

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General Information
The Type 2A61 is a high sensitivity differential amplifier
designed for use with all Tektronix 560-series Oscilloscopes.
The amplifier provides calibrated deflection factors from
10 pV/division to 20 mV/division in a 1-2-5 sequence.
The upper and lower cutoff frequencies of the amplifier
are adjustable. In addition, a line frequency notch filter can
be switched in to minimize signals in the power line fre-
quency region.
SECTION I!
Type 2A61
CHARACTERISTICS
A trace restorer pushbutton returns the trace if it is driven
off the CRT graticule area by a large transient.
A front panel control, the Input Selector, can connect the
amplifier input to an internal common mode signal for a
check of common mode rejection; ground the inputs of the
amplifier for a no-signal reference display; select either
Input A or B; or select A minus B.
The following characteristics apply over an ambient tem-
perature range of 0° C to 50° C. Warm-up time to reach
specified performance is 20 minutes at 25° C, 5° C.
Characteristic
Performance Requirement
Supplemental Information
Deflection Factor
0.01 mV/div to 20 mV/div, =5%
GAIN AD§J control properly set and VARI-
ABLE at CALIB. Steps in 1-2-5 sequence.
VARIABLE range
>2.5 to 1
Maximum Bandwidth
<0,06 Hz to >0.3MHz, at 20 mV/div
<0.1 Hz to >0.1 MHz, at 0.01 mV/div
FREQ RESPONSE TO
switch (High Frequency —3dB
point)
Selectable in fixed steps. Within 30%
of indicated frequency at 300-100 kHz (de-
pending upon mV/div switch setting}, 60
kHz, 6kHz, 600 Hz and 60 Hz.
Risetime (10-90%}. (Based on cutoff fre-
quency within +30% of front-panel fre-
quency indication.)
60 kHz 4.5 to 8.3 ps
6 kHz 45 to 83 ys
600 Hz 0.45 to 0.83 ms
60 Hz 45 to 8.3 ms
FROM switch (Low Frequency —3
dB point)
Selectable in fixed steps. Within +30%
of indicated frequency at 0.06 to 0.1 Hz (de-
pending upon mV/div switch setting}, 0.6
Hz, 6Hz, 60Hz and 600 Hz.
Decay Time Constant (Based on cutoff fre-
quency within +30% of front-panel fre-
quency indication).
600 Hz 0.21 to 0.37 ms
60 Hz 2.1 to 3.7 ms
6 Hz 21.0 to 37.0 ms
0.6 Hz 0.21 to 0.37 s
POSITION control Range
> +6 divisions
Short Circuit Noise
<3.5 uV (RMS), referred to input, (0.01 mV/
div and maximum frequency response).
2220 uN peak to peak. Input at GND.
Grid Current
A, —B input grid current, 10nA (nano-
amperes) max.
Input R and C
10 Megohms, +1%, paralleled by approxi-
mately 100 pF
For each input to ground
Line Filter
Attenuation at 60 Hz
>50:1 (with 60Hz filter}
Insertion Loss
<15% at and above 1 kHz (with 60 Hz
filter)
Filters for 50Hz and 400Hz available
(60 Hz standard)
Common Mode Rejection Ratio
50,000:1 below 10 kHz, 5V peak to peak
common mode sine-wave input.
MILLIVOLTS/DIV switch set to .05
Maximum Common-Mode
Voltage
Input
+5V
A—B mode
1-1

Characteristics—Type 2A61
Characteristic
Performance Requirement
Supplemental Information
Dynamic Range
+15 divisions minimum, about the zero-
input
level, with the VARIABLE control at
CALIBRATED
For other settings of the VARIABLE, the
dynamic range is reduced in proportion to
the gain, and on-screen distortion may ap-
pear.
Differential Input DC Bias
+0.1 V maximum
May be superimposed on the AC input sig-
nal with no change in instrument perform-
ance.
Overload Recovery Time
2 minutes maximum {with +0.1 V DC bias)
A recovery period of at least 2 minutes
should be allowed after applying maximum
allowable bias, before any measurement is
attempted.
DC Balance DC BAL adjustable for no trace shift with | Inputs at ground
operation of MILLIVOLTS/DIV Control
throughout its range
1-2

Type 2A61
SECTION 2
OPERATING INSTRUCTIONS
Introduction
This section gives a brief description or front panel con-
nectors, controls and signal overload indicator.
INPUT CONNECTOR The INPUT connector is designed to
mate with the low noise input cable
supplied with the instrument. Pin
connections are shown on the front
panel.
The INPUT SELECTOR is a five posi-
tion rotary switch that selects the
method of coupling A and B Inputs
to the amplifier.
A—B: Connects the Amplifier to A
and —B inputs for differential oper-
ation.
GND: Connects the Amplifier to
ground without grounding A and
—B Inputs.
INPUT SELECTOR
CM: Connects the Amplifier to a com-
mon 5 volt peak to peak sine-wave
at power line frequency for a com-
mon-mode rejection check.
A: Connects the + side of the Ampli-
fier to the A Input and connects the
— side of the Amplifier to ground.
—B: Connects the — side of the
Amplifier to the B Input and connects
the + side of the Amplifier to
ground.
NOTE
A signal applied to the A Input will be displayed
in the same polarity, positive upward, with the
INPUT SELECTOR switch in the A position, The same
signal applied to —B Input will be displayed
inverted with the INPUT SELECTOR switch in the
—B position.
MILLIVOLTS/DIV. Selects calibrated deflection factors
when VARIABLE is in the CALIB.
position.
MILLIVOLTS/DIV. Provides continuously variable de-
VARIABLE flection factor to at least 2.5 times
the setting of MILLIVOLTS/DIV.
switch.
CAUTION
When the VARIABLE control is in the minimum gain
(fully counterclockwise) position and the POSI-
TION control is at either end of its range, severe
signal distortion may appear on the screen for cer-
tain settings of the MILLIVOLTS/DIV. control.
©i
DC BAL. Front panel screwdriver adjustment.
Adjusts for minimum trace shift as the
MILLIVOLTS/DIV. switch is rotated
throughout its range.
DIFF BAL. Front panel screwdriver adjustment.
Sets optimum common-mode rejec-
tion.
LINE FREQUENCY With FILTER switch set to IN, signals
FILTER at power line frequency are attenu-
ated approximately 34 dB, The chart,
Fig. 2-1, shows —3 dB points of the
Filter.
FREQ RESPONSE FROM: (Black knob) Sets the approxi-
mate lower 3dB down frequency
limit. See Chart, Fig. 2-1.
TO: (Red knob) Sets the approximate
upper 3 dB down frequency limit. See
Chart, Fig. 2-1.
GAIN ADJ. Screwdriver adjustment for changing
the gain of the Type 2A61 to com-
pensate for differences in deflection
factors of different indicator units.
POSITION Provides vertical positioning of the
display. Clockwise rotation moves
the trace up. Counterclockwise rota-
tion moves the trace down.
TRACE RESTORER The red button in the center of the
POSITION knob. Returns the trace
to its original position if the trace is
driven off the CRT screen by signal
transient or control operation. Neon
indicator lights whenever the display
is driven beyond the range of the
POSITION control.
GENERAL OPERATING INFORMATION
Signal Connections
Whenever possible, make all signal connections to the
Type 2A61 INPUT connector with the low-noise input cable
supplied with the instrument. If signal connections are made
to the Type 2A6] with leads other than those supplied, be
sure to use shielded cable.
When making single-signal measurements be sure to
establish a common ground between the chassis of the device
under test and the Type 2A61. The common ground provides
a reference for the voltage measurements. No ground con-
nection is required in differential measurements, but one
should be maintained as a safety measure.
2-1

Operating Instructions—Type 2A61
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Make certain that any signal carrying leads are positioned
away from stray electrostatic or AC magnetic fields that
might induce voltages into the input circuit of the Type 2A61.
This ‘is important because the Type 2A61 is a low-level, high
gain system in which stray signal of a millivolt or less can
cause serious measurement error.
The circuits under test should operate as closely as possible
to normal operating conditions. The amount of loading that
the input circuit of the Type 2A61 imposes on the circuit
under test must be considered. This is especially important
when the circuit under test offers a high source impedance.
Each input of the Type 2A61 has a resistance of 10 MQ,
+1%, shunted by about 100 pF. Due to the capacitive
component in the input circuit of the Type 2A61, loading
on the circuit increases with frequency. For example, the
input impedance of the Type 2A61 with an input frequency
of 3kHz is about 530kQ but with an input frequency of
50 kHz, input impedance decreases to about 32 k0.
It is important to note that shielded cable adds capacit-
ance to the input circuit of the Type 2A61. A 42-inch length
of 509 coaxial cable with BNC connectors, such as Tek-
tronix part number 012-0057-00, will add about 110 pF to the
input capacitance.
Setting the GAIN ADJ Control
The GAIN ADJ control should be checked and/or adjusted
each time the Type 2A61 is used with a new or different
indicator unit, and occasionally during regular operation,
to insure reliability of measurement.
To set the GAIN ADJ contro! proceed as follows:
1. Plug the Type 2A61 into the left-hand plug-in compart-
ment of a Tektronix 560-series Oscilloscope and tighten the
securing knob. Turn on the power and allow about 5 minutes
for the system to reach operating temperature.
2. Free run the oscilloscope sweep at 2ms/diy and con-
nect a suitable patch cord between the oscilloscope calibrator
and the A terminal of the Type 2A61 INPUT connector. Set
the
oscilloscope calibrator for a 50 mV (0.05 volt} input.
3. Set the Type 2A61 front panel controls as follows:
INPUT SELECTOR A
MILLIVOLTS/DIV 10
VARIABLE CALIB
FREQ RESPONSE
FROM .06 to .1 Hz
TO 1 to .3 MHz
FILTER : OUT
4. Check for exactly five major graticule divisions of
deflection. If the deflection is not correct set the GAIN ADJ
control for five major divisions. Use the Type 2A61 POSI-
TION control as necessary to keep the display centered.
Setting the DIFF BAL Control
The DIFF BAL control (differential balance) should be set
occasionally during regular operation of the TYPE 2Aé6l,
particularly prior fo making critical differential measurements.
Either of two methods may be used to set the DIFF BAL
control. The first of the methods to be described uses the
®
Operating Instructions—Type 2A61
internal 5 volt peak to peak line frequency source. The
second uses an external 5 volt peak to peak signal.
To set the DIFF BAL control by the first method, proceed
as follows:
Method 1
1. Plug the Type 2A61 into the left-hand plug-in compart-
ment of a Tektronix Type 560 Series oscilloscope and tighten
the securing knob. Turn on the oscilloscope power and
allow about 5 minutes for the system to warm up.
2. Set the Type 2A61 front panel controls as follows:
INPUT SELECTOR CM
MILLIVOLTS/DIV |
VARIABLE CALIB
FREQ RESPONSE FROM—.6 Hz
TO—.1 MHz to .3 MHz
FILTER OUT
3. Trigger the oscilloscope time base from the line and set
the sweep rate to 10 ms/div.
4, Position the trace vertically to the graticule center line,
using the Type 2A61 POSITION control and/or the Trace
Restorer button. Adjust the DIFF BAL control for minimum
vertical deflection. One major division of deflection with
the mV/DIV switch set at .1 indicates a common mode rejec-
tion ratio of 50,000:1.
If the 1 division requirement for 50,000:1 rejection ratio
cannot be met by adjusting the DIFF BAL control, the Type
2A61 should be recalibrated. See Section 6.
Method 2
1. Plug the Type 2A61 into the left-hand plug-in compart-
ment of a 560-series Oscilloscope and tighten the securing
knob. Turn on the oscilloscope power and allow 5 minutes
for the system to reach operating temperature.
2. Set the Type 2A61 front panel controls as follows:
INPUT SELECTOR A—B
MILLIVOLTS /DIV J
VARIABLE CALIB
FREQ RESPONSE
FROM .06 Hz
TO .1 MHz to 3 MHz
FILTER OUT
3. Apply 5 volts peak to peak signal (preferably the signal,
or a signal similar to the signal to be rejected) to the A and
B INPUT terminals of the Type 2A61.
4. Set the oscilloscope time base at a sweep rate con-
sistent with frequency of the signal applied. Set the oscillo-
scope triggering for a stable display. Position the trace as
necessary with the Type 2A61 POSITION and/or Trace
Restorer button.
5. Adjust the Type 2A61 DIFF BAL control for minimum
vertical deflection. One major division of deflection with
the MILLIVOLTS/DIV at .1 indicates a common mode rejec-
tion ratio of 50,000:1 with a 5 volt peak to peak signal
applied.
2-3

Operating Instructions—Type 2A61
(B) Signal applied to —INPUT.
(C) Resultant display.
Fig. 2-2. Typical CRT displays showing differential rejection of common mode signal. Resultant waveform (C) displays the difference in
signals A and B.
Setting the DC BAL Control
The DC BAL control may need to be set occasionally dur-
ing regular operation of the Type 2A61. Proper DC balance
minimizes trace shift when rotating the MILLIVOLTS/DIV
switch from one position to another.
To set the DC BAL control, proceed as follows:
1. Plug the Type 2A61 into the left-hand plug-in compart-
ment of a Tektronix 560-series Oscilloscope and tighten the
securing knob. Turn the oscilloscope power on and allow
about 5 minutes for the system to reach operating tempera-
ture.
2. Free run the oscilloscope time base at a sweep rate
of 2 ms/div.
3. Set the Type 2A61 front panel controls as follows:
INPUT SELECTOR GND
MILLIVOLTS/DIV 5
VARIABLE CALIB
FREQ RESPONSE
FROM 60 Hz
TO 60 Hz
FILTER OUT
4. Vertically position the trace to the CRT graticule center
line using the Type 2A61 POSITION control.
5. Reset the MILLIVOLTS/DIV switch to .01.
6. Adjust the DC BAL control to return the trace to the
graticule center line.
2-4
7. Reset the MILLIVOLTS/DIV switch to .5 and repeat steps
4, 5 and 6 until there is no trace shift while rotating the
MILLIVOLTS/DIV switch between .5 and .01.
Fig. 2-3. Example of a measurement situation in which CMRR is only
1000:1.