Instruction manual

Subject to change without notice
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signal, the maximum frequency to be captured is 20MHz
(50ns per period).
Alias signal display
If, due to the time base setting, the sampling rate is too low,
the display of an alias signal may occur. For example: A sine
wave signal may be sampled only once per period. If the
sine wave signal frequency is accidentally in phase with the
sampling frequency and each sample is taken at the positive
peak value, a straight line is displayed in this position.
Alias signal display may also occur in the form of an apparent
untriggered waveform display of different frequency from
the true signal. Another aliasing condition is the display of
signals seeming to be amplitude modulated.
The easiest way to recognize alias signals is to switch to
analog mode, where the true waveform is displayed. Transfer
from analog to store mode without changing time base range
must produce the same frequency display.
Operating modes of the vertical amplifiers
In principle, the instrument can operate in digital storage mode
with the same operating modes as in analog mode. Thus,
the following can be displayed:
• Channel I by itself
• Channel II by itself
• Channel I and II simultaneously
• The sum or difference of both channels
• XY mode
Storage mode differs from analog operation of the scope in
the following respects:
• In DUAL mode (for simultaneous display of both channels),
both input signals are also simultaneously captured, since
each channel has its own A/D converter. In contrast to
analog mode therefore, it is not necessary to switch bet-
ween chopped and alternating mode.
• Because of the high display rate (approx. 80Hz) of the
screen, no flicker can occur.
• The storage time base is also active in XY storage mode.
Test Instructions
General
These Test Instructions are intended as an aid for checking
the most important characteristics of the instrument at regular
intervals without the need for expensive test equipment.
As with the First Time Operation instructions, care should be
taken that all variable functions are set to their calibrated
positions. This is the case if AUTO SET had been pressed. It
is recommended to switch on the instrument for about 20
minutes prior to the commencement of any check.
Cathode Ray Tube:
Brightness and Focus,
Linearity, Raster Distortion
Normally, the CRT of the instrument has very good brightness.
Any reduction of this brightness can only be judged visually.
Decreased brightness may be the result of reduced high
voltage. In this case the sensitivity of the vertical amplifiers
is significantly increased .
The control range for maximum and minimum brightness
(intensity) must be such that the beam just disappears before
reaching the left hand "stop" of the INTENS. control (parti-
cularly when in XY mode), while with the control at the right
hand stop the focus and the line width are just acceptable.
With maximum intensity the time base fly-back must on no
never be visible. Visible trace faults without input signal:
• bright dot on the left side or decreasing
• brightness from left to right or shortening of the baseline.
(Cause: Incorrect Unblanking Pulse.)
It should be noted that with wide variations in
brightness, refocusing is always necessary. Moreover,
with maximum brightness, no “pumping” of the dis-
play must occur. If pumping does occur, it is normally
due to a fault in the regulation circuitry for the high
voltage supply. The presetting pots for the high vol-
tage circuit, minimum and maximum intensity, are
only accessible inside the instrument.
A certain out-of-focus condition in the edge zone of the screen
must be accepted. It is limited by standards of the CRT
manufacturer. The same is valid for tolerances of the
orthogonality, the undeflected spot position, the non-linearity
and the raster distortion in the marginal zone of the screen in
accordance with international standards (see CRT data book).
These limit values are strictly supervised by B&K. The
selection of a cathode-ray tube without any tolerances is
practically impossible.
Astigmatism Check
Check whether the horizontal and vertical sharpness of the
display are equal. This is best seen by displaying a square-
wave signal with the repetition rate of approximately 1MHz.
Focus the horizontal tops of the square-wave signal at normal
intensity, then check the sharpness of the vertical edges. A
certain loss of marginal sharpness of the CRT is unavoidable;
this is due to the manufacturing process of the CRT.
Symmetry and Drift of the Vertical Amplifier
Both of these characteristics are substantially determined
by the input stages of the amplifiers.
The symmetry of both channels and the vertical final amplifier
can be checked by inverting (depress the corresponding INV
pushbutton). The vertical position of the trace should not
change by more than 0.5div. However, a change of 1div is
just permissible. Larger deviations indicate that changes have
occurred in the amplifier. A further check of the vertical
amplifier symmetry is possible by checking the control range
of the Y-POS. controls. A sine-wave signal of 10-100kHz is
applied to the amplifier input. When the Y-POS. control is
then turned fully in both directions from stop to stop with a
display height of approximately 8div, the upper and lower
positions of the trace that are visible should be approximately
of the same height. Differences of up to 1div are permissible
(input coupling should be set to AC).
Checking the drift is relatively simple. 20 minutes after
switching on the instrument, set the baseline exactly on the
horizontal center line of the graticule. The beam position must
not change by more than 0.5div during the following hour.
Calibration of the Vertical Amplifier
A square-wave voltage of 0.2Vpp ±1% is present at the output
socket of the calibrator (CAL.) If a direct connection is made
between the 0.2V output and the input of the vertical amplifier
(e.g. using a x1 probe), the displayed signal in the 50mV/div
position (variable control to CAL.) should be 4div ±.16div high
Test Instructions