35
Subject to change without notice
With the start of each recording, the signal voltage at the
input(s) of the analog-to-digital converters is briefly measured
(sampled), converted to an 8-bit value and written to an
address in RAM. The next sampled value is converted in the
same way, but stored at the next RAM address.
The maximum sampling rate is 100MS/s. This yields a sampling
interval of 10ns. Assuming that 10 measurements (samples)
per signal period are sufficient for a sinewave signal, the
maximum frequency to be captured is 10MHz (100ns 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 sinewave signal may be sampled only once per period. If
the sinewave 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 between
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.
Resulting corrections and readjustments inside the instrument,
indicated by the following tests, are described in the “Service
Manual”. They should only be undertaken by qualified personnel.
A chargeable “Service Manual” is available from
HAMEG
. The
“Service Manual” describes the complete adjustment of the
instrument in English language and contains the circuit diagrams
as well as the component locations.
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 (particularly
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
account be visible. Visible trace fault 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 display 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 voltage 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
HAMEG
. 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. If it is possible
to improve this vertical sharpness by turning the FOCUS
control, then an adjustment of the astigmatism control is
necessary. A potentiometer of 47k Ohm is provided inside the
instrument for the correction of astigmatism. 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 Channel I and II (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-
Test Instructions
