7
R18. SINE INTERPOLATION
When the displa mode is set to MAG (magnif ) the number of
true samples shown across the screen is reduced to 102. Al-
though 1,024 samples are displa ed, 9 samples out of each block
of 10 are mathematicall calculated using linear interpolation.
Linear interpolation provides a good reconstruction of the original
waveform provided that either the number of samples per c cle
of the waveform is reasonabl high (i.e. ten or more) or, if the
number of samples is low, that the original waveform is made up
mainl of linear elements (e.g. pulse, sawtooth, triangle
waveforms).
When the waveform is basicall sinusoidal and the number of
samples per c cle is low, linear interpolation will not provide a
satisfactor reconstruction and sine interpolation should be used
instead. Sine interpolation uses a mathematical curve fitting al-
gorithm and can provide a near perfect reconstruction of a sine
wave from onl four samples per c cle. See Appendix C for a
detailed discussion of interpolation.
Pressing the SINE INTERPOLATE ke toggles the function on or
off as indicated b the lamp above the ke . Interpolation is onl
active when the SEARCH mode is set to MAG. Sine interpolation
slows down the displa update rate considerabl .
R19. AVERAGE
The RUN AVERAGE function allows a number of digitisations of
the waveform to be summation averaged before being displa ed.
This improves the signal to noise ratio of a nois signal and can
often enable a signal to be observed that would otherwise have
been masked b large amounts of HF or LF noise. The average
function reduces the size of the digitising memories to 1K words
per channel.
The number of waveform digitisations which are averaged can be
Set to an number between 2 and 256. To change the number of
readings averaged press SHIFT (ke 8) followed b SET AVG (ke
4). The current averaging value is shown on the displa and can
be incremented or decremented using the REF CURS ke s (ke s 2
and 3). When the required number is displa ed, press ESCAPE.
Pressing the RUN AVERAGE ke enables averaging, pressing the
ke again disables it again, the status is indicated b the lamp
above the ke . Acquisitions continue to be controlled b the
RUN/HOLD and SINGLE ke s.
When the displa update rate is set to NORM the displa is
updated onl whenever the total number of averages has been
completed. When the update rate is set to SLOW the displa is
updated continuousl as averaging takes place. The displa
shows the number of averages currentl accumulated.
Note that averaging slows down the displa update rate. This is
particularl so at fast timebase speeds in "repeat" mode and slow
timebase speeds in normal mode.
R20. A= AXB (TRACE MULTIPLY)
This function provides a displa of the Trace A waveform multi-
plied b the Trace B waveform. It is particularl useful for ob-
taining power waveforms.
Press SHIFT (ke 8) followed b AxB (ke 2). The two waveforms
will be multiplied together and displa ed on Trace A (Trace B
continues to be displa ed). To return to a normal displa press
ESCAPE.
The multiplied waveform is scaled using units of one screen
division with the centre of the screen being zero. The traces are
multiplied exactl as the appear on the oscilloscope displa in-
cluding offsets introduced b the trace position controls. Thus at
a point where the position of Trace A is +2 divs and of Trace B is
—1.5 divs, the value of the multiplied waveform will be — 3 divs.
It should be noted that if either trace has values greater than ± 1
divs then overflows could occur on the multiplied waveform. The
displa ed waveform is limited to ± 4.25 divisions and conse-
quentl —3 divs multiplied b —2.5 divs will be displa ed as +
4.25 divs instead of + 7.5 divs.
The AxB function cannot operate while digitising is taking place.
Pressing the AxB ke will set RUN/HOLD to HOLD. The function
multiplies the traces exactl as the appear at the moment that
the AxB ke is pressed. When the function is enabled all of the
trace controls are inoperative with the exception of the Trace A
position control which can be used to adjust the position of the
multiplied waveform.
R21. LINE OR DOTS DISPLAY
In line displa mode the 1,024 individual points of the displa
are joined with angled lines to simulate one continuous line.
This gives a displa which is easier to view and less subject to
visual aliasing than one made up of dots (see Appendix B).
Sometimes, however, it is desirable to see the exact points on
the waveform where samples have been taken, along with their
levels. Pressing SHIFT (ke 8) followed b DOTS (ke 7) causes
the points to be joined b right-angled lines which form a stair-
case. Since the horizontal lines are brighter than the vertical
ones, this gives an impression of dots and makes it easier to
see each sample point and its associated level.
Also the line displa ma reduce the amplitude of waveforms
where successive samples can have ver large vertical
separations. Such a waveform will also appear brighter if a dots
displa is used.
To return to a line displa press LINE (ke 7). The unit defaults
to a line displa after RESET.
R22. PLOT
The waveforms shown on the oscilloscope displa can be sent
either to a digital X-Y plotter (HP-GL compatible) using the RS-
423 interface; or to an analog X-Y plotter or Y-T chart recorder
using the analog plotter interface.
The plot function can be directed either to the RS-423 interface
for use with a digital plotter, or to the analog plotter interface.
The DSA is factor initialised for use with a digital plotter. To
change to use with an analog plotter ke in "function 26" (press
FUNCTION NN, followed b 2 followed b 6). To return to use
with a digital plotter ke in "function 27".
a) Digital Plotter
The digital plotter must be compatible with HP-GL plotting com-
mands and must be fitted with a serial interface for connection
to the RS-423 interface socket of the DSA. For details of making
connections and of setting the baud rate see Section R25
’RS-423 Interface’. Alternativel , if the DSA has the IEEE-488
interface option fitted, a plotter with an IEEE-488 interface ma
be used.
To commence plotting press PLOT (ke 6), the time and voltage
per division and a graticule will be plotted followed b Trace A
followed b Trace B. If the plotter is a multi-pen plotter the text
and the graticule will be plotted using pen 1, Trace A will be
plotted using pen 2, and Trace B will be plotted using pen 3.
The entire plot fits onto an A4 sized sheet. To abandon plotting
at an stage press ESCAPE.
b) Analog Plotter
The analog plotter interface is mounted on the rear panel and
comprises five 4mm sockets marked X, YA, GND, YB, and PL
(pen lift).
The waveforms for Trace A and Trace B are sent simultaneousl
to their separate output sockets along with an X axis signal for
use with X-Y plotters. The 10.24 divisions of the displa are
plotted using a level of 100mV/div (for both Y and X) at a rate
selectable as 1, 2, 5 or 10 seconds per division. The pen-lift
signal is normall "low to lift" but can be inverted.
To set the plot rate, press SHIFT (ke 8) followed b SET PLOT
(ke 5)., The oscilloscope displa will show the present plot
rate. Press one of the REF CURS ke s (ke 2 or 3) to select a
new value, then press ESCAPE.
