11. DIGITAL STORAGE OSCILLOSCOPE AND
ARBITRARY WAVEFORM GENERATOR
11.1. Tasks of the measurement
11.1.1. Make yourself acquainted with block diagrams of digital storage oscilloscope
(Fig. 11.2) and arbitrary function generator (Fig. 11.3).
11.1.2. Derive the conditions for frequency independent division ratio of the passive
oscilloscope probe 10:1 – see Fig. 11.1.
11.1.3. Using arbitrary waveform generator (settings: frequency 1 kHz, amplitude 1 V,
rectangle) perform a compensation of the probe 10:1 model (connected to
oscilloscope Channel 1). Draw the observed waveform to your notebooks for both
limit positions of the tuning knob (“overcompensated”/”undercompensated” probe)
and evaluate the signal parameters (amplitude, frequency, rise time, fall time and
overshot where relevant). Use cursor functions for this evaluation. Observe and draw
also the waveform from Channel 2 (the same signal acquired by professional probe).
11.1.4. Switch the generator to the “Burst” mode. Measure the overshot in the case of
“overcompensated” probe. Use triggering on negative edge and explain the
“pretrigger” oscilloscope mode that is being used in this case.
11.2. Block diagram
Rp= 9 M
G
GENERATOR
Ro = 1 M
Co = 14 pF(*)
Cp
PROBE
OSCILLOSCOPE
Fig. 11.1 Circuit connection ((*) value of C o is given for the used type of digital oscilloscope)
11.3. List of the equipment used
OSCILLOSCOPE - type ...;
G
- arbitrary waveform generator, type ...;
PROBE
- oscilloscope passive voltage 10:1 with frequency compensation.
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11.4. Theoretical background
a) Digital oscilloscope description can be found in [1], Chap. 5.2.1.2, Fig. 5.13, arbitrary
waveform generator - Chap. 5.5.4, Fig. 5.41. Only block diagrams are given below.
INPUT
(4)
INPUT
(1)
CHANNEL 4 (DI, S/H, ADC, M)
MICROPROCESSOR
DI
EXT.
TRIG.
S/H
COMPARATOR
ADC
M
CLOCK
TIMER
RAM
VIDEOPROCESSOR
STD. INTERFACE
IEEE 488
(RS-232)
Fig. 11.2 Block diagram of the digital oscilloscope (DI – input divider and amplifier, S/H – sample and
hold circuit, ADC – analogue-to-digital converter, M - memory)
DIGITAL
INPUT
(N x k bit)
MEMORY
Nxk
DAC1
(k bit)
FILTER
ANALOG
OUTPUT
UA
COUNTER
“1 to N”
fS
DAC2
AMPLITUDE
(DIGITAL INPUT)
UN
Fig. 11.3 Block diagram of arbitrary waveform generator
(DAC1, DAC2 – digital-to-analog converters)
b) To minimize signal distortion during measurement, it is necessary to increase input
impedance of the oscilloscope and assure the input signal path shielding. Therefore,
passive (event. active) voltage probes are used (see Fig. 11.1). Transit impedance of
the probe is given by parallel combination R p || C p and it creates together with input
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oscilloscope impedance (parallel combination R o || C o ) a voltage divider (with division
ration 0.1 or 0.01). To assure frequency independent division ratio, the ratio of real parts of
denominator and numerator must be equal to the ratio of imaginary parts. After
recalculation it gives
Rp
Ro

Co
Cp
(11.1)
To achieve the constant frequency response, the variable capacitor C p is used both in our
model case and in real probes. In the case of real probe, a miniature capacitance trimmer is
used, which is to be set by proper insulating tool (plastic screwdriver). The real probe that
is available on CH 2 is compensated already. There is no need to compensate it again.
Usually, there is a calibration source of the rectangular signal available on the oscilloscope
for probe compensation in the field (we will not use it in our exercise).
c) Definition of rise time is available in Task No. 1. Overshot is defined as difference
between signal maximum (peak) and its steady-state value (before new pulse arises). It is
usually expressed in % of the steady state value.
c) The term „burst“ means a group of pulses. Since the default number of pulses in the
group is 1 in the case of our generator, the burst mode will be used for generation of
short pulse (its length is the same as in the previous task, it means 500 s) with
repetition rate app. 1 Hz. In this case, we cannot trigger on the signal rising edge (with
overshot) since we need to display the rising edge fully. Therefore the falling edge has
to be used for trigger and, consequently, a special mode „pre-trigger“ must be used
for displaying the waveform before trigger event. The detailed description of this mode
is available in [1], Chap. 5.2.1, see also Fig. 5.15 there.
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