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Examples:
Displayed wavelength
L
= 7div.,
set time coefficient
= 0.2 ^is/div.,
required period T
= 7 0.2-10"® =
1.4(is
required rec. freq. F
= 1 ;(1.4-10"®) =
714 kHz.
Signal period
T
= 0.5s,
set time coefficient
= 0.2s/div.,
required wavelength L
= 0.5:0.2 =
2.5div..
Displayed ripple wavelength
L
= 1 div.,
set time coefficient
= 10 ms/div.,
required ripple freq. F
= 1 : (1 -lO-IO"®) =
100 Hz.
TV-line frequency
F
= 15 625 Hz,
set time coefficient Tc = lO^rs/div.,
required wavelength L
= 1 :(15 625 -10"®) =
6.4div..
Sine wavelength
L
= min. 4div., max. 10div.,
Frequency
F
= 1 kHz,
max. time coefficient T;, = 1: (4-10®) = 0.25 ms/div.,
min. time coefficient Tc = 1 :(10-10®) = 0.1 ms/div.,
set time coefficient
= 0.2 ms/div.,
required wavelength L
= 1 (10® 0.2 10"®) =
5div.
Displayed wavelength
L
= O.Sdiv.,
set time coefficient T,, = 0.5pis/div.,
pressedX-MAG. xIO
button:
= 0.05^is/div.,
required rec. freq. F
= 1: (0.8-0.05-10"®) =
25 MHz,
required period T
= 1: (25 -10®) =
40 ns.
If the time is relatively short as compared with the complete
signal period, an expanded time scale should always be
applied
(X-MAG. xIO
button pressed). In this case, the
ascertained time values have to be divided by 10. The time
interval of interest can be shifted to the screen center using
the
X-POS.
control.
The following figure shows correct positioning of the oscil¬
loscope trace for accurate risetime measurement.
100
%
90%
10
%
0
With a time coefficient of 0.2ns/div. and pushed
X-MAG
xIO
button the example shown in the above figure results
in a measured total risetime of
t,ot = 1.6div.-0.2ns/div.: 10 =
32ns
When very fast risetimes are being measured, the rise-
times of the oscilloscope amplifier and of the attenuator
probe has to be deducted from the measured time value.
The risetime of the signal can be calculated using the fol¬
lowing formula.
In this ttot is the total measured risetime, tosc is the risetime
of the oscilloscope amplifier (approx. 17,5 ns), and tp the
risetime of the probe (e.g. = 2 ns). If ttot is greater than
100 ns, then t^t can be taken as the risetime of the pulse,
and calculation is unnecessary.
Calculation of the example in the figure above results in a
signal risetime
When investigating pulse or square waveforms, the critical
feature is the risetime of the voltage step. To ensure that
transients, ramp-offs, and bandwidth limits do not unduly
influence the measuring accuracy, the risetime is generally
measured between
70%
and 90% of the vertical pulse
height. For peak-to-peak signal amplitude of 6div. height,
which are symmetrically adjusted to the horizontal center
line, the internal graticule of the CRT has two horizontal dot¬
ted lines ±2.4div. from the center line. Adjust the Y
attenuator switch with its variable control together with the
Y-POS.
control so that the pulse height is precisely aligned
with these 0 and 100 % lines. The 10 % and 90 % points of
the signal will now coincide with the two lines, which have
a distance of ±2.4div. from the horizontal center line and an
additional subdivision of 0.2 div. The risetime is given by
the product of the horizontai distance in div. between
these two coincidence points and the time coefficient
setting. If magnification is used, this product must be
divided by 10. The /a/Zf/meof a pulsecan also be measured
by using this method.
tr = y 32® - 17.5® - 2® =
26.27 ns
The measurement of the rise or fall time is not limited to the
trace dimensions shown in the above diagram. It is only par¬
ticularly simple in this way. In principle it is possible to
measure in any display position and at any signal amplitude.
It is only important that the full height of the signal edge of
interest is visible in its full length at not too great steepness
and that the horizontal distance at 10% and 90% of the
amplitude is measured. If the edge shows rounding or over¬
shooting, the 100% should not be related to the peak val¬
ues but to the mean pulse heights. Breaks or peaks
(glitches) next to the edge are also not taken into account.
With very severe transient distortions, the rise and fall time
measurement has little sense. For amplifiers with approxi¬
mately constant group delay (therefore good pulse trans¬
mission performance) the following numerical relationship
between rise time
tr
{in ns) and bandwidth
B
(in MHz)
applies;
tr =
3|0
B
tr
Subject to change without notice
M5 203-7
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