Subject to change without notice
10
With a time coefficient of 10 ns/div (X x10 magnification active),
the example shown in the above figure results in a total measured
risetime of
t
tot
= 1.6 div x 10 ns/div = 16 ns
When very fast risetimes are being measured, the risetimes of
the oscilloscope amplifier and of the attenuator probe have to be
deducted from the measured time value. The risetime of the
signal can be calculated using the following formula.
t
r
=
√
t
tot2
– t
osc2
– t
p2
In this t
tot
is the total measured risetime, t
osc
is the risetime of the
oscilloscope amplifier (approx. 7 ns), and t
p
the risetime of the
probe (e.g. = 2 ns). If t
tot
is greater than 100 ns, then t
tot
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:
t
r
=
√
16
2
– 7
2
– 2
2
= 14.25 ns
The measurement of the rise or fall time is not limited to the trace
dimensions shown in the above diagram. It is only particularly
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 overshooting, the 100% should not be related to the
peak values 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 meaning. For amplifiers with approximately
constant group delay (therefore good pulse transmission
performance) the following numerical relationship between rise
time tr (in ns) and bandwidth B (in MHz) applies:
Connection of Test Signal
In most cases, briefly depressing the
AUTOSET
causes a useful
signal related instrument setting. The following explanations
refer to special applications and/or signals, demanding a manual
instrument setting.
The description of the controls is explained
in the section ”controls and readout”
.
Caution:
When connecting unknown signals to the oscilloscope input,
always use automatic triggering and set the input coup-
ling switch to AC. The attenuator should initially be set to
20 V/div.
Sometimes the trace will disappear after an input signal has been
applied. Then a higher deflection coefficient (lower input sensitivity)
must be chosen until the vertical signal height is only 3 – 8 div.
With a signal amplitude greater than 160 V
pp
and the deflection
coefficient (
VOLTS/DIV.
) in calibrated condition, an attenuator
probe must be inserted before the Y input. If, after applying the
signal, the trace is nearly blanked, the period of the signal is
probably substantially longer than the set time deflection
coefficient (
TIME/DIV.
). It should be switched to an adequately
larger time coefficient.
Examples:
Displayed wavelength L = 7 div,
set time coefficient Tc = 100 ns/div,
thus period T = 7 x 100 x 10
-9
= 0.7 µs
thus freq. F = 1/(0.7 x 10
-6
) = 1.428 MHz.
Signal period T = 1s,
set time coefficient Tc = 0.2 s/div,
thus wavelength L = 1/0.2 = 5 div.
Displayed ripple wavelength L = 1 div,
set time coefficient Tc = 10 ms/div,
thus ripple freq. F = 1/(1 x 10 x 10
-3
) = 100 Hz.
TV Line frequency F = 15625 Hz,
set time coefficient Tc = 10 µs/div,
required wavelength L = 1/(15,625 x 10
-5
) = 6.4 div.
Sine wavelength L = min. 4 div, max. 10 div,
Frequency F = 1 kHz,
max. time coefficient Tc = 1/(4 x 10
3
) = 0.25 ms/div,
min. time coefficient Tc = 1/(10 x 10
3
) = 0.1 ms/div,
set time coefficient Tc = 0.2 ms/div,
required wavelength L = 1/(10
3
x 0.2 x 10
-3
) = 5 div.
Displayed wavelength L = 0.8 div,
set time coefficient Tc = 0.5 µs/div,
pressed X-MAG. (x10) button: Tc = 0.05 µs/div,
thus freq. F = 1/(0.8 x 0.05 x 10
-6
) = 25 MHz,
thus period T = 1/(25 x 10
6
) = 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. (x10)
active). In this case, the time interval of interest
can be shifted to the screen center using the
X-POS.
control.
Rise Time Measurement
When investigating pulse or square waveforms, the critical
feature is the rise time of the voltage step. To ensure that
transients, ramp-offs, and bandwidth limits do not unduly influence
the measuring accuracy, the rise time is generally measured
between 10% and 90% of the vertical pulse height. For
measurement, adjust the Y deflection coefficient using its variab-
le function (uncalibrated) together with the Y-POS. control so that
the pulse height is precisely aligned with the 0% and 100% lines
of the internal graticule. The 10% and 90% points of the signal
will now coincide with the 10% and 90% graticule lines. The
risetime is given by the product of the horizontal distance in div
between these two coincident points and the calibrated time
coefficient setting. The fall time of a pulse can also be measured
by using this method.
The following figure shows correct positioning of the oscilloscope
trace for accurate rise time measurement.
Type of signal voltage
Summary of Contents for HM504-2
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