10
Subject to change without notice
Examples:
Displayed wavelength
L
= 7div.,
set time coefficient
T
c
= 0.1µs/div.,
required period
T
= 7x0.1x10
−
6
=
0.7µs
required rec. freq. F
= 1:(0.7x10
−
6
) =
1.428MHz.
Signal period
T
= 1s,
set time coefficient
T
c
= 0.2s/div.,
required wavelength L
= 1:0.2 =
5div.
.
Displayed ripple wavelength
L
= 1div.,
set time coefficient
T
c
= 10ms/div.,
required ripple freq. F
= 1:(1x10x10
−
3
) =
100Hz
.
TV-line frequency
F
= 15625Hz,
set time coefficient
T
c
= 10µs/div.,
required wavelength L
= 1:(15 625x10
−
5
) =
6.4div.
.
Sine wavelength
L
= min. 4div., max. 10div.,
Frequency
F
= 1kHz,
max. time coefficient
T
c
= 1:(4x10
3
) = 0.25ms/div.,
min. time coefficient
T
c
= 1:(10x10
3
) = 0.1ms/div.,
set time coefficient T
c
= 0.2ms/div.,
required wavelength
L
= 1:(10
3
x0.2x10
−
3
) =
5div.
Displayed wavelength
L
= 0.8div.,
set time coefficient
T
c
= 0.5µs/div.,
pressed X-MAG. (x10) button: T
c
= 0.05µs/div.,
required rec. freq. F
= 1:(0.8x0.05x10
−
6
) =
25MHz
,
required period T
= 1:(25x10
−
6
) =
40ns
.
If the time is relatively short as compared with the
complete signal period, an expanded time scale should
always be applied (
X-MAG. (x10)
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.
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
10%
and
90%
of the vertical
pulse height. For measurement adjust the Y attenuator
switch with its variable control 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 coincidence points and the time coefficient
setting
. If X x10 magnification is used, this product must
be divided by 10. 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 risetime measurement.
t
r
=
√
t
tot
2
-
t
osc
2
-
t
p
2
t
r
=
√
32
2
- 12
2
- 2
2
=
29.6ns
350
B
t
r
=
350
t
r
B =
With a time coefficient of 0.2µs/div. and pushed X-MAG
x10 button the example shown in the above figure results
in a measured total risetime of
t
tot
= 1.6div·0.2µs/div.:10 =
32ns
When very fast risetimes are being measured, the risetimes
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 following
formula.
In this t
tot
is the total measured risetime, t
osc
is the risetime
of the oscilloscope amplifier (approx. 12ns), and tp the
risetime of the probe (e.g. = 2ns). If t
tot
is greater than
100ns, 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
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
Caution:
When connecting unknown signals to the oscillo-
scope input, always use automatic triggering and set the
DC-AC
input coupling switch to
AC
. The attenuator switch
should initially be set to
20V/div
.
