8
Subject to change without notice
Displayed wavelength L = 0.8div,
set time coefficient Tc = 0.5µs/div,
pressed X-MAG. (x10) button: Tc = 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) active). In this case, 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 deflection coefficient using its
variable 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 risetime measurement.
With a time coefficient of 10ns/div (X x10 magnification
active), the example shown in the above figure results in a total
measured risetime of
t
tot
= 1.6div x 10ns/div = 16ns
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. 8.75ns), and t
p
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
t
r
=
√
16
2
- 8.75
2
- 2
2
= 13.25ns
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 AUTO SET 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 a x10 probe, automatic triggering and
set the input coupling switch to DC (readout). The
attenuator should initially be set to 20V/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 3-8div. With a signal amplitude greater than 160Vpp and
the deflection coefficient (
VOLTS/DIV.
) in calibrated condition,
an attenuator probe must be inserted before the vertical 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.
The signal to be displayed can be connected directly to the Y-
input of the oscilloscope with a shielded test cable such as
HZ32
or
HZ34
, or reduced through a x10 or x100 attenuator
probe. The use of test cables with high impedance circuits is
only recommended for relatively low frequencies (up to approx.
50kHz). For higher frequencies, the signal source must be of
low impedance, i.e. matched to the characteristic resistance
of the cable (as a rule 50
Ω
). Especially when transmitting
square and pulse signals, a resistor equal to the characteristic
impedance of the cable must also be connected across the
cable directly at the Y-input of the oscilloscope. When using a
50
Ω
cable such as the
HZ34
, a 50
Ω
through termination type
HZ22
is available from
HAMEG
. When transmitting square
signals with short rise times, transient phenomena on the
edges and top of the signal may become visible if the correct
termination is not used. A terminating resistance is sometimes
recommended with sine signals as well. Certain amplifiers,
generators or their attenuators maintain the nominal output
voltage independent of frequency only if their connection
cable is terminated with the prescribed resistance. Here it
must be noted that the terminating resistor
HZ22
will only
dissipate a maximum of 2Watts. This power is reached with
10Vrms or at 28.3Vpp with sine signal. If a x10 or x100
attenuator probe is used, no termination is necessary. In this
case, the connecting cable is matched directly to the high
impedance input of the oscilloscope. When using attenuators
probes, even high internal impedance sources are only slightly
loaded (approx. 10M
Ω
II 12pF or 100M
Ω
II 5pF with
HZ53
).
Therefore, if the voltage loss due to the attenuation of the
Type of signal voltage
