11
Subject to change without notice
risk destruction of the capacitor bridging the input resistor with
possible ensuing damage of the scope input!
If the residual ripple of a high voltage is to be measured, a high
voltage capacitor may be inserted in front of a 10:1 probe, it will
take most of the voltage as the value of the probe’s internal
capacitor is very low, 22 to 68 nF will be suffi cient.
If the input selector is switched to Ground the reference trace
on the screen may be positioned at graticule center or else-
where.
DC and AC components of an input signal
The dashed curve shows an AC signal symmetrical to zero. If
there is a DC component the peak value will be DC + AC peak.
Timing relationships
The repetition frequency of a signal is equal to the number of
periods per second. Depending on the TIME/DIV setting one or
more periods or part of a period of the signal may be displayed.
The time base settings will be indicated on the readout in s/cm,
ms/cm, μs/cm und ns/cm (1 cm is equivalent to 1 div.). Also the
cursors may be used to measure the frequency or the period.
If portions of the signal are to be measured use delayed sweep
or the magnifi er x 10. Use the HORIZONTAL positioning control
to shift the portion to be zoomed into the screen center.
Pulse signals are characterized by their rise and fall times
which are measured between the 10 % and 90 % portions. The
following example uses the internal graticule of the CRT, but
also the cursors may be used for measurement.
Measurement:
– Adjust the rising portion of the signal to 5 cm.
– Position the rising portion symmetrically to the graticule
centre line, using both Y and X positioning controls.
– Notice the intersections of the signal with the 10 and 90 %
lines and project these points to the centre line in order to
read the time difference.
In the example it was 1.6 cm at 5 ns/cm equals 8 ns rise time.
When measuring very short rise times times proceed mainly
as outlined above, however rise times may be measured any-
where on the screen. coming close to the scope rise time it is
necessary to subtract the scope’s (and if used the probe’s) rise
times geometrically from the rise time as seen on the screen.
The true signal rise time will become:
t
tot
is the rise time seen, t
osc
is the scope’s own rise time (2.3
ns with the HM1500-2), t
t
is the rise time of the probe, e.g. 2 ns.
If the signal’s rise time is
>
22 ns, the rise times of scope and
probe may be neglected.
For the measurement of rise times proceed mainly as outlined
above, however rise times may be measured anywhere on the
screen. It is mandatory that the rising portion of the signal be
measured in full and that the 10 to 90 % are observed. In case
of signals with over- or undershoot the 0 and 100 % levels are
those of the horizontal portions of the signal, i.e. the over- re-
spectively undershoots must be disregarded for rise and fall
time measurements. Also, glitches should be disregarded. If
signals are very distorted, however, rise and fall time measu-
rements may be of no value.
For most amplifi ers, even if their pulse behaviour is far from
ideal, the following relationship holds:
350
350
t
a
= —— B = ——
B
t
a
tr/ns = 350/Bandwidth/MHz
Connection of signals
In most cases pressing the AUTOSET button will yield a satis-
factory display (see AUTOSET). The following relates to special
cases where manual settings will be advisable. For a description
of controls refer to ”Controls and Readout“.
STOP
Take care when connecting unknown signals to the
inputs!
It is recommended to use probes whenever possible. Without
a probe start with the attenuator set to its 20 V/cm position. If
the trace disappears the signal amplitude may be too large,
overdriving the vertical amplifi er or/and its dc content may
be too high. Reduce the sensitivity until the trace reappears
on screen. If calibrated measurements are desired it will be
necessary to use a probe if the signal becomes
>
160 V
p
. Check
the probe specifi cations in order to avoid overstressing. If the
time base is set too fast the trace may become invisible, then
reduce the time base speed.
If no probe is used at least screened cable should be used,
such as HZ32 or HZ34. However, this is only advisable for low
impedance sources or low frequencies (
<
50 kHz). With high
frequencies impedance matching will be necessary.
Nonsinusoidal signals require impedance matching, at both
ends preferably. At the scope input a feed through – 50 Ω-ter-
mination will be required. HAMEG offers a HZ22 termination. If
proper terminations are not used sizeable pulse aberrations will
B a s i c s i g n a l m e a s u r e m e n t
t
a
=
8
2
- 2.3
2
- 2
2
= 7.4 ns
t
a
=
t
tot
2
– t
osc
2
– t
t
2
voltage
peak
AC
DC
DC
AC
DC + AC
peak
= 400 V
max
5 cm
t
tot
100%
90%
10%
0%
