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B a s i c s i g n a l m e a s u r e m e n t
Basic signal measurement
Signals which can be measured
The following description pertains to analog and digital ope-
ration. The different specifi cations in both operating modes
should be kept in mind.
The oscilloscope HM1508-2 can display all repetitive signals
with a fundamental repetition frequency of at least 150 MHz.
The frequency response is 0 to 150 MHz (-3 dB). The vertical
amplifi ers will not distort signals by overshoots, undershoots,
ringing etc.
Simple electrical signals like sine waves from line frequency
ripple to hf will be displayed without problems. However, when
measuring sine waves, the amplitudes will be displayed with an
error increasing with frequency. At 100 MHz the amplitude error
will be around –10 %. As the bandwidths of individual instru-
ments will show a certain spread (the 150 MHz is a guaranteed
minimum) the actual measurement error for sine waves cannot
be exactly determined.
Pulse signals contain harmonics of their fundamental fre-
quency which must be represented, so the maximum useful
repetition frequency of non sinusoidal signals is much lower
than 150 MHz (5 to 10 times). The criterion is the relationship
between the rise times of the signal and the scope; the scope's
rise time should be <1/3 of the signal's rise time if a faithful
reproduction without too much rounding of the signal shape
is to be preserved.
The display of a mixture of signals is especially difficult if it
contains no single frequency with a higher amplitude as the
scope's trigger system normally discriminates by amplitude.
This is typical of burst signals for example. Display of such
signals may require using the HOLD OFF control.
Composite video signals may be displayed easily as the instru-
ment has a tv sync separator.
The maximum sweep speed of 5 ns/cm allows suffi cient time
resolution, e.g. a 100 MHz sine wave will be displayed one period
per 2 cm.
The vertical amplifi er inputs may be DC or AC coupled. Use DC
coupling only if necessary and preferably with a probe.
Low frequency signals when AC coupled will show tilt (AC low
frequency – 3 dB point is 1.6 Hz), so if possible use DC coupling.
Using a probe with 10:1 or higher attenuation will lower the
–3 dB point by the probe factor. If a probe cannot be used due
to the loss of sensitivity, DC coupling the scope and an external
large capacitor may help which, of course, must have a suffi cient
DC rating. Care must be taken, however, when charging and
discharging a large capacitor.
DC coupling is preferable with all signals of varying duty cycle,
otherwise the display will move up and down depending on the
duty cycle. Of course, pure DC can only be measured with DC
coupling.
The readout will show which coupling was chosen: = stands for
DC, ~ stands for AC.
10 Subject to change without notice
Amplitude of signals
In contrast to the general use of rms values in electrical engi-
neering oscilloscopes are calibrated in Vpp as that is what is
displayed.
To derive rms from V
: divide by 2.84. To derive V
pp
multiply by 2.84.
Values of a sine wave signal
V
p
V
rms
V
= rms value
rms
V
= pp – value
pp
V
= momentary value, depends on time vs. period.
mom
The minimum signal for a one cm display is 1 mV
ded 1 mV/cm was selected and the variable is in the calibrated
position.
The available sensitivities are given in mV
let you read the amplitudes of the signals immediately on the
readout as the attenuation of probes is automatically taken into
account. Even if the probe attenuation was selected manually
this will be overridden if the scope identifies a probe with an
identification contact as different. The readout will always give
the true amplitude.
It is important that the variable be in its calibrated position.
The sensitivity may be continuously decreased by using the
variable (see Controls and Readout). Each intermediate value
between the calibrated positions 1–2–5 may be selected. Thus
a maximum of 400 V
may be displayed without using a probe
pp
(20 V/div x 8 cm screen x 2.5 variable).
Amplitudes may be directly read off the screen by measuring
the height and multiplying by the V/div. setting.
Please note: Without a probe the maximum permis-
sible voltage at the inputs must not exceed 400 Vp
irrespective of polarity.
STOP
In case of signals with a DC content the peak value DC + AC
peak must not exceed + or – 400 V
is permissible.
If probes are used their possibly higher ratings are
only usable if the scope is DC coupled.
In case of measuring DC with a probe while the scope input is
STOP
AC coupled the capacitor in the scope input will see the input
DC voltage as it is in series with the internal 1 MΩ resistor.
This means that the maximum DC voltage (or DC + peak AC) is
that of the scope input, i.e. 400 V
DC and AC the DC content will stress the input capacitor while
the AC content will be divided depending on the AC impedance
from rms:
pp
V
mom
V
pp
±5 % provi-
pp
or V
. The cursors
pp
pp
. Pure AC of up to 800 V
p
pp
! With signals which contain
P
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