Basic Signal Measurement; Nature Of The Signal Voltages; Amplitude Of Signals; Time Measurements - Hameg HM400 Handbuch

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

Nature of the signal voltages

The oscilloscope HM400 displays in real time most repetitive signals
containing frequencies from DC to beyond 40 MHz (- 3 dB). The vertical
amplifi er is designed for minimum overshoot.
The display of simple electrical waveforms like LF or HF sine waves or
mains frequency ripple is no problem. When measuring the amplitude
of sine waves, the frequency response of the oscilloscope has to be
taken into account which begins to fall off at fairly low frequencies.
At 25 MHz the amplitude error will amount to appr. –10 %. Due to the
tolerance of the –3 dB frequency the exact amount of the amplitude
error may vary.
Square wave or pulse signals, in general all nonsinusoidal signals,
contain frequencies well above their repetition frequency, depending
on their shape and rise resp. fall times. This oscilloscope has a rise
time of 8.5 ns and will reproduce signals fairly well if their rise times
remain 3 to 5 times slower. It follows that the repetition rate of such
nonsinusoidal signals must remain considerably lower than the –3 dB
frequency of 40 MHz, otherwise their harmonics will be attenuated too
much, i.e. the edges will be rounded.
It is more diffi cult to display socalled mixed signals unless there is a
repetition frequency with outstanding amplitudes, so the scope can
trigger on them. This may be the case with burst signals. In order to
obtain a stable display, it may be necessary to vary the hold-off time. The
active TV sync separator will allow stable triggering on video signals.
The fastest time base speed using the magnifi er is 10 ns/div which
allows to spread a period of 40 MHz over 2.5 divisions, consequently,
time resolution is no problem.
The vertical amplifi er is DC coupled, when AC coupling is selected, a
capacitor is switched in series with the signal input. The normal coupling
mode is DC; if the DC content of the input signal is too high, AC coupling
will be required. In this case, however, two effects need to be considered.
Signals with a very low frequency content may be distorted, e.g. low
frequency square waves will show tilt (appr. 1.6 Hz – 3 dB) . Signals with
varying duty cycle will be displayed with a vertical shift depending on the
duty cycle corresponding to their DC content. The low frequency limit
could be reduced by selecting DC coupling and connecting an external
larger capacitor of suffi cient voltage rating, but use of this method is
discouraged, a 10:1 probe will reduce the low frequency –3 dB point
to 0.16 Hz. Due to their internal circuit, 100:1 and 1000:1 probes do not
reduce the lower frequency –3 dB frequency. As outlined in more detail
later, oscilloscopes are rarely used without probes.

Amplitude of signals.

In electrical engineering, ac voltages are given in rms units. Oscilloscopes
show the actual peak-to-peak voltages, hence they are calibrated in V
In order to arrive at the RMS value of a sine wave, its pp – value must be
divided by 2.83. RMS voltages will be displayed larger by that factor.
The highest sensitivity of this scope is 1 mV/DIV, a signal of 1 division
will amount to 1 mV ±5 % unless the variable is activated. Calibrated
pp
measurements require that the „variable" is off. The variable allows to
decrease the sensitivity by a factor of appr. 2.5 to a lowest of appr. 50
V/DIV. The variable also allows to bridge the 1 – 2 – 5 steps of the input
attenuators. Without a probe signals of up to 400 V
(50 V/DIV x 8 divisions). In order to measure the amplitude of a signal,
it is only necessary to read the height of the display and multiply it by
the sensitivity selected in V/DIV.
32 Subject to change without notice
In case the signal consists of DC and AC, the DC plus peak AC must not
exceed + or – 400 Vp. A pure ac voltage may reach 800 Vpp (of which STOP
only 400 Vpp can be displayed on the screen.)
STOP
Considering the foregoing, HAMEG HZ154 10:1 probes allow to measure
DC up to 400 V and pure ac voltages up to 800 V
100:1 probes dc voltages up to 1200 V and pure ac voltages up to 2400
V . Please observe the decrease of the permissible input voltage with
pp
increasing frequency for each probe type, see the respective probe ma-
nuals. Risking the measurement of excessive voltages with a standard
10:1 probe may cause a short of the probe's input capacitor which could
destroy the scope input circuitry!
It is possible to measure the ripple on a high voltage by inserting a high
voltage capacitor in series with a 10:1 probe, but it is mandatory to switch
the input to DC; in order to avoid excessive transients, the input must
fi rst be switched to ground, then the high voltage applied, then the input
switched to DC. The high voltage capacitor has to be discharged with
proper care using a resistor of suffi cient voltage rating!
The GND position of the input coupling selector is used to set the base
line using the POSITION control as desired before switching to DC.

Time measurements

As a rule, scopes are used to display repetitive signals, the designation
period is used here for simplicity. The repetition frequency is equal
to the number of periods per second. Depending on the setting of
the TIME/DIV switch one or more periods may be displayed or just
portions of one period. The time base speeds are indicated by the
LEDs around the circumference of the TIME/DIV knob in us/DIV,
ms/DIV, s/DIV.
In order to measure the period or portions of a signal, read the number
of divisions and multiply this by the time base speed selected.
The HORIZONTAL position knob allows to shift the horizontal position
of the trace. Rise and fall times are defi ned between 10 and 90 % of
the full amplitude.

Applying the signal voltages

Use AUTOSET for a quick automatic selection of suitable display
parameters (see AUTOSET). The following paragraph applies to ma-
nual operation. The function of the controls is detailled in the chapter
„Controls".
.
pp
Without a probe set the VOLTS/DIV switch to 20 V/DIV and use AC coup-
STOP
ling. If the trace disappears after application of the signal, it is possible
that the signal amplitude is much too large and overdrives the vertical
amplifi er. Decrease the sensitivity (increase the VOLTS/DIV setting) until
the signal remains fully within the screen area. If portions of the signal
fall outside this area, they may still overdrive the amplifi er which can
may be displayed cause distortions! With calibrated 20 V/DIV a probe will be required if
p
the signal exceeds 160 V
be displayed without a probe. The probe used must be specifi ed for the
maximum voltage applied.
Without a probe the maximum input voltage at both inputs
is + or – 400 Vp.
If 10:1 probes are used, their possibly higher maximum
voltages may only be made use of if the scope input is
switched to DC coupling. This does not apply to 100:1 or
1000:1 probes.
Be careful when applying unknown signals to the vertical
amplifi er.
, if the variable is activated up to 400 V
pp
, and HAMEG HZ53
pp
may
pp