Amplitude Of Signals; Time Measurements; Applying The Signal Voltages - Hameg Instruments HM 400 Manuel

B a s i c s i g n a l m e a s u r e m e n t
3.2

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.
Without a probe the maximum input voltage at both inputs
is + or – 400 Vp.
In case the signal consists of DC and AC, the DC plus peak AC must not
exceed + or – 400 V . A pure ac voltage may reach 800 V
p
400 V can be displayed on the screen.)
pp
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.
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
first 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 sufficient 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.
3.3

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 defined between 10 and 90 % of the
full amplitude.
3.4

Applying the signal voltages

Use AUTOSET for a quick automatic selection of suitable display para-
meters (see AUTOSET). The following paragraph applies to manual ope-
ration. The function of the controls is detailled in the chapter „Controls".
32 Subject to change without notice
. Without a probe set the VOLTS/DIV switch to 20 V/DIV and use AC cou-
pp
pling. If the trace disappears after application of the signal, it is possible
that the signal amplitude is much too large and overdrives the vertical
amplifier. 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 amplifier which can
cause distortions! With calibrated 20 V/DIV a probe will be required if
the signal exceeds 160 V
be displayed without a probe. The probe used must be specified for the
may be displayed maximum voltage applied.
p
Please note that the display of signals with a low repetition rate at high
sweep speeds will cause the trace to dim, the intensity may be increased
until the trace starts to blur. In such case the time base speed must be
decreased so far that the trace remains visible.
The signals may be connected to the scope either through shielded cables
or by using probes. The use of cables is restricted to low frequencies and
low impedance signal sources because they add typically 100 pF/m load
(of which only capacitance. At higher frequencies cables with standard characteristic
pp
impedances like 50 Ω can be used if they are correctly terminated at both
ends. HAMEG HZ22 feedthrough terminations at the scope can be used
together with HAMEG 50 Ω cables such as HZ34. Incorrect or missing
terminations will cause massive pulse distortions. Generators, ampli-
fiers etc. will only perform to specifications if they are feeding properly
terminated cables. The HZ22 is specified for a maximum of 2 W which is
reached if the signal increases to 10 V
, and HAMEG HZ53 With probes no terminations are needed nor allowed, the probes are
pp
directly connected to the scope's BNC connectors. Probes load high
impedance sources only moderately (10:1 probes with 10 MΩ II 12 pF,
100:1 with 10 MΩ//5 pF), but this applies only up to appr. 100 KHz, above
the loading increases with increasing frequency, see the probe manuals
for details, also for the necessary derating. Passive probes are unsuited
for measurements on high Q HF circuits!
As mentioned, in most applications probes are used, at least as long
as the loss in sensitivity can be compensated by increasing the scope's
sensitivity. Also, a probe offers protection for the scope's input circuit.
Because probes are manufactured separated from the scopes, they
are only coarsely preadjusted, it is absolutely necessary to adjust each
probe to the input it is used on (see Probe Adjustment).
Probes may decrease the bandwidth of a scope considerably if they
are the wrong type! We recommend to use the HAMEG HZ51 (10:1),
HZ52 (10:1 HF), HZ154 (1:1 and 10:1) probes. Replacement parts may
be ordered from HAMEG and may be exchanged by the user. The
probes mentioned have a HF adjustment in addition to the basic 1 KHz
adjustment. By using the 1 MHz probe adjust signal, the HF adjustment
corrects for group delay aberrations near the –3 dB frequency. With
these probes the HM400 rise time/bandwidth remain nearly constant.
The probe HF adjustment also allows for an optimum pulse response
of the combination probe and scope.
As mentioned if AC coupling is used, the 1.6 Hz –3 dB frequency comes
into play which causes distortions with low frequency signals, e.g.
square waves are displayed with tilt. With a 10:1 probe the low frequency
response is improved by a factor of ten (0.16 Hz). If the sensitivity with
this probe is insufficient, DC coupling and an external capacitor can
be used e.g. for ripple superimposed on a high dc potential. First the
input must be switched to GND, then the voltage applied, then the input
switched to DC.
The measurement of small voltages requires proper ground connec-
tions as close to the measuring point as possible. Use short ground
cables.
Be careful when applying unknown signals to the vertical
amplifier.
, if the variable is activated up to 400 V
pp
or 28.3 V
rms
With a 10:1 or 100:1 probe DC coupling has to be used if
the signal voltage exceeds 400 Vp.
may
pp
.
pp