Basic Signal Measurement; Nature Of The Signal Voltages - Hameg HM400 Manual

18 LINE (LED button)
Selects the mains as the trigger source as indicated by the LED.
19 EXT (LED button)
Selects the external input as the trigger source as indicated by
the LED.
20 AC (LED button)
Selects AC coupling for the trigger source as indicated by the LED.
21
DC (LED button)
Selects DC coupling for the trigger source as indicated by the
LED.
22 LF (LED button)
Switches a low pass filter into the trigger channel as indicated
by the LED
23 TV (LED button)
Selects TV signal triggering as indicated by the LED.
24
DC/AC; CH1 + CH2 (LED buttons)
Selects DC or AC coupling for channel 1 or channel 2 resp.. If
AC is selected, the LED will light up.
25
GND; CH1 + CH2 (LED buttons)
Disconnects the input of channel 1 or channel 2 resp. and
connects it to ground internally as indicated by the LED.
26
INV (LED button)
Inverts the signal of channel 2 (CH 2) as indicated by the LED.
(Inversion of channel 1 is not available.)
27
HOLD OFF / ON (LED button)
By pressing this button a hold-off time can be selected, the
amount of hold-off can be adjusted with the knob TIME/DIV.
28 Z–INP (LED button)
Activates the external Z axis input
as indicated by the LED.
29 INPUT CH 1 + CH 2 (BNC connectors)
Signal input for channel 1 or channel 2 resp.. In XY mode the
CH1 input will control the horizontal movement (X) of the trace.
30 PROBE ADJUST
(contact)
1 KHz/1 MHz square wave output for the adjustment of probes
other than 1:1.
PROBE ADJUST
(contact)
Ground connection for the probe adjustment.
31
EXT. TRIG/Z-INP (BNC connector)
Input for external trigger or intensity modulation signals.
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Mode select buttons with LED:
CH 1: Activates the channel 1 (CH 1) input or selects access to
the settings memory 1 as indicated by the LED.
CH 2: Activates the channel 2 (CH 2) input or selects access to
the settings memory 2 as indicated by the LED.
DUAL:Selects dual channel operation or access to the settings
memory 3 as indicated by the LED.
ADD:Selects the add mode of the vertical amplifier or access
to the settings memory 4 as indicated by the LED.
XY: Selects the XY mode or access to the settings memory 5
as indicated by the LED.
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for intensity modulation
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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
COMP: Activates the COMPONENT tester or selects access to
the settings memory 6 as indicated by the LED.
33 COMPONENT TESTER (two 4 mm test jacks)
Terminals of the component tester, the left one is connected to
the chassis and thus to the safety earth connector of the mains
cable.

3 Basic signal measurement

3.1

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
amplifier 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 difficult 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 magnifier 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 amplifier 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 sufficient 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.
Subject to change without notice
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