Hameg HM504-2, Manual

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27
Subject to change without notice
Figure 1:
Amplitude and frequency spectrum for AM display (m = 50%)
The display of an amplitude modulated HF oscillation can be
evaluated with the oscilloscope provided the frequency spectrum
is inside the oscilloscope bandwidth. The time base is set so that
several cycles of the modulation frequency are visible. Strictly
speaking, triggering should be external with modulation frequency
(from the AF generator or a demodulator). However, internal
triggering is frequently possible with normal triggering using a
suitable trigger level setting and possibly also using the time
vernier (variable) adjustment.
Oscilloscope setting for a signal according to figure 2:
I
Y: CH. I; 20mV/div; AC.
I
TIME/DIV.: 0.2ms/div.
I
Triggering: Normal; with LEVEL-setting; internal
(or external) triggering.
Figure 2: Amplitude modulated oscillation
(F = 1 MHz; f = 1 kHz; m = 50 %; U
T
= 28.3 mV
rms
)
If the two values a and b are read from the screen, the modulation
factor is calculated from
where
a = UT (1+ m) and b = UT (1- m) .
The variable controls for amplitude and time can be set arbitrarily
in the modulation factor measurement. Their position does not
influence the result.
Triggering and time base
All controls regarding trigger and time base are located on the
right of the
VOLTS/DIV.- knobs. They are described in the section
”Controls and Readout”.
Time related amplitude changes on a measuring signal (AC
voltage) are displayable in Yt mode. In this mode the signal
voltage deflects the beam in vertical direction (Y) while the time
base generator moves the beam from the left to the right of the
screen (time deflection = t).
Normally there are periodically repeating waveforms to be
displayed. Therefore the time base must repeat the time deflection
periodically too. To produce a stationary display, the time base
must only be triggered if the signal height and slope condition
coincide with the former time base start conditions. A DC voltage
signal can not be triggered as it is a constant signal with no slope.
Triggering can be performed by the measuring signal itself
(internal triggering) or by an external supplied but synchronous
voltage (external triggering).
The trigger voltage should have a certain minimum amplitude.
This value is called the trigger threshold. It is measured with a
sine signal. Except when external trigger is used the trigger
threshold can be stated as vertical display height in div, at which
the time base generator starts, the display is stable, and the
trigger indicator LED lights or flashes.
The internal trigger threshold of the oscilloscope is given as £
0.5div. When the trigger voltage is externally supplied, it can be
measured in Vpp at that input. Normally, the trigger threshold
may be exceeded up to a maximum factor of 20.
The instrument has two trigger modes, which are characterized
as Automatic Peak and Normal triggering.
Automatic Peak (Value) Triggering
Instrument specific information can be drawn from the items NM
- AT - [9], LEVEL [11] and TRIG. MODE [20] in the section
”Controls and Readout”.
This trigger mode is automatically selected after the AUTO SET
pushbutton is pressed. As the peak value detection makes no
sense in combination with
DC and TV (television) signals, it is
switched off automatically in DC , TVL and TVF trigger coupling
conditions as well as in alternate trigger mode. In this case the
automatic is still present, but a wrong trigger level setting causes
an untriggered display.
In automatic trigger mode the sweep generator can run without
an input signal or external trigger voltage. A base line will always
be displayed even with no signal. With an applied AC signal, peak
value triggering enables the user to select the trigger point on the
displayed signal, by the adjustment of the trigger level control.
The control range depends on the peak to peak value of the signal.
This trigger mode is therefore called Automatic Peak (Value)
Triggering.
Operation of the scope needs only correct amplitude and time
base settings, for a constantly visible trace. Automatic mode is
recommended for all uncomplicated measuring tasks. However,
automatic triggering is also the appropriate operation mode for
the ”entry” into difficult measuring problems, e.g. when the test
signal is unknown relating to amplitude, frequency or shape.
Presetting of all parameters is now possible with automatic
triggering; the change to normal triggering can follow thereafter.
The automatic triggering works above 20Hz. The failure of
automatic triggering at frequencies below 20Hz is abrupt.
However, it is not signified by the trigger indicator LED which may
still be blinking. Break down of triggering is best recognizable at
the left screen edge (the start of the trace in differing display
height).
Triggering and time base