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or expressed in radians
Relatively small phase angles at not too high frequencies
can be measured more accurately in the X-Y mode with Lis-
sajous figures.
Measurement of an amplitude modulation
The momentary amplitude u at time f of a HF-carrier volt¬
age, which is amplitude modulated without distortion by a
sinusoidal AF voltage, is in accordance with the equation
Figure 2
Amplitude modulated oscillation:
F=
1 MHz;
f=
1 kHz;
m = 50%;
Ut= 28.3
mV,„„.
If the two values a and
bare
read from the screen, the mod¬
ulation factor is calculated from
m= -—-
bzw.
m = -—-
•100[%]
a + b
a + b
where
a = Uj (1+m)
and
b = Uj (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
u = Uj- sinSlt + 0,5m •
l/j- •
cos(Q—o>)t — 0,5m • Uf • cos(S2-hco)t
where
Uj =
unmodulated carrier amplitude
Si
=
= angular carrier frequency
(a
= 27rf
= modulation angular frequency
m
=
modulation factor (g 1 ^ 100%).
The lower side frequency F—fand the upper side frequency
F-/-f arise because of the modulation apart from the carrier
Figure 1
F-f
F
F+f
Amplitude and frequency spectrum for AM display (m = 50%)
The display of the 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 wave of the modulation fre¬
quency are visible. Strictly speaking, triggering should be
external with modulation frequency (from the AF generator
or a demodulator). However, internal triggering is fre¬
quently possible with normal triggering (AT-NORM. button
depressed) using a suitable LEVEL setting and possibly also
using the time variable adjustment.
Oscilloscope setting for a signal according to figure 2:
Depress no buttons.
Y.
CH. I; 20mV/div.; AC.
TIME/DIV.; 0.2ms/div.
Triggering: NORMAL; with LEVEL-setting; internal (or
external) triggering.
A signal can be displayed only if the time base is running or
triggered. To produce a stationary display, triggering must
be synchronous with the test signal. This is possible by
using the test signal itself or by an externally supplied but
synchronous signal voltage.
The trigger voltage should have a certain minimum
amplitude. This value is called the
trigger threshold.
It is
measured with a sine signal. When the trigger voltage is
taken internally from the test signal, the trigger threshold
can be stated as vertical
display height in mm,
through
which the time base generator starts, the display is stable,
and the trigger LED lights.
The internal trigger threshold of the HM 203-7 is given as
^5 mm. When the trigger voltage is externally supplied, it
can be measured in Vpp at the TRIG. INP. socket. Normally,
the trigger threshold may be exceeded up to a maximum
factor of 20.
The HM 203-7 has two trigger modes, which are charac¬
terized in the following.
Automatic Triggering
If the AT/NORM, pushbutton in the X field is in the out posi¬
tion AT, the sweep generator is running without test signal
or external trigger voltage. A base line is always displayed
even without a signal applied. This trigger mode is therefore
Ml 0 203-7
Subject to change without notice
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