15
Subject to change without notice
centre, thus overdriving the inputs resulting in sharper zero
crossings. Also, it is possible to use half a period over the full
10 cm. As the time base is quite accurate, increasing the time
base speed after adjusting for e.g. one period = 10 cm and
positioning the first crossing on the first graticule line will also
give better resolution.
Measurement of amplitude modulation
Please note: Use this only in analog mode because in digital
mode alias displays may void the measurement! For the display
of low modulation frequencies a slow time base (TIME/DIV) has
to be selected in order to display one full period of the modula-
ting signal. As the sampling frequency of any digital oscilloscope
must be reduced at slow time bases it may become too low for
a true representation.
The momentary amplitude at time t of a hf carrier frequency
modulated by a sinusoidal low frequency is given by:
u = U
T
·
sin
Ω
t + 0,5 m
·
U
T
·
cos (
Ω
-
ω
) t - 0,5 m
·
U
T
·
cos (
Ω
-
ω
) t
where: U
T
= amplitude of the unmodulated carrier
Ω
=
2
π
F = angular carrier frequency
ω
=
2
π
f = modulation angular frequency
m
= modulation degree (
≤
1
v
100%)
In addition to the carrier a lower side band F – f and an upper
side band F + f will be generated by the modulation.
Picture 1: Amplitudes and frequencies with AM (m = 50 %) of
the spectra
As long as the frequencies involved remain within the scope’s
bandwidth the amplitude modulated HF can be displayed. Pre-
ferably the time base is adjusted so that several signal periods
will be displayed. Triggering is best done from the modulation
frequency. Sometimes a stable displayed can be achieved by
adjusting the time base variable.
Picture 2: Amplitude modulated hf. F = 1 MHz, f = 1 kHz,
m = 50 %, U
T
= 28,3 mV
rms
Set the scope controls as follows in order to display the picture
2 signal:
CH1 only, 20 mV/cm, AC
TIME/DIV: 0.2 ms/cm
Triggering: NORMAL, AC, internal.
Use the time base variable or external triggering.
Reading a and b off the screen the modulation degree will
result:
a – b
a – b
m = —— bzw. m = ——
·
100 [%]
a + b
a + b
a = U
T
(1 + m) and b = U
T
(1 – m)
When measuring the modulation degree the amplitude and time
variables can be used without any infl uence on the result.
Triggering and time base
The most important controls and displays for these functions
are to be found in the shaded TRIGGER area, they are described
in „Controls and Readout“.-
In YT mode the signal will defl ect the trace vertically while the ti-
mebase will defl ect it horizontally, the speed can be selected.
In general periodic voltage signals are displayed with a peri-
odically repeating time base. In order to have a stable display,
successive periods must trigger the time base at exactly the
same time position of the signal (amplitude and slope).
STOP
Pure DC can not trigger the time base, a voltage
change is necessary.
Triggering may be internal from any of the input signals or
externally from a time related signal.
For triggering a minimum signal amplitude is required which
can be determined with a sine wave signal. With internal trigge-
ring the trigger take off within the vertical amplifi ers is directly
following the attenuators. The minimum amplitude is specifi ed
in mm on the screen. Thus it is not necessary to give a minimum
voltage for each setting of the attenuator.
For external triggering the appropriate input connector is used,
thus the input amplitude necessary is given in V
pp
. The voltage
for triggering may be much higher than the minimum, however,
it should be limited to 20 times the minimum. Please note that
for good triggering the external voltage should be a good deal
above the minimum. The scope features two trigger modes to
be described in the following:
Automatic peak triggering (MODE menu)
Consult the chapters MODE
20
>
AUTO, LEVEL A/B
19
, FILTER
21
and SOURCE
22
in ”Controls and Readout“. Using AUTOSET
this trigger mode will be automatically selected. With DC cou-
pling and with alternate trigger this mode will be left while the
automatic triggering will remain.
Automatic triggering causes a new time base start after the
end of each foregoing sweep and after the hold off time has
elapsed even without any input signal. Thus there is always
a visible trace in analog or digital mode. The position of the
trace(s) without any signal is then given by the settings of the
POSITION controls.
As long as there is a signal, scope operation will not need more
than a correct amplitude and time base setting. With signals
<
20 Hz their period is longer than the time the auto trigger
T r i g g e r i n g a n d t i m e b a s e
F – f
F
F + f
0,5 m · U
T
0,5 m · U
T
U
T
b
a
m · U
T
U
T
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