Rohde & Schwarz Hameg HM8150, User Manual

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27
Operation of the HM8150
This is done in the same way as the setting of the other
parameters. The sweep parameters can also be changed
while sweep mode is active (online) and the changes be-
come visible immediately. In this case the sweep currently
in progress is interrupted and a new sweep is started. At
the same time, the activated parameter is shown in the
display. As soon as sweep mode is enabled, the left part
of the display shows “sweep“, unless another parameter is
selected. The sweep proceeds linearly from the start fre-
quency to the stop frequency and can go either from low
to high frequencies or vice versa. A sawtooth signal corre-
sponding to the sweep pattern is available at the BNC sok-
ket SWEEP OUT
24 at the rear panel. The output voltage
range is 0 V (start frequency) to +5 V (stop frequency).
4.11 Controlling the output voltage
The output signal of the HM8150 can be controlled by
means of an applied external DC voltage. The BNC socket
MODULATION INPUT
23 at the rear panel is used to apply
the control voltage. A signal between 0 V and +5 V applied
to this input attenuates the output voltage of the HM8150
and changes the set output voltage to maximum 0 V.
If the output is not terminated (no load), the output voltage
can be calculated using the following equation:
V
out(pp)
= V
display
x K with K = (5 V - external DC voltage) / 5
The output voltage of the HM8150 is varied within the
previously set voltage range. By applying an external
voltage of 5 V, it is possible to achieve an output voltage of
approx. 0 V at the output of the HM8150.
4.12 Amplitude modulation
A modulated waveform consists of a carrier waveform and
a modulating waveform. In amplitude modulaton (AM), the
amplitude of the carrier is varied by the instantaneous vol-
tage of the modulating waveform.
Attention:
The displayed output voltage remains unchanged.
The function generator HM8150 will accept only an ex-
ternal modulation source. However, the BNC socket MO-
DULATION INPUT
23
on the rear panel can be used for
this purpose. An external signal for amplitude modulation
can be applied there. Modulation factors up to 100% can
be achieved. Since a bipolar signal is needed for this mo-
dulation, it is necessary to superimpose the input voltage
with a DC offset of 2.5 V. Ideally, this should be obtained
from a function generator with an offset function (e.g. the
HM8030). In this cases, the amplitude shown in the display
of the HM8150 is greater than the actual output voltage.
To set the external DC voltage for optimum amplitude mo-
dulation symmetry, proceed as follows:
1
st
Do not apply any signals to the external input.
2
nd
Set the HM8150 to the desired output voltage (V
out(pp)
).
3
rd
Measure the amplitude of this signal.
4
th
Apply a DC signal to the MODULATION INPUT
23
. In-
crease this voltage until the output voltage of the
HM8150 is attenuated to 50% of its previous amplitude.
5 th Apply the AC voltage for setting the desired
modulation.
The modulation factor will now remain constant, re-
gardless of the amplitude of the generator output voltage
is changed. The generator output signal is modulated in-
vers to the external modulation signal.
4.13 Arbitrary function
In addition to the fixed signal shapes, the HM8150 allows
the generation of user-defined waveform, too. When de-
fining this signal, certain rules and limiting specifications
must be observed. These are described below.
Arbitrary signals are digitally generated and can therefore
be defined with a very good resolution. The frequency and
amplitude of a waveform defined in this way can be varied
like with the ”hard-wired“ signals. Besides the constraints
imposed by the equipment specifications (due to the in-
tegrated D/A converter), it must always be taken into ac-
count that freely defined and digitally generated curve sha-
pes are accompanied by harmonics situated far above the
Fig. 4.5: Arbitrary signal
Fig. 4.4 : Sinewave with AM