Digital Signal Processing Reference
In-Depth Information
Time domain
Saw tooth
VCO
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Illustration 169:
DASYLab VCO as a frequency modulator
Using the example of a periodic sawtooth signal as a source signal the function of the generator module as
a VCO or FM generator is shown here. In the experiment corresponding to this diagram the “instanta-
neous frequency” can be measured by means of the cursor. The result is: the level of the instantaneous
frequency corresponds exactly to the instantaneous value of the sawtooth. According to this the instanta-
neous frequency (left) is 100 Hz at t = 0 ms and at the highest value of the sawtooth 250 Hz etc.
As this example shows it is very easy to set the desired frequency range. The source signal values -here
from 0 to 250 - do not physically represent a voltage in V; rather these are digital signals, that is strings of
numbers which only when they are transmitted to the D/A module - output via a multi-function card - have
a realistic range of values.
• The
generator module
is capable of generating an FM signal, an AM signal or a mixed signal of these two.
A further possibility of generating an FM signal can be carried out by means of the
module “formula interpreter”, but requires mathematical skills (mathematical model
of an FM signal).
•
Programmable function generators: digital VCO for the range 0 to 50 MHz
:
Using a signal function chosen via the menu, a string of numbers - i.e. a digital signal
- which corresponds to an FM signal is generated in a digital function generator. A
subsequent D/A converter produces an analog FM signal from this. In the final
analysis a program generates an FM string of figures via the signal processor.
The highest FM frequency is primarily determined by the maximum clock pulse fre-
quency of the D/A converter. It should be noted that at least 20 “support positions”
(figures) per period are necessary to generate an FM signal with a sine-like carrier. In
