User manual

page 72
Analog System Lab Kit PRO
C1
R1
R 1k DAC7821
R
FB VDD
GND
VREF
IOUT1
IOUT2
VDD
VOUT
TL082
TL082
C 1u
TL082
R2
experiment 14
To design a digitally controlled oscillators where the oscillation frequency of
the output square and triangular wave forms is controlled by a binary pattern.
Such systems are useful in digital PLL and in FSK generation in a MODEM.
In Experiment 6, we used an analog multiplier in conjunction with an integrator to
build a VCO. In this experiment, we will use a multiplying DAC7821 (instead of a
multiplier) and an integrator to implement a digitally controlled square and triangular
wave generator. See Figure 14.1 for the circuit schematic of a digitally programmable
square and triangular wave generator. VOUT is the square wave output and the
output of the integrator is the triangular waveform.
Design a Digitally Programmable Oscillator that can generate square and triangular
waveforms with a maximum frequency of 400 Hz.
Implement the Digitally programmable Square and Triangular wave generator using
the circuit as shown in Figure 14.1.Observe the frequency of Oscillations of system
and vary it by varying bit pattern input to the DAC.
Simulate the circuit using any simulator and observe the frequency of oscillation
of the square and triangular waveforms. See Figure 14.2 for the result of
simulation. The typical simulation waveforms are of the form shown in Figure
14.3. For this simulation, we used the macro-model of MV95308 since the
macro-model for the DAC is not available at the time of writing.
VarythebitpatterninputtotheDACinmannerspeciedinTable14.1and
note down the change in the frequency of oscillations and compare the
practical results with the simulation results.
Plot a graph where the x-axis shows the analog equivalent of the bit pattern
and the y-axis shows the frequency of oscillations. Note that the 12-bit input
to the DAC is interpreted as an unsigned number.
14.1 Brief theory and motivation
14.2Specications
14.3 Measurements to be taken
14.4 What Should you Submit
Goal of the experiment
Figure 14.1: Circuit for Digital Controlled Oscillator
Frequency of oscillations of digital programmable oscillator is given by
Table 14.1: Varying the bit pattern input to the DAC
S.No. BIT Pattern Peak to Peak Amplitude of the output
1 100000000000
2 010000000000
3 001000000000
4 000100000000
1
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