Digital Signal Processing Reference
In-Depth Information
Tutorial 51
Q: Design a digital controlled oscillator (DCO) with a clock rate of 1 MHz and a
center frequency of 10 kHz. Draw the block diagram and the output signal for an
input sequence of 0, 30, -30, 70,…. Indicate sampling times.
(Fixed Number)
M
o
f
c
(Clock)
Digital Subtracter
Programm−
able
Counter
2
′
s compl−
ement
Zero
Detector
Full Adder
M
o
−
N
Load
M
o
−
N
N
(Input Number)
DCO Pulses
Solution: We have f
c
= 1 MHz, hence, T
c
= 1/f
c
= 1l s.
The center frequency is f
o
= f
c
/M
o
= 10 kHz.
Hence, the free-running input number is M
o
= f
c
/f
o
= 100.
For the given sequence, the values of the counter initial number are R
o
= 100 -
0 = 100; 100 - 30 = 70; 100 - (-30) = 130;
and
100 - 70 = 30;
respectively.
Hence, the sampling times are
0 (initial), 100 T
c
= 100l s, 170 l s, 300 l s, and 330 l s.
An illustrative (not to scale) diagram is shown below.
130
100
70
30
T
c
=1/f
c
Time,
t
DCO
Pulses
t
,
μ
s
0
100
170
300
330
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