Rainbow Electronics DS1086L User Manual
Page 11
DS1086
DS1086 Spread-Spectrum EconOscillator
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11
must be read from the RANGE register (last five bits). In
this example, 12h (18 decimal) was read from the
RANGE register. OS - 2 for this case is 10h (16 deci-
mal). This is the value that is written to the OFFSET reg-
ister.
Finally, the two-byte DAC value needs to be deter-
mined. Since OS - 2 only sets the range of frequencies,
the DAC selects one frequency within that range as
shown in Equation 3.
f
MASTER OSCILLATOR = (MIN FREQUENCY OF SELECTED OFFSET
RANGE) + (DAC value x 10kHz)
Valid values of DAC are 0 to 1023 (decimal) and 10kHz
is the step size. Equation 4 is derived from rearranging
Equation 3 and solving for DAC.
Since the two-byte DAC register is left justified, 655 is
converted to hex (028Fh) and bit-wise shifted left six
places. The value to be programmed into the DAC reg-
ister is A3C0h.
In summary, the DS1086 is programmed as follows:
PRESCALER = 03h (4% peak dither) or 13h (2% peak
dither)
OFFSET = OS - 2 or 10h (if range was read as 12h)
DAC = A3C0h
Notice that the DAC value was rounded. Unfortunately,
this means that some error is introduced. In order to
calculate how much error, a combination of Equation 1
and Equation 3 is used to calculate the expected out-
put frequency. See Equation 5.
The expected output frequency is not exactly equal to the
desired frequency of 11.0592MHz. The difference is
450Hz. In terms of percentage, Equation 6 shows that the
expected error is 0.004%. The expected error assumes
typical values and does not include deviations from the
typical as specified in the electrical tables.
Example #2: Calculate the register values needed to
generate a desired output frequency of 100MHz.
Since the desired frequency is already within the valid
master oscillator frequency range, the prescaler is set
to divide by 1, and hence, PRESCALER = 00h (for 4%
peak dither) or 10h (for 2% peak dither).
f
MASTER OSCILLATOR
= 100.0MHz x 2
0
= 100.0MHz
Next, looking at
Table
2, OS + 1 provides a range of
frequencies centered around the desired frequency. In
order to determine what value to write to the OFFSET
register, the RANGE register must first be read.
Assuming 12h was read in this example, 13h (OS + 1)
is written to the OFFSET register.
Finally, the DAC value is calculated as shown in
Equation 8.
The result is then converted to hex (0110h) and then
left-shifted, resulting in 4400h to be programmed into
the DAC register.
In summary, the DS1086 is programmed as follows:
PRESCALER = 00h (4% peak dither) or 10h (2% peak
dither)
OFFSET = OS + 1 or 13h (if RANGE was read as 12h)
DAC = 4400h
f
MHz
kHz
MHz
MHz
OUTPUT
(
.
)
(
)
.
.
=
+
×
=
=
97 28
272 10
2
100 0
1
100 0
0
DAC VALUE
MHz
MHz
kHz STEP SIZE
decimal
(
.
.
)
.
(
)
=
−
=
100 0
97 28
10
272 00
%
%
.
.
.
.
.
%
ERROR
f
f
f
ERROR
MHz
MHz
MHz
Hz
MHz
EXPECTED
DESIRED
EXPECTED
DESIRED
EXPECTED
=
−
Ч
=
−
Ч
=
Ч
=
100
11 0592
11 05875
11 0592
100
450
11 0592
100
0 004
f
MIN FREQUENCY OF SELECTED OFFSET
RANGE
DAC VALUE x
kHz STEP SIZE
prescaler
f
MHz
x
kHz
MHz
MHz
OUTPUT
OUTPUT
(
)
(
)
(
.
)
(
)
.
.
=
+
=
+
=
=
10
81 92
655
10
8
88 47
8
11 05875
DAC VALUE
f
MIN FREQUENCY OF SELECTED
OFFSET RANGE
kHz STEP SIZE
DAC VALUE
MHz
MHz
kHz STEP SIZE
decimal
MASTER OSCILLATOR
(
)
(
.
.
)
.
(
)
=
−
=
−
=
≈
10
88 4736
81 92
10
655 36
655
(4)
(8)
(7)
(6)
(5)
(9)