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.

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).

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

ff

f

ERROR

MHz

MHz

MHz

Hz

MHz

EXPECTED

DESIRED

EXPECTED

DESIRED

EXPECTED

=

−

×

=

−

×=

×

=

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)