Low-Frequency, Spread-Spectrum
EconOscillator
MASTER OSCILLATOR FREQUENCY vs.
EXTERNAL RESISTOR SELECTION
IF DITHER AMOUNT = 0%
(+ 1, 2, or 4% of f
)
MOSC
DITHER
AMOUNT
(2, 4, OR 8%)
Programmed f
MOSC
9
8
7
6
5
4
3
(- 1, 2, or 4% of f
)
MOSC
1
MOD
f
TIME
Figure 2. Center Frequency Dither Diagram
Factory-Programmed Prescaler
The prescaler divides the frequency of the master oscil-
lator by 1, 2, 4, 8, 16, or 32 to generate the square-
50
80
RESISTANCE (kΩ)
40
60
70
90
R
SET
wave output clock (f
is an ordering option.
). This divisor is factory-set and
OSC
Figure 1. Master Oscillator Frequency
Dither Generator
Detailed Description
Spread-spectrum functionality is achieved by a user-
configurable divider (determines dither rate), a triangle
generator, and a user-configurable dither amplitude cir-
cuit (see Block Diagram).
The DS1090 is a center-dithered, spread-spectrum sili-
con oscillator for use as an external clock in reduced-
EMI applications. With a combination of factory-
programmed prescalers and a user-selected external
resistor, output frequencies from 125kHz to 8MHz can
be achieved. The output center frequency can be
dithered by selecting the desired dither rate and ampli-
tude with discrete inputs J0, J1, JC0, and JC1.
The input to the triangle-wave generator is derived from
the internal master oscillator and is fed through a user-
configurable divider. The settings of control pins JC0
and JC1 determine this dither rate divisor setting (see
Table 1), dividing the master clock by 4, 8, 16, or 32.
The clock signal is further divided by 128 in the
triangle-wave generator, which results in a triangle-
wave signal of either 1/512th, 1/1024th, 1/2048th, or
The DS1090 contains four basic circuit blocks: master
oscillator, factory-programmed prescaler, dither gener-
ator, and the voltage-bias circuit that provides the feed-
back path to the master oscillator for frequency control
and dithering functions.
1/4096th of the master oscillator (f
upon the user’s divisor setting.
), depending
MOD
The dithering frequency can be also expressed as the
result of
Master Oscillator
The master oscillator is programmable in the applica-
tion by the use of an external resistor (R
) tied to
SET
f
MOSC
f
=
ground (GND). Resistor values of 45kΩ to 91kΩ vary
MOD
Divisor×128
the square-wave output frequency of the voltage-con-
trolled master oscillator (f
4MHz (see Figure 1).
) from 8MHz down to
MOSC
where Divisor is 4, 8, 16, or 32.
The master oscillator (Hz) frequency can be stated as
Table 1. Dither Rate Divisor Settings
3.6461E+11
Resistor
DITHERING PERCENTAGE
DIVISOR
SETTING
f
≅
JC1
JC0
MOSC
(f
MOSC
/n)
0
0
1
1
0
1
0
1
F
/ 512
/ 1024
/ 2048
/ 4096
4
8
MOSC
F
MOSC
MOSC
MOSC
f
f
16
32
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