LTC6903 Linear Technology, LTC6903 Datasheet - Page 6

• Manufacturer Part Number: LTC6903
• Description: (LTC6903 / LTC6904) 1kHz - 68MHz Serial Port Programmable Oscillator
• Manufacturer: Linear Technology

BLOCK DIAGRA
LTC6903/LTC6904
The LTC6903/LTC6904 contain an internal feedback loop
which controls a high frequency square wave VCO operat-
ing between 34MHz and 68MHz. The internal feedback
loop frequency is set over an octave by a 10-bit resistor
DAC. The VCO tracks the internal feedback loop frequency
and the output frequency of the VCO is divided by one of
sixteen possible powers of two.
Higher VCO frequencies and lower output divider settings
can result in higher output jitter. Random jitter at the lower
APPLICATIO S I FOR ATIO
THEORY OF OPERATIO
6
Frequency Setting Information
The frequency output of the LTC6903/LTC6904 is deter-
mined by the following equation:
where DAC is the integer value from 0-1023 represented
by the serial port register bits DAC[9:0] and OCT is the
integer value from 0-15 represented by the serial port
register bits OCT [3:0].
f
2
OCT
•
2078
2
–
1024
DAC
U
( )
Hz
W
U
+
–
W
V
8
+
DAC
I
U
SET
+
–
V
U
A1
SET
GND
1
SDI
f
MO
2
= 68MHz • k
SERIAL PORT
OSCILLATOR
frequency ranges is very low because of the high output
divisor.
The higher frequency settings will display some determin-
istic jitter from coupling between the control loop and the
output. This shows up in the frequency spectrum as spurs
separated from the fundamental frequency by 1MHz to
2MHz.
Use the following two steps to choose binary numbers
“OCT” and “DAC” in order to set frequency “f”:
1) Use Table 1 to Choose “OCT” or use the following
formula, rounding down to the integer value less than or
equal to the result.
2) Choose “DAC” by the following formula, rounding DAC
to the nearest integer:
MASTER
SCK
OE
7
3
OCT
DAC
V
+
I
– V
SET
SEN (LTC6903)
ADR (LTC6904)
SET
4
3 322
2048
.
–
log
PROGRAMMABLE
CLK
OCT
6
2078
DIVIDER
1039
CLK
f
( ) •
5
Hz
6903 BD
f
2
(
10
OCT
)
69034fa