Datasheet
LTC6903/LTC6904
8
69034fe
Theory oF operaTion
applicaTions inForMaTion
The LTC6903/LTC6904 contain an internal feedback loop
which controls a high frequency square wave VCO oper-
ating 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 frequency ranges is very low because of the high
output divisor.
The higher frequency settings will display some deter-
ministic 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.
Frequency Setting Information
The frequency output of the LTC6903/LTC6904 is deter-
mined by the following equation:
f
Hz
DAC
OCT
=
2
2078
2
1024
•
()
–
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].
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.
OCT
f
=
3 322
1039
. log
2) Choose “DAC” by the following formula, rounding DAC
to the nearest integer:
DAC = 2048 –
2078(Hz)• 2
(10+OCT)
f
For example, to set a frequency of 6.5MHz, first look
at Table 1 to find an OCT value. 6.5MHz falls between
4.25MHz and 8.5MHz yielding an OCT value of 12 or
1100. Substituting the OCT value of 12 and the desired
frequency of 6.5MHz into the previous equation results in:
DAC = 2048 –
2078(Hz)• 2
(10+12)
6.5e6(Hz)
= 707.113
Rounding 707.113 to the nearest integer yields a DAC
value of 707 (or a 10-bit digital word of 1011000011.)
Table 1. Output Frequency Range vs OCT Settling
(Frequency Resolution 0.001 • f)
f ≥ f < OCT
34.05MHz 68.03MHz 15
17.02MHz 34.01MHz 14
8.511MHz 17.01MHz 13
4.256MHz 8.503MHz 12
2.128MHz 4.252MHz 11
1.064MHz 2.126MHz 10
532kHz 1063kHz 9
266kHz 531.4kHz 8
133kHz 265.7kHz 7
66.5kHz 132.9kHz 6
33.25kHz 66.43kHz 5
16.62kHz 33.22kHz 4
8.312kHz 16.61kHz 3
4.156kHz 8.304kHz 2
2.078kHz 4.152kHz 1
1.039kHz 2.076kHz 0