The Smart Timing Choice
The Smart Timing Choice
SiT3907
High Precision Digitally Controlled Oscillator (DCXO)
Rev. 1.2 Page 7 of 10 www.sitime.com
Programmable Drive Strength
The SiT3907 includes a programmable drive strength feature
to provide a simple, flexible tool to optimize the clock rise/fall
time for specific applications. Benefits from the programmable
drive strength feature are:
• Improves system radiated electromagnetic interference
(EMI) by slowing down the clock rise/fall time
• Improves the downstream clock receiver’s (RX) jitter by de-
creasing (speeding up) the clock rise/fall time.
• Ability to drive large capacitive loads while maintaining full
swing with sharp edge rates.
For more detailed information about rise/fall time control and
drive strength selection, see the SiTime Applications Note
section; http://www.sitime.com/support/application-notes.
EMI Reduction by Slowing Rise/Fall Time
Figure 9 shows the harmonic power reduction as the rise/fall
times are increased (slowed down). The rise/fall times are
expressed as a ratio of the clock period. For the ratio of 0.05,
the signal is very close to a square wave. For the ratio of 0.45,
the rise/fall times are very close to near-triangular waveform.
These results, for example, show that the 11th clock harmonic
can be reduced by 35 dB if the rise/fall edge is increased from
5% of the period to 45% of the period.
Figure 9. Harmonic EMI reduction as a Function of
Slower Rise/Fall Time
Jitter Reduction with Faster Rise/Fall Time
Power supply noise can be a source of jitter for the
downstream chipset. One way to reduce this jitter is to
increase rise/fall time (edge rate) of the input clock. Some
chipsets would require faster rise/fall time in order to reduce
their sensitivity to this type of jitter. The SiT3907 provides up
to 3 additional high drive strength settings for very fast rise/fall
time. Refer to the Rise/Fall Time Tables to determine the
proper drive strength.
High Output Load Capability
The rise/fall time of the input clock varies as a function of the
actual capacitive load the clock drives. At any given drive
strength, the rise/fall time becomes slower as the output load
increases. As an example, for a 3.3V SiT3907 device with
default drive strength setting, the typical rise/fall time is 1.15ns
for 15 pF output load. The typical rise/fall time slows down to
2.72ns when the output load increases to 45 pF. One can
choose to speed up the rise/fall time to 1.41ns by then
increasing the drive strength setting on the SiT3907.
The SiT3907 can support up to 60 pF or higher in maximum
capacitive loads with up to 3 additional drive strength settings.
Refer to the Rise/Tall Time Tables to determine the proper
drive strength for the desired combination of output load vs.
rise/fall time
SiT3907 Drive Strength Selection
Tables 1 through 5 define the rise/fall time for a given capac-
itive load and supply voltage.
1. Select the table that matches the SiT3907 nominal supply
voltage (1.8V, 2.5V, 2.8V, 3.3V).
2. Select the capacitive load column that matches the appli-
cation requirement (5 pF to 60 pF)
3. Under the capacitive load column, select the desired
rise/fall times.
4. The left-most column represents the part number code for
the corresponding drive strength.
5. Add the drive strength code to the part number for ordering
purposes.
Calculating Maximum Frequency
Based on the rise and fall time data given in Tables 1 through
4, the maximum frequency the oscillator can operate with
guaranteed full swing of the output voltage over temperature
as follows:
Where Trf_10/90 is the typical rise/fall time at 10% to 90% Vdd.
Example 1
Calculate f
MAX
for the following condition:
• Vdd = 1.8V (Table 1)
• Capacitive Load: 30 pF
• Typical Tr/f time = 5 ns (rise/fall time part number code = G)
1357911
-80
-70
-60
-50
-40
-30
-20
-10
0
10
Harmonic number
Harmonic amplitude (dB)
trise=0.05
trise=0.1
trise=0.15
trise=0.2
trise=0.25
trise=0.3
trise=0.35
trise=0.4
trise=0.45
=
1
3.5 x Trf_10/90
Max Frequency
