The Smart Timing Choice



The Smart Timing Choice



SiT5000

Standard Frequency MEMS (VC)TCXO

Rev. 1.0

Page 4 of 7

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SoftEdge™ Option

The SoftEdge™ output is available as a standard option for the SiT500x family of MEMS (VC)TCXOs. It is typically used for EMI

reduction similar to that of the clipped sinewave output common to many quartz based TCXOs.

In the SoftEdge™ mode, the slower rise/fall edges of the output waveform reduce the higher clock harmonics in a digital clock

signal, minimizing EMI radiation at these harmonics. The table below show the actual rise/fall time in relation to the desired output

frequency and the supply voltage with a 10 k

 / 10 pF load. Rail-to-rail swing of the output is maintained for these supported

frequencies.

Rise/Fall Time for SoftEdge™ Option

Parameter

Symbol

Min.

Typ.

Max.

Unit

Condition

Rise/Fall Time

Tr, Tf

4.0

6.5

9.5

ns

1-26 MHz, 1.8V, 3.0 and 3.3V, MHz 10k and 10 pF, 20%-80% Vd

2.5

4.0

6.0

ns

1-26 MHz, 2.5V and 2.8V, MHz 10k and 10 pF, 20%-80% Vdd

1.5

3.5

5.0

ns

26-40 MHz, 1.8V, 3.0V and 3.3V, MHz 10k and 10 pF, 20%-80% Vdd

1.5

2.5

4.5

ns

26-40 MHz, 2.5V and 2.8V, MHz 10k and 10 pF, 20%-80% Vdd

SoftEdge™ Waveform Examples and Corresponding Harmonics Reduction

Figures below illustrate the harmonic power reduction as the rise/fall times are slowed from the standard squarewave output to

that of the SoftEdge™ output. In general, the 1.8V device shows the lowest harmonics and provides best EMI performance

comparing to devices with higher operating voltages.

Time (ns)

LVCMOS and SoftEdge™ Outputs,

VDD = 3.3V

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

0

0.5

1

1.5

2

2.5

3

3.5

SoftEdge

LVCMOS

Harmonic Number

Odd Harmonic Power Comparison for LVCMOS and SoftEdge™ Outputs,

VDD = 3.3V

0

2

4

6

8

10

12

14

16

18

20

-40

-30

-20

-10

0

10

20

SoftEdge

LVCMOS

-10dBm

-24.4dbm

Time (20 ns/Div)

LVCMOS and SoftEdge Outputs,

VDD = 2.5V

SoftEdge

LVCMOS

0

2

4

6

8

10

12

14

16

18

20

-40

-30

-20

-10

0

10

20

Harmonic Number

Odd Harmonic Power Comparison for LVCMOS and SoftEdge Outputs,

VDD = 2.5V

SoftEdge

LVCMOS

-11 dBm

-20 dBm

LVCMOS and SoftEdge Outputs,

VDD = 1.8V

Time ( 20ns/Div )

SoftEdge

LVCMOS

Odd Harmonic Power Comparison for LVCMOS and SoftEdge Outputs,

VDD = 1.8V

0

2

4

6

8

10

12

14

16

18

20

-40

-30

-20

-10

0

10

20

Harmonic Number

SoftEdge

LVCMOS

-17 dBm

-22

dBm