Si5317-EVB

Rev. 0.1

3

2. Si5317-EVB Input and Output Clocks

2.1. Input Clocks

The Si5317-EVB has a differential clock input that is ac terminated and ac coupled before being presented to the

Si5317. If the input clock frequencies are low (below 10 MHz), there are extra considerations that should be taken

into account. The Si5317 has a maximum clock input rise time specification of 11 ns that must be met (see CKNtrf

in the Si5317 data sheet). Also, if the input clock is LVCMOS, it might be advantageous to replace the input

coupling capacitors (C7, C12, C16. and C18) with 0

 resistors. When using LVCMOS inputs, the user should

consider removing the ac termination and using source series termination located at the driving source.

Regardless of the input format, if the clock inputs are not approximately 50% duty cycle, it is highly recommended

to avoid ac coupling. For input clocks that are far off of 50% duty cycle, the average value of the signal that passes

through the coupling capacitor will be significantly off of the midpoint between the maximum and minimum value of

the clock signal, resulting in a mismatch with the common mode input threshold voltage (see Vicm, in the Si5317

data sheet).

2.2. XA-XB Reference

To achieve a very low jitter generation and for stability during holdover, the Si5317 requires a stable, low jitter

reference at its XA-XB pins. To that end, the EVB is configured with a 114.285 MHz third overtone crystal

connected between pins 6 and 7 of the Si5317. However, the Si5317-EVB is also capable of using an external XA-

XB reference oscillator, either differential or single-ended. For details concerning the allowed XA-XB reference

frequencies and their RATE settings, see the Si5317 data sheet. J1 and J2 are the SMA connectors with ac

termination. AC coupling is also provided that needs to be installed at C6 and C8. Table 1 explains the component

changes that are needed to implement an external XA-XB reference oscillator.

Table 1. XA-XB Reference Connections

Mode

Xtal

Ext Ref

Ext Ref In+

NC

J1

Ext Ref In-

NC

J2

C6, C8

NOPOP

install

R8

install

NOPOP

MH

MM

Notes:

1. Xtal is 114.285 MHz.

2. NC - no connect.

3. NOPOP - do not install.

4. J12 jumper, see Table 3.

5. C6 on bottom of the board.