Si5345-44-42-D-RM
Rev. 1.0
61
11. XAXB External References
11.1. Performance of External References
An external standard non-pullable crystal (XTAL) is used in combination with the internal oscillator (OSC) to
produce an ultra low jitter reference clock for the DSPLL and for providing a stable reference for the free-run and
holdover modes. A simplified diagram is shown in Figure 35. The device includes internal XTAL loading capacitors
which eliminates the need for external capacitors and also has the benefit of reduced noise coupling from external
sources. Although the device includes built-in XTAL load capacitors (CL) of 8 pF, crystals with load capacitances up
to 18 pF can also be accommodated. Frequency offsets due to CL mismatch can be adjusted using the frequency
adjustment feature which allows frequency adjustments of ±200 ppm. The recommended crystal suppliers is
provided in Table 34 with PCB layout recommendations for the crystal to ensure optimum jitter performance.
Figure 35. Crystal Resonator and External Reference Clock Connection Options
The Si5345/44/42 accepts a clipped sine wave, CMOS, or differential reference clock on the XA/XB interface. Most
clipped sine wave and CMOS TCXOs have insufficient drive strength to drive a 100
or 50
load. For this
reason, place the TCXO as close to the Si5345/44/42 as possible to minimize PCB trace length. In addition, ensure
that both the Si5345/44/42 and the TCXO are both connected directly to the ground plane. Figure 36 shows the
recommended method of connecting a clipped sine wave TCXO to the Si5345/44/42. Because the Si5345/44/42
provides dc bias at the XA and XB pins, the ~800 mV peak-peak swing can be input directly into the XA interface of
the Si5345/44/42 once it has been ac-coupled. Because the signal is single-ended, the XB input is ac-coupled to
ground. Note that when using a single-ended XO, the XO signal must be driven on XA. If XA is not driven, the
device will report an LOSXAXB alarm. Figure 37 illustrates the recommended method of connecting a CMOS rail-
to-rail output to the XA/XB inputs of the Si5345/44/42. The resistor network attenuates the rail-to-rail output swing
to ensure that the maximum input voltage swing at the XA pin is less than 1.6 V pk-pk. The signal is ac-coupled
before connecting it to the Si5345/44/42 XA input.
If an external oscillator is used as the XAXB reference, it is important to use a very low phase noise external
oscillator because there is essentially no jitter attenuation (up to 1 MHz) from the XAXB pins to the outputs. Before
selecting an external oscillator at XAXB, it is important to first test the output jitter to determine if the output jitter
degradation from the use of the external oscillator is acceptable.
To achieve the lowest output jitter the best approach is usually to use a crystal in the range of 48-54 MHz at XAXB.
OSC
XB
XA
48-54MHz
XTAL
2xC
L
2xCL
Crystal Resonator
Connection
OSC
XB
XA
48-54MHz
XO
100
Differential XO
Connection
OSC
XB
XA
48-54MHz
XO
Single-ended XO
Connection
2xC
L
2xC
L
2xC
L
Si5345/44/42
2xC
L
Si5345/44/42
Si5345/44/42
÷PXAXB
÷PXAXB
÷PXAXB
OSC
XB
XA
48-54MHz
XO
Split Differential XO
Connection
(Highest Performance)
2xC
L
2xC
L
Si5345/44/42
÷PXAXB
50
50