Silicon Laboratories SI5367, SI5369, SI5365, SI5366, SI5374, SI5375 Reference vs. Output Frequency

Si53xx-RM

Reference vs. Output Frequency

Because of internal coupling, output frequencies that are an integer multiple (or close to an integer multiple) of the XA/XB reference frequency (either internal or external) should be avoided. Figure 62 illustrates this by showing a

38.88MHz reference being used to generate both a 622.08 MHz output (which is an integer multiple of 38.88 MHz) and 696.399 MHz (which is not an integer multiple of 38.88 MHz). Notice the mid-band spurs on the 622.08 MHz output, which contribute to the RMS phase noise for the SONET jitter masks. Their effect is more pronounced for the broadband case. For more information on this effect, see "Appendix G—Near Integer Ratios" on page 162.

155.52MHz in, 622.08 MHz out, 696.399 MHz out

	0
	-20
(dBc/Hz)	-40
	-60
Noise	-80
	-100
Phase
	-120
	-140
	-160
	100	1000	10000	100000	1000000	10000000	100000000

Offset Frequency (Hz)

Yellow—696.399 MHz output

Blue—622.08 MHz output

Figure 62. Reference vs. Output Frequency

Table 62. Jitter Values for Figure 62

	696.399 MHz Out	622.08 MHz Out
Jitter Bandwidth	Yellow, fs RMS	Blue, fs RMS
SONET_OC48, 12 kHz to 20 MHz	379	679
SONET_OC192_A, 20 kHz to 80 MHz	393	520
SONET_OC192_B, 4 MHz to 80 MHz	210	191
SONET_OC192_C, 50 kHz to 80 MHz	373	392
Broadband, 800 Hz to 80 MHz	484	1,196

The crystal frequency of 114.285 MHz was picked for its lack of integer relationship to most of the expected output frequencies. If, for instance, an output frequency of 457.14 MHz (= 4 x 114.285 MHz) were desired, it would be preferable not to use the 114.285 MHz crystal as the reference. For a more detailed study of this, see "Appendix G—Near Integer Ratios" on page 162.

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