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How to select the right timing device--A case for MEMS—Part II

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steve5500
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re: How to select the right timing device--A case for MEMS—Part II
steve5500   4/19/2012 5:32:55 PM
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..and response to the 2nd, more recent comment: 2) Phase noise contributions and low-frequency offsets: It is correct that jitter contribution at low offset frequencies cannot be ignored, and that was not the intended point in this article. The confusion seems to be in definition of “corner frequency”. The corner frequencies cited in the article refer to the 3dB upper and lower cutoff frequencies of a bandpass filter. The attenuation in stop-band is 20dB/dec for most standards, as the reader noted. The integration range is typically much larger than pass-band frequency range of the filter, for example, 100Hz to 50MHz (or half the clock rate). The full detail can be found in SiTime application note AN10012: http://www.sitime.com/support2/documents/AN10012_SerialIO_PhaseJitterReguirement_rev1.2.pdf. The 12kHz to 20MHz integration range does not assume any smooth filtering of phase noise outside that range; it totally ignores any phase noise outside that range. It is true that for cases that 12kHz to 20MHz phase jitter is low, the filtering response of the application can be ignored because the noise outside the application filter pass-band would be small. Nevertheless, the application filter response is the one governing the jitter budgeting of the application and 12kHz to 20MHz integrated phase jitter is merely a proxy that has been widely used in practice. SiTime's new class of LVCMOS, LVPECL, and LVDS devices meet low typical phase jitter of 0.5ps (0.85ps max) integrated from 12kHz to 20MHz. The user can easily use these parts regardless of analysis method used. The standard governing phase jitter filtering and budgeting has been developed for a long time with careful attention to different trade-offs. One of important jitter documents for serial-IOs is MJSQ that was generated for Fibre Channel applications. To the author’s knowledge, the MJSQ does not make any assumption about the oscillator source.

steve5500
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re: How to select the right timing device--A case for MEMS—Part II
steve5500   4/19/2012 5:31:37 PM
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I'd like to reply to a couple of recent comments submitted: 1) The need for low wander (phase jitter below 10Hz) in telecom (SDH/SONET) applications and convergence of telecom and networking through Synchronous Ethernet is expected to increase demand for parts with low wander. This is true. The topic of wander was not addressed becasue it's beyond the scope of this article. The question is whether MEMS oscillators can deliver on that, and the answer is yes. SiTime has published phase noise plots on their website (starting from 1kHz offset) for specific frequencies. In addition, SiTime provides phase noise plots down to 1Hz for any specific frequency that a customer may request. As a guideline, the RMS wander for SiTime's SiT8208/9 and SiT912x single-ended and differential MEMS-based oscillator products is 1.5UI rms (phase jitter between 1Hz to 10Hz). This easily meets the 2490UI tolerance limit the reviewer mentions. Furthermore, SiTIme's upcoming MEMS-based Stratum-3 devices will meet all MTIE and TDEV requirement for synchronous networks.

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