[time-nuts] Why a 10MHz sinewave output?

Hal Murray hmurray at megapathdsl.net
Tue Feb 7 04:58:15 UTC 2012


magnus at rubidium.dyndns.org said:
> Oh... nothing really beats "it's what customers traditionally asks for"
> Squarewave out provides high slew-rate which reduces the effect of
> additional noise. 

Right.  But if you have a single frequency you can easily filter out most of 
the noise.

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As clock speed has increased over the years, a new field has emerged.  A good name is signal integrity.  That covers clock distribution, data distribution, and power supply bypassing/regulation/decoupling.  It's basically all the analog stuff needed to make digital logic work in the real world.

The technology for distributing more bits is also useful for reducing noise/jitter.

There are whole families of chips for clock distribution.  Many include PLLs which can correct for trace length, make other frequencies, and/or do the spread spectrum thing to "reduce" EMI.  The ones I'm familiar with are targeted at digital applications.  Jitter within a small fraction of a bit cell is fine.  The target market doesn't care about time-nut class super clean clocks.

There are other families of chips (or parts of big chips) for driving/receiving clocks and data between boards/boxes.  Most of those are now differential so I assume twisted pair is cheaper than coax.  SATA between your motherboard and hard disk is a good example.  It runs at 1.5, 3, or 6 gigabits/second.  Wiki says up to 1 meter.

If you want a good example of the technology in this area, check out gigabit ethernet over CAT5.  It's 5 level encoding (2 bits/baud) at 125 megabaud/sec over 4 pair in both directions over each pair.


Optical stuff is still single-ended.  :)

There are some very low cost optical links for distribution of audio.  The key idea is to use plastic rather than glass.  Bandwidth is limited but cost is low.



-- 
These are my opinions, not necessarily my employer's.  I hate spam.






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