[time-nuts] Comparison of Logic Standards for Clock Distribution

[Stephan Sandenbergh]

Hi Bruce,

Thank you for the elaborate answer covering different logic types. Funny
enough, I have just read the excellent book you recommended cover to cover -
probably the origin of many of my questions.

As I said in reaction to Said's response - I am not surprised that analog
(sine wave) transmission is superior. But, it takes a lot more effort to do
it well.

You mentioned that locking to a crystal at the receiving end as an option.
Does this mean that signal transmission is primarily plagued by short term
noise? 

I have never really touched the topic of optical fibre, but I realise that
it is superior to conventional methods. The superiority of optic fibre is
probably not as pronounced at short distances, is it?

I realise that a better reference clock will only improve a system's
performance up to the point where the jitter and phase noise of the other
components in the system begins to dominate. However, I would like to have a
good grip on the basics. Are there any good books you can recommend on the
topic of clock distribution?

Kind regards,

Stephan Sandenbergh  

> >
> Differential signalling using current mode drivers (open collector long
> tailed pair or equivalent) can be more effective than ECL and LVDS in
> that the output common mode range may be significantly larger. This
> allows larger differences in ground potential between the transmitter
> and receiver and hence greater immunity to this. ECL can have problems
> when the ground potential differences between the receiver and
> transmitter are large enough. Whre a current mode driver will work well
> even with several volts of difference in ground potentials. Transformer
> coupling is also effective when dc coupling isnt necessary, however the
> transsformer characteristics will degrade the signal rise and fall times.
> 
> It is difficult to preserve the subnanosecond rise and fall times of
> digital signals when transmitted over a significant length of  circuit
> board trace (particularly when using an FR4 or equivalent board
> substrate).
> 
> 
> Distributing a standard frequency  using a well screened dedicated
> shielded balanced transmission line presents fewer difficulties (for
> distances of a few hundred meters or so) than attempting to distribute a
> relatively small amplitude logic level signal. RF transformers at each
> end can be used to provide good common mode rejection and it is
> relatively easy to transmit higher power signals than is feasible with
> logic signals. If noise is perceived as a problem then one can always
> phase lock a crystal oscillator at the receiving end to the transmitted
> signal.
> 
> Eventually cable losses associated with long cables limits the bandwidth
> and hence the signal risetime.
> Optical fibre is used when a reference frequency has to be transmitted
> over several kilometers as in Radio telescope interferometer arrays.
> The fibre is relatively immune to differences in ground potential, and
> other noise sources.
> Fibre can also have a significantly lower propagation delay temperature
> coefficient.
> Fibre bandwidth degrades less rapidly with length than cable bandwidth.
> 
> Analog transmission techniques also have the advantage of degrading the
> signal short term stability less than digital transmission techniques.
> The jitter of a digital device adds more phase noise than a well
> designed analog amplifier.
> 
> A good reference on the problems of high speed digital design is:
> 
> HIGH-SPEED DIGITAL DESIGN
> A Handbook of Black Magic
> Howard W Johnson
> Martin Graham
> PTR Prentice Hall
> ISBN 0-13-395724-1
> Bruce
> 
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