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

[Dr Bruce Griffiths]

Stephan Sandenbergh wrote:
> Hi All,
>
>  
>
> Can anyone give me the low-down on logic standards for clock distribution in
> digital systems?
>
>  
>
> It seems that ECL, PECL and LVDS are the most widely used. After a quick
> glance at the specs for the above mentioned standards I noted the following
> differences between ECL/PECL and LVDS: Although ECL/PECL is faster and can
> be distributed along longer lengths of cable it requires more power and
> produce more noise. Also the input sensitivity of LVDS is twice that of
> ECL/PECL. So logically, I can conclude that one should use ECL/PECL for
> clock distribution along long lengths of cable and LVDS for shorter ones.  
>
>  
>
> However, there are a number of questions that spring to mind. Even when
> distributing a low frequency digital signal, for example a 10MHz frequency
> reference, one is in reality faced with a high-speed digital problem. One
> explicitly wants the signal to have fast edges so that it is effectively
> immunized against amplitude/temperature/power supply variations. The faster
> the edge (or rise time) the lower the delta t (or jitter) in response to the
> mentioned variations. However, faster edges give rise to various problems
> such as crosstalk, EMI and reflections&ringing. I guess that another
> approach would be to differentially transmit the clock in sinusoidal format
> where after it is heavily filtered and squared at the receiving end. Now,
> one would have less transmission line problems but also less noise immunity.
>
>  
>
> It seems that there must be a trade-off between the digital and analog
> worlds here. I grasp that more bandwidth (for faster rise times) allows
> faster data throughput but where is the middle ground for conveying
> frequency references? (i.e. at which rise time will the side-effects out
> weigh the benefits)
>
>  
>
> Kind regards,
>
>  
>
> Stephan Sandenbergh.       
>
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>   
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