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

Ulrich Bangert df6jb at ulrich-bangert.de
Wed Oct 25 10:21:09 EDT 2006


Hi Folks,

> 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? 

While it is not exactly locking a crystal to a input frequency HP has
used crystal filters in the 'reference input stages' of many devices
like the one shown on the JPG. The 10 Ohms trimmer has no noticeable
influence on the circuit. Today the circuit is more easily built with
integrated amplifiers as the MAX477.

Best Regards
Ulrich Bangert, DF6JB

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Stephan Sandenbergh
> Gesendet: Mittwoch, 25. Oktober 2006 13:07
> An: 'Discussion of precise time and frequency measurement'
> Betreff: Re: [time-nuts] Comparison of Logic Standards for 
> Clock Distribution
> 
> 
> 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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> > time-nuts at febo.com 
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> 
> 
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