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

[Dr Bruce Griffiths]

Ulrich Bangert wrote:
> 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
>>>
>>> _______________________________________________
>>> time-nuts mailing list
>>> time-nuts at febo.com 
>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>       
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>>     
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But the input voltage noise of the MAX477 is 5nV/rtHz, the corresponding 
value for a discrete transistor circuit may be as low as 1nV/rtHz.

Bruce