[time-nuts] Comparison of Logic Standards for Clock Distribution
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Thu Oct 26 00:24:11 UTC 2006
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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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
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