[time-nuts] Of rubidium life and piggy-bank anemia....

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Dec 1 17:56:07 EST 2007

Bruce Griffiths wrote:
> Poul-Henning Kamp wrote:
>> In message <4751DAF0.3070700 at xtra.co.nz>, Bruce Griffiths writes:
>>> The thorny issues of avoiding the noisy environment of a PC and its
>>> unstable PCI clock whilst still allowing a PC to be synchronised to an
>>> external timebase may perhaps be adressed by:
>>> Producing a simple PCI (or PCIe, or even ISA bus card - still widely
>>> used in industry)
>> You don't need that.
>> If you have an external timestamping device you can just generate
>> any relevant signal from the PC (I prefer parallel ports, but they're
>> going out of vogue) and time that relative to whatever clock your
>> OS uses, then read back from your timestamping device (via USB ?)
>> and calibrate accordingly.
>> Modules cable-length and slope of your generating signal (EMI
>> filtering), there is no difference in the resulting precision.
>> BTW: I belive the NI PCI-66xx series of cards can be used also,
>> but I've never actually tried, I only read the low-level doc.
> That certainly sidesteps the PC bus obsolescence issue until suitable
> interfaces such as parallel ports and serial ports vanish never to be
> replaced.
> It would be relatively easy to include an optically isolated (or
> functional equivalent such as the Analog Devices chip scale transformer
> isolators) port on the external time stamp device for such an application.
> Since USB and Firewire ports may persist a little longer, is it possible
> to use similar techniques with these interfaces?
> Do you mean that the PCI66XX boards could be used to implement the
> multichannel timestamp function?
> If so then most of the issues associated with PC noise may be addressed
> by using a small external board that incorporates RF transformers and to
> break LF ground loops and has clock conditioning circuity to produce
> logic level clocks for the PCI66XX board from sinewave inputs.
> Bruce
After reading the PCI 66xx datasheets, it would appear that the external
board should also include a synchronous programmable PPS divider for
each channel.
Since the number of available clocks is limited in an FPGA it may be
best to use a single FPGA per channel, this also avoids crosstalk
between channels allowing the dividers to be used effectively with
higher resolution time stamp instruments when this is useful.


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