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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Dec 1 23:06:53 UTC 2007


Poul-Henning Kamp wrote:
> In message <4751E2B2.4000009 at xtra.co.nz>, Bruce Griffiths writes:
>   
>> Poul-Henning Kamp wrote:
>>     
>
>   
>> Since USB and Firewire ports may persist a little longer, is it possible
>> to use similar techniques with these interfaces?
>>     
>
> For talking to the timestamping device: certainly.  For signalling
> the computers time-edge: no way, they're serial protocols.
>
>   
>> Do you mean that the PCI66XX boards could be used to implement the
>> multichannel timestamp function?
>>     
>
> Yes, they have counter and latches that I belive a suitable for
> at least two signals, but I'm not sure there were any usable
> prescalers.  It was a long time ago.
>
>   
My understanding is that the prescalers are only useful for dividing the
clock frequency down to a frequency that is sufficiently low for the
counters.
These boards are now available in 2, 4 and 8 channel versions. Their
industrial counter timer boards have a maximum external clock frequency
of 400Khz and a minimum pulse width of 1us, so thes may be less suitable.
> I'm not sure what kind of flanks plastic fibers (as used for instance
> from CD-players) have, I might go that route to get complete
> electrical separation.
>
>   
I have 3 HP (now Avago technologies) 5 Mbaud glass fibre
transmitter/receiver evaluation kits, I must get around to measuring
their jitter and delay tempcos.
However for low resolution time stamping (10ns resolution) they (and
their plastic fibre cousins) should be perfectly adequate. Jitter may be
a little high (for some purposes) when the timestamp resolution is 100ps
or so.
> I saw a HP eval kit with 3' of plastic fiber and transmitter and
> receiver that would work great.
>
>
>   
Bruce




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