[Gb-ccb] forwarded message from Martin Shepherd

[Richard Lacasse]

Martin,

You could make up a 1-channel prototype fairly cheaply.  You could it to test 
and optimize the circuitry, and we could check it for RFI.

Rich

Martin Shepherd wrote:
> On Thu, 21 Aug 2003, John Ford wrote:
> 
>>Remember that the purpose of the opto-isolated lines is to drive
>>switching signals, and so Rich's pulse shaping method is probably more
>>appropriate than creating a perfect match.
> 
> 
> I believe that the cable is too short for cable skew to be a
> problem. The filters are more of a concern. I'm aiming at 100ns rise
> times, so frequencies much higher than 10MHz aren't significant. The
> 32MHz RFI filters have insignificant attenuation up to about 20MHz, so
> these filters alone presumably aren't a problem either. However, what
> I don't know is the frequency response of the 2 RFI filters loaded by
> a section of cable and two impedance matching circuits, especially if
> the latter include pulse-shaping capacitors. I don't have enough
> details of either the cable or the filters to figure this out. As
> such, I am thinking of simply specifying a circuit that ignores pulse
> distortion concerns, along with a caveat that the PCB make allowance
> for the possible addition of ceramic capacitors in parallel with the
> matching resistors, and that the resistors used for matching be of a
> form that can be easily desoldered and replaced.
> 
> Martin
> 
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