[time-nuts] 1 PPS 50-ohm driver

[Bob Camp]

Hi

If the resistors are typical thick film SMT parts, they will begin to have issues below
1 GHz. Thin film resistors will get you a bit higher than thick films. In both cases board
layout to achieve 1 GHz performance is going to be a challenge. 

All that said, for a 1 PPS output the waveform looks fine. A 5 ns rise time is equally 
fine for a 1 pps. The faster edge will bother radios a bit more. Both are plenty fast
enough to properly trigger anything you have “down stream” of the device. Since faster
than 5 is trivial to do, why not get the faster speed. 

When run through normal lab cables and connectors, the open termination at the far end will be
more of an issue than a 2 to 5 ns rise time. Your “got a great deal” cables and connectors 
are not perfect into the 100’s of MHz.  If you are completely wired with hard line and APC-7’s 
this does not apply to you :)

Bob

> On Apr 17, 2016, at 12:58 AM, Stewart Cobb <stewart.cobb at gmail.com> wrote:
> 
> Here's a scope photo from a PPS driver built exactly to the description in
> my earlier post.  It's a 74ACT04 in a TSSOP package, with five parallel
> outputs driving five 220-ohm resistors (0402 SMT) to form a 50-ohm output.
> 
> The photo shows 2 ns rise time for the leading edge of the pulse. The scope
> bandwidth is around 1 GHz, so this measurement is pushing its limits.  The
> pulse looks fairly clean, but with a bit more care in layout and cabling,
> it might get even better.
> 
> Cheers!
> --Stu
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