[ntp:questions] Re: TAC-2 internal delay

Tim Shoppa shoppa at trailing-edge.com
Mon Oct 20 16:10:05 UTC 2003

test <test at iinet.net.au> wrote in message news:<ge92pv0ta14fj5u9o7djco6pp4ga5jl9tg at 4ax.com>...
> I'm currently setting up a TAC-2 and Oncore VP based reference clock
> for connection to a FreeBSD server's serial port and am trying fine
> tune the accuracy of the 1PPS signal as much as possible.  Working out
> antenna and measurement cable delays was fairly easy, but the TAC-2
> operations manual contains the following text:
> "Note: This screen also shows a value of 8 nsec for the
> internal receiver delay. This is a provisional value subject
> to more detailed calibrations of the TAC and is based on
> the propagation delay thru two 74AC14 gates."
> Looking at the TAC-2 schematic, the 1PPS signal from the GPS module
> gets to the DCD serial pin via one 74AC04 and one MAX232.  The
> datasheets I have for these chips give their propagation delays as 8ns
> for the 74AC04 and 1.3us for the transmit section of the MAX232.
> So, the question I have is about the relevance of the 8ns receiver
> delay.  Shouldn't it be closer to 1.3us, or am I just missing the
> point completely?

The 1.3 us number for the MAX232 includes the effect of driving a
long capacitive line.  When driving a short line, the actual delays that
I've measured are much shorter.

For a "typical" linux nano-kernel NTP server, pulse-to-pulse jitters
are in the very low microsecond range and are dominated by the RS-232
receiving hardware and interrupt latency.  That doesn't put the 1.3us
completely in the noise (after all, it's systematic and not random),
but after you figure in network delays to your clients then 1.3us
is way down in the noise.  

If 1.3 us is important to you, you should undoubtedly be using well-
characterized differential transmitters/receivers and not RS-232.


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