[time-nuts] Advantages of GNSS ???

[Tom Van Baak]

Let's put the Bert vs. Dana misunderstanding aside.

To me the key feature in Bert's photo is the Dallas/Maxim digital delay 
chip. Look carefully and see the DS1023-50, which is an 8-bit 
programmable delay line (~0 to ~127 ns in 0.5 ns steps). This is a 
technique used to remove sawtooth error without requiring a ns or sub-ns 
TIC and a PC.

The trick: before each 1PPS the delay line is latched with the 
appropriately signed and scaled sawtooth correction number so that when 
the 1PPS arrives the leading edge is physically (electronically) delayed 
by exactly the right number of compensating ns. If you look inside one 
of Rick Hambly's GPS clocks [1] you will see this. Each second a PIC 
reads the binary sawtooth message from the receiver and programs the 
delay line just in time for the next 1PPS. The result is a sawtooth-free 
1PPS without requiring a TIC or a PC. The idea has been around for 20 
years, the era of the Motorola Oncore VP receiver.

The performance of this "hardware" solution to sawtooth correction is 
simpler and nearly as good as the more complex "software" solution 
that's used today. For comparison plots see page 35 of:

https://www.haystack.mit.edu/workshop/TOW2017/files/Seminars/tow-time2017.pdf

The plot is beautiful. The reason this delay line technique isn't used 
much anymore is that AFAIK the Dallas chips are no longer produced. So 
almost every uses s/w sawtooth correction now.

/tvb

[1] https://www.cnssys.com/cnsclock/CNSClockII.php