[GPS_Standard] TIC (Time Interval Counter) board available for Bert's board

[Bob Stewart]

A TIC (Time Interval Counter) measures the interval between two pulses, and as I discovered, is pretty much a necessary component for a PLL.  I made a simple TIC daughterboard for Bert's board to finally get the PLL I was looking for.  It has a sensitivity of better than 250ps, but I am only using whole nanosecond values from it.  I have four extras available, if anyone wants one.  You'll have to upgrade to an 18F2320, as the code got too large for the 18F2220.  I'll put the new source code up on  my site as necessary.  It uses the PD part of the PID control system, and is now in a pretty mature state; meaning I haven't had any errors in many weeks.  If there is interest, I can probably work up a small program for the PC to grab the sawtooth from your timing board and forward it to the PIC; though this would be targeted for Linux.

A picture of the board installed on my machine is here: http://evoria.net/AE6RV/TIC/2014-03-27%2018.40.12.jpg


The schematic is here if you want to make your own:  http://evoria.net/AE6RV/TIC/TIC1.1.bmp

A sample plot is here: http://evoria.net/AE6RV/TIC/fll.png

In the sample plot, the cyan is the measured phase in nano-seconds of the 1PPS from my Adafruit.  The blue is the DAC value.  The red (almost buried in the cyan) is the filtered 1PPS.  The Adafruit doesn't supply a sawtooth value, so I've done the best I could with a digital filter.  All things considered, it does a great job, as the Adafruit wanders around a bit more than the manufacturer says they do.  The smear at the bottom is a debug value for me, showing the contents of the filter.  


As you can see, I have locked the phase to 50ns, or 180 degrees.  The spikes in the plot are due to the fact that the Adafruit is not a timing receiver, so there are phase changes as it wrongly senses position changes.  Note that I have a 4:1 divider on the EFC line, so any changes will be 4 times larger than if the EFC were wired directly to Bert's board.


Anyway, these 4 are extras from my purchase from ExpressPCB.  The installation isn't as clean as I would have liked, but my options were limited.  If you'd like one, let me know.  I only ask that you send me a datafile for a few hours of operation.

I am starting the design of a new GPSDO that incorporates this TIC and uses a dsPIC33 as the processor.  I chose the dsPIC33 because I have other projects I want to do on one, but more importantly, it has 2 UARTs.  This will allow my new board to talk directly to the GP receiver to get the sawtooth figure.  Otherwise, it will look a lot like Bert's board, though it will use different chips due to the fact that the dsPIC runs on 3 volts.  Hopefully it will fit on the ExpressPCB form factor.  I plan to code it in C, which will make it easier for others to fiddle with.


Bob - AE6RV