[time-nuts] PICTIC II

[Richard H McCorkle]

Fellow Time-Nuts,
When I first uploaded the Simple PICTIC interpolating time interval
counter to the K04BB site in 12/08 and presented it to the group as
a Christmas present my goal was to get amateurs building their own
interpolating time interval counters for GPS monitoring and making
improvements to my design. The interpolator in the PICTIC was
“borrowed” from an early HP counter with minor modifications so I
didn’t design it and had no personal attachment to it. My forte is
writing PIC assembly code for time and frequency applications and
I make no claims of being a hardware design engineer. I am just
an amateur interested in writing PIC code and was building my own
PICTIC. I wanted to share my design, as I couldn’t afford multiple
commercial high-resolution time interval counters to monitor my
standards and thought there might be others with similar needs that
could benefit from a low cost TIC design.
  Testing the HP interpolator with my code showed it had sufficient
capability to be useful in a low cost TIC for GPS monitoring where
a modest resolution increase was desired. In response to my posting
I was subjected to a barrage on the shortcomings of the interpolator
with few comments on the rest of the design. I responded to the
comments based on test results showing it was sufficient for purpose
and suggested trying the design before saying it wasn’t usable. The
response returned was “I wouldn't waste my time with a circuit that
is inherently as non-linear as yours.” so the rest of the design was
discounted by others as not being worth the time to evaluate. I began
a discussion on line about the interpolator, which was destined to
turn into a long and annoying thread similar to the recent TPLL
discussion. Rather than getting into it on line I elected to put a
stop to it by saying publicly I was at fault and would begin further
study, as I am not as thick skinned as Warren. Subsequent independent
testing of the PICTIC by William Riley showed the interpolators were
linear to the 10-bit ADC resolution over the measurement range with
suitable accuracy for the intended application of GPS monitoring
as originally stated.
  Over the last 18 months I have developed a new diode switched
interpolator based on the comments made on line and have thoroughly
tested it. Some suggestions made improvements in the performance and
some resulted in poorer performance. I incorporated those suggestions
that made improvements, eliminated those that made things worse, and
once I was satisfied I sent the revised interpolator design directly
to Bruce off line. Based on comments and suggestions he returned
during a long series of emails I incorporated additional changes in
the code, front-end, and interpolator designs and tested those
changes until I was satisfied with the performance and he had no
further comments. I am finally satisfied with the new design and
admit that by incorporating the majority of the suggestions made
the new interpolator has significantly better linearity than the
HP interpolator used in the PICTIC and it is now suitable for
higher resolutions.
  I was reluctant to post the PICTIC II in this forum, as I don’t
want to get in another public discussion of its faults without any
discussion of the merits of a $50 serial output interpolating TIC
on a 3.8” x 2.5” thru-hole board designed for amateur construction.
The K04BB WIKI PICTIC page was recently updated to include the
PICTIC II code and ExpressPCB board layout and schematic files for
those that might be interested.

http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:pictic

The PICTIC II incorporates the new interpolator, requires a delay
between the inputs, and uses a low stability XO timebase with
software peak detection for calibration with provision for an
external timebase like the PICTIC to minimize size and cost.

Richard