>Eric Williams wrote:
>I just saw Lars' postings on his design and thought I'd throw mine in since
>I've been working on a similar low-end goal but doing it a different way.
>Not really an Arduino, either, but it's based on an AVR processor and
>should be portable to an Arduino.
Good to see someone else experimenting and sharing it (same for Bob Stewart!)
Do you use the Arduino bootloader and environment (code)?
>I don't consider it a PLL because I have no way to measure phase
>differences within a cycle.
I prefer to use time instead of phase as it is confusing to use phase with two
different clock speeds (5MHz /1Hz). I also should say that it took me a while
to figure out that Bob Stewart’s graphs showed a range of 100ns for an ADC
range of about 400 (100-500). Maybe using both TIC value and ns scales would be
good (or explained in a header or the file name).
I consider HP5328, 5334, 5335, 5370 etc counters in time interval mode to be
very similar to my and your TIC or what Bob Camp recently named TDC. TIC is an
acronym for Time Interval Counter and TDC Time to Digital Counter. In fact I
used a 5328A as TIC in my first trials to do a GPSDO long ago.
Having a clock of 5 MHz and counter to 5000 for me is the same as a TIC range
of 5000 with an input frequency of 1 kHz. Think of a counter in time interval
mode with start from the 1PPS and stop from a 1 kHz (1ms) clock. It doesn´t
matter if it is an analog integrator+ADC with 5000 “bits” (counts) or a digital
counter with a 5MHz clock the resolution will be 200nsecs. In my Arduino GPSDO
you will find two TIC´s, one analog and one digital. The analog has a clock of
1MHz and a range of about 1000 (slightly less than 10bits) so the resolution is
about 1ns and range 1uS. The other is like yours with a 5MHz clock to Timer 1.
As I also count overflows the range is 5000000 (+-2500000) and up to 1sec (uses
something like a modulo 5000000 in the program). This also gives a resolution
of 200ns as yours. I decided to have some overlap between the analog and
digital TIC as the synchronization to the clock in the ATmega328 is a problem.
This gives some margin between the ranges of the analog and digital TIC.
A note maybe more to those working with an analog TIC with a simple RC net: In
my Arduino GPSDO the ADC value and ns is about the same. For low ADC values
1LSB is about 0.88ns and for high values 1.12ns. The range I see from the ADC
for 1000ns is 15-1005 (after fine trimming of the R). The relatively small
deviation from 1ns/LSB is due to that I only use about 20% of the RC range due
to the 1.1V ref instead of 5V ref. From the beginning I used the 5V ref and
about 600 out of the 1024 range. This gave a factor of 2.5 in resolution change
over the range. For a normal deviation of +-10-100nS (depending on different
GPS receivers) the TIC range is also a parameter for the linearity. With a
1000ns TIC range the linearity around +-50ns from the midpoint is very good. A
TIC range of 100nS +-50ns cover all of the range. But in a GPSDO the
nonlinearity may not be a problem at all (especially if you correct for it).
Most probably the nonlinearity only gives a small time offset as the values
that are above the midpoint have a slight different gain to the ones below.
Lars
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