[time-nuts] Experiment in lowering the TAPR TICC noise floor

[John Ackermann N8UR]

Hi Attila --

Just a couple of corrections -- the "coarse clock" in the TICC runs at 
10 kHz (100 us), not 1 kHz, and therefore the TDC never sees a 
measurement interval longer than 100 us, not 1 ms.

More importantly, the chart in Figure 17 of the datasheet is for 
operation in "Mode 1" of the TDC, which is recommended for time 
intervals of 500 nanoseconds or less.  But the TICC uses "Mode 2" which 
doesn't have that limitation, and Figure 17 doesn't apply.

It would be possible to lower the noise slightly by using a 16 MHz clock 
rather than 10 MHz (but if you look at Figure 15, the improvement 
wouldn't be very great).  That would require reprogramming the PIC 
divider chip, and may some Arduino code changes as well.  (I *think* the 
clock speed is set as a constant in the code that could be changed at 
compile time, but I never tested to see if that would work without 
breaking anything.)

John
----

On 10/7/20 2:29 PM, Attila Kinali wrote:
> On Sat, 03 Oct 2020 10:37:59 +0200
> Matthias Welwarsky <time-nuts at welwarsky.de> wrote:
> 
>> When I started to look more into the software side of the TICC and especially
>> the ominous "time dilation" parameter, I set up an experiment where I feed the
>> same event into both channels of the TICC, for evaluating the sensitivity of
>> the measurements to this parameter (spoiler: there is a measurable influence
>> but it's not as critical as I originally thought).
> 
> That is to be expected. There are two resons for this:
> 
> First, the major limit to the measurement is the noise within
> the TDC7200. If you want to get lower, then you have to reduce
> this noise. If you look at Figure 17 in the TDC7200 manual, you
> will see that the noise of the TDC is highly dependent on the
> length of the measurement. Shortening the measurement will
> decrease the noise. For this you need to use a higher clock
> of the stop signal to measure against, than the 1ms that the TICC
> does. But that will not work with the Arduino. You can get around
> this if you use a faster µC like an STM32F4. See Tobias Pluess GPSDO
> design for an example how to do this.
> 
> Second, both inputs of the TICC measure against the same divided
> 1kHz clock with a modified half-Nutt interpolator. I.e. most of
> the measurement time will be common to both input signals and thus
> most of the noise seen due to the TDC and the reference clock are
> common.
> 
> 
> On Wed, 07 Oct 2020 18:34:00 +0200
> Matthias Welwarsky <time-nuts at welwarsky.de> wrote:
> 
>> the noise is likely not white, but it really depends on what is the dominant
>> noise source in the system. I guess there is some correlation but still enough
>> entropy to make a difference. I'll try with different cable lengths next to
>> see if it makes a measureable difference, but ideally you'd use two TICCs and
>> two non-coherent reference clocks. But they'd need to be somehow frequency
>> locked.. You'd need some mechanism that causes enough jitter to break the
>> correlation. A delay line controlled by some noise source?
> 
> Adding noise will not break any correlation. It will only mask it.
> I.e., the correlation will pop up once again, when you start
> using methods to remove the added noise.
> 
> Adding noise helps only if your noise is mostly quantization noise,
> then it acts as a dithering mechanism which allows you to average
> over the quantisation (and added) noise, which wouldn't be possible
> otherwise.
> 
> 
> 			Attila Kinali
>