[time-nuts] Re: Seeking feedback on a HW Architecture for a DIY two channel timer/counter and frequency reference

[Magnus Danielson]

Hi Erik,

On 7/16/22 16:31, Erik Kaashoek via time-nuts wrote:
> Some time ago I have shared some info on the timer/counter project I'm 
> working on.
> Based on the excellent feedback from this community I've made some 
> changes to the HW architecture.
> The goal is a simple and cheap HW, possibly leading to a product that 
> can be sold for around $100.
> An architecture level overview can be found here: 
> http://athome.kaashoek.com/time-nuts/Architecture_2.JPG
> Not shown is a touch LCD display that runs the user interface.
> The device will have two inputs (channels A and B) with AC or DC 
> coupling, user definable trigger levels and up or down trigger edge.
> One input will have an optional pre-scaler
> A third input can be used either for a GPS antenna for the internal 
> 10MHz GPSDO or for input of an external 10MHz reference.
This is useful. If you also can make it accept 5 MHz it will be even 
better. Depending on where you are, 5 or 10 MHz is the base frequency.
> A separate output can provide either the internal 10MHz reference or a 
> frequency derived by integer divide from this internal reference, such 
> as 1MHz or 1Hz. The phase of the output is locked by the GPSDO to the 
> GPS PPS
> Given the form factor, 4 inputs/output is the maximum possible.

Each additional input after 2 will give additional capability to measure 
things.

> Does this setup of inputs/output make sense?
> Are there any suggestions for improving this architecture?

While not directly visible in the architecture, there is two things I'd 
like to high-light:

First is the input side, it needs to have as low noise as possible, but 
also amplification around the trigger point to increase slew-rate with 
as little bandwidth as needed. This is the traditional wisdom at least, 
for best single-shot resolution performance. Then you have people like 
me that points out more subtle points, but those becomes valid only after:

The maximum sample rate of the TIC is important. For good result, you 
want to decimate the samples before or integrated with phase, frequency 
and linear drift estimation. The gain of that depends heavily on the 
hardwares ability to do high rate of samples, since this can then 
increase the amount of samples that goes into each estimation.

With such decimation in place on high enough sample rate, some noise can 
actually be beneficial. This will reduce the single-shot resolution for 
the benefit of decimated resolution. This technique was in commercial 
use in the 70thies, but before it could be fully analyzed.

Cheers,
Magnus