[time-nuts] The amazing $5 timestamper with 6ns resolution (next stop: 184ps)

[Bob kb8tq]

Hi

You can indeed do some pretty fancy things with easy to get 
chips !!!

If you take a look at JLPCB

https://jlcpcb.com <https://jlcpcb.com/>

They seem to have a range of STM32’s (though not all of them …. :) )
in stock. You can do up an assembled PCB very cheaply through them. 

One thing you might want to try:

Run the *same* clock into the timebase on the STM32 *and* into the 
DUT input. Collect a few million samples and take the standard deviation. 
That will give you a pretty good idea of what’s what. 

At least on a conventional HP counter, the test comes up with one number
for the “LSB” of the measurement and another (much larger) number for 
the one sigma average. Just what term you use to refer to that one sigma
seems to be up to the marketing guys. 

With a time interval counter, you get to divide the measured one sigma
by sqrt(2) to get the per channel number. With a time tagger, doing that
can generate some level of debate. Usually it’s back to the fine print on 
this or that spec sheet to see what marketing decided to do this time ….

Fun !!

Bob

> On Feb 15, 2021, at 9:50 PM, Jeremy Elson <jelson at gmail.com> wrote:
> 
> I spent this past weekend building a little gadget I'm proud of and wanted
> to share.
> 
> I use STM32-based microcontrollers for a variety of personal projects, and
> realized that I could use one to acquire reasonably high-precision
> timestamps. The STM32G431
> <https://www.st.com/en/microcontrollers-microprocessors/stm32g4x1.html>,
> which costs about $5
> <https://www.digikey.com/en/products/detail/stmicroelectronics/STM32G431C6T6/10326704>,
> can
> run up to 170Mhz and has counters that run at the same speed. It has an
> input capture mode that can record counter values at the moment a GPIO
> signal goes high. So this weekend I put together all the pieces needed to
> do high precision timestamping:
> 
> * A Rubidium oscillator I bought on Ebay some time ago (an LPRO-101) with a
> diode clamper circuit to shift its voltage up, so it can drive the clock of
> the STM32. This serves as the frequency standard for the timestamper. I
> configured the STM32's internal PLL to multiply the Rb XO's 10Mhz x17 to
> give a 170MHz system clock.
> * Software on the STM32 that waits for input capture interrupts, reads the
> captured counter value and writes the values over a serial port, where it's
> logged on a PC
> * A basic Python script that reads the data and does some statistical
> analysis and draws graph
> * A GPS receiver as a test signal generator -- the STM32 timestamps the PPS
> signal coming from the GPS.
> 
> The results of an 18-hour run are in the attached graph. The blue points
> are the phase error between the Rubidium oscillator and the GPS PPS over
> time after some outliers were rejected. The orange line is the linear
> regression line, which has a slope (and thus implied frequency error
> between Rb and GPS) of about 2e-10. My guess is that my Rubidium oscillator
> needs some calibration since I think the performance of these things is
> supposed to be closer to 1e-11. I suspect the larger discontinuities (and
> the short term frequency error) are the GPS receiver gaining and losing
> lock as birds come and go. My GPS receiver does not have a very good sky
> view and is single-constellations, so is usually about 30 meters incorrect
> about its location. It's actually the GPS inside of Nick Sayer's GPSDO
> <https://www.tindie.com/products/nsayer/gps-disciplined-ocxo/>, though in
> this case I am just using the PPS output directly from the GPS and not
> Nick's conditioned and disciplined 10mhz output.
> 
> I've also attached a second graph of a shorter run (tau=3000s) which gives
> you a better view of the short-term jitter.
> 
> I'm pretty pleased with the results, and all done with a single $5 chip.
> RIght now it's just a mess of wires on a breadboard, but I'll probably make
> a PCB out of it. With the cost of PCB fabrication and connectors, I suspect
> the BOM would still be under $15 even in small quantities, which is
> remarkable considering that such a device would have cost tens of thousands
> of dollars a few decades ago.
> 
> The higher-end, $10 version of this chip -- the STM32G474 -- has a special
> high resolution timer that can run at 32x the system clock rate, for an
> incredible 184ps resolution. I might try to get that running on some future
> weekend; I didn't use it this past weekend because the '431 happened to be
> what I had in the house already when I had the idea. Honestly, though,
> anything higher resolution will be tougher to test unless I have a more
> stable time standard to test against; maybe I should get a second Rb XO.
> 
> The firmware
> <https://github.com/jelson/rulos/blob/master/src/app/timestamper/timestamper.c>
> and graph generator
> <https://github.com/jelson/rulos/blob/master/src/app/timestamper/eval.py>
> are both available on github. If there's interest, I will probably turn
> this into a PCB and make that available too.
> 
> Cheers,
> -Jeremy
> <gpsdo-pps-vs-rubidium-feb14.time.plot.png><gpsdo-pps-vs-rubidium-feb15.time.plot.png>_______________________________________________
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