[time-nuts] Re: Current-day GPS timing receivers

[glen english LIST]

For those of you interested in rolling your own a-la the Brooks Shera 
style, which is essentially adigital readout phase detector , these days 
, with almost a single chip....

with modern microcontrollers you can do better than the Shera hardware, 
I describe how I do it :

The modern micros like 168 MHz $10 STM32F4 have 16 and 32 bit counters 
running at full core speed of 168 MHz or core div2 at 84 MHz depending 
on what you buy.

The core clock pin  receives it input clock from the 10 MHz (or whatever 
you like) oscillator you wish to disclipine. Internally this 10 MHz is 
routed to the clock of a  counter. The microcontroller has a PLL that 
multiplies this up to 160 MHz ish.

If you are willing to accept a little extra noise from the internal PLL 
multiplier , the 160 MHz core clock also sent to the counter. (much 
higher PD resolution)

The 1PPS is connected to the external input counter gate ON OFF- all 
hardware gates internally - so the 1PPS will gate on and gate off the 
timer(s). There are several gate modes to get either emulated 
phase/frequency detectors. Now you have the Shera hardware emulated.

The processor has a fast 1MHz 12 bit DAC and internal bandgap DAC 
reference. nice !.... The 12 Bit DAC is oversampled up to 18 bits, There 
will be a limit due to the ADEV of references etc etc but its all pretty 
good.

I use the LVDS DS90LV....  family of differential receivers, non 
internally terminated inputs,  for my squaring of input sine waves into 
nice sharp logic inputs for the processor. Some processors have schmitt 
and glitch removal  inputs already.

The C code is nice an easy to write and debug , fast processor, and 
plenty of UARTs to drive serial outputs for status strings .

-glen


On 20/02/2022 12:36 pm, Bob kb8tq wrote:
> HI
>
> Here’s the basic problem:
>
> If you don’t already know that everything you are doing is perfect, you need
> something better than what you GPSDO does to compare to. If you are happy
> with the performance of an Rb then … why not just use the Rb? The grubby issues
> are all out past a number of hours and stop at however long you decide to look.
>
> Why “how ever long?”. There are a *ton* of cor