[time-nuts] Re: Returnee, interested in Lars GPSDO

[Bob Camp]

Hi

My point was, you have a CPU and various other chunks of hardware running around. 
There are a wide range of ways to convert that pulse to “something else” ( like a voltage
to feed the A/D) . 

You also could go over to a <$10 RP2040 and get a lot more mileage out of your “something
else”. 

Bob

> On Dec 23, 2022, at 8:13 AM, Jim Harman via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> (I originally sent this just to Bob, repeating for the group)
> The "something else" in Lars's design is provided by 5 MHz derived from the
> oscillator, which feeds a timer in the processor.  The 1 pps triggers a
> "timer capture" which records the number of 5 MHz cycles since the last pps
> with a resolution of 200 nsec.  (The Arduino processor clock is 16 MHz and
> the maximum timer frequency is Fclk/2, so you can't drive the timer with 10
> MHz.)
> 
> This coarse detector is used to extend the lock-in range and achieve the
> initial lock. It would be possible to make a GPSDO with just this detector,
> but the phase sensitivity would be low. The 4046 followed by the pulse
> width to voltage converter and the processor's 10 bit  A/D converter reads
> the phase with a resolution of about 1 nsec.
> 
> 
> On Thu, Dec 22, 2022 at 8:40 PM Bob Camp <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> Since you only get one sample per second, no matter what you do, why
>> bother with
>> the 4046 at all? 1 us will roll over at 1 ppm off frequency ( which is
>> very possible with
>> an OCXO ). You still need “something else” to get things close.
>> 
>> Bob
>> 
>> 
> --Jim Harman
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