[time-nuts] advice sought on basic gpsdo

[Robert LaJeunesse]

While I'm no expert at time, or Arduino yet, the first thing I would look at is getting the bootloader to run with not 16MHz, but a 10MHz clock. Not having looked at it I don't know if it requires modifying and re-flashing, but it might. Now the VCTCXO can clock the Arduino and the Arduino IDE would work unmodified. Code the Arduino chip to lock the VCTXCO to GPS 1PPM, and you can accurately keep time of day in RAM. As long as you have a valid battery backup 5V supply just detect the loss of 1PPM (or GPS signal) and freeze the VCTXCO control voltage. With some effort the clock can auto-set itself from the GPS when the GPS is in a good state. Not simple coding, but doable.

Bob L.

> Sent: Friday, May 08, 2020 at 7:06 PM
> From: "Robert Melville" <bobmelville1 at gmail.com>
> To: time-nuts at lists.febo.com
> Subject: [time-nuts] advice sought on basic gpsdo
>
> Fellow time nutters:
>
> Any input or advice on the following project would be appreciated.
>
> I want to make a small, portable, batter-operated clock that will
> spend most of its time docked into a charging bay with access
> to a signal from a GPS receiver -- either 1 PPs or 10.00000000 MHz.
> The device will have a voltage-controlled TXCO that will be disciplined
> by the GPS input unless the unit is traveling.
>
> I have seen several designs for disciplined oscillators using a uP,
> such as an Arduino or a PIC. The main decision seems to be between
> 1 PPs or 10 MHz as the input. 1 PPs might be preferable because
> not all cheap GPS receivers seem to provide 10.000000 MHz out.
> Mostly, I want accurate time but a frequency reference with
> decent phase noise would be useful.
>
> 1. I am tending towards a published design from Lars Walenius, in part
> because
> it uses a familiar uP (an Arduino) but am open to other designs.
>
> 2. What advice to people have on glitch-free switching when
> docking/undocking the unit from the GPS???
> This seems to be something like the de-bouncing problem for a
> push-button.
>
> 3. Has anyone used the Arduino time library withOUT the Dallas RTC chip --
> i.e., some other source of time such as the from the locked oscillator?
>
> 4. Can anyone share experience with conditioning the power going into the
> TXCO -- to what extent can digital noise from the uP or counters
> contaminate the phase noise of the TCXO? Does a separate isolating buffer
> help for the "osc out" port? I have had good success in the past with
> so-called "active bypassing" to deliver very clean power to an oscillator.
>
> Thanks to all for your attention to this message -- I am glad to look at any
> and all possible designs.
> Happy time-keeping!
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