[time-nuts] WWVB PM Time Questions

[Tim S]

>From a signal processing point of view this problem interests me, so I
wanted to float an idea.

If I read the WWVB document correctly, the phase shifts are always going to
be +/-180° out of phase, but still the exact same frequency.  It would
stand to reason that if one wanted to detect this - one would want to start
by comparing two signals to the incoming clock, one normal and one inverted
(essentially, a differential signal).  Once the received RF clock frequency
is locked in phase with the local oscillator, adding the incoming signal to
both normal and inverted would be illuminating to see which phase is used
by simple RMS math.

180° out of phase added to the incoming signal would be zero ((+1) + (-1) =
0, while ((+1) + (+1) = 2 - then do your absolute RMS window over 1.5
cycles (three peaks).  Note that the out-of-phase signal would be zero when
correctly locked but would be greater than zero (absolute) if not locked.
These two analog outputs for each, can then have a hysteresis applied to
them to produce a logical output by feeding them into a comparator op-amp -
the phase tuning can then be steered from sampling the valid output phase
based on the binary selection (additional windowing two gate when phase
shifts are possible can steer the hysteresis values for noise robustness).

For example, bit transitions may only occur on bit edge transitions - so
detecting a phase shift and counting the center- to center of that phase
shift should help reject mid-bit-period errors.  You'd need to know this
window anyway to know when to clock in a phase bit state (middle 1/3rd or
middle 3/5ths).  Should be doable in hardware with some
counters/digital-comparators...

-Tim

On Thu, Jul 30, 2020 at 9:00 AM <time-nuts-request at lists.febo.com> wrote:

>
> Date: Wed, 29 Jul 2020 20:33:41 -0700
> From: <rcbuck at atcelectronics.com>
> To: "Discussion of precise time and frequency measurement"
>         <time-nuts at lists.febo.com>
> Subject: Re: [time-nuts] WWVB PM Time Questions
> Message-ID:
>         <
> 20200729203341.db13e9c5ce513c85d5e788cef9f0c738.7489e885d3.wbe at email06.godaddy.com
> >
>
> Content-Type: text/plain; charset="utf-8"
>
> Paul,
> "The new de-psk-r I built has no raw wwvb outputs." What do you mean by
> raw?
>
> I have been thinking about how the phase shift could be detected in
> software instead of hardware. Could something like this maybe work:
>
> If a micro is able to detect the zero crossing of a sine wave it should
> be able to determine if the phase shifts. When a new second starts
> (which is easy to determine), delay for 300 msec and then watch for the
> next zero crossing. Store the time at that point. Then wait 1000 msec
> which would put you at the same point in the next second. Wait for the
> next zero crossing and determine the time between that crossing and the
> first crossing. You can compute the phase difference based on the time
> difference. If the phase difference is between 100 degrees and 180
> degrees, you know a phase shift has taken place. The reason for using
> 100 as the low number is in case the zero crossing on one sine wave was
> at the leading edge and it was on the trailing edge of the other one.
>
> You would want to use the output of the PLL to perform those operations
> since it is local and not subject to ionospheric interference or delays.
> A 100 MHz STM32 should be able to easily handle the calculations in a
> couple of microseconds.
>
>
> Ray,
> AB7HE
>