[time-nuts] WWVB PM Receiver

Jim Lux jimlux at earthlink.net
Thu Sep 27 04:14:42 UTC 2012


On 9/26/12 9:11 PM, Peter Monta wrote:
>> Have you actually tried it and gotten it working, except possibly in a
>> very strong signal area?
>
> This is precisely the issue.  Squaring the WWVB signal results in a
> significant SNR penalty.  At high SNR it doesn't matter that much; at
> low SNR you are in a world of hurt.
>
> I had suggested to John Lowe that they consider retaining some carrier
> in the signal, which would be trivial to do---instead of a modulation
> index of 180 degrees for BPSK, make it 120 degrees or 150 degrees, so
> that there remains a little bit of pure carrier at 60 kHz that's
> trackable with PLL receivers.  He said something about being receptive
> to the idea, but apparently it was not adopted, since the latest
> document still says antipodal BPSK.
>
> Ironically, GPS is heading in the opposite direction.  The legacy C/A
> and P(Y) signals have no unmodulated pilot spreading codes, while the
> newer ones at L2 and L5 have strong pilots that allow much better
> tracking.
>

I'm not sure about residual carrier aiding the tracking process.  A 
Costas loop recovers the carrier pretty well, and a symbol aided loop 
(where the I channel has a hard limiter, for instance) does even better.

After all, the energy is still the same.







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