[time-nuts] WWVB PM Receiver

[paul swed]

I am feeling a bit slow here.
There is a carrier always. Thats how the AM works. So somehow we are
speaking about a semi non coherent carrier perhaps??
So whats the nickle solution and it is not squaring in a low s/n
environment. Been there done that. Very bad results on the east coast.
Regards
Paul
WB8TSL

On Thu, Sep 27, 2012 at 1:15 AM, Peter Monta <pmonta at gmail.com> wrote:

> > 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.
>
> Yes, these work (and a soft tanh() limiter improves on the hard
> limiter a little bit), but I think they don't work as well as a PLL
> with a pure carrier, where performance is measured as the variance of
> the phase estimate at a given SNR.
>
> > After all, the energy is still the same.
>
> True, but information has been lost as a result of introducing these
> unknown phase transitions.  Now if the phase transitions are known,
> one can certainly wipe them off by multiplying by a noiseless replica
> of the known phase modulation, and then you're back to pure carrier.
> But if you don't know the transitions ahead of time, you need the
> Costas loop to find them for you, and that costs SNR.
>
> In WWVB's case, many of these phase transitions probably can be
> predicted.  But the point is not so much that good timing receivers
> for the new signal are problematic.  On the contrary, they're no
> problem at all with a little DSP.  But for the sake of backward
> compatibility, putting 5 or 10 percent of the signal power into a
> carrier seems a small price to pay.
>
> Using a Costas-loop preprocessor to a legacy phase receiver is almost
> to the point where you're better off tossing the legacy receiver and
> just using the preprocessor.
>
> I don't want to sound too negative here.  I'm glad WWVB is getting
> these improvements, and the clarification from John Lowe earlier today
> about the openness of the signal is helpful.  But backward
> compatibility would have been so easy to put in.
>
> Cheers,
> Peter
>
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