> I once used a pll to respond to sonar signals in noise, it worked with a
> signal 6db lower than the noise. on a scope you could not even see the
> signal in the noise, and this was with no extra filtering.

The signal was 6 dB lower than the noise floor, but not in the capture
band of the PLL, otherwise it would not lock ? Afair the magic 'getting
signals from below noise level' done by PLLs is due to their capture
bandwidth being a sort of filter. This correspondingly improves the S/N
ratio for the wanted signal. A scope will not show the singal in the noise
but I am almost certain that a scope with a prefilter with the same
bandwidth as the PLL capture loop will show it.

Peter

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