Re: FLL Band-Edge

[Marcus Müller]

Pretty much.

The FLL band edge system in GNU Radio does the following:

1. have a filter that extracts the energy "around the place where the
upper edge of where the signal is supposed to be", and around the lower edge
2. compares energies in both
3. multiplies the signal with e^jft, while adjusting the f until there's
equal energy in both edge filters.

The idea behind 2. is that you want your signal centered, smack in the 0
Hz middle of your baseband.

The principle in 3. is just a control loop to control the frequency shift

The approach for 1. is that it's mathematically very elegant to use the
derivative of the transmitter's pulse shaping filter. It still works
(though with higher variance) if you just use any symmetric low-pass
filter that lets through roughly the signal's bandwidth.

For more info, fred harris himself has done a presentation of FLL band
edge filters at GRCon'17[1], and you might find that enlightening. Note
that GNU Radio's FLL band edge is nowhere as smart – it doesn't do
anything with the phase information it could be getting, for free.

Best regards,
Marcus

[1] https://www.youtube.com/watch?v=Zmjk9NE-3k0

On 15.01.21 02:33, Jeff Long wrote:
> I think the general idea is that some signals have no carrier that a PLL
> could lock onto. The FLL Band-Edge centers a signal using the shape of a
> specific signal (the matched filter being used).
> 
> On Thu, Jan 14, 2021 at 6:55 PM Kristoff <kristoff@skypro.be
> <mailto:kristoff@skypro.be>> wrote:
> 
>     Hi all,
> 
> 
>     I have been reading some more on PSK demodulation.
> 
> 
>     One of the PSK signals I can easily pick and and is available
>     24h/day is
>     the telemetry signal on QO100.
>     I found a flowgraph from Daniel Estevez that decodes the QO100
>     telemetry, so I started examining how it works.
> 
>     In that flowgraph (which for some reason I do not seams to find any
>     more), the first block in the flow (i.e. right after downconverting and
>     a AGC) is a "FLL_Band-edge" block.
> 
> 
>     Can somebody explain what exactly this block does?
> 
>     I researching this, I found that it seams to shift the received signal
>     slightly up in frequency (at least, that is what it did in my tests).
> 
>     I tried reading the documentation, which kinds-of explain how it does
>     work, .. but not what it does and why it is there.
>     Can somebody explain the purpose of a "FLL with band-edge filter" block?
> 
> 
> 
>     Thx :-)
> 
>     73
>     kristoff - ON1ARF
> 
>

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