Re: FLL Band-Edge

[Kristoff]

Hi Marc, Jeff, all


Thanks for your reply.
(Also thanks to everybody who replied off-list.)


Hum.. looks like a good mental overview of all the blocks useful for PSK 
would be good start for myself.


I did know about the "PLL Carrier-tracking block", but I had not yet 
heard of this "FLL band-edge block".

Doesn't a PLL also track the frequency of the source (in addition to its 
phase)?
So what would be the reason to use a FLL and not a PLL?


Kristoff


On 15/01/2021 9:56 a.m., Marcus Müller wrote:
> 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