For educational purposes, I am investigating a carrier recovery scheme
for BPSK demodulation.
I have synthesized a BPSK modulated signal, assembled a hardware carrier
recovery scheme which
works well (squaring the modulated signal, bandpass filtering and
dividing, aka Costas loop) which
I now want to implement as SDR in GNURadio. To do so, I acquire the BPSK
modulated signal, extract
the carrier by squaring the I/Q data flow and bandpass filtering, all
this work fine. Before squaring
the signal, I insert a Xlating FIR filter whose NCO frequency will act
as the tunable VCO in the
hardware version of the carrier tracking circuit.
Question: how can I feedback the output of the lowpass filter (ie error
signal of the PLL) to the
NCO of the Xlating FIR Filter ?
Additional question: since gnuradio-companion 3.7.5 I see a grey “freq”
tab in the Xlating FIR
filter block, but cannot find any associated documentation or what the
use of this input is. Is
this the functionality I am looking for ?
Thank you, JM
–
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l’Epitaphe,
25000 Besancon, France
the signal, I insert a Xlating FIR filter whose NCO frequency will act as
like to implement it
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l’Epitaphe, 25000
Besancon, France
GNU Radio cannot do loops in the flowgraph. Because it passes hundreds
to
thousands of samples at a time between blocks – necessary to reduce
scheduler overhead and context switching – it doesn’t make any signal
processing sense. There’s a paper that I’d have to dig up that shows you
what happens to a control loop when you’re decisions are more than 1
sample
away. Spoiler: it’s not good. So while it’s possible to structure the
process graph such that loops are allowed, it complicates things for
something that’s unworkable mathematically. For loops like this, we do
so
inside of a block, which can do feedback and all of that.
Instead, the freq xlating block takes in a message that can reset the
frequency. That’s what that grey tab is in GRC, a message port. You can
make a decision elsewhere in the graph and then post a message to that
block to change the frequency. Note that this is done asynchronously. So
again, you won’t use this to control the frequency on a sample-by-sample
basis. You’d have to rethink your algorithm for frequency adaptation
than
the traditional Costas loop concept.