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[Discuss-gnuradio] feedback loop for bpsk carrier recovery ?

From: jean-michel . friedt
Subject: [Discuss-gnuradio] feedback loop for bpsk carrier recovery ?
Date: Thu, 23 Oct 2014 12:54:19 +0200

For educational purposes, I am investigating a carrier recovery scheme for BPSK 
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 ?

(I have found, following 
the Costas loop block which works very well with my real signal, but I'd like 
to implement it
manually -- or can't gnuradio-companion close a feedback loop ?)

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

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