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Re: Problem with the PLL Carrier Tracking Block
From: |
Daniel Estévez |
Subject: |
Re: Problem with the PLL Carrier Tracking Block |
Date: |
Wed, 17 Mar 2021 22:22:48 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 |
El 16/3/21 a las 19:17, George Edwards escribió:
Hello GNURadio Community,
I am having trouble getting the PLL Carrier Tracking Block to work
properly. I designed the GRC with a qpsk constellation modulator at a
sample rate of 40 k Hz. I designed the PLL Carrier Track block to
correct incoming waveforms that are off centered from DC by +/-500 Hz
(setting max/min freq with the formula +/-2*pi*f(=500)/samp_rate). The
eye test at the PLL outputs seems to show that the input signal freq is
recentered around DC, however, the signal level coming out of the PLL is
greatly reduced and appears to be smeared. So obviously, when I follow
this block with a Symbol Sync block it does not lock and the
constellation points continue to rotate.
Hi George,
The PLL Carrier Tracking block expects an unmodulated carrier at its
input. It won't work with a QPSK signal. For a QPSK signal you need to
use the Costas Loop block.
In more detail: The PLL Carrier Tracking block uses what is sometimes
called as a pure (or four-quadrant arctan()) discriminator, which only
locks to an unmodulated carrier. You would use that for residual carrier
signals. For a usual suppressed-carrier QPSK signal you should use a PLL
with a Costas discriminator (of order 4, in the case of QPSK). In GNU
Radio, this is implemented by the Costas Loop block.
Best,
Dani.
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