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Re: [Discuss-gnuradio] frequent phase slip with the new digital.costas_l

From: Kyle Zhou
Subject: Re: [Discuss-gnuradio] frequent phase slip with the new digital.costas_loop_cc
Date: Fri, 21 Sep 2012 09:18:19 +1000

On 21/09/2012, at 12:45 AM, Tom Rondeau wrote:

> On Thu, Sep 20, 2012 at 9:56 AM, Kyle Zhou <address@hidden> wrote:
>> I adapted my codes from gnuradio-3.3 to the new 3.6 version.
>> One of the changes is the new digital.costas_loop_cc is now implemented 
>> based on control_loop.
>> The costas loop is used for phase recovery of a QPSK signal.
>> But I noticed that with the new version, the output experiences a lot of 
>> phase slips.
>> To be exact, the phase at the output rotate 90 degree from time to time.
>> I changed it back to the old gr.costas_loop_cc and this did not happen.
>> The setting for the old loop is alpha=0.01, beta=alpha*alpha/4, 
>> max_freq=2*pi*0.1, min_freq=-max_freq.
>> The setting for the new loop is loop bandwidth = 2*pi/100
>> The new version is only good when SNR is very high, say 13dB+
>> Any has encountered the same problem? or am I doing something wrong?
>> KZ
> Hi Kyle,
> I was using this just last week in a demonstration and hadn't noticed
> any problems. If you can pin-point what's going wrong, though, I'll be
> sure to fix it.
> Tom

Thanks Tom.
I am trying to write a small test case to reproduce the problem reliably, and 
will get back to you shortly.
At the same time, what was the SNR you tried in your demo? The problem would 
not happen when you have a high SNR say 13dB.

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