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Re: [Discuss-gnuradio] PBF clock sync tag propagation

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] PBF clock sync tag propagation
Date: Sat, 21 May 2016 11:36:04 -0400

On Sat, May 21, 2016 at 1:33 AM, Laur Joost <address@hidden> wrote:

I have a funny issue with PFB clock sync tag propagation.

I'm using the corr_est block to detect the start of a burst. It works fine (at least when I have a decent-enough frequency lock). However, the PFB clock sync block shifts the tags to way before the burst, causing the following Costas loop to go off lock.

The first picture shows the tags as they're supposed to be:

Second shows how they are after PFB and Costas (and yes, I checked that it's the PFB which introduces the lag).

Scanning the mailing list shows that there were issues (supposedly fixed) with tags getting lost due to rate changes. I do not have that issue, all tags come through fine.

Has anyone encountered this before and is there a workaround?

Thanks in advance


Yes, this is a known problem. The tag placement is updated based on the sample rate change of a block. For clock sync blocks like this, that rate change is not constant, even within a work function. The work-around is to stop tag propagation (TPP_DONT [1]) and then handle the tag placement within the work function.

I have corrected this behavior in my packet2 branch [2] commit [3]. We haven't moved this into the mainline tree (maint/master) because it might just be a case of 'works for me' and needs more testing.


[1] http://gnuradio.org/doc/doxygen/page_stream_tags.html#stream_tags_propagation
[2] https://github.com/trondeau/gnuradio/tree/packet2
[3] 8b5169b933cb4d9443ed3a8e242a22bb981c1267

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