Re: [Discuss-gnuradio] M&M clock recovery for async digital signal

[Johnathan Corgan]

On Thu, Feb 26, 2015 at 1:41 AM, Tomaž Šolc <address@hidden> wrote:

 

I'm trying to synchronize on the bits in an asynchronous digital signal
using M&M clock recovery block. For instance, roughly 7 samples of +1.
represent a "1" and roughly 7 samples of -1. represent a "0".

In general, timing recovery algorithms attempt to locate and track the "center" of a symbol and output an estimate of what the symbol value is at that point.  They also work best when they are preceded by a symbol matched filter; this both improves signal-to-noise ratio and results in a peak near the center that the recovery loop is designed to find.

In your case, your symbol is a rectangular pulse 7 samples wide, of bipolar amplitude.  A matched filter for this symbol shape is simply a moving average filter of the same period. If you add one of these ahead of the MM block, it will sync up quite easily.

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Johnathan Corgan
Corgan Labs - SDR Training and Development Services
http://corganlabs.com