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

[Tomaž Šolc]

On 26. 02. 2015 17:24, Johnathan Corgan wrote:
> On Thu, Feb 26, 2015 at 1:41 AM, Tomaž Šolc <address@hidden
> <mailto: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.

Thank you for your answer Jonathan. I suspected that the algorithm
doesn't work on rectangular pulses. This is why I performed my test with
a sine wave as well as a rectangular wave (see the block diagram I
linked to)

I think a sine wave of the same frequency as a rectangular wave should
be very close to what is at the output of a symbol matched filter you
mention. However in the setup I described, the MM block doesn't sync to
a sine wave either.

Best regards
Tomaž