Re: [Discuss-gnuradio] ASK demodulation help

[John Malsbury]

I don't know if I would call it overkill.  It is just one of several methods you can use to achieve synchronization.  Other options for synchronization include correlate and sync (probably needs modification), or possible the polyphase resync.  Others on the list would have more expertise on these blocks.

In my experience 10 samples per symbol seems to work well with M&M.  I've heard very high samp/sym (ie. >20) can cause problems, but I haven't seen this myself. 

The amplitude going into M&M should be as close as possible to +/- 1.0, which is why you want the scaling functions before this block.  The AGC block assures you're working with something constant amplitude for demodulation.

-John

On Wed, Apr 9, 2014 at 2:04 PM, Francois Gervais <address@hidden> wrote:

Thanks guys for the information,

I looked a little about the M&M recovery block but it seemed to me like and advance algorithm, overkill for what I'm trying to achieve. I'm I mistaken?

If I'm using the M&M clock recovery block what is the quality of input signal I should aim to avoid translation errors? Should my signal be filtered with a really narrow band and should I allow more harmonics in to the signal is more square? Can the input signal have too much sample per bit? Right now I'm at 16. Is more better? Is it better to have more amplitude?

Thanks

On Wed, Apr 9, 2014 at 4:31 PM, John Malsbury <address@hidden> wrote:

Depending on various factors the implementation may vary, but you could probably start with a chain that looks something like this:

I/q source -> filter -> AGC -> AM demod (complex to mag) -> scaling for am depth -> m&m clock recovery -> slicer -> do something with the data

Other, more advanced implementations might use correlation for synchronization.

-John

On Wed, Apr 9, 2014 at 1:27 PM, Francois Gervais <address@hidden> wrote:

Hi,

I'm new to gnu radio and I'm trying to demodulate a 125kpbs ASK signal from a device I have, as a first project. I'm using RTL-SDR as the input device.

I'm slowly getting there. I receive the signal, at 2Msample/s, I low-pass filter it to 300khz, I send it through the AM demodulation block and then through the DC blocker. 

From there I have my signal and it looks fine i.e I could retrieve the information manually by looking at it. 

Now I think the goal is to somehow synchronize with the bits and re-sample to get 1 sample per bit. This could then be sent to a file. Is that it?

At first glance I'm thinking I should have a PLL which ouputs a clock at about 250khz (twice the bit rate) and synchronize the rising edge with every bit transitioning from 0 to 1 so unless I receive only ones ou zeros I should be quite in sync. Then I could toggle a sample every falling edge of the clock which should be at about the middle of the bit. 

Is this a viable solution? Can it be done with gnuradio? Other alternatives?

Thanks

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