Re: [Discuss-gnuradio] A medley of questions

[Brian Padalino]

On 3/14/07, Steven Clark <address@hidden> wrote:
> GMSK is a constant amplitude modulation. So unless I am misunderstanding
> something (which is entirely possible), if you plot the complex baseband
> signal with real for X and imag for Y, the result should be a
> constant-amplitude circle regardless of where in the symbol you are
> sampling, and regardless of any carrier freq discrepancy between the tx and
> rx. A carrier freq offset will make the phasor spin faster or slower, but it
> should still have constant magnitude.

I'd be interested if you could point me to some of your plots.  It
sounds interesting.

> I'd seen the 50ppm number before, and that's part of the reason I asked the
> second question about adding improved carrier recovery. I was noticing a
> freq difference of ~5 kHz @ somewhere in the 200-300MHz range (that's about
> 20ppm, so in line with our expectations), and it was really making the M&M
> clock recovery block in gmsk.py struggle. I'm guessing that M&M depends on
> zero crossings for clock recovery, so when it's input (which is normally in
> the [-1, 1] range) gets a DC offset such that it's now [-0.5, 1.5] for
> example, it has problems.
>
> I'm interested in your IIR suggestion. It seems like what we want to do is
> essentially high-pass filter the output of the FM demod to get rid of the DC
> offset. Were you recommending low-pass filtering the FM demod output to
> estimate the DC bias and then subtracting that off from the original signal?
> Why an IIR filter rather than a FIR? Which IIR block would you choose?

You said before you were looking at using just a little bit of the
bandwidth, so why not sample such that you have an image of your
signal at some higher frequency, and use a DDC in the PC to get it
down to baseband - shifting your DC offset by that higher frequency
and then apply the LPF?

> Thanks again for your continued patience.

I'm looking forward to seeing those pictures.

Brian