Re: [Discuss-gnuradio] Basic Bandwidth math

[Jeffrey Karrels]

Hello all,  I found that the 7 samples per symbol cap was from a
pick_bitrate example code I was using. No matter what I changed the
interpolation to, it used 7 samples per symbol. I added support for
higher samples per symbol in that file, and seem to be good to go now.
Other than some random spikes spurious showing up in my FFT that seem
to be appearing when I use a lower interpolation number.

Jeff


On 9/7/07, Jeffrey Karrels <address@hidden> wrote:
> Thanks Eric. So my math was right. Yesterday I seemed to be having
> issues because I was running out of memory. I since have upgraded
> boxes.  Now I think I am down to 1 issue and I believe it is a basic
> RF one that everyone is going to jump over me for!
>
> Let me give a picture tutorial of my problem.
>
> 1) If I transmit a modulated signal with two additional signals mixed
> with -400kHz and 400kHz, I get what I expected. (see image 400.png)
>
> 2) If I transmit a modulated signal with two additional signal mixed
> with -400kHz and 800kHz, I get the +800kHz showing up with some
> additional power down at  -1200kHz. (see image 800)
>
> 3) If I transmit... mixed with -400kHz and 1200kHz, I get something at
> + and - 1000kHz.
> (see image 1200)
>
> 4) If I transmit... mixed with -400kHz and 1600kHz, I get the same as
> #3.  (see image 1600).
>
>  I was messing with samples per symbol, but it will not go any higher
> than 7 without an error.
>
> Anyone have any suggestions on where to start looking or things to
> try? Filters?
>
> Thanks all for your help!
> Jeff
>
>
> On 9/6/07, Eric Blossom <address@hidden> wrote:
> > On Thu, Sep 06, 2007 at 11:56:30AM -0400, Jeffrey Karrels wrote:
> > > Ok. I am having a world of problems today, the main one being
> > > determining if it is my issues causing the headache or the headache
> > > causing the issues. ;)
> > >
> > > I have a basic bandwidth limit that I am hitting and for some reason I
> > > cannot do basic math today.
> > >
> > > I am trying to see how many carriers I can transmit on. I am mixing a
> > > single gmsk modulated source with LOs in software and seeing how many
> > > times I can do that. A single channel is 250kHz wide and the channels
> > > are 400kHz apart center to center. In other words my LOs are 400e3,
> > > 800e3, ...
> > >
> > > How many carriers would you expect that I could transmit on?
> > >
> > >
> > > Thanks
> > > Jeff
> > > P.S. Sorry in advance, I am nuts today.
> >
> > In theory, 20.  In reality probably about 16, unless you run out of
> > CPU first.
> >
> > You can get 8Mz of IF bandwidth across the USB.
> > This is 8MS/s * 2 * 2 = 32MB/s (16-bit I & Q).
> >
> > 8e6/4e5 = 20 "bins"  (ignoring roll off)
> >
> > Allow for roll off because of CIC interpolator in FPGA.  Say that 80%
> > is usable (depends on your app, whether you preemph signal etc).
> >
> > 20 * .8 = 16 bins.
> >
> > There's a fence post problem here ;)
> >
> > So, 1 carrier needs 250kHz
> >
> > 3 carriers needs 2 * 400 + 2 * 125
> >
> > 5 carriers needs 4 * 400 + 2 * 125
> >
> > N carriers needs (N-1) * 400 + 250 (for odd N)
> >
> >   15 -> 5.85MHz
> >   17 -> 6.65MHz
> >   19 -> 7.45MHz
> >
> > Put the middle carrier at 0 Hz,
> > then arrange the others symmetrically about it.
> >
> > Eric
> >
>
>