Basic Bandwidth math

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. :wink:

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.

On Thu, Sep 06, 2007 at 11:56:30AM -0400, Jeffrey K. wrote:

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 :wink:

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

The adjacent channel interference will be horrendous since one channel
overlaps the next door neighbor by 50 Hz.

Bob

Eric B. wrote:

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.


AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
“If you’re going to be crazy, you have to get paid for it or
else you’re going to be locked up.” Hunter S. Thompson

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)
  1. 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

Never mind. My math mind is as bad as yours today. There is 150 Hz of
separation between what I assume is the 3 dB points.

Sorry

Bob

Robert McGwier wrote:

causing the issues. :wink:
How many carriers would you expect that I could transmit on?

Thanks
Jeff
P.S. Sorry in advance, I am nuts today.


AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
“If you’re going to be crazy, you have to get paid for it or
else you’re going to be locked up.” Hunter S. Thompson

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

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