GMSK MOD question

Hi Everyone,
My name is VIvian and I am working with GMSK modulation. I have a little
doubt about it.

I am transmitting a cosine signal using UHD sink and source blocks,
daughter board RFX 2400. The spectrum that I transmit is the one that
can see in the attached files.

I use a network analyzer to see the real spectrum that I am sending and
spectrum that I obtain can be seen in the other photo that I attach.

Finally I look the spectrum received in GNU Radio, which is attached to
this email too.

I found that the spectrum that I send is the half of the size that I see
the network analyzer. I mean the spectrum that I send has a bandwidth of
200 KHz ( I see this in GNU Radio) but the one that I obtain looking at
the network analyzer is approx. 400 KHz. When i see the received signal
GNU Radio is perfect, its bandwidth is the same that I send ( 200 KHz).
dont know why I see a different bandwidth in the network analyzer.
knows what is happening?

I would appreciate any help you can give me. Thanks in advance.


On Tue, Jun 12, 2012 at 4:03 PM, Vivian Paola Triana G.
[email protected] wrote:

I would appreciate any help you can give me. Thanks in advance.


Hi Vivian,

Just remember that the GNU Radio scope plots don’t have to tell you
the truth. Everything in GNU Radio really just runs off a unitless
sample rate of 1, so samples/second are actually meaningless to it. We
tell the scope what the sample rate is and it sets the x-axis scale
accordingly, but we can easily lie to it. My guess is that there is a
factor of two in the sample rate of the system that you aren’t
accounting for when creating the plot.