# GMSK MOD question **URL:** \ **Category:** GNU Radio\ **Created:** [June 12, 2012, 10:04pm UTC](https://www.ruby-forum.com/t/gmsk-mod-question/220540 "2012-06-12T22:04:51Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![aris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/258eb7/32.png) [@aris](https://www.ruby-forum.com/u/aris)\ **Post date:** [June 12, 2012, 10:04pm UTC](https://www.ruby-forum.com/t/gmsk-mod-question/220540/1 "2012-06-12T22:04:51Z")
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, USRP1, daughter board RFX 2400. The spectrum that I transmit is the one that you can see in the attached files. I use a network analyzer to see the real spectrum that I am sending and the 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 in 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 in GNU Radio is perfect, its bandwidth is the same that I send ( 200 KHz). I dont know why I see a different bandwidth in the network analyzer. Anyone knows what is happening? I would appreciate any help you can give me. Thanks in advance. Vivian. ---
**Author:** ![Vivian\_Paola\_Triana](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/a6a055/32.png) [@Vivian\_Paola\_Triana](https://www.ruby-forum.com/u/Vivian_Paola_Triana)\ **Post date:** [June 13, 2012, 3:42pm UTC](https://www.ruby-forum.com/t/gmsk-mod-question/220540/2 "2012-06-13T15:42:25Z")
On Tue, Jun 12, 2012 at 4:03 PM, Vivian Paola Triana G. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I would appreciate any help you can give me. Thanks in advance. > > Vivian. 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. Tom