I’m new in this forum and this is my first message!
It’s more or less 2 months that I bought and playing with my new ettus
usrp2 B100 with a WBX radio board.
I admit that I have a discrete knowledge of RF transmission, but I’m not
an expert (so, if I write something “strange”…)
I’m trying to build a 2 sidelobe digital transmission (USB and LSB)
centered @145.025Mhz and separated by 50 Khz (so LSB centered @145Mhz
and USB @145.05Mhz). The sidelobe transport 2 different message and are
modulated with a GMSK modulation @9600 bps and 25 KHz BW.
My idea was to generate two sidelobe separately in baseband, then
frequency shift each one by +/- 25KHz and in the end sum togheter. This
shift is done with a mixer: in gnu radio companion I put 2 sine signals
sources @+/-25Khz and then multiplied with my digital modulated signals.
I used a sum block with 2 input and connected the output to the UHD.
UHD has a center frequency of 145 MHz and 10 dBm power output.
I analyzed the RF output with my spectrum analyzer and I saw a nasty RF
signal, full with intermodulation spike spaced by 50 KHz. (The signal in
input to the UHD block seem’s ok, I have 2 good “hills” centered at 0 Hz
and with 25 KHz BW. Other signal are just confused in the floor noise)
Surfing on the web, I found an option to set and implement some sort of
superetherodyne transmitter with the LO option in the uhd.tune_request_t
function. This seem’s to solve the intermodulation problem but
- the main signal is strongly attenuated
- If I amplifie the RF output from the B100 with an external amplifier
I get back the same spectrum analisys with intermodulation.
So, my final questions are: is it possible to remove all this
intermodulation signal and generate a 2 sidelobe transmission (USB/LSB)?
I’m doing that right in Gnu radio companion? I mean mixing with multiply
a sine signal source? is it enough? The only way to remove
intermodulation spike is to use an external filter?
THANK YOU VERY MUCH TO ALL!!!