Hi, i’m experimenting about signal transmission with LimeSDR mini and SoapySDR drivers.
Program is written in C++.
I have an Audio signal sampled at 22.050 Khz and i want to generate an Amplitude modulated signal.
So i upsampled the signal 20 times at a rate of 441000 Hz and i used a FIR interpolation filter (119 taps) designed with Matlab to interpolate the samples.
Then i generated with a software DDS Generator at 441 khz a 50 Khz carrier samples sequence and built the am signal with 0.4 modulation index.
When trasmitting the signal i set the limesdr transmit frequency to 120Mhz (Air Band) and i send a complex flow CF32 stream to limesdr passing samples on I component and 0s as the Q component.
When listening to the signal I found the nice modulated audio at 120.050 Mhz (as expected)
but also i have audio at 119.950 Mhz (image frequency) and i also have 120Mhz blank carrier.
I don’t guess this is normal … there is no reason why the up conversion leaves so strong DC component (120 Mhz in this case is DC). Should i have to do some IQ balancing before transmission ?