I am transmitting a 1 kHz cosine wave with a HackRF One. I receive it with a NI2921 USRP at a sample rate of 200kHz. All the process is on GNURadio Companion.
I changed the sample rate at emission to see the impact. I started at 200kHz and lowered of 1kHz each time. The sample rate at reception stays the same : 200kHz. Here are the results :
From 200kHz to 96kHz, nothing appeared. Then, at a sample rate of 95kHz at emission, the cosine spectrum showed two peaks at 13kHz and 87kHz, centered on 50kHz (DC component) giving a cosine of frequency 37kHz. All the frequencies on the table above are the only ones showing something on the FFT of the received signal.
My questions are :
- Why does the sample rate at emission impact the frequency seen at the receiver?
- Why are there frequencies that do not make the cosine spectrum appear? (Only the frequencies present in the table work). For example, if I have the two same sample rates at emission and reception (200kHz for our example), it does not work.
- Why are the frequencies seen on the FFT plot different from 1kHz, the real cosine frequency emitted with the HackRF One?