Performance degradation at lower Frequencies

#1

Dear All,
We are using the Lime mini for TX and sending a sine wave (sampled at 2MS/s) using GNU RADIO and gr-limesdr.
At the RX side we use the LIMESDR-USB and configure it using LimeSuite 19.04.
We use the"Example.ini" file to configure the LIMESDR-USB.
At 800 MHz the constellation looks like a thin Ring as shown in the attachment.
At 200 MHz the constellation looks like a FAT Ring and there are fluctuations in amplitude as shown in the attachment.
At 70 MHz the constellation looks like a exploding Ring and there are heavy fluctuations in amplitude .
(we applied the EasyFIX1 but it didn’t help)
We don’t know if its the fault of the Lime-mini gr-limesdr atTX or the LIMESDR-USB Limesuite combo’s fault.

#2

I think that the lowest LPF(Low Pass Filter) for TX is 5MHz, I’d probably start by increasing the IQ sample rate to 5MSPS and see if the problem persists.