In a SDR TX/RX the IQ balance of the LSB/USB TX can be inspected with the tinySA using a 2.7kHz test tone. The TX is a Softrock and the LO is set 7.1MHz, equal to the TX, to create a worst case setup. The frequency span of the tinySA is set to 20kHz
Untuned this looks like this, below
Marker 1 is the wanted USB and marker 2 is the unwanted LSB
After a bit of tuning it already looks better
but due to the noise at the left side of the resolution filter of the tinySA it is not possible to see below -35dBc with an offset of only 5.4kHz.
This can be solved by shifting the TX frequency 5kHz from the Softrock LO and this shows we are not done yet. The span is increase to 50kHz.
Tuning the IQ balance a bit better shifts the unwanted sideband into the noise below -40dBc
There is still a fair amount of LO leakage (marker 3) but this can not be solved with IQ balance tuning.