Electronic – Why the mean squared value of a random noise signal at a single precise frequency is zero

The mixer output should not be sensitive to small variations in amplitude on the LO path. So, any amplitude error in the measurement will be dominated by the amplitude error from source SG2. In general, the amplitude error of a good signal generator is very low. So, the error you measure is almost entirely a combination of the phase errors from both sources.

As a check, you can measure the amplitude error by itself using a single signal generator. Split the output and apply the signal to both the LO and RF inputs.

They do not contribute the the noise because of the degeneration. Having emitter/source degeneration in effect introduces negative feedback, greatly improving the performance of the transistors. For a more quantitative discussion see the end of this presentation.

You end up with the following equation for the noise: $$\frac{i_D}{1 + g_m r_o} \approx 0$$ Where \$r_o\$ is the degeneration. For high frequencies \$C_{gs}\$ provides an alternative current path, reducing the degeneration.