In the modular sideband-separating mixers that we built over the last years, we observe a clear anti-correlation
between the image rejection ratio obtained with a certain block and its noise performance, as well as strong
correlations between the image rejection and imbalances in the pumping of the mixer devices.
We report on the mechanisms responsible for these effects, and conclude that the reduction of the image
rejection is largely explained by the presence of standing waves. We demonstrate the rejection ratio to be
very sensitive to those. In principle, all potential round-trip paths should be terminated in matched loads, so no
standing waves can develop. In practice, the typical high reflections from the SIS mixers combined with imperfect
loads and non-negligible input/output reflections of the other components give many opportunities for standing
waves. Since most of the loss of image rejection can be attributed to standing waves, the anti-correlation with
the noise temperature can be understood by considering any excess loss in the structure, as the waveguides start
acting as distribured loads. This reduces the standing waves, and thereby improves the rejection ratio, at the
expense of noise temperature.
Based on these experiences, we designed a new waveguide structure, with a basic waveguide size of 400×200 μm
and improved loads. Strong emphasis was placed on low input and output reflections of the waveguide components,
in some places at the cost of phase or amplitude imbalance. For the latter there is ample margin not to
impair the performance, however. Apart from further details of the design, we present the first results of the
new mixers, tested in a modified production-level ALMA Band 9 receiver, and show that even in an unfinished
state, it simultaneously meets requirements for image rejection and noise temperature.