Radio Frequency Integrated Circuit Design - Webs

228 Radio Frequency Integrated Circuit Design
Figure 7.29 Three-dimensional plot of image rejection versus phase and amplitude mismatch.
7.11.1 The Moore Mixer
In a receiver, the noise produced by the mixer is sometimes very important. If
the mixer is to use resistive degeneration and it is to have its phase shifts in
the IF and LO paths, then there is a way to interleave the mixers, as shown in
Figure 7.30, such that the noise produced by the degeneration resistors R E is
also image rejected. Here, the noise due to these resistors is fed into both paths
of the mixer rather than just one; thus, it gets image rejected, and its effect is
reduced by 3 dB. Since noise due to degeneration resistors is often very important,
this can have a beneficial effect on the noise figure of the mixer.
7.11.2 Mixers with Transformer Input
Figure 7.31 shows a mixer with a transformer-coupled input and output [2].
Such a mixer has the potential to be highly linear, since a transformer is used
in place of the input transistors. In addition, this mixer can operate from a low
power supply voltage, since the number of stacked transistors is reduced compared
to that of a conventional mixer. We note that for a downconversion
mixer, the input transformer could be on-chip, but for low IF, the output
transformer would have to be off-chip.