Radio Frequency Integrated Circuit Design - Webs

Mixers
make this one a second-order polyphase filter. Compare the design to one using
only a first-order polyphase filter. Design the mixer so that it is able to drive
100-� output impedance. Explore the achievable image rejection over process
tolerances of 20%.
Solution
For the LO path there is little additional design work to be done. For this
example, we will ignore the square wave buffers that would normally be used
to guarantee that the mixer is driven properly. We add a simple phase-shifting
filter to the circuit like the one shown in Figure 7.25 to provide quadrature
LO signals. Since this filter must be centered at 1.95 GHz, we choose R =
300� fairly arbitrarily, and this makes the capacitors 272 fF. Both of these are
easily implemented in most technologies.
Next we must design the IF stage that will follow the mixers using the
polyphase filter to achieve the second phase shifter. In order to prevent loading
of the mixers by the polyphase filter, we need buffers at the input and we will
need buffers at the output to drive the 100� load impedance. A polyphase
filter with buffers is shown in Figure 7.36. Note that this circuit implements
both IF paths as well as the summing-component shown in Figure 7.22. The
polyphase filter components must be sized so that the impedance is large to
minimize buffer current. However, if the impedance is made too large, then it
Figure 7.36 IF stage of an image reject mixer.
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