Radio Frequency Integrated Circuit Design - Webs

204 Radio Frequency Integrated Circuit Design
7.6.2 Picking the LO Level
The differential pair will require an input voltage swing of about 4 to 5 vT for
the transistors to be hard-switched one way or the other. Therefore, the LO
input to the mixer should be at least 100 mV peak for complete switching. At
50�, 100-mV peak is −10 dBm. Small improvements in noise figure and
conversion gain can be seen for larger signals; however, for LO levels larger
than about 0 dBm, there is minimal further improvement. Thus, −10 to 0 dBm
(100 to 300 mV) is a reasonable compromise between noise figure, gain, and
required LO power. If the LO voltage is made too large, then a lot of current
has to be moved into and out of the bases of the transistors during transitions.
This can lead to spikes in the signals and can actually reduce the switching
speed and cause an increase in LO feed-through. Thus, too large a signal can
be just as bad as too small a signal.
Another concern is the parasitic capacitance on node Vd , as shown in
Figure 7.5. The transistors have to be turned on and off, which means that
any capacitance in the emitter has to be charged and discharged. Essentially,
the input transistors behave like a simple rectifier circuit, as shown in Figure
Figure 7.5 Large-signal behavior of the differential pair: (a) schematic representation; (b)
diode rectifier model; and (c) waveforms illustrating the problem of slewing.