TABLE OF CONTENTS The following three sections discuss three techniques to reduce phase noise upconversion: bank switching, differential tuning, and simultaneous matching of transconductance and impedance.
To cover the required frequency band and the effects of process variations, a particular K VCO is required. However, if the band is broken into many subbands, then K VCO is reduced, and phase noise up conversion is reduced. For example, by breaking a band into three subbands, potentially K VCO is reduced by a factor of three, and phase noise up conversion can be reduced by a factor of about 10 dB. A better way to implement bank switching with n banks of varactors as shown in Figure 8.34 is to use ( n - 1) AMOS varactors as switches and one bank as a continuously variable capacitor. Because of the shape of the AMOS varactor curve (as shown in Figure 8.10) when operated as a switch, AMOS varactors have a low gain when fully switched. This will result in minimal low-frequency noise upconversion. Note that, to get full switching, both positive and negative control voltages must be supplied. This is conveniently obtained when the oscillator has both NMOS and PMOS cross coupling, since the output nodes are nominally biased between the power-supply voltages. However, for the case where there is only a single polarity of cross-coupling, the output voltage will be nominally at the rail voltage [e.g., with NMOS cross...
