The Design Of Single-Ended & Differential MMIC VCOs

This paper describes design techniques for both single-ended and differential VCOs, with examples shown fabricated in GaAs MMIC processes. The potential uses and advantages of the differential over the conventional single-ended approach are discussed. Introduction Voltage Controlled Oscillators (VCOs) are widely used as signal sources in RF and microwave communication systems. Fully monolithic VCO ICs are rarely encountered as stand-alone components, mainly due to the low Q of the resonators and poor tuning performance of the available varactor technology. Their use in multi-function ICs is, however, now relatively common, as pressure to reduce the number of external components needed by the IC increases. This paper aims to show that the use of monolithic differential VCOs has several advantages in transceiver ICs over a conventional single-ended approach. In monolithic transceiver circuits the use of double balanced mixers also offers important advantages (Ref. 1) These include: • Rejection of the LO signal • Suppression of even-order products of the LO and/or RF • Inherent isolation between all three ports • Improved linearity However, double balanced mixers also require differential, rather than single-ended, drives. Figure 1 shows a block level schematic of a double balanced quad-ring resistive FET mixer driven with a single-ended LO (VCO). A balun (or differential amplifier) is required at all three ports.