RF CMOS Varactors for Wireless Applications



This Industrial PhD project deals with CMOS integration of the RF VCO, located in the receiver part of a GSM-900/GSM-1800 dual band handset. The major challenge is to meet the GSM phase noise specifications, which are calculated from the GSM blocking requirements. From the Leeson equation, predicting the relative phase noise power spectral density, four important parameters are identified. These are the loaded quality factor QL, the output power Po, the flicker noise corner frequency fc and the noise figure F. Of these parameters, QL is the most important, since it is squared in the Leeson's equation. High Q-values for on-chip LC-tanks are very difficult to obtain in a standard CMOS process. Usually spiral inductors are preferred over bond wire inductors due to process simplicity and cost. In a standard CMOS process spiral inductors with a Q of 10 are difficult to make, if not impossible - typical values ranges from 3 to 6. To fulfil the GSM phase noise requirements, a QL of 5-10 is required. Hence, it is very important to ensure that other components have a sufficiently high Q, to prevent them from loading the inductor Q. In this context an overlooked component seems to be the varactor, on which only few research results have been reported. Consequently, a lot of effort in this project is dedicated to elaborate the knowledge about CMOS varactors. Of special interest is the Q, the capacitance tuning range, the self-resonance frequency, and the capacitance-versus-voltage-characteristic. The VCO type considered in this project is using two crosses coupled n-channel MOSFET's as the active element. The LC-tank consists of a balanced inductor with center tap and two Standard Mode nMOS varactors in series back to back connection. Different versions of this VCO type are designed and tested. The VCO's are designed using an empirical design method. The key results of the work are: (i) Phase noise specification for the RF VCO, located in the receiver part of a GSM dual band handset, (ii) Performance evaluation of three different types of RF CMOS varactors, (iii) Optimum circuit configuration for the three evaluated types of RF CMOS varactors, (iv) Layout of the Standard Mode nMOS varactor for capacitance scalability, (v) Layout of the Standard Mode nMOS varactor for optimum Q and capacitance tuning range, and (vi) Empirical design method for cross coupled n-channel MOSFET VCO's. (Erik Pedersen, Torben Larsen; Lars Bonde, Maxon Telecom)
Effektiv start/slut dato31/12/200331/12/2003