Combined VCO/Mixer for satellite receiver in CMOS

Kingma, J. (2009) Combined VCO/Mixer for satellite receiver in CMOS.

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Abstract:In this thesis research is reported studying the possibility to directly drive a switching mixer with a Voltage Controlled Oscillator without buffers in between. The feasibility research is carried out with an LC-oscillator and a Balun/LNA I/Q mixer (BLIXER) [1] topology. At first the VCO is modeled as an LC-tank with an ideal compensation network to study the effect on the oscillation frequency when loaded with a capacitive load. Spread of capacitances in the oscillator will demand a certain tuning range of the tuneable capacitance in the oscillator to compensate for the spread and keep the oscillation frequency fixed. The required tuning range of the tuneable capacitance in the tank is studied as function of the load capacitance and spread of the load capacitance and tank capacitance. Secondly a practical CMOS oscillator is used with an NMOS cross-coupled pair with a parallel LC-tank for simulation of the performance of the VCO when loaded with at first a fixed capacitive load and later on with the BLIXER topology. Effects on performance of the BLIXER driven with sine wave oscillator signals have been compared to the performance of the BLIXER driven with square wave signals. The mixer will be driven with sine wave oscillator signals, which will give a small decrease in conversion gain for the mixer. Finally the effects on performance of the BLIXER have been analyzed when driven directly with the CMOS LC-oscillator without buffers in between. Leaving the buffers between a VCO and a mixer can give benefit in power consumption but appear to give a number of difficulties when combining the VCO and mixer. It is shown that interaction between the VCO and mixer appears to give an increase in noise in both the VCO as well as the mixer. The switching transistors of the mixer appear to be part of the oscillator and since their capacitance varies in time, also the total capacitance will vary in time. Since the oscillation frequency is dependent on the total capacitance in the tank, this will give an increase in phase noise of the oscillator. The VCO appear also become susceptible for low frequent noise sources within the mixer.The capacitances of the switching transistors will be dependent on the applied gate-source voltage and also low-frequent noise sources within the mixer can slowly vary these capacitances. Careful design of the mixer will be necessary to prevent low frequent noise contribution of the mixer to be up converted to phase noise of the VCO. Induced gate noise of the switching transistors has a dominant noise contribution in the degraded phase noise of the VCO. The noise figure of the mixer will be degraded for low IF-frequencies as a result of the degraded phase noise of the VCO.
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:54 computer science
Programme:Computer Science MSc (60300)
Link to this item:http://purl.utwente.nl/essays/58613
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