|M.Sc Thesis||Department of Electrical Engineering|
|Supervisor:||Prof. Ritter Dan|
|Full Thesis text - in Hebrew|
Voltage controlled oscillators (VCOs) are used in various radiation systems enabling frequency up and down conversion when used as local oscillators, and as a reference source for system synchronization. The increasing demand for both bandwidth and power in data communication systems has motivated research on new types of high-speed devices and circuits. Heterojunction bipolar transistors (HBTs) combine outstanding high frequency properties with the inherent low l/f-noise which are necessary for frequency sources.
This work describes the design and measurements of two types of oscillators. One is an X-band InP HBT VCO fabricated in different technologies: with separate varactor layers and base-collector varactor layers. The second is an X-band InP HBT oscillator monolithically integrated with an on chip photodiode (PD) for optical applications.
In previously published InP HBT VCOs, the base collector junction was used as the varactor diode. This results in tradeoffs between transistor performance and varactor characteristics and resulting wide tuning range at the expense of phase noise. In order to improve the performance of InP HBT VCOs, separate and optimized varactor epitaxial layers were added underneath the transistor layers, allowing separate optimization of each element. This work presents a fully integrated Colpitts InP HBT VCO with separate varactor layers, and performance comparison to a VCO fabricated using the base collector layers as the varactor.
Oscillator's measurements were performed on-wafer using low noise power supplies and a spectrum analyzer. Minimum phase noise of -94 dBc/Hz at 100 kHz offset from the carrier was achieved. Phase-noise model parameters were extracted from phase noise measurements. The VCO with BC junction varactor layers achieved only 3.5% tuning range with 13% linearity. The VCO with the separate layers achieved 12% tuning range with 2% linearity with frequencies range from 7.8 to 8.8 GHz. The output power of both VCOs was -3 dBm and was obtained with a supply voltage of 5V and current consumption of 6mA.