I've have several questions on making a 400 MHz VCO with about a 5% tuning range and -130 dBc/Hz @ 10 kHz offset. I decided on using a quarter wavelength ceramic resonator to get a high Q tank circuit.
- How do I determine the resonant frequency (fres) of the coaxial resonator? Given the required tuning range, should I use the resonator near its fres or should I use the resonator as a high-Q inductor in which fosc < fres?
- If the resonator is used as a high Q inductor, how far below fres should it be used at? For a lossless, shorted quarter wavelength line, the impedance is X=Zo*tan((PI*fosc)/(2*fres)), where fosc is the frequency of operation. At fosc/fres < 0.5, the plot of X vs. fosc/fres appears linear, which means the inductance is constant. At fosc/fres >
0.5, the plot is rises almost exponentially and the inductance is nonlinear vs. frequency.
- I often see Colpitts oscillator designs using either a emitter follower amplifier or common base amplifier. How do I choose which amplifier topology to use? I've seen the diagrams where simplified ac models attempt to show equivalence between CB, CC, and CE - based oscillators. It seems over-simplified to me. If things were really equivalent, then why is there a differences in oscillator schematics?
Thanks for any advice..