Coaxial Resonator VCO design

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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.

1. 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?


2. 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.


3. 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..

 

[John Larkin]

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.

1. 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?


2. 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.


3. 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..

Get this:

http://www.amazon.com/Oscillator-Design-Computer-Simulation-Randall/dp/1884932304

It has some coaxial resonator circuits.

In my experience, conventional topologies like Colpitts don't work
well here. We did a nice cr oscillator using a MiniCircuits ERA-series
mmic as the active element. And all you can do to pull the frequency
is add capacitive loading. It behaves not so much like an inductor as
like a parallel LC!

400 MHz is a little low for coaxial resonators, but maybe you can find
a big 500 MHz part or something you can tune down to 400. It probably
doesn't make a lot of sense to try to pull a GHz cr down this low.

John