VCO Design

All circuits contain stray capacitance and inductance. The higher you go in frequency, the more significant these strays become. Transistors have junction capacitance. These strays and parasitic reactances can be hard to predict, but they can be minimised, and they should be small. It's common practice to include a small trimmer capacitor to correct for them. The circuit you posted recently in s.e.b included a trimmer. Be aware though: it's hard to make a stable, accurate LC-controlled oscillator at frequencies much above 10 MHz.

An empirical approach is often taken to RF design. Why not make a prototype? It could save you a lot of time and you will learn from it. Don't try to work out every last detail before soldering two components together.

in designing vco with negative resistance analysis , the output reactance of the resonator should be equal to the input reactance of the reflection amplifier, so the resonant frequency will be the same as the resonant frequency of the resonator, am i right? what about the value of Q, i have no specific phase noise requirement, what value of Q should i take?

The magnitude of the negative resistance produced by the amplifier needs to exceed the loss resistance of the inductor.

Any reactance seen looking into the amplifier will appear in series with the tuned circuit and will slightly alter its resonant frequency, but this should not be a significant proportion of the total reactance. Resonant frequency should primarily be governed by the tuned circuit components.

Inductor Q is the ratio of its reactance to loss resistance. You want this to be as high as possible. Plucking figures out of the air, for an oscillator, with no specific application, I would say 20 is rubbish;

50 is OK; 100 is good.

[snip]

Colpitts and Clapp circuits can be analysed as negative resistance oscillators, and the capacitive divider is shown as part of the negative resistance amplifer, even though it is part of the tuned circuit. If the dotted line - where tuned circuit meets "reflection amplifier" - is drawn to the left of the capacitive divider, there would be equal and opposite reactances either side of the line. But the oscillator frequency is not the same as the resonant frequency of the series LC components to the left alone, because the capacitive divider contributes significant reactance.

I am glad to hear you say that. I have always had a problem with the concept of the negative resistance connected to a tank circuit. My problem is that the transistor by itself cannot create the negative resistance, it needs the impedance transformation created by the tank circuit to create the negative resistance. So it really is all about where you "put the dotted line" as you say. The negative resistance may be one concept for the thought process but there is no actual physical way that you can disconnect the tank circuit from the transistor, and connect a network analyzer, and measure a negative resistance. The tank circuit must also be attached to create the negative resistance because it creates the impedance transformation. So the tank circuit and transistor TOGETHER create the negative resistance, but this is just another way of saying that you connect an amplifier to a tank circuit and the amplifier makes up for the losses of the tank and for the output power so it oscillates. To me, the amplifier concept is much more straight forward compared to the negative resistance concept.

Mark

i am going to use this resonator:

i have a coil that has Q of about 48 at the specified center frequency

950MHz, should i find another coil with higher Q to have a high effective Q, why does the Q of the coil affects Qe that much? Qe=-92.3 for QL=100 , Qu=48

i searched for another inductor but the maximum Q that i found for an inductor at this frequency is about 70, may be i didn't search well ,but if i want to have higher Q what should i do? is there anyway to increase the Q of the vco?

i searched for another inductor but the maximum Q that i found for an inductor at this frequency is about 70, may be i didn't search well ,but if i want to have higher Q is there anyway to increase it? what is the meaning of negative Q

A broad band amplifier can be shown as a negative resistance. Start with the assumption that there is zero phase across the ampliier i/p to o/p if it had a negative resistance of -51r then it would just oscillate a series LC with a 50r loss. The stability of the oscillator will be the ratio of the Qs ie the resonant Q and the rate of change of phase through the amplifier.

In message , redhat writes

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dd