lc q factor

Hi all,

I'm wondering how to increase Q factor of an LC tank,

If i change the reactance of the LC tank, won't be this affecting the frequency of interest?

Thanks in advance

LC resonant frequency is related only to L and C values.
http://en.wikipedia.org/wiki/LC_circuit
http://www.deephaven.co.uk/lc.html

Adjusting value on L and C affect frequency, but not if you make lower resistance in circurit, Q = reactance/resistance

Only way to make higher Q-value is make lowest possible resistance in inductance (capacitance have already low loss in most case, but use High Q-capacitanse if you can)

air-winding inductance with round copper wire can give Q around 200-300 as best.

if you wind inductance with copper folie (ex. 2-3 mm wide 35 um strip take from unetched PCB) you can make around Q = 1000 - 1500 depend of thin thickness and wide area 'wire' make lower skin effect loss in inductance. we talk now of 10-20 turns single layer inductance - not multilayer.

in high valued inductance (many nH ) with many multilayer turns, you must using litz-thread (expensive...)


you can also using inductance with magnetic core to shorten legth of copper wire, but in this case is always compromissing between usable frequency range, size, risk for magnetic saturate in core, non liniarity and loss in core.

You can also try using an active circuit to supply you with negative conductance that can reduce the effect of the losses of the LC circuit .But make sure that the total resistance is still positive to avoid instability and that this does NOT decrease the noise generated by the circuit but may increase it by the active circuit .

To make high Q coil is easy & not so expensive. Take big diameter winding & thick guage wire. But to buy hi Q capacitors r quite expensive.

Thanks guys, it really helps. What i did was connect a resistor parallel to the inductor to decrease the resistance.

My goal was to increase the Q so that the frequency deviation in oscillator will increase when i feed in different level of input voltage

?
i can not agree with u
the method u used will decrease Q

yes, the method you use will decrease Q