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How to get the Q of my inductor

  1. Sep 30, 2009 #1

    I'm trying to construct a simple Class A Common emitter Colpitts oscillator. The Electronics book that I have tells me that I need to compute the Rcoil of the inductor. To do that, I need the Q factor of the inductor at 1 MHz, which is the frequency I'm trying to generate. How do I get the Q in the first place?

    I do have an LCR meter which gives me what I think is the series resistance, the inductance and the Q, which it says is around 1.12 (at 1Khz), of the inductor. It also has a 120Hz option. Is this Q something I can use to compute the Q at 1MHz?

    I need the correct value of Q to run the Colpitts oscillator at 1Mhz.

    Any help, including formulas or circuits to compute Q, would be much appreciated.


  2. jcsd
  3. Sep 30, 2009 #2
    Hi rsr life-
    There are four things that will determine the effective Q at 1 MHz. The dc wire resistance, the ac wire resistance (skin losses-does it have Litz wire windings?), the ac losses in ferrite or other magnetic core at 1 MHz, and inter-turn capacitance that will cause the coil impedance to have resonances below or near 1 MHz. The inductance of the coil should be linear all the way up to 1 MHz. Q = wL/R for a tank circuit, where w = 2 pi frequency.
    Bob S
  4. Sep 30, 2009 #3
    Hello Bob,

    The inductor that I have is wrapped in some kind of plastic/rubber shielding, so I can't tell what's inside, unfortunately. I do know the L value and the resistance I can compute from the LCR meter (at 1KHz). I think that resistance that the meter gives is the series resistance, but am not sure. The formula for Q that you stated uses the parallel R (inductive element in parallel with the coil resistive element). But is there some way I can calculate this Q/R given the Q and L at 1Khz?

    I imagine people who design the Colpitts oscillator in class labs either know their inductor's Q at the desired oscillator frequency or have some straightforward method of finding this out.

    Here are the readings from the LCR meter:

    120 Hz L=21uH Q=0.1 R=0.117 ohms
    1 KHz L=20.8uH Q=1.13 R=0.26 ohms​

    Hope this makes sense.


  5. Sep 30, 2009 #4


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    You can try to attract the coil with a magnet. If it attracts, the coil has a magnetic core and all measurements at 1 KHz are meaningless at 1 MHz because magnetic cores change their properties dramatically with frequency.

    Air cored coils have fairly constant inductance if they are used well below their self resonant frequency. So, you could get an idea of their inductance with the LCR meter.

    Capacitance measured on the LCR meter will also be much the same at 1 MHz.

    If you have a signal generator or function generator that works at 1 MHz you can try the circuit arrangement below to check the coil. If the coil and capacitor are resonant, there will be a big increase in output signal at that frequency.

    If your coil has an inductance of 21 uH at 1 MHz it should resonate with about 1200 pF.

    Attached Files:

  6. Oct 1, 2009 #5
    Thanks vk6kro,

    I will try that.
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