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#1
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resonant coil with high phase stability
Hi, all, my current project requires a high phase stability (1 mill
degree @ 10 MHz / oC) resonant surface coil. I am trying to use a LC resonant circuit to drive this surface coil, but its phase stability is really poor (about 40-80 mill degree @ 10 MHz / oC ). I guess the main problem comes from the used capacitor whose value is easily changed by temperature. so my question is that can the crystal oscillator help me more? or is there any other method to achieve this requirement? Thanks a lot. |
#2
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resonant coil with high phase stability
On Apr 30, 8:54 am, huiliu wrote:
Hi, all, my current project requires a high phase stability (1 mill degree @ 10 MHz / oC) resonant surface coil. I am trying to use a LC resonant circuit to drive this surface coil, but its phase stability is really poor (about 40-80 mill degree @ 10 MHz / oC ). I guess the main problem comes from the used capacitor whose value is easily changed by temperature. so my question is that can the crystal oscillator help me more? or is there any other method to achieve this requirement? Thanks a lot. If it is as you suspect, that the capacitor is causing the trouble, why not use a capacitor with low temperature coefficient? C0G dielectric ceramic capacitors are rated at +/-30ppm/C maximum, and I have found some to be very much better than that. I have no idea what the circuit is that you are driving, but I would expect that if you are driving a high-Q resonant tank, the phase shift will be very rapid with frequency, and that implies the need to use a tank circuit with very good temperature stability. If I'm not mistaken, the rate of phase change at resonance of the impedance of a series RLC, expressed as radians per fractional frequency change, is 2*Q. So for a milliradian change, you need to keep the resonant frequency within f(center)/(2000*Q). A milliradian is 57 millidegrees, so to keep such a tank within 1 millidegree if the Q is only 10, you need to hold the resonant frequency (relative to the excitation) to a little better than one part per million. I'd say that's not a very reasonable goal if the inductor dimensions change with temperature. It will take a decent oscillator just to maintain the excitation frequency to that sort of accuracy. You might actually find a capacitor that will give you that sort of stability, by selecting from C0G caps (or possibly by making your own by plating electrodes on fused quartz...). But that doesn't solve the problem of the inductor itself. Could you characterize the tank circuit frequency shift versus temperature and compensate by causing the excitation frequency to change synchronously with that? Cheers, Tom |
#3
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resonant coil with high phase stability
On May 1, 2:04*am, K7ITM wrote:
On Apr 30, 8:54 am, huiliu wrote: Hi, all, my current project requires a high phase stability (1 mill degree @ 10 MHz / oC) resonant surface coil. I am trying to use a LC resonant circuit to drive this surface coil, but its phase stability is really poor (about 40-80 mill degree @ 10 MHz / oC ). I guess the main problem comes from the used capacitor whose value is easily changed by temperature. so my question is that can the crystal oscillator help me more? or is there any other method to achieve this requirement? Thanks a lot. If it is as you suspect, that the capacitor is causing the trouble, why not use a capacitor with low temperature coefficient? *C0G dielectric ceramic capacitors are rated at +/-30ppm/C maximum, and I have found some to be very much better than that. I have no idea what the circuit is that you are driving, but I would expect that if you are driving a high-Q resonant tank, the phase shift will be very rapid with frequency, and that implies the need to use a tank circuit with very good temperature stability. *If I'm not mistaken, the rate of phase change at resonance of the impedance of a series RLC, expressed as radians per fractional frequency change, is 2*Q. *So for a milliradian change, you need to keep the resonant frequency within f(center)/(2000*Q). *A milliradian is 57 millidegrees, so to keep such a tank within 1 millidegree if the Q is only 10, you need to hold the resonant frequency (relative to the excitation) to a little better than one part per million. *I'd say that's not a very reasonable goal if the inductor dimensions change with temperature. *It will take a decent oscillator just to maintain the excitation frequency to that sort of accuracy. *You might actually find a capacitor that will give you that sort of stability, by selecting from C0G caps (or possibly by making your own by plating electrodes on fused quartz...). *But that doesn't solve the problem of the inductor itself. Could you characterize the tank circuit frequency shift versus temperature and compensate by causing the excitation frequency to change synchronously with that? Cheers, Tom Thanks again. I am trying to use temperature compensated crystal quartz to take next measurement. It is a great idea to compensate the frequency shift caused by temperature. Hope it can work. |
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