The tunnel diode resonator (TDR) is centered on the tunnel diode. This is a semiconducting device that has an interesting I-V curve. For a certain range of bias voltages, a tunnel diode shows negative differential resistance. If a TD is connected to an LC circuit properly, what you get is a spontaneously resonating circuit. The advantage of such a set-up is the oscillation frequency is locked onto the resonant frequency. Any change in the inductance or capacitance leads to a change in the frequency. This is what we exploit.

If the circuit is designed properly and thermally stabilized the time resolved uncertainty in the resonant frequency can be on the order of 0.005-0.05 Hz. We typically operate at about 20 MHz. Just taking orders of magnitude, 10 MHz is 10⁷ Hz and 0.01 Hz is 10^-2 Hz. This means we can know our resonant frequency to about 1 part in a billion. If we are only concerned with frequency changes on the order of 1%, then the change in frequency is directly proportional to the change in inductance. The change in inductance is directly proportional to the change in the magnetic susceptibility of the sample. When all the constants of proportionality are taken into consideration, the final sensitivity of our system is on the order of 10 parts per billion. This means if the susceptibility of the sample changes by 1 part in 100 million, we can detect it. To give some perspective on these numbers (if this is possible) imagine a room with 1 million dollars in pennies. Equivalent sensitivity would be if someone came in a left 1 more penny while we were away, our system would be able to determine that.

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Updated 18 September 2007 from home.