FM demodulator for high frequency FM-AFM

Category: Research Topics Posted by: dai

We developed a new type of FM demodulator for high frequency FM mode AFM.

Based on direct frequecny conversion and vector demodulation techniques, it shows good stability and wide range of applicability. We call the demodulator Symmetrical Quadrature Demodulator (SQD).

The SQD is designed to be used with our superheterodyne system for high frequency FM-AFM. In the superheterodyne system, the center frequency of cantilever vibration in the range from 250 k to 100 MHz is converted to 10.7 MHz intermediate frequency (IF), and noise filtering, amplitude regulation and phase adjustment are done to the IF signal. The SQD is also connected to IF signal line. The IF signal is frequency converted to the cantilever?vibration frequency again, and fedback to a cantilever actuation device.

The SQD converts the IF signal to a pair of zero-Hz-centered signals (I and Q) with mutual phase difference of 90 degrees. By taking time derivatives of I and Q, their amplitude become proportional to their frequency, that is equal to the frequency debviation of IF signal from its center frequency. The vibrating component in derivatives of I and Q are cancelled out by cross multiplying with Q and I and a mutual subtraction, providing an output signal proportional to the frequency deviation of IF.

The SQD was implemented in analog circuit with digital offset canceler.

Compared with PLL demodulator, the SQD has advantages as follows.

- Center frequency is stable, because it depends not on crystal oscillator's property.

- Response is fast, because the SQD does not include feedback loop in its algorithm.

- The scale factor can be changed in wide range only by changing the time constant of time differentiation.

Keywords: circuit, control, AFM, measurement_technique

Japanese Journal of Applied Physics

Vol. 43, No. 7B, 2004, pp. 4566-4570