Patent ID: 7395044

Claim:
A signal processing apparatus of a differential transformer for obtaining a measurement signal with a fast response speed from a differential transformer having a low excitation frequency, the apparatus comprising: a high-frequency oscillator which generates a high-frequency signal having a frequency higher than the excitation frequency of the differential transformer; a frequency divider which divides the high-frequency signal to generate a driving signal for the differential transformer; frequency reduction means which reduces the frequency of the high-frequency signal by the frequency of the driving signal; a mixer which mixes an output from the frequency reduction means with an output from the differential transformer; a high-pass filter or a band-pass filter which cuts a low-frequency component of an output from the mixer; means which synchronously detects the output from the high-pass filter or the band-pass filter with the use of the high-frequency signal; a first PLL circuit which extracts and holds a frequency component of an output from the high-pass filter or the band-pass filter; and a first phase shifter which matches the phase of the high-frequency signal to a phase of an output from the high-pass filter or the band-pass filter.