Signal Demodulation
Alternating current signals are multiplied by a reference waveform to extract the amplitude of a specific frequency component. Utilizing phase-sensitive detection allows sensors to measure tiny changes in impedance while ignoring out-of-phase noise. This process generates a direct current output that corresponds to the in-phase signal component.
Phase Alignment
Synchronous timing between the reference signal and the input waveform is required to achieve accurate measurement results. Any phase error reduces the output voltage and introduces cross-talk between the real and imaginary components. Adjustment of the reference phase is performed during initial instrument calibration.
Interference Rejection
Narrowband filtering achieved through synchronous demodulation removes noise that does not match the reference frequency. This technique acts as an extremely sharp bandpass filter with a bandwidth that can be adjusted by the post-demodulation low-pass filter. Noise signals at other frequencies are averaged to zero over the measurement period.
This filtering is effective for recovering weak sensor signals buried in environmental and electromagnetic noise.
Signal Recovery
Low-noise amplification of the demodulated signal produces a stable voltage for the analog-to-digital converter. This step is necessary when the raw sensor output is in the microvolt range. The recovery stage prevents signal degradation before digital processing occurs.
