Frequency Mixing
Nonlinearities in an electronic amplification or mixing stage generate spurious signals at sums and differences of the input frequencies. In high-frequency sensor interfaces, intermodulation signal folding occurs when these mixing products fall within the active bandwidth of the analog-to-digital converter. This phenomenon occurs when high-frequency out-of-band interferers mix with each other or with the sensor excitation signal.
Sensing Interference
The generated difference frequencies cannot be separated from the genuine sensor output during digitization. Systems affected by intermodulation signal folding display a raised noise floor and false spectral peaks that mimic actual mechanical vibrations. This interference degrades the signal-to-noise ratio and can lead to incorrect diagnostic readings.
A system designer must use highly linear amplifiers to minimize the generation of these unwanted frequency components. The use of fully differential signal paths also helps suppress even-order intermodulation products.
Calibration Evaluation
Testing with two-tone sinusoidal generators identifies the intermodulation performance of the sensor receiver chain. Measuring the third-order intercept point helps quantify the susceptibility to intermodulation signal folding before deployment. This evaluation ensures that the hardware can operate in environments with strong electromagnetic noise.
Filtering Technique
Bandpass or lowpass filters placed before the active gain stages suppress the out-of-band interferers before they can mix. This preventative filtering reduces the amplitude of the signal components that cause intermodulation signal folding. This hardware shielding is necessary because digital filtering cannot remove these folded signals once they have entered the digitized band.
