Demodulating AM Square Signals via a Digital Timer for Sensor Applications

Abstract

This paper evaluates theoretically and experimentally the performance of a timer-based demodulator applied to low-frequency amplitude-modulated (AM) square signals coming from sensor circuits. The demodulator extracts the amplitude of the AM square signal by measuring the period of a reference triangular signal that is altered by the AM signal itself, as already suggested in a previous paper but for AM sinusoidal signals. The demodulation and digitization are carried out simultaneously via a digital timer and without requiring a rectifier, a mixer, a low-pass filter, or an analog-to-digital converter, thus resulting in a simple and low-cost design solution. Unlike the sinusoidal case, this paper proposes to infer the amplitude of the AM square signal by measuring the bias of the period measurement with respect to the ideal (known) value. Experimental results show a non-linearity error lower than 0.03% full-scale span and a resolution of 13.3 bits.

Using a Digital Timer to Demodulate AM Triangular Signals for Sensor Applications

“…The performance of the circuit in Fig. 2 strongly depends on the ratio fc / f0 selected, as already proven for the demodulation of AM sinusoidal [10] and square [13] signals.…”

“…According to [10], the optimal performance when demodulating AM sinusoidal signals is achieved when fc / f0 = (m + 0.5)•(1+Δ) with |Δ| << 1, m being any positive integer including the zero. However, from [13], the optimal response when demodulating AM square signals is obtained when fc / f0 is exactly equal to m + 0.5, especially when m = 0. In the sinusoidal case, the AM-signal amplitude was estimated by computing the standard deviation (STD) of a set of period measurements, whereas in the square case, by measuring the bias with respect to the ideal value of period.…”

“…A similar approach has also been suggested to measure sensors that provide a voltage-modulated output [8], [9], but this is assumed of very low frequency or quasi static. The measurement of amplitude-modulated (AM) signals, with a carrier frequency in the range of kilohertz, via a digital timer has also been recently proposed for sinusoidal [10], [11] and square [12], [13] carrier signals. Unlike what occurs in conventional demodulators [14], [15] that require a rectifier/mixer, a low-pass filter (LPF), and an ADC, the demodulator proposed in [10]- [13] extracts and digitizes the AM-signal amplitude mainly through a digital timer, thus resulting in a simple and low-cost design solution.…”

“…The measurement of amplitude-modulated (AM) signals, with a carrier frequency in the range of kilohertz, via a digital timer has also been recently proposed for sinusoidal [10], [11] and square [12], [13] carrier signals. Unlike what occurs in conventional demodulators [14], [15] that require a rectifier/mixer, a low-pass filter (LPF), and an ADC, the demodulator proposed in [10]- [13] extracts and digitizes the AM-signal amplitude mainly through a digital timer, thus resulting in a simple and low-cost design solution. Some examples can be found in the literature where the sensor is excited by a triangular wave, instead of a sinusoidal or a square wave.…”

“…For the case reported in [16], the AM triangular output signal was first demodulated using an asynchronous topology based on a peak detector and then digitized by an ADC. This work aims to demonstrate that the timer-based demodulator previously suggested for AM sinusoidal [10], [11] and square [12], [13] signals is also valid to demodulate AM triangular signals. The proposed circuit and operating conditions are the same as those applied to AM square signals, but the performance is significantly different.…”