ES2639476B1 - CIRCUIT AND METHOD FOR DEMODULAR SIGNALS THROUGH A DIGITAL TIMER - Google Patents

Abstract

Circuit and method for demodulating signals by means of a digital timer. # The present invention relates to a circuit and a method for the demodulation of signals modulated in amplitude by means of a digital timer, without using any rectifier, mixer, low pass filter or analog converter. digital. The modulated signal is assumed with a carrier frequency in the kilohertz range, as occurs in conditioning circuits for capacitive and inductive sensors. It is proposed that the modulated signal act as an interference superimposed on the threshold voltage used to measure the period of a known triangular signal. Thus, the standard deviation of a set of N period measurements subject to the effects of the modulated signal contains information about the amplitude.

Description

CIRCUIT AND METHOD FOR DEMODULAR SIGNALS THROUGH A DIGITAL TIMER

Technical sector:

 5

The present invention relates to a circuit and a method for demodulation of amplitude modulated signals by means of a digital timer. The sector of the technique to which it refers is that of electronic instrumentation.

State of the art: 10

In the field of electronic instrumentation, the reading of certain sensors is carried out by means of electronic conditioning circuits that excite the sensor with an alternating signal, this being generally sinusoidal or square. For example, the reading of capacitive and inductive sensors (which are used to measure displacement, position, distance, pressure and level) requires an alternating excitation. Some resistive sensors (for example, electrolytic sensors) also require an alternating excitation in order to avoid electrolysis. A common conditioning circuit for these sensors is the alternating excited bridge type circuit, also known as the alternating bridge. 20 In the literature we can find different alternating bridges and their modifications through operational amplifiers (to obtain a linear response and minimize the effects of parasitic components) designed for capacitive and inductive sensors, such as the Wien bridge, Maxwell-Wien [1 ], Maxwell with inductances [2, 3], and Hay. 25

In the event that the sensor excitation is sinusoidal, the frequency (faith) of the excitation signal is generally worth units of kilohertz for inductive sensors [1-3], and tens [4] or hundreds [5-6] of kilohertz for capacitive sensors The output signal of the conditioning circuit is also sinusoidal with a carrier at the frequency fe and an amplitude modulated according to the physical quantity to be measured. The demodulation of this signal can be implemented with: (a) an asynchronous demodulator (or envelope detector) that incorporates a rectifier and a low-pass filter that provides the average value of the signal

rectified [2,6], or (b) a synchronous (or coherent) demodulator that incorporates a mixer and a low pass filter [4-5]. If the modulated signal does not have a carrier suppression, both asynchronous and synchronous demodulation can be applied, taking into account that the second offers a greater capacity to reject interference at the expense of greater design complexity. On the other hand, if the modulated signal has a carrier suppression and it is necessary to detect phase changes, only synchronous demodulation can be applied. Once the demodulation (whether asynchronous or synchronous) is performed in the analog domain, the resulting low frequency signal is digitized by an analog-digital converter. Therefore, the reading of the modulated signal requires a rectifier or a mixer, plus a low pass filter and an analog-digital converter. The functions of synchronous demodulation (mixing and filtering) can also be implemented in the digital domain using first an analog-digital converter that works at a higher sampling rate [3].

 fifteen

In the event that the excitation of the sensor is square, as occurs in a relaxation oscillator [7], the output signal of the conditioning circuit has a modulation in the temporal domain, for example: a signal whose period (or frequency) ) varies with the physical quantity to be measured. The demodulation of these signals can be done directly with a digital timer (for example, integrated in a microcontroller) without using an analog-digital converter since the information of interest is not in the amplitude of the signal. The period measurement by means of a digital timer is based on the following operating principle. When the input signal crosses a certain threshold voltage, the digital timer starts counting high frequency pulses 25 from a reference oscillator. After a period of the input signal, it crosses the threshold voltage again with the same edge and the digital timer stops. Therefore, the period of the input signal can be calculated as the number of accounts registered in the timer multiplied by the period of the reference oscillator signal. 30

According to the theoretical and experimental study documented in [8], the period measurement of a non-square signal (that is, with a slow rate of change) is susceptible to sinusoidal interference superimposed on both the input signal

as at the threshold voltage. This interference causes the timing of the start and end of the timing to be wrong and, therefore, the period measurement has a variability. This variability (quantified from the standard deviation of the measurements) increases proportionally with the amplitude of the interference and depends on the relationship between the frequency of the interference (fi) and the frequency of the input signal (f0) . The effects of the interference are null when fi = k · f0, while they are maximum when fi = (k + 0.5) · f0, where k is any positive integer including zero. This behavior is similar to that presented by an integrating analog-digital converter that is capable of completely rejecting interference superimposed on the analog input signal as long as the interference period is a submultiple of the converter integration time.

[1] S. Chattopadhyay, S.C. Bera, "Modification of the Maxwell-Wien bridge for accurate measurement of a process variable by an inductive transducer", IEEE 15 Trans. Instrum Meas. 59 (9) (2010) 2445-2449.

[2] S.C. Bera, N. Mandal, R. Sarkar, "Study of a pressure transmitter using an improved inductance bridge network and Bourdon tube as transducer", IEEE Trans. Instrum Meas. 60 (4) (2011) 1453-1460.

[3] P. Kumar, B. George, J. Kumar, “A simple signal conditioning scheme for 20 inductive sensors”, Int. Conf. Sensing Technology (2013) 512-515.

[4] W.Q. Yang, "A self-balancing circuit to measure capacitance and loss conductance for industrial transducer applications", IEEE Trans. Instrum Meas. 45 (6) (1996) 955-958.

[5] C. Kolle, P.O. Leary, “Low-cost, high-precision measurement system for 25 capacitive sensors”, Meas. Sci. Technol. 9 (1998) 510-517.

[6] J.C. Lötters, W. Olthuis, P.H. Veltink, P. Bergveld, “A sensitive differential capacitance to voltage converter for sensor applications,” IEEE Trans. Instrum Meas. 48 (1) (1999) 89-96.

[7] E.G. Bakhoum, M.H.M. Cheng, “High-sensitivity inductive pressure sensor”, 30 IEEE Trans. Instrum Meas. 60 (8) (2011) 2960-2966.

[8] F. Reverter, M. Gasulla, R. Pallàs-Areny, "Analysis of interference effects on period-to-digital conversions", Meas. Sci. Technol. 16 (2005) 2261-2264.

Description of the invention:

The circuit and method proposed in the present invention are intended to demodulate amplitude modulated signals by means of circuits specific to the demodulation of modulated signals in the time domain, that is: a digital timer. The amplitude modulated signal is assumed without carrier suppression and with a frequency faith in the kilohertz range, as occurs in conditioning circuits for capacitive and inductive sensors. The proposed circuit does not require the typical blocks of an amplitude demodulation (such as a rectifier or mixer, and a low pass filter) or an analog-digital converter 10, since demodulation and digitization are both implemented by the digital timer , thus obtaining a low cost and low consumption demodulator circuit.

The proposed method uses the amplitude modulated signal as an interference 15 in the period measurement of a known triangular signal. The effects (initially considered adverse) of a sinusoidal signal superimposed on the threshold voltage during the period measurement of a known signal are exploited here to extract the amplitude value. To be precise, the amplitude of the modulated signal is quantified by the standard deviation (SD) of a set of N period measurements of the triangular signal. Assuming that faith is known, the frequency (f0) of the triangular signal must be adjusted to meet the relation f0 = fe / (k + 0.5). In this way, a maximum DE value is obtained which increases with the amplitude of the modulated signal. For example, if the electronic circuit has an excitation with a frequency fe = 1 kHz, the triangular signal 25 to be timed should have an f0 equal to 2 kHz, 666 Hz, 400 Hz,… corresponding to k equal to 0, 1, 2 , ... respectively. The main limitation of this method is the measurement time since it is necessary to measure N times the period of the triangular signal to subsequently extract the standard deviation. To reduce this measurement time, the case k = 0 resulting in a higher f0 30 is the most appropriate. For the previous example and assuming an N = 100, the measurement time is 50 ms, which is acceptable for industrial applications.

Brief description of the drawings:

Figure 1 presents a preferred embodiment of the invention for demodulating amplitude modulated signals by a digital timer.

Figure 2 shows the voltage waveforms at the inputs (5) and (7), and 5 at the output (9) of the comparator of Figure 1 for an amplitude A1 of the modulated signal, assuming k = 0 and , therefore, f0 = 2fe.

Figure 3 shows the voltage waveforms at the inputs (5) and (7), and at the output (9) of the comparator of Figure 1 for an amplitude A2 (> A1) of the modulated signal, assuming k = 0 and, therefore, f0 = 2fe. 10

Description of a preferred embodiment:

The present invention proposes to demodulate amplitude modulated signals by means of the circuit of Figure 1, where a unipolar supply is assumed from the VDD voltage. The modulated sinusoidal signal (1), with a carrier frequency fe, is alternately coupled through the capacitor (2) to a voltage divider formed by two resistors (3) and (4) of the same value. In this way, a voltage (5) equal to VDD / 2 (acting as a threshold voltage in the period measurement) is obtained with the alternating component of the signal (1) superimposed. On the other hand, an oscillator (6) provides a triangular signal (7) between 0 V and VDD, and with a frequency f0 = fe / (k + 0.5), where k is a positive integer including zero. The signals (5) and (7) are subsequently compared by means of a comparator (8), which provides the output (9) with a high logic level (i.e., VDD) when (7) is greater than (5), and a Logic level low (i.e. 0 V) 25 when (5) is greater than (7). Next, the period of this square signal generated at the output (9) of the comparator is measured by a digital timer (10) integrated, for example, in a microcontroller.

In the event that the modulated signal (1) has an amplitude equal to zero, the voltage (5) is free from interference and, therefore, the timing of the start and end of the timing are correct. Under these conditions, the comparison between (5) and (7) results in a square signal with a period (T0) equal to that of the triangular signal and with zero variability. However, if the modulated signal (1) has a certain amplitude (A1), the comparison between (5) and (7) generates erroneous start and end times of the timing, as shown in Figure 2, thus obtaining a square signal with a period equal to T'0 instead of T0. The measurement of N periods under these conditions gives rise to N values of T’0 whose DE is proportional to the amplitude of the modulated signal (1). Note in Figure 3 how a larger amplitude of the modulated signal (ie A2> A1) causes more disparate T’0 values and, consequently, a greater DE of the N period measurements.

Claims (2)

1. A circuit to demodulate amplitude modulated signals by means of a digital timer characterized in that it comprises: - an oscillator that generates a triangular signal; 5 - an alternating coupling circuit that provides a threshold voltage with the modulated signal in superimposed amplitude; - a comparator that compares the two previous signals and that provides at the output a square signal whose period contains information on the amplitude of the modulated signal; and 10 - a digital timer that measures the period of the square signal of the comparator output. 2. A method to demodulate amplitude modulated signals by a digital timer characterized by - use the modulated signal as an interference superimposed on the threshold voltage when measuring the period of a known triangular signal; - use a frequency of the triangular signal f0 = fe / (k + 0.5), being the carrier frequency of the modulated signal and k a positive integer including zero; and 20 - use the standard deviation of a set of N measures of the triangular signal period subject to the effects of the modulated signal to quantify the amplitude.