RU2594918C2 - Method for digital generation of width-modulated signals for hydroacoustics - Google Patents

Abstract

FIELD: information technology.

SUBSTANCE: invention relates to digital form of modulated pulse signals for amplifier and generator devices of hydroacoustic transmitting channels of ultrasonic range. This is achieved due to that at provision of integer ratio γ= T_s/T_p= 3÷5 period T_S digital input signal to period T_P frequency-modulated sawtooth digital signal forming two symmetrical antiphased saw digital signals, as a result of comparison with digital input signal is formed of two modulated pulse sequences corresponding to two-channel symmetrical PWM.

EFFECT: reduced distortions of ultrasonic signals with minimum energy losses by reducing frequency of PWM.

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Description

The invention relates to methods for generating modulated pulsed signals for amplifying and generating devices for sonar transmission paths of the ultrasonic range.

A modern trend in the development of transmission paths in the ultrasonic frequency range, in particular for near-lighting conditions of sonar systems, is the introduction of digital methods for generating signals based on the digital generation of modulated pulse sequences with subsequent key amplification of the pulses and the allocation of excitation signals with a given amplitude and phase distribution on the emitting antenna [one]. In this area, methods for digitally generating pulse-width modulated signals, described, for example, in patents [2-4], used to implement key power amplifiers [5], are becoming more widespread.

Known devices [2, 3] use the conversion of the digital code of the input signal into the duration of modulated pulses with a constant repetition rate. A pulse-width modulated (PWM) signal is generated in known devices [2, 3] using clock-counters with clock frequencies that form time intervals proportional to the digital code of the input signal at the recording clock times.

In the device for functional conversion of PWM signals [2], one-sided modulation of the second kind is formed, characterized by a significant level of harmonic distortion, especially with a small ratio of the PWM frequency ω to the frequency of the amplified signal Ω. So, for the ratio ω / Ω <10, harmonic distortions exceed 10% [5].

The digital pulse-width pulse modulator according to the patent [3], which implements two-sided PWM of the second kind, is characterized by an improved spectral composition of the modulated pulse signal due to the absence of even harmonics, which leads to a decrease in harmonic distortion. For ω / Ω = 6 ... 10 in the known device, the harmonic coefficient does not exceed 2-3%.

The main disadvantage of the known devices [2, 3] is the significant penetration of the Raman components of the spectrum of pulsed signals with frequencies (ω-nΩ) into the frequency range of the amplified signal with decreasing PWM frequency. With a small frequency ratio ω / Ω <6, the Raman coefficient exceeds 10% [5].

To reduce the Raman distortion of the modulated pulse voltage in the device described in the patent [4], allows the use of a two-channel PWM to control a two-channel key power amplifier (CMC), made, for example, according to a bridge circuit. The addition of two pulse signals with a PWM in the diagonal of the KUM bridge circuit generates a total pulse voltage with a double switching frequency. The combination components of the spectrum of the total pulse voltage are grouped around the harmonics of the total frequency equal to 2ω, which leads to a more than twofold decrease in the coefficient of Raman distortion. However, a further increase in the frequency of the amplified signal in the known device [4] leads to a significant increase in combinational and harmonic distortions, which prevents its use in the CMC of the ultrasonic frequency range.

Closest to the proposed invention is a method of digital generation of PWM signals described in the patent of the Russian Federation 2308800 [6]. In a known technical solution, the principle of implementing a digital pulse-width modulator with a distributed spectral characteristic is proposed, which is achieved by changing the switching frequency of the PWM signals.

The known method of digitally generating PWM signals is based on the formation of a frequency-modulated sawtooth digital signal, its comparison with a digital input signal, and generation of a sequence of pulse-width modulated pulses by comparing the frequency of the sawtooth signal according to a pseudo-random law, which achieves the distribution of the Raman spectral characteristics spectrum components of the modulated PWM signal.

The advantage of the prototype method is the implementation of the possibility of additional frequency modulation of the PWM signals, which allows for a change in the frequency of the combination components of the spectrum.

However, when the frequency of the sawtooth digital signal changes according to a pseudo-random law, the distribution of the spectral characteristics of the Raman components is accompanied by an increase in the Raman distortion of the PWM signal. This circumstance is aggravated in the prototype method by the formation of a one-sided (increasing) sawtooth digital signal, which leads to an increase in both combinational distortions and, in the case of modulation of the second kind [5], to significant harmonic distortions, the magnitude of which exceeds 10% at the frequency ratio ω / Ω≤10.

The prototype method can be implemented in a known device [6], containing a series-connected frequency-modulated pulse generator, the frequency of which varies according to a pseudo-random law, a memory register and a digital comparator, the other input of which is connected to the digital signal bus, and the output - with the output of the device moreover, the synchronization input of the frequency-modulated pulse generator is connected to the synchronization frequency bus.

The frequency-modulated pulse generator can be implemented algorithmically and hardware using a pseudo-random number generator, two digital switching units, a digital adder and an additional memory register.

The frequency of the frequency-modulated pulses varies according to the pseudo-random law in the band from f _min to f _max , which corresponds to a change in the frequency of the sawtooth digital signal and, accordingly, the frequency of the PWM signal in the range ω _min <ω <ω _max for ω _min = 2πf _min / β; ω _max = 2πf _max / β, where β - clock frequency dividing ratio set in the memory register pulse counter mode.

The number β determines the discreteness of change of the digital sawtooth voltage and, accordingly, the discreteness of change and the dynamic range of the PWM of the digital input signal. When the dynamic range over 40 dB integral value should be greater than 100. In a digital pulse width modulator [6] is set for each PWM period T _{of PWM} according to a pseudorandom. Moreover, with a decrease in the period T of the _PWM , the number β decreases and, accordingly, for the given amplitude of the digital sawtooth voltage, the PWM sampling step increases. The latter circumstance leads to an additional decrease in the accuracy of the formation of pulse-width modulated pulse signals.

The disadvantages of this method, associated with reduced quality indicators of the formation of modulated width-modulated signals, especially in the upper frequency range, with a small ratio of the PWM frequency and the frequency of the generated signal, limit the capabilities of the prototype method in sonar transmission paths. These disadvantages are especially pronounced when using the prototype method in amplification and generator devices in GAC OBO, since a feature of the library of signals generated in OBO modes is the use of harmonic frequency-modulated oscillations with a relative frequency band of 0.3-0.5 octaves. This is explained by the fact that for the implementation of the paths of the ultrasonic frequency range, for example, for hydroacoustic stations (GAS) of near-field lighting (OBO), further development of the method for digitally generating pulse-width modulated pulse signals is necessary, aimed at reducing distortion while minimizing the frequency ratio of the modulating effect ( input digital signal) and pulse repetition rate with PWM.

Digital synthesis of such signals for sonar transmission paths implies a constant frequency in each individual oscillation period according to a known digital code, the use of which is possible to optimize PWM.

The objective of the invention is to improve the quality characteristics of the formation of pulse-width modulated pulse signals while increasing the energy efficiency of the ultrasonic-generating devices, which is especially important in object-based sonar complexes.

The technical result from the use of the invention is to reduce the distortion of ultrasonic signals while minimizing energy losses in the generator devices by reducing the frequency of the PWM.

To achieve this technical result, in a known method for generating a pulse-width modulated pulse signals based on the formation of a frequency-modulated sawtooth digital signal, comparing it with a digital input signal and generating a sequence of pulse-width modulated pulses by comparing it, new features have been introduced, namely: -modulated sawtooth signal is changed in frequency in proportion to the frequency of the digital input signal from the condition of the integer ratio h astoth.

In this case, the combination spectrum components whose frequencies do not coincide with the harmonic frequencies of the input digital signal are excluded in the spectrum of the sequence of pulse-width modulated pulses, which suppresses the combination distortion and improves the quality of the formation of the pulse-width modulated signal.

An additional improvement in the quality of the formation of a pulse-width modulated pulse signal under conditions of lowering the PWM frequency is achieved in the claimed method by introducing additional new features, namely: a frequency-modulated digital signal is formed in the form of symmetrical direct and inverse sawtooth signals, which, when compared with a digital input signal, are formed a sequence of pulse-width modulated pulses in the form of the first and second one-cycle sequences of pulse-width modulated pulses in, addition key which ensures the formation of a push-pull bipolar pulse sequence modulated pulses.

The combination of new features in the present method allows for the digital generation of a width-modulated signal with an integer ratio of the PWM frequency ω and the frequency of the digital input signal Ω in the range from 10 to 3 with harmonic distortion of not more than 0.3 ... 3% with a dynamic range of 40 dB. For the region of ultrasonic frequencies, the most interesting is the digital formation of two sequences of pulses with a symmetric PWM, when amplifying and summing them with the well-known KUM bridge circuit [4], the total switching frequency of the resulting push-pull pulse sequence is doubled. In this case, a satisfactory quality of the resulting pulse voltage is achieved (non-linear distortion of not more than 3%) for a minimum frequency ratio ω / Ω = 3.

As a result of the application of the proposed method under conditions of a minimum frequency ratio, it is possible to reduce the energy loss by switching at least 3 times in comparison with the known technical solutions. The highlighted circumstance is of fundamental importance for increasing the energy efficiency of the paths of the ultrasonic frequency range used in the GAS OBO.

The invention is illustrated in FIG. 1, 2, which shows the structural diagrams of devices that implement the inventive method for digitally generating a pulse-width modulated pulse signal, and FIG. 3, 4, in which phonograms of signals are presented explaining their operation. The spectrograms of the pulse signals generated according to the claimed method for a synchronous and fixed value of the PWM frequency are illustrated in FIG. 5a, b.

A device for digitally generating a pulse-width modulated pulse signal (Fig. 1) comprises a memory register 1, a frequency-modulated generator 2, a digital counter 3, and a digital comparator 4.

The device is connected to the digital signal conditioner via the clock bus with frequency F, via the digital input signal bus (Code Input), via the bus code of the input signal frequency (Code Frequency).

The memory register 1 is connected by an input to the Code Frequency bus and provides registration of the code by pulse from the transfer output of the digital counter 3, which is fed to the input of the memory register 1 at the end of each digital sawtooth voltage generation cycle.

Digital signal Code The frequency is set proportional to the period of the input digital signal and is supplied from the output of the memory register 1 to the control input of the frequency-modulated generator 2.

The output signal of the frequency-modulated generator 2 is formed by dividing the frequency of the clock in accordance with the dimension of the digital signal Code Frequency. Thus, the synchronization input of the digital counter 3 connected to the output of the frequency-modulated generator 2 receives a pulse with a period T _c proportional to the period T _{S of the} digital input signal. The digital counter 3 generates a digital sawtooth signal with a resolution set by a constant division factor β and a given constant amplitude AP.

As a result, the period T _{P of the} digital sawtooth signal is set equal to:

where α is the division coefficient of the frequency-modulated generator proportional to the value of the signal Code Frequency;

β is the constant division coefficient of the digital counter;

T ₀ - the period of the clock pulses.

The implementation of the proposed method is based on the condition of an integer relation of the period of the digital input signal to the period of the digital sawtooth signal of a given value:

The selected condition determines the restrictions for setting the period of the digital input signal for given values of α, β, γ and the common clock bus with a period T ₀ :

For constant values of γ, β, the accuracy of installation T _S should be determined by the dimension of the digital signal Code Frequency.

A feature of the implementation of the device (Fig. 2) is the formation of a digital counter of two antiphase symmetric digital sawtooth signals, each of which is compared with a digital input signal. As a result, two sequences of pulse-width modulated pulses are formed, the summation of which according to the well-known rules [4] allows one to obtain a push-pull sequence of bipolar pulses with a double switching frequency.

It is advisable to consider the method of digitally generating latitudinal-modulated signals for hydroacoustics together with a description of the operation of the devices implementing it (Fig. 1, Fig. 2) and signal diagrams explaining their principle of operation.

In the device of FIG. 1, for the implementation of the proposed method provides the formation of a one-sided rising sawtooth digital signal VP from the condition:

where t _k, t _{k + 1} define the beginning and end of the formation interval digital sawtooth signal.

For digital signals, the function of the current time is represented by a discrete function of changing time intervals in the form:

Where

t _N - the initial moment of time;

T _pk is the k-th period of the sawtooth digital signal;

K is the number of periods of the sawtooth digital signal in the period of the digital signal in the period of the digital input signal;

N is the number of discrete steps in the period of the sawtooth digital signal;

k, n are the current values of discrete intervals.

The fulfillment of conditions (1), (2), (3) provides an integer number of periods T _pk sawtooth digital signal in the period of the digital signal, which corresponds to the features of the proposed method. In this case, the equalities N = β, K = γ are ensured.

Thus, as a result of comparing the sawtooth digital signal VP with a periodic input digital signal, a sequence of PWM pulse-width modulated pulses is formed:

Due to the integer ratio T _s / T _p = γ, the period of the modulated pulse sequence is equal to the period of the input digital signal.

For an input digital signal that varies according to harmonic law, the noted condition excludes combined components from the spectrum of pulse-width modulated signals with frequencies that do not coincide with the frequency of harmonics, thereby improving the quality of formation of the pulse-width modulated pulse signal.

Realization of additional advantages of the proposed method for generating pulse-width modulated pulse signals, which minimizes the switching frequency when using a two-channel symmetric PWM, is achieved in the device shown in FIG. 2. The device (Fig. 2), which contains a memory register 1, a frequency-modulated generator 2, a digital counter 3, a comparator 4, has an additional comparator 5, and the digital counter 3 is reversible, has direct and inverse outputs connected respectively to inputs of comparators 4 and 5, other inputs that are connected to the digital input signal bus. The outputs of the comparators 4 and 5 provide the formation of the first and second sequences of pulse-width modulated pulses (PWM1 and PWM2).

According to the timing diagrams shown in FIG. 4, the digital formation of two sequences of pulse-width modulated pulses with symmetrical PWM is as follows. A reversible digital counter 3, under conditions (1), (2), (3), which determine the integer ratio of the PWM frequency and the frequency of the digital signal, generates two symmetric antiphase sawtooth digital signals VP1, VP2 (Fig. 4).

Given the discrete function of changing the time intervals (4), the symmetrical sawtooth signals VP1 and VP2 are determined by the following relation:

As a result of comparing the digital signals VP1 and VP2 with the digital input signal S, the formation of two modulated pulse sequences PW1, PW2, corresponding to a symmetrical two-channel PWM (figure 4):

The generated pulse signals PW1, PW2 can be used according to well-known rules for controlling a two-channel CMC, for example, made according to a bridge circuit [4]. As a result, the total pulse voltage VPWM is formed with a pulse repetition rate twice as high as the frequency of the initial signals PWM1 and PWM2,

Implementation of additional features of the proposed method for the digital generation of pulse-width modulated pulse signals provided in the device (Fig. 2), allows to minimize the integer ratio of the PWM frequency with improved quality indicators of the total pulse signal. Given the doubling of the frequency of the total pulse voltage, the minimum frequency ratio ω / Ω = 3 can be realized. Moreover, the spectrum of the total modulated pulse sequence contains only odd harmonics of the modulating harmonic signal, and the relative level of the third harmonic, which determines the main harmonic distortion, does not exceed 3-5% in the dynamic range of 30-40 dB.

The highlighted advantages of the proposed method provide a reduction in the distortion of ultrasonic signals while minimizing energy losses in the generating devices by minimizing the frequency of the PWM, which makes it appropriate to introduce the proposed technical solution in the transmitting paths of the GAS OBO.

At our enterprise, a prototype of a multi-channel digital generator device has been manufactured and is being tested, in one of the operating modes of which the proposed method for digital generation of pulse-width modulated pulse signals is implemented. The spectral composition of a pulsed signal with a two-channel symmetric PWM was estimated by modulating a harmonic frequency-modulated signal with a relative modulation band of 0.3 octave under the conditions of using the method with a constant PWM frequency and the inventive method with synchronizing the PWM frequency with the frequency of the modulating signal from the condition of an integer frequency ratio. The spectrograms of pulsed signals at a frequency of ω≈5Ω are shown in FIG. 5 for a synchronous PWM frequency (Fig. 5.a) according to the claimed method and for a fixed PWM frequency (Fig. 5.b).

A comparative analysis of the spectra of pulsed signals convincingly shows the advantages of the proposed technical solution.

In the case of a PWM with a fixed frequency, the nearest harmonic (Ω3 = 352) reaches -30 dB from the level of the useful signal (Ω), the spectrum contains 5 harmonics -50 dB and 7 harmonics -16 dB, and there are also background spectral components in the entire band frequencies at the level of -60 ... -70 dB.

The use of synchronous PWM can significantly improve the structure of the spectrum of the pulse signal: the relative value of the 3rd harmonic does not exceed -42 dB; 5th harmonic is absent; The 7th harmonic is less than -27 dB. An important circumstance is the absence of background Raman spectrum components.

The given example confirms the possibility of reducing nonlinear distortion of pulsed signals by 3-4 times when introducing the proposed method of digital formation in conditions of minimizing the switching frequency, which is important for implementation in ultrasonic sonar transmission paths, for example, used in the GAS OBO.

INFORMATION SOURCES

1. Alexandrov V.A., Potapov A.I. A new type of genset for near lighting mode. - Proceedings of the X All-Russian Conference Applied Technologies for Hydroacoustics and Hydrophysics, St. Petersburg: Nauka, 2010, p. 160-163.

2. RF patent No. 2237920. Device for the functional conversion of PWM signals, publ. 10/10/2004.

3. RF patent No. 2172062. Digital pulse width modulator, publ. 08/10/2001.

4. US Patent No. 20044212524. PWM signal generator and PWM signal generating method, publ. 10/28/2004.

5. Kibakin B.M. The basics of key gain methods. - M .: Energy, 1980, 232 p.

6. RF patent No. 2308800. Digital pulse-width modulator with distributed spectral response, publ. 10.20.2007.

Claims (1)

A method for digitally generating a pulse-width modulated pulse wave signal, including generating a frequency-modulated sawtooth digital signal, comparing it with a digital input signal, and generating a sequence of pulse-width modulated pulses by comparing the frequency-modulated sawtooth waveform in the form of symmetrical direct and inverse sawtooth signals, with a period of T _P, with an integer ratio $$\gamma$$

= 3 ... 5 to the period T _S = $$\gamma$$

T _{P of the} digital input signal, and the sequence of pulse-width modulated pulses is formed in the form of the first and second one-stroke sequences, the key addition of which ensures the formation of a push-pull sequence of bipolar pulse-width modulated pulses.

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