RU2810899C1 - Method for bandpass discrete analog filtering - Google Patents

Abstract

FIELD: signal processing.

SUBSTANCE: filtering (detection) of a signal in conditions of strong interference for spectrum analysis. To do this, in the method of bandpass discrete-analog filtering: set the frequency of the clock generator, which determines the tuning frequency of the bandpass discrete-analog filtering; provides an input analog signal; supplying clock pulses from the clock generator to the input switch; the input signal is switched in the input switch in accordance with the clock pulses; the output signal is removed, characterized in that: clock pulses are supplied from the clock generator to the input switch and the output switch; in the input switch, in accordance with the clock pulses, the input signal is switched one by one to all low-pass filters; the output signal in the output switch is removed from the low-pass filters, synchronously and in phase with the switching in the input switch; remove the output signal from the output switch.

EFFECT: improving the separation of periodic signals from the signal-noise mixture.

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Description

The invention relates to the field of filtering analog signals and can be used for filtering (detection) of a signal in conditions of powerful interference or for spectrum analysis.

The proposed method of bandpass discrete analog filtering (BAF) is based on switched analog filtering.

This technical solution is primarily intended for separating periodic signals from the signal-noise mixture.

The problem with signal reception in radar and data transmission is the low signal-to-noise ratio. The useful signal is lost against the background noise and cannot be isolated by simple means, for example, such as a simple bandpass filter. There are two main methods for isolating weak signals from a signal-noise mixture: the accumulation (integration) method and the method based on the use of complex signals. The accumulation method is used for periodic or quasi-periodic signals of long duration.

Filtering is one of the most common signal processing operations. The purpose of filtering is to suppress interference located in adjacent frequency ranges relative to the specified one.

When solving problems of separating periodic signals from a signal-noise mixture, various types of accumulations are usually used. In properly constructed averaging devices, over time, a voltage level sufficient for further processing is formed, due to the useful signal. The noise component from the mixture being processed is filtered out.

One of the most powerful methods for extracting a signal from a signal-to-noise mixture is synchronous filtering. Its foundations were laid by K.V. Vladimirsky [1] in the early fifties and were further developed in the works of M.I. Finkelshtein [2], A.A. Kharkevich [3] and a number of other authors.

The primary analogue of the proposed Method of bandpass discrete-analog filtering can be considered the Method of synchronous filtering on switched capacitors, the operating principles of which are described in [1, 2, 3].

The essence of this method is as follows.

The filter consists of a set of capacitors C1, C2,..., C _n , which, using switches K ₁ , K ₂ ,..., K _n , are alternately connected over a period T _k to a common integrating resistor R for a time Δt each. A prerequisite for the correct operation of a synchronous filter is the equality of the switching period of the keys T _k with the signal period T _c .

In this operating mode of the device, each capacitor is connected through a resistor R to the signal source in the same phase, that is, each switch always has the same voltage level. If there is noise in the signal, then its period does not coincide with the period of the filter, which means that the voltage for it on each capacitor is 0, therefore, it does not pass through the filter.

The analogue method has a serious drawback, namely the impossibility of using low-pass filters of any order, both passive and active, which limits the selective properties.

The prior art method [4] of active filtering is known, in which the filtering characteristics are changed by switching capacitors. This filter consists of capacitors, resistors, transistor switches and operational amplifiers.

The disadvantage of the known device is its low noise immunity, since the determination of the input signal level at which the filter should change its characteristics is carried out using comparators that are sensitive to external interference signals.

A discrete matched signal filter is known from the prior art, described in application for invention No. 2006141281, class. N03N 17/00, 2008.06.10, including a multiplier, low-pass filter, signal delay line, adder and decision device connected in series. In this case, the output of the adder is connected to the input of the device for removing the constant component of the output signal, forming an autocorrelation function of the pseudonoise signal.

A discrete matched signal filter is known, described in patent for invention No. 2310978, class. N03N 17/00, 2007.11.20, related to devices for optimal asynchronous signal reception, providing increased noise immunity to message transformation and trigger sensitivity. This discrete matched filter also contains a signal delay line, an adder, a multiplier and a decision device, the response threshold of which is selected according to the Neyman-Pearson criterion, based on the requirements for the probability of false alarms and operational reliability.

The disadvantage of all the discrete matched signal filters described above is their low noise immunity, due to the lack of taking into account the influence of constant signal distortions caused by external additive and multiplicative noise, as well as distortions caused by technical features of the implementation of delay lines, especially on capacitive elements. This leads to a decrease in the correlation response of existing discrete matched signal filters and, consequently, to a decrease in noise immunity. In addition, the discrete matched signal filters presented above significantly lose noise immunity when irreducible errors occur in the signal, since their multipliers reflect the impulse characteristics of the expected signal and are fixed, and in the event of distortion of the received signal, the response of such filters is significantly reduced, reducing the noise immunity of the discrete matched filter.

The closest in technical essence to the claimed method and chosen as a prototype is the Bandpass Filtering Method [5], containing several cascade-connected ladder links.

The essence of this prototype method is as follows.

Parallel circuits are included in the transverse branches of the filter, and capacitors are included in the longitudinal branches. By changing the reactive components of the transverse branches of this filter, a frequency-tunable bandpass filter is obtained. Capacitive-coupled filters, which use capacitors connected parallel to an inductor for frequency tuning, are quite technologically advanced, since all tunable capacitors are connected to a common bus, and in addition, such circuits contain a minimum number of inductive elements, which makes it possible to provide highest filter transmission coefficient.

The prototype method has the following disadvantages.

1. Do not use low-pass filters of any order, both passive and active.

2. Do not adjust the filtering frequency by changing the frequency of the clock generator.

3. They use nonlinear elements, which does not allow achieving a high dynamic range, limited only by the dynamic range of the switch keys, which does not make it possible to use it when solving problems of identifying periodic signals against a background of powerful interference.

4. Do not use an output switch.

Elimination of these shortcomings is possible through the use of filtering adjustment based on changes in the frequency of the clock generator, which controls the operation of the input and output switches, connecting the input signal in turn to low-pass filters.

Considering the above, the technical objective of the invention is to filter analog signals while providing:

output signal shape approaching the input signal shape;

bandwidth equal to twice the bandwidth of low-pass filters and independent of their number;

the shape of the frequency response is symmetrical with respect to the tuning frequency and repeats the shape of the frequency response of the low-pass filters used;

using low-pass filters of any order, both passive and active, to increase frequency selectivity;

adjustments by changing the frequency of the clock generator while maintaining unchanged the bandwidth and shape of the frequency response;

high dynamic range to isolate periodic signals from strong background noise.

The technical result of the proposed method consists in filtering (detection) of analog signals against a background of powerful interference.

The operation of the invention is illustrated by the following graphic materials:

Fig. 1 - functional diagram of the Method of bandpass discrete-analog filtering.

Figure 1 shows a functional diagram of the Bandpass Discrete Analog Filtering Method, which includes the following elements:

1. Clock generator.

2. Input analog signal.

3. Input switch.

4. Output switch.

5. Analog output signal.

6. Low pass filter No. 1.

7. Low pass filter No. 2.

8. Low pass filter No. 3.

9. Low pass filter No.N.

To solve the stated problem, a method of bandpass discrete-analog filtering is proposed, which consists in the following:

1. Set the frequency of clock generator 1, which determines the tuning frequency of bandpass discrete analog filtering.

2. The input analog signal 2 is supplied to the input switch 3.

3. Clock pulses are supplied from clock generator 1 to input switch 3 and output switch 4.

4. In the input switch 3, in accordance with the clock pulses, the input signal is switched one by one to all low-pass filters 6-9, and in the general case the number of filters is not limited and is selected based on the required accuracy of reconstructing the signal shape after filtering.

Input switch 3, consisting of N switches, provides alternate connection of an analog signal from its input to the inputs of each of the N low-pass filters.

Unlike the method of synchronous filtering on switched capacitors, where the shape of the frequency response, and therefore the selectivity, is determined by a first-order RC filter, in the method of bandpass discrete-analog filtering, low-pass filters of any order, both passive and active, can be used, which allows you to significantly (almost unlimitedly) increase its frequency selectivity.

5. The output signal in the output switch 4 is removed from the low-pass filters 6-9, synchronously and in phase with the switching in the input switch 3.

In this case, the operation of the input and output switches is synchronous and in-phase, that is, the frequency and phase of switching the keys coincide. As a result, the frequency and phase of the input and output signals at the tuning frequency are the same.

6. Remove the output signal 5 from the output switch 4.

In this case, with an increase in the number of low-pass filters N used, the signal shape at the output switch will approach the shape of the input signal.

The voltage transfer coefficient is defined as K _0LPF /N, where K _0LPF is the transmission coefficient of low-pass filters.

The bandwidth is equal to twice the bandwidth of the low-pass filters used and does not depend on their number N.

The shape of the frequency response is symmetrical with respect to the tuning frequency and follows the shape of the frequency response of the low-pass filters used.

The “industrial applicability” of the method is due to the possibility of implementing it on a domestic element base using both analog and digital switches controlled by clock pulses.

The resulting frequency response corresponds to the response obtained using the Synchronous Filtering Method on switched capacitors and has a comb character, that is, the signal passes at frequencies that are multiples of the main tuning frequency.

However, a comparison of the claimed Bandpass Discrete Analog Filtering Method and the prototype shows that the claimed method differs significantly from the prototype.

General features of the proposed method and prototype:

1. Use alternate connection of the input signal to low-pass filters.

2. A frequency response with a comb character is obtained.

3. Use analog input signal.

Distinctive features of the proposed solution:

1. Use low-pass filters of any order, both passive and active.

2. The filtering frequency is adjusted by changing the frequency of the clock generator, while the bandwidth and shape of the frequency response remain unchanged.

3. They do not use nonlinear elements, which makes it possible to achieve a high dynamic range, limited only by the dynamic range of the switch keys and makes it possible to use it when solving problems of identifying periodic signals against a background of powerful interference.

4. Use an output commutator that operates in-phase and synchronously with the input commutator at the frequency and phase specified by the generator.

Thus, the claimed Method of bandpass discrete analog filtering through the use of an input and output switch consisting of N switches and alternate synchronous and in-phase connection of an analog signal to the inputs of each of the N low-pass filters allows for:

a signal shape that approaches the input signal shape at the output as the number of low-pass filters N increases;

bandwidth equal to twice the bandwidth of low-pass filters and independent of their number;

the shape of the frequency response is symmetrical with respect to the tuning frequency and repeats the shape of the frequency response of the low-pass filters used;

the use of low-pass filters of any order, both passive and active, which in turn allows for a significant (almost unlimited) increase in frequency selectivity;

frequency response of a comb nature (that is, the signal passes at frequencies that are multiples of the main tuning frequency);

restructuring by changing the frequency of the clock generator, while the bandwidth and shape of the frequency response remain unchanged;

high dynamic range, limited, due to the absence of nonlinear elements, only by the dynamic range of the switch keys, which makes it possible to use it when solving problems of identifying periodic signals against a background of powerful interference.

In fig. 2, 3 and 4 show the timing diagrams of the nodes when using the Bandpass Discrete Analog Filtering Method with the number of low-pass filters N=8 at the frequency and phase of the input signal coinciding with the frequency and phase of the generator tuning, and:

fig. 2 - input signal;

fig. 3 - switch key control signals;

fig. 4 - output signal when the frequency and phase of the signal and the switches match.

INFORMATION SOURCES

1. Vladimirsky K.V. About the synchronous filter. JETP, 1951, no. 1.

2. Finkelshtein M.I. Methods for constructing comb filters. Tr./RII GFV, 1964, No. 38.

3. Kharkevich A.A. Fighting interference. Science, 1965

4. Gausi M., Lacker K. Active filters with switchable capacitors: Transl. from English - M.: Radio and communication, 1986. - 168 p.

5. Znamensky A.E., Popov E.S. Tunable electrical filters. Ed. "Communication", M.: 1979

Claims (1)

A method of bandpass discrete-analog filtering, which consists in: setting the frequency of a clock generator that determines the tuning frequency of bandpass discrete-analog filtering; provides an input analog signal; supplying clock pulses from the clock generator to the input switch; the input signal is switched in the input switch in accordance with the clock pulses; an output signal is removed, characterized in that: clock pulses are supplied from the clock generator to the input switch and the output switch; in the input switch, in accordance with the clock pulses, the input signal is switched one by one to all low-pass filters; the output signal in the output switch is removed from the low-pass filters, synchronously and in phase with the switching in the input switch; remove the output signal from the output switch.

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