DE3920168A1 - FILTER - Google Patents

Abstract

In a filter, filter input signals (u(t)) are modulated on a carrier signal (p(t)) with a carrier frequency (fo) via a multiplier (M1) and taken to an input (E1) of a SAW convolver (C). A sine-wave signal (v(t)) with a predeterminable frequency is also modulated on the carrier signal (p(t)) via a second multiplier (M2) and taken to a second input (E2) of the convolver (C). The output signal (A) of the convolver (C) represents the folding product of the input signals. The sine-wave input signal (v(t)) is taken via a retarder (VZ) to an input (5) of a further multiplier (M4) which also receives at its input the synchronously demodulated output signal (g(t)) of the convolver (C). The filter has a band pass characteristic with a very rapidly variable median frequency in wide ranges.

Description

The invention relates to a filter with a first Multiplier that applies filter input signals to one beatable input and another with a carrier signal has an input to which a carrier frequency can be applied, and with a second multiplier, the one with a predetermined The input signal can be acted upon and a wide input Ren with the carrier signal with the carrier frequency baren input, the multiplier on the output side each connected to a signal input of a convolver are.

Such a filter is from an article by Dr. H.-P. Graßl in "Electronics" 6 / 22.03.1985, pages 61 ff. At the Sem known filter is a surface wave device as Convolver used, which has a signal input that with is connected to an output of a first multiplier. This first multiplier is used on the input side Filter input signals and applied with a carrier signal, whose carrier frequency is chosen so that the frequency of the modulated output signal of the first multiplier inside is half of the working range of the convolver. An entrance to a second multiplier is by one of a code generator Formed code acted as a predetermined input signal and Another input of the second multiplier is also used loaded with the carrier signal; the exit of the second Mul tiplizers is with another signal input of the Convol verse connected. The output signal of the convolver represents the convolution product of the input signals, which is due to the phy the convolver's physical structure has a factor of 2. With the help of the known filter signals are recognized in the filter input signal, which with the  predeterminable input signal (code) match. In which known filter is thus a fix on a whole certain signal to be specified. It is not possible, signals with frequencies from the filter input signals to filter out using within a certain bandwidth a specifiable center frequency. To be a complete Convolution between the filter input signals and the predefinable To ensure input signal, the so-called timing Be condition must be met. This timing condition can be formulate as follows:

For the time interval in which the output signal of the convolver the convolution product of the two input signals of the convolver should correspond to the convolution result Contributing signal components of the input signals completely converted as surface waves under the integration electrode of the convolver.

This condition is only due to time-limited input signals or by repetitively making the predefinable available Input signal (code). Will not be time limited Input signals used, no phase reference of the Output signal of the convolver to the input signals put.

The invention has for its object a filter type mentioned in a simple manner bandpass filter to give properties that make it possible to filter out filter signals, their frequencies within one that can be determined by a specifiable center frequency Frequency range, with a fixed phase relationship between the output signal of the filter and the filter signals should exist.

According to the invention, this object is achieved in that the predefinable input signal is a sinusoidal input signal with a predetermined frequency is that the predetermined Input signal also to an input of a delay  is guided, the output side with an input of a another multiplier is connected, and that another Input of the further multiplier with the synchronous demodu gated output signal of the convolver is applied. Under synchronous demodulation is a demodulation of the output signals of the convolver with a demodulation carrier stand, whose frequency is exactly twice the carrier frequency of Carrier signal corresponds, creating a fixed phase relationship between demodulation carrier and that due to the time compression doubling the effect of the convolver in its carrier frequency pelten carrier signal exists.

The filter according to the invention thus advantageously a phase relationship of the output signal to the input signals without time limits of the input signals and measures to undo the time compri mation of the output signal are necessary. Over other Bandpass filters conventionally constructed with LC elements the filter according to the invention is characterized by an exception neatly fast - almost in real time - and in one wide adjustment range variable center frequency. The Center frequency is namely exclusively from the specifiable Frequency of the sinusoidal input signal is determined. In order to there are no temperature-related drifts in the midrange quenz. Because the filter function of the physical structure of the convolver used is determined according to the Invention appropriate filters for all center frequencies the same filter function on; this means that the filter has the advantage of a con constant bandwidth for all center frequencies. A Another advantage is the linear phase of the filter.

An advantageous development of the filter according to the invention provides that the further multiplier on the output side Low pass filter is subordinate. This will remove the filter advantageously clears the input signal from disturbance variables, the multiplication in the further multiplier  arise and possibly the subsequent signal can affect processing.

The finite carrier suppression of the first and the second Multipliers and self-convolution effects of the convolver can lead to a constant DC voltage that the output signal of the convolver is superimposed. An influence of this DC voltage on the output signal of the invention Filters can be easily excluded by the fact that between the further input of the further multiplier and an AC coupling element at the output of the convolver is arranged.

It is particularly advantageous if the further multiplier is formed by a ring mixer. A ring mixer works particularly favorable due to its high dynamics and mixed steepness on the quality of the filter according to the invention.

In the event that the filter input to be examined signals are in a frequency range within the Working frequency range (input working band) of the used Convolvers lies on the first and the second multi multiplier and on the synchronous demodulation of the output signals of the convolver are waived, which is in view to those by the first and second multipliers and the synchronous demodulation advantageously caused interference affects.

To explain the invention is in the single figure Drawing a signal diagram of an embodiment of the Filters according to the invention shown in a schematic representation.

The signal diagram of a filter F according to the figure shows filter input signals u (t) which are fed to an input 1 of a multiplier M 1 . Another input 2 of the multiplier M 1 is charged with a carrier signal p (t) which has a carrier frequency f _o of 350 MHz. The output of the multiplier M 1 is fed to a signal input E 1 of a convolver C. A predefinable sinusoidal input signal v (t) with a variable frequency f _v is fed to an input 3 of a second multiplier M 2 . Another input 4 of the second multiplier M 2 is supplied with the carrier signal p (t); on the output side, the second multiplier M 2 is routed to a further signal input E 2 of the convolver C. An output A of the convolver C is passed through a bandpass filter BP with a center frequency f _m of 700 MHz and a bandwidth B of 200 MHz to an input of a synchronous demodulation stage DEM, which has a demodulation carrier, the frequency of which corresponds to twice the carrier frequency f _o . A synchronously demodulated convolver output signal g (t) occurs at the output of the demodulation stage DEM. An input 5 of a further multiplier M 4 is acted upon by the sinusoidal input signal v (t) after it has passed through a delay element VZ. The electrical input signals applied to the signal inputs E 1 and E 2 are converted into surface acoustic waves by interdigital converters of the convolver C. The transit time T of the surface waves is calculated from the quotient of the spatial distance between its interdigital transducers and the speed of propagation v _{o of} the surface waves on the convolver substrate. In an analogous manner, an integration time T _{i of} the convolver C is calculated from the quotient of the length of the integration electrode of the convolver C, which taps off polarization charges occurring on the convolver substrate, and the propagation speed v _{o of} the surface waves. The surface acoustic waves have a much lower propagation speed than electromagnetic waves. This results in a time delay of the output signal of the Con volvers C compared to its input signals. The delay time Ver of the delay element VZ is set so that it corresponds to half the running time T of the convolver C. The same time delay is thus impressed on the sinusoidal, predefinable input signal v (t) fed to the input 5 of the further multiplier M 4 , so that no phase shift of the signals to be multiplied occurs in the multiplier M 4 .

In the exemplary embodiment shown, an output of the demodulation stage DEM is connected to a further input 6 of the further multiplier M 4 via an AC coupling element C 1 formed by a capacitor. An output signal b '(t) of the multiplier M 4 reaches an input of a low-pass filter TP which has a cut-off frequency f _g . A filter output signal b (t) of the filter F can be tapped at the output of the low-pass filter TP.

The signal processing of the filter F is brief below explained, with a simplified view only for positive Loop frequencies made and the convolver-related runtime delay of the output signal g (t) by an additive term of T / 2 is taken into account in the argument of the output signal.

The output signal

of an ideal convolver to:

with: k _o Convolver constant.

The convolver constant k _o describes the convolution efficiency of the convolver, the amplification of the amplifiers used in the filter and the influence of the multipliers. By transforming and standardizing the convolver constants k _o to 1 and taking into account the uniformity function e (t) (a more detailed explanation of the uniformity function of a convolver is, for example, the essay "Small-Aperture Focusing Chirp Transducers vs. Diffraction-Compensated Beam Compressors in Elastic SAW-Convolvers "by H.-P. Graßl and H. Engan in" IEEE Transactions on Sonics and Ultrasonics ", Vol. SU-32, No. 5 Sept. 1985) of the convolver results with the sinusoidal predeterminable input signal

u (t): filter input signals,
ω _v : angular frequency of the input signal v (t).

By inserting the Fourier transformers for u (t) in Equation Gl-1 results in:

ψ = ω + ω₁;
Δ = ω - ω₁.

The following uniformity function applies to an ideal convolver:

that is, the ideal convolver has a square-wave pulse of integration duration T _i as the output signal if one input is subjected to a constant signal of 1 and the other input is subjected to a Dirac pulse.

The output signal is obtained by multiplying in the multiplier M 4 by the predeterminable signal v (t) = cos (ω _v · t), which experiences a delay of T / 2 via the delay element VZ

The low-pass filtering with the low-pass filter TP suppresses, with a suitable choice of its cut-off frequency f _g, interfering signal components with an angular frequency of 2ω _v . The output signal filtered via the low-pass filter TP

of the filter F then has the Shape:

If ω _v is much larger than the reciprocal of T _i , the function F (jψ) is approximately zero. In practice, this condition is usually sufficiently met. This results in the output signal

of the filter F shown in the Shape:

The filter function F _BP (jω) of the filter F shown is therefore determined too

This function can be divided into an all-pass component Ph (jω) and the transfer function F _BPÄ (jω):

Thus, the filter F can be described by a bandpass filter with the filter function F _BPÄ (jω) and an all-pass component Ph (jω). The linear phase of the all-pass component can be seen from equation Gl-11. The transfer function of the filter F is a si function, the center frequency of which is determined by the angular frequency ω _{v of} the sinusoidal input signal v (t).

The demodulation of the output signal of the convolver C in the Demodulation level DEM is not necessary in every case but depends on the frequency band in which the Output signals b (t) of the filter F for further processing be desired. In general, the synchronous demodule takes place lation in the demodulation stage DEM so that the output signal b (t) of the filter F lies in the so-called baseband in which the filter input signals u (t) are also located.

By multiplying the sinusoidal definable one output signal v (t) with the demodulated output signal g (t) of Convolvers C is a phase relationship between the input signals and the output signal b (t) of the filter.

The AC voltage coupling element C 1 keeps away from the downstream multiplier M 4 DC components that can occur due to self-convolution effects of the convolver C and the finite carrier suppression of the multipliers M 1 and M 2 .

The cut-off frequency f _{g of} the low-pass filter TP is chosen in the embodiment example to f _g = 2.5 f _v . In order to ensure a minimum attenuation of interference spectra, the frequency f _{v is} limited in practice to a minimum value of approximately f _vmin = 4 MHz. The maximum center frequency f _vmax is determined by the band limitation of the convolver C and is approximately 50 MHz in the exemplary embodiment. For filter input signals u (t), the frequencies of which are in the range from 0 to 50 MHz, the center frequencies of the filter F can be set via f _v in the range from 4 to 50 MHz, the multiplication by the carrier frequency f _{o of} the carrier signal p (t) of 350 MHz results in frequencies of approximately 354 to 400 MHz; these frequencies lie in the convolver used in the initial work band. For filter input signals u (t), the frequencies of which are above 50 MHz, a band limitation to 50 MHz is to be provided and, in addition, the carrier frequency f _{o of} the carrier signal p (t) should be varied such that one sideband of the multiplication products of u ( t) or v (t) with the carrier signal p (t) lies in the working band of the convolver C. Since the frequency f _{v of} the predeterminable input signal v (t) can be freely selected in wide ranges _, filter input signals u (t) of a wide frequency range can be filtered by correspondingly adapting the carrier frequency f _o .

Claims (5)

1. Filter

• - With a first multiplier (M 1 ) which has an input ( 1 ) to which filter input signals (u (t)) can be applied and a further input ( 2 ) which can be applied to a carrier signal (p (t)) with a carrier frequency (f _o ) , and
• - With a second multiplier (M 2 ) which has an input ( 3 ) to which a predefinable input signal (v (t)) can be applied and a further input ( 4 ) which can be applied to the carrier signal (p (t)) by the carrier frequency (f _o ) ) having,
• the multipliers (M 1 , M 2 ) are each connected on the output side to a signal input (E 1 , E 2 ) of a convolver (C),

characterized in that the predetermined input signal (v (t)) is a sinusoidal input signal with a predetermined frequency (f _v ), that the predetermined input signal (v (t)) is also routed to an input of a delay element (VZ) which is on the output side is connected to an input ( 5 ) of a further multiplier (M 4 ), and that a further input ( 6 ) of the further multiplier (M 4 ) with the synchronously demodulated output signal of the convolver (C) is applied. 2. Filter according to claim 1, characterized in that a further low-pass filter (TP) is arranged downstream of the further multiplier (M 4 ). 3. Filter according to claim 1 or 2, characterized in that an AC voltage coupling element (C 1 ) is arranged between the further input ( 6 ) of the further multiplier (M 4 ) and the output of the convolver (C). 4. Bandpass filter according to one of the preceding claims, characterized in that the further multiplier (M 4 ) is formed by a ring mixer.