DE2659967C2 - High-pass filter that can be commutated in multiplex operation - Google Patents

Description

marked by

- a shunt resistor connected between the common output line (312) and earth (315 ').

2. Hoci'-pass filter according to claim 1, characterized in that the capacitors (123 ') all have the same capacitance.

3. High-pass filter according to claim 1 or 2, characterized by a dwell setting circuit (360) for setting the ratio of dwell time (D), ie the time in which a certain switching element (316) is closed, to channel switch-off period (T), ie the time in which this switching element is open

30th

The invention relates to a high-pass filter according to the Generic term of the patent claim L

Such high-pass filters for multiplex operation are preferably used in the seismic exploration of the Structure of underground layers or used in the detection of schools of fish or ships. It is with a seismic sound source such. B. an air cannon or a gas explosion device an acoustic signal generated and reflected on or near the surface of the earth or under water Signals are received by sensors, which are usually equidistantly spaced along a cable are distributed, e.g. a length of 30 to 100m having. The output of each sensor or several sensors is connected to a central signal processing device tied together. The individual sensors are controlled one after the other in multiplex mode. Each input connection to the multiplexer is connected in such a way that it receives the output signal of an analog channel, in the example picks up the signal from a seismic sensor. An output trunk from the Sensors is connected to an amplifier. It is often desirable to use the DC component of the To eliminate analog signal, for which purpose the high-pass filter is used. Such a high-pass filter for multiplex operation has a series capacitance and a shunt resistance and requires the multiplexer trunk terminated with a high impedance load. According to previous practice, the Multiple line connected to a high input impedance amplifier.

The structure of such a known high-pass filter for multiplex operation and its insertion into a simplified analog-digital conversion system is illustrated below with reference to FIGS. 1 explained in more detail. A multiplexer 122 with a plurality of switching elements 316 is connected to a corresponding plurality of input channels C1, CZ, Cn . To the input channels are z. B. seismic sensors 11 connected. The input channels are all connected to a multiplexer output trunk 312 via coupling capacitors 123, resistors 315, and switching elements 316. Each resistor 315, in conjunction with its capacitor 123, represents a high-pass AC filter for its channel. The switching elements 316 are very fast-responding field-effect transistor switches.

The output of multiplexer trunk 312 is connected to the non-inverting input of a buffer amplifier 320 with gain 1 and high input impedance. The output of the buffer amplifier 320 is connected to a test and hold circuit 322 which has a bypass capacitor 324 and a series switch 326. The output of the Circuit 322 is coupled to a binary variable gain amplifier 124

The state controller, which is the control network 132, is connected to the switches 316, 326 by a control trunk 341 which has a plurality of control lines. Controller 132 sequentially switches multiplexer 122 to connect signal input channels Q-C _n to output trunk 312 in sequence during a multiplexer scan cycle in the manner described above. The control device 132 also controls the mode of operation of the binary amplifier system 124 via the control lines 350.

The amplifier system 124 is with the analog-to-digital converter 126, which converts the tested analog signals into corresponding digital signals. Other networks which do not contribute to the essence of the invention are shown in Figs. 1, 2 and 3 of the has been omitted for a better overview.

In the circuit of FIG. 1, each /? C high pass filter has a series cord set 123 and a shunt resistor 315 connected to ground. on. For use in seismics, each high-pass filter has a low cut-off frequency ft, on the order of 1 Hz, although other cut-off frequencies can also be provided.

The capacitance C is related to the resistance R and the cutoff frequency F ₀ by the following equation:

c = \ / [2RnF ₀ )

for practical design reasons, R must be relatively small, on the order of 10,000 ohms. Thus, for k = 1 Mz, the value of C = 16 microfarads. Such large capacitors are relatively expensive and bulky. The high-pass toilet filters according to FIG. 1 are therefore not only expensive, but also stand in the way of miniaturization.

The invention is based on the object of specifying a high-pass filter of the type mentioned above, which is particularly simple and in particular with an unchanged cut-off frequency with smaller capacitors gets by.

According to the invention, this object is achieved by the subject matter of the main claim. It will not as before, a resistor is used in each channel in front of the switching elements, but only one only shunt resistance between the common output line of the switching elements and earth used. The structure according to the invention does not lead

only to a considerable reduction in the number of resistors to be used, but also to the usability of capacitors with considerably lower capacitance than before. It has been found that the capacitance value given above for a capacitor in a circuit according to the invention still has to be multiplied by the factor D / T , the dwell time D being the time in which a certain switching element is closed and the channel switch-off time T is the time in which this switching element is open. If, for example, sixteen channels are used and each switching element is only closed for ¹ A _{6 of} a total time and ^{open for 15} A _{6 of} the total time, then capacitors can be used whose capacitance is only one fifteenth of the capacitance of the capacitors previously used.It is obvious that this is the case leads to a considerable reduction in size and cheaper than previously known high-pass filters

According to an advantageous development of the invention is provided to use a Verweileinstellschaltung, by means of which the residence time D is variable, since the residence time D exerts a EinflutJ on the effective capacitance of the capacitors used, can be achieved by varying the residence time D in a simple manner, the Change the cutoff frequency of the high-pass filter.

The invention is explained in more detail below with reference to exemplary embodiments illustrated by figures explained it shows

F i g. 1 is a schematic block diagram of a known Filter circuit for an analog-digital signal processing system;

F i g. 2 shows a schematic representation of an embodiment of a high-pass filter according to the invention for Multiplex operation;

3 is a schematic block diagram of a system according to FIG. 1, but with the filter according to FIG. 3 and with a dwell setting circuit;

F i g. 4 shows a simplified block diagram of a further known system;

5 shows a simplified schematic block diagram of a system according to FIG. 4, which is modified to adapt to the invention by a Eliminate DC deviation; and

6 shows a simplified schematic block diagram of a system according to FIG the input channels.

FIG. 2 shows the possibility of using a single common shunt resistor 315 'on the output line of multiplexer 122 instead of an individual shunt resistor 313 for each channel. As the multiplexer 122 is advanced through its channel selection cycle, ie, sample cycle, capacitor 123 ', which is in series with each channel selected by the multiplexer, e.g. B. Q, Ci etc., in conjunction with the common shunt resistor 315 'represents the desired KC filter for this channel.

When the circuit of Fig. 2 was tested, posed however, experimentally (which was later proved theoretically) it is found that the capacitance C determines is through:

That is, the length of time that each switch 316 is open. Dwell time D is a function of the number of analog channels and the total time required to scan all of those channels. For a multiplexer with sixteen channels and a sampling duration of 1000 microseconds, the maximum dwell time is D & 1 microseconds per channel and the channel switch-off duration is 93 / .5 microseconds.

The dwell time D can be changed with the aid of the dwell setting circuit 360 according to FIG. The dwell adjustment circuit 360 can be a one-shot circuit, such as National Semiconductor DM 74 121. A one-shot circuit is a circuit or device that can be used to modify the duration of a control pulse by stretching or shortening the pulse width. The pulse width adjustment is achieved with the aid of a resistance-capacitance feedback circuit of a known type

The desired dwell time remains set to the same value via a multiplexer sampling cycle
Equation (2) can be written as follows:

JtRCJ] - [DfT]

(3)

= [1 / (2JtRf ₀ )] - [D / T]

where D is the dwell time, ie the length of time each switch 316 is closed, and T the At constant values for R, C, T , the cutoff frequency of the filter can be changed by changing the dwell time. In the numerical example given above, the maximum dwell time for a sampling period of t milliseconds is 62 microseconds. The shortest dwell time is determined by the detection time of the test and hold circuit 322 '. For example, the detection time is 8 microseconds. The dwell time setting circuit 160 must be programmed so that a selectable dwell time range is obtained, for example from a minimum dwell time of 8 microseconds to a maximum dwell time which is determined by the sampling time, which is 623 microseconds in the above example. Thus, the cutoff frequency of the filter can be adjusted to a desired value over a range of almost 8: 1.
The manner in which the improved, multiplexer-commutated / C filters are used in the seismic data processing device according to the present invention is shown in FIG. 3 shown. Since the F i g. 1 and 3 are very similar, the description of FIG. 1 is limited. 3 only on the differences between the

so both representations.

The most important feature of exploiting the channel selection cycle of the multiplexer used in the multi-channel seismic data logger is the combination with a common shunt resistor applied to the output of the multiplexer. Not only has the number of shunt resistors been reduced, but the size of the capacitors 123 'required has also been reduced by a factor of 15: 1 in the case of sixteen input channels can be set to almost 8: 1.
F i g. 3 shows that it is convenient to use a low impedance operational amplifier 320 '. Series resistor 31V, which corresponds to resistor 140 in Figure 6, is the input resistance to the inverting input of amplifier 320 '. Not that

inverting input of amplifier 320 'is connected to ground. Because both inputs have the same potential must, the connection point 333 is practically at ground potential. Therefore the filger according to Fig. 3 is that Equivalent to the filter according to FIG. 2.

Accordingly, the operational amplifier 320 'with a low input impedance in which the common Resistor 315 'of the filter is used as the input resistor, the preferred filter circuit. the Use of operational amplifier 320 'eliminates the major design limitations imposed by the amplifier 320 relatively high input impedance (Fig. 1) are given.

It was found that the use of a common resistor 315 '. which is successively brought by switch 316 with capacitors 123 connected, results in a considerable saving in the number of required resistors and in which of the resistors and the capacitors besns ⁿ ?? n smell space. These advantages are of particular importance in miniaturized systems for purposes of the present invention.

For a better understanding of the feature to be explained, reference is made to FIG. 4, which is a simplified schematic representation of the arrangement according to FIG. 1 is. Thus, FIG. 1 - as mentioned - the multiplexer 122 with a plurality of recording channels C), Cj ... C _n . all of which are connected to multiplexer trunk 312 through DC coupling capacitor 123, resistors 315 and switches 316. The signal reception channels are coupled to seismic sensors 11. Each resistor in conjunction with its capacitor 123 forms a high pass /? C filter for its channel. The filter removes the DC components of the incoming analog signals. The switches 316 are high speed FET switches of known construction.

The output from the multiplexer 122 on the multiple line 312 is connected to a signal conditioning and amplifier network (SCAN) 3011, which * o schematically represents the circuit arrangement shown in FIG. 3 and thus, for example, the amplifier 320 'and the test and hold circuit 322' having. This network conditions and amplifies the tested analog signals before they are sent to an evaluation device 3033 * '> . The network 3011 can have a buffer amplifier with a fixed gain factor, a test and hold circuit and other circuits for preconditioning a signal sample prior to transmission to the evaluation device 3033, the evaluation device in practice being the amplification system 124 and the analog-to-digital converter 126 of the above Description is Internally generated erroneous noises in multiplexer 122 appear on each capacitor 123 as interference voltage K ₁ , which can have one of the two polarities with respect to ground, and the internally generated interference noise _{voltage V 2} on network 3011 occurs at the output and is generalized to the voltage Vi added to it If the voltage amplitudes of the incoming desired signals can be compared with the interference voltages K ₁ and K ₂ , the noise-signal ratio becomes too high. In seismics there must be a trace that. is too noisy, often erased, thereby losing valuable seismic information

The stray voltages Vi and V ₂ have various causes, z. E.g. thermoelectric influences, Peltier effects, displacement drift of the amplifier stages in the network 3011 etc. Each of the FET switches 316 which is used in the multiplexer 122 induces the noise voltage V \ on a capacitor 123. It is known that FET switches are used as semiconductor switches Feed-through capacitors and bleeder resistors between their control and switching elements (not shown). In addition, each control element in each FET switch is activated by a relatively large control pulse, which is provided by the network 3011, to a control line 3021. This control pulse reaches each DC coupling capacitor 123 via the feed-through capacitor and the bleeder resistor of the associated FET switch 316.

The capacitors 123 are identical, as are the feed-through capacitors and bleeder resistors the FET switches are also identical to each other. That is why the stray voltages are on the capacitors 123 uniform.

According to FIG. 5 is, according to an improved modification, one of the input channels to the multiplexer

122 grounded. This channel, referred to as the CO or "test" channel, is similar in all respects to the signal receiving multiplexer input channels Ci-C _n -I, where π is the number of channels. except that the input connection to the channel CO is connected to ground, so that no external signals can be given to the test channel CO. The test channel has a capacitor 123 'which has the same capacitance value as the capacitors

123 owns. A multiplexer switch 3025 is assigned to the capacitor 123 '.

The test and hold (SH) network 3024 is between network 3011 and evaluation device 3033 turned on, the series capacitor 3025 and the normally open shunt FET switch 3026, which is connected to earth.

The output from multiplexer 122 on trunk 312 is to network 3011 connected, which typically includes amplifier 320 ' with a gain factor of ONE, the output of which is connected to the signal test and hold circuit 3024 (Fig. 3) connected is.

The condition monitoring unit 132 controls all the operating networks of the switching arrangement according to Figure 5 via the control lines 3021. For example, switches the control device 132 sequentially multiplex channels Co-C _n - / a Multiplexerabtastzyklus and controls the Signalauswertvorrichtung 3033 having the amplification system 124, which with the Analog-to-digital converter 126 is coupled in the manner best described above. The converter 126 converts the multiplexed analog signals from the channels QC _n -I into corresponding digital numbers. Other networks, which are not important in connection with the feature discussed here, have been omitted from FIG. 5 for the sake of clarity.

In the operation of the system according to FIG. 5, at the beginning of a multiplexer sampling cycle, the multiplexer 122 is reset to the test channel CO , and at this point in time the switch 3023 and a further switch 3026 are closed. The stray voltage Vi that is generated across the capacitor 123 'is algebraically added to the stray voltage V ₂ that is generated on the network 3011. The voltage Vi + K ₂ combined in this way is transmitted to a capacitor 3025 so that a test voltage V is generated there, which when the

Switch 3026 has the opposite polarity to voltage Vi + Vi . Whenever the multiplexer is reset to the test channel CO , the test and hold network 3024 receives the stray voltages from the test channel CO and from the network 3011. After this, the switches 3023 and 3026 remain open, while the multiplexer 122 sequentially scans the channels C 1, C 1... C _n - ι receiving the active signals.

If the capacitors 123 and 123 'have the same capacitance values, and if the overall gain of the network 3011 remains constant, then V = V, + V ₂ . When the multiplexer 122 scans the channels d - C _n - ι , each channel at the output of the network 3011 has a signal voltage V _c " as well as the scattered noise voltage Vi + Vj, which is added successively and algebraically to the test voltage V, which is generated by the capacitor 3025 is held according to the following equation:

V "+ V ₁ + V ₂ + (- V) = V _c "

so that every stray voltage Vi + V ₂ from every channel receiving signals is largely deleted by the test voltage V.

F i g. 6 shows the principle of extinguishing j v> r Sireuspai nungen as it is the circuit according to FIG. i be abandoned. The single resistor 140 is used in place of the individual resistors 315 in FIG. 5 used. For reasons given above, the capacitors 123 have a substantially smaller Kapazitiitsweri than the corresponding capacitors of Fig. ^R Accordingly, generates a given leakage current of a switch 316, for example, a greater voltage on the capacitors 123 to F i g. 6 than on the corresponding larger capacitors in FIG. 5.

For this purpose 3 sheets of drawings

Claims (1)

Patent claims: 1. High-pass filter for multiplex operation with - A plurality of inputs (11) for analog signals, - A corresponding number of with a common output line (312) Switching elements (316) connected to condensers (123 ') and - a control device (132) controlling the switching elements