US2880316A - Active filters - Google Patents

Description

United States Patent ACTIVE FILTERS John M. Wozencraft, Washington, D.C., assignor to the United States of America as represented by the Secretary of the Army Application March 21, 1955, Serial No. 495,833 6 Claims. (Cl. 250-27) (Granted under Title 35, U. S. Code (1952), sec. 26.6)

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

The invention generally relates to a new and useful active filter circuit. More specifically, the present invention is directed to an otherwise passive filter circuit in which the initial circuit conditions are set at a predetermined time and the output is sampled at a later predetermined time whereby the filter operation has desired response characteristics.

One type of use to which the present invention is adapted is the reception of a signal which is made up of a succession of C.W. pulses, any one pulse of which may or may not exist, the pulse repetition rate being constant and the carrier frequency of the pulse being specified. This type of signal occurs, for example, in the operation of radio Teletype systems, and the invention will be described in connection with such a system, although it is to be understood that it is not limited to such use.

By way of explanation and background, the operation of a simple passive filter network consisting of a series resistor and a shunt parallel resonant network will be described. The filter elements are series connected to an input source and have an output circuit connected across the parallel resonant circuit. If this type of filter circuit has a C.W. pulse of alternating current applied to its input circuit the output voltage at the instant of application of the pulse will be substantially equal to zero. The output voltage, however, builds up with the passage of time and by a proper choice of constants this increase in output voltage can be made to bear, within limits, a substantially linear relationship to the elapsed time from the application of the pulse. At the termination of the pulse the output voltage of the filter does not drop immediately to zero. The voltage across the parallel resonant circuit follows a decay characteristic which has a duration comparable to that which would have been required for the build-up of voltage across it to go to completion.

In certain types of systems where the time factor becomes irnportant the passage filter network described above possesses definite limitations. When it is desired, for example, to examine with a single narrow-band filter input C.W. pulses occurring one right after the other, the decay time of the filter prevents itsoutput voltage from presenting a true picture of the voltage resulting from the second pulse due to carryover from the first pulse. This problem has been solved in prior art by using a plurality of passive filter networks with a synchronous switching system, whereby the input voltage during a first discrete time interval is applied to a first passive filter network, the input voltage is then switched to a second passive filter network during the second discrete time interval, and so on. A rotating drum, or other equivalent arrangement, carrying a sufficient number of passive filter networks used in sequence, allowed sufficient time for the output voltage of each filter to decay before input voltage was again applied thereto. In this manner a relatively true picture of voltage conditions over each time interval could be obtained.

The present invention utilizes only a single filterto obtain the same results derived from the multiple filters a fie and synchronous switching system described above. The active filter disclosed is set to an initial circuit condition at the beginning of each discrete time interval. Thefilter then acts as a passive filter whose output voltage buildsup in a linear manner with respect to time and is sampled at the end of that discrete time interval. The filter circuit is then reset to the initial circuit condition at the beginning of the next following time interval.

It is a principal object of the present invention to provide a new and useful active filter circuit capable of evaluating input voltages over discrete and immediately successive intervals of time.

It is a further object of the present invention to provide an active filter circuit whose impulse response is matched to the input signal at certain specified times, that is, provides the maximum signal to noise ratio when the signal is corrupted by white Gaussian noise.

It is a still further object of the invention to provide a filter circuit which functions as an interval integrator to evaluate input voltages thereto over discrete and immediately successive intervals of time.

Other objects, and many attendant advantages of the novel filter system, will become apparent as the same becomes better understood from the following detailed description when considered in conjunction with the accompanying drawings, wherein:

Fig. l is a schematic drawing, partially in block form,

of a system for the reception of pulsed C.W. signals which incorporates a filter system constructed in accordance with the principles of the invention, and

Figs. 2 through 6 are diagrams of voltage waveforms occurring at particular locations in the circuit of Fig. 1,

which are used to explain the operation of the circuit.

Referring first to Fig. l of the drawings, a circuit is shown which incorporates the novel filter system as a part thereof. This circuit comprises a filter 11 having a pair of input terminals 13 and 15, a pair of output terminals 17 and 19 and a control input terminal 21. The input terminal 15 is connected through a resistor 23 to one terminal of a parallel resonant circuit 24 and the output terminal 17. The other terminal of the parallel resonant circuit 24 is connected to the input terminal 15 and the output terminal 19.

The parallel resonant circuit is made up of a condenser 25 and an inductance 27. The circuit 24 is effectively shunted by a resistance element 29 which is shown in dotted form to indicate that it may or may not have a physical existence. It may be representative of the resistive components of the condenser and inductance elements of the circuit or it may actually be a physical resistor. In still other cases where a very selective circuit is desired, the element 29 may actually include a Q multiplier such as the positive feedback, negative resistance circuits, known to the prior art. The requirements for element 29 of the circuit 24 are determined by the nature of the input signal to the filter and will be discussed more fully later.

The circuit 24 is further shunted by a circuit which includes a diode 31 and a resistor 33 connected in series. The junction point between the cathode of the diode 31 and the resistor 33 is connected to one terminal of a art and may, for example, be of the type commonly used in television receivers. The synch separator or filter is connected to the filter input terminals and passes only signals of a particular frequency applied thereto. Pulses of this frequency received at the input are passedto'the automatic frequency and phase control unit where they are compared with the output voltage generated by the local oscillator 45. Any shift in phase between the locally generated signal and the received pulses produces a control voltage in element 43 which is applied to a frequency control element such as a reactance tube contained within the oscillator 45. This circuit operates in a known manner to retain synchronism and phase position of the locally generated and received synchronizing pulses.

The output voltage of the oscillator 45 is fed to a wave shaping generator 49. The wave shaper and harmonic generator circuits operate in a known manner to convert the output voltage of the oscillator to generally squared output trigger voltage of the shape shown in Fig. 3. The wave contains relatively narrow negative-going trigger pulse portions, a, accurately synchronized with the incoming synch pulses. The output voltage of the wave shaping network is applied directly to the coil 56 of a sampling device, in this case a biased relay 55, and through a time delay multivibrator circuit 51 to the control input terminal 21 of the filter network. The delay multivibrator reproduces the input pulse with a time delay small compared to the pulse duration. This time delay is illustrated at C in Fig. 5.

The voltage at the output terminals 17 and 19 of the filter network 11 is applied to a detector circuit 53 which is illustrated as a conventional diode detector. The rectified output of the diode detector 53 is applied at the end of each pulse through the contacts of the biased sampling relay 55 to the input of a holding circuit 58, whose output in turn is applied to the system output terminals 59 and 61. The holding circuit 58 may be a bi-stable multivibrator circuit which operates in response to the sampled detector output voltage in a manner which will be more fully described hereinafter.

The operation of the circuit of Fig. 1 will now be described. Attention is directed to Fig. 2, which illustrates the waveform of a typical voltage input to the filter terminals 13 and 15. The instantaneous voltage magnitude is plotted along a time axis. It will be noted that the voltage waveform is divided into discrete intervals of equal time duration T, commencing at a time T and running through a time T Over certain of the intervals of duration T, e.g., from T to T T to T T to T and T to T the voltage is of the form:

e=E cos 21rft where,

E =the maximum voltage f=the frequency of the carrier voltage, and t=time Over other intervals of duration T, e.g., T to T and T to T the voltage is of the form:

The filter operates to evaluate the voltage input in each of the intervals of duration T separately and distinctly from each other interval. The input to terminals 13 and 15 also includes synchronizing or synch pulses, of a frequency other than that of the carrier 1, which are not illustrated in Fig. 2. These synch pulses, in a manner known to the art, occur at definite time intervals with respect to the time intervals T and may, for example, occur at the start of every sixth time interval. These pulses are taken from the input terminals through the synch filter circuit 41 and are applied to the automatic frequency and phase control unit 43. The local oscillator 45 is free running and normally generates one cycle of output in each period T. The phase and frequency control unit 43 compares the phasing of the output pulses from the oscillator 45 with that of the incoming synch pulses from the filter 41 and develops a control voltage which is applied to a control element such as a reactance tube included in the oscillator 45 to control its output frequency. As is well known in the art, even though a phase comparison is made only at spaced intervals, the phasing of the oscillator output voltage can be accurately controlled in this manner.

The output voltage from the oscillator 45 is fed through the harmonic generators and wave shaping networks 49 to convert it to the waveform illustrated in Fig. 3. The output voltage of the harmonic generators and wave shaping networks has a relatively high positive D.C. level over the greater portion of the time, with negative going pulses, a, occurring at times T T etc., spaced by time intervals T. As may be seen by a comparison of Figs. 2 and 3 which are plotted to the same time scale, the negative pulses, a, occur at the start of each CW pulse interval.

Referring back to Fig. l, the output of the harmonic.

generator and wave shaping network 49 is applied by way of the delay multivibrator and the gaseous discharge tube 35 to the cathode of the switching diode 31. The

output voltage of the delay multivibrator maintains the same waveform as that illustrated in Fig. 3, but is shifted along the time axis by an amount small compared to the pulse duration. When the output of the delay multivibrator is at a high D.C. level the gaseous discharge tube 35 is conducting and the cathode of diode 31 is maintained at a relatively high positive potential. The diode 31 cannot conduct under these conditions. When, however, a negative-going pulse, a, such as that at time T occurs the potential applied to the gaseous discharge tube 35 falls below the critical or threshold level of the tube, shown at b (see Fig. 3), and the discharge tube ceases to conduct, becoming effectively an open circuit. The cathode of diode 31 is no longer maintained at its relatively high potential and the diode becomes conducting, connecting resistor 33 across the parallel resonant circuit in shunt with elfective resistance 29 of the circuit.

The resistor 33 is very small in comparison to the resistor 23, and when it is connected by the diode any voltage existing across the parallel resonant circuit is rapidly discharged through the low resistance path. The filter output voltage is thus quickly reduced to zero. This action is shown in Fig. 4. After the delayed negative-going pulse, a, the diode returns to its non-conducting state and any voltage of the frequency f, for which the filter is designed, subsequently applied to the filter causes the voltage across the parallel resonant circuit 24 to build up with respect to time in the manner shown in Fig. 4. At the time T the negative-going pulse, a, occurs to again reduce the filter output voltage to zero and the filter thereafter responds to any input voltage of frequency f applied thereto in the next succeeding pulse interval T. The filter system is reset at the start of each interval.

In order that the envelope of filter output during each pulse be substantially equal to the integral of the envelope of the voltage input, the bandwidth of the resonant circuit must be very much less than the reciprocal of the pulse duration T. For example, for a pulse duration of 20 milliseconds, a bandwidth less than 10 cycles is sufiiciently small. The exact value of bandwidth is not critical.

The voltage output of the filter is applied to the detector circuit 53 where it is rectified to produce the detector output waveform illustrated in Fig. 5 of the drawing. The carrier voltage is eliminated, the positive D.C. envelope voltage only remaining.

The output of the harmonic generators and wave shaping network 49 is also applied directly to a sampling device illustrated in Fig. 1 as a biased relay. The biased relay includes an energizing coil 56 and a bias coil 57 connected to a source of voltage (not shown). So long as the voltage output of element 49 is maintained at its high positive D.C. level the flux of the coil 56 cancels the flux of the bias coil 57 and the contacts of relay 55 remain open. When a negative-going pulse, a, occurs the flux of the bias coil 57 is no longer cancelled and the armature is actuated by the fiux of the bias coil to close its associated contacts and connect the output of the detector 53 to the input of the holding circuit 58 for the duration of the pulse, a. Since the voltage of the harmonic generators and wave shaping networks 49 is applied directly to the coil 56 of the sampling relay 55 without passing through the delay multivibrator 51, it will be apparent that the contacts of the sampling relay are closed just prior to the resetting of the filter output voltage to zero. In other words, the output voltage of the filter is sampled at the end of one pulse interval and the filter is reset at the beginning of the next pulse interval.

The detector output voltage sampled in the manner described above is applied as the input to the holding circuit 58. The holding circuit may be, as previously mentioned, a bi-stable multivibrator circuit developing an output voltage determined by a pulsed input thereto and holding such an output voltage until the condition of the circuit is altered by a succeeding pulsed input. This action is illustrated in Fig. 6. The filter output is sampled at a time just prior to T At this time the filter and detector outputs are at a high level due to the fact that a C.W. input voltage existed between times T and T The sampled detector output applies a positive pulse to the holding circuit, tripping it to produce a positive output voltage at the instant of sampling. This output voltage is held by the holding circuit until a time just prior to T when the detector output is again sampled. The detector output is at zero level at this time and this output applied to the holding circuit trips it back again to reduce its positive output voltage. The output voltages of the holding circuit thus persist for intervals equal to T, but are shifted along the time axis due to the fact that they are dependent upon sampling which occurs at the termination of the C.W. pulse intervals. The resultant output voltage of the holding circuit is shown in dotted lines with a small square at the head of each line indicating the time of occurrence of the sampling pulse initiating the particular output level.

It will be apparent that each pulse level in the output wave of Fig. 6 is dependent upon, and is determined, by the integral of the envelope of the C.W. input voltage to the filter during a corresponding time interval. The single active filter circuit used responds only to input voltages of the proper frequency occurring within this particular time interval, the periodic resetting action of the filter eliminating the efiect of any voltages outside this interval.

While the novel filter system has been disclosed in connection with a particular form of pulsed C.W. pulse input thereto, it is clear that the invention is not limited to such systems. For example, with aninput composed of discrete pulses of direct current the parallel resonant circuit could be replaced by a condenser and the timing wave for the resetting and sampling circuits generated in any desired fashion so long as they are properly coordinated with the intervals between successive pulse intervals.

Furthermore, while there has been described what are at present deemed to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore the aim of the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In an active filter system adapted to respond to a voltage which is generated in successive discrete intervals of time with a random probability of the existence of voltage in any particular time interval, the combination comprising a pair of filter input terminals, a series impedance and a shunt impedance connected in series across said input terminals, a pair of filter output terminals connected across said shunt impedance, switching means actuated in synchronism with the start of each discrete time interval to momentarily shunt said output terminals and reduce the filter output voltage to zero, and sampling means synchronously actuated at the termination of each discrete time interval to momentarily connect the said filter output terminals to a utilization device.

2. In an active filter system adapted to respond to a voltage which is generated in successive discrete intervals of time with a random probability of the existence of voltage in any particular time interval, the combination comprising a pair of filter input terminals, a series impedance and a shunt impedance connected in series across said input terminals, a pair of filter output terminals connected across said shunt impedance, a source of trigger voltage synchronized with said discrete intervals of time, synchronous switching means responsive to said trigger voltage to momentarily shunt said filter output terminals at the start of each time interval, and a second switching means responsive to said trigger voltage to momentarily connect the output terminals of said filter to a utilization device at the termination of each discrete interval of time.

3. In an active filter system adapted to respond to an alternating voltage having a particular frequency which is generated in successive discrete intervals of time with a random probability of the existence of voltage in any particular time interval, the combination comprising a pair of filter input terminals, a series impedance and a shunt impedance connected in series across said input terminals, a pair of output terminals connected across said shunt impedance, a source of trigger voltage synchronized with said discrete intervals of time, synchronous switching means responsive to said trigger voltage to momentarily shunt said filter output terminals at the start of each time interval, and second switching means responsive to said trigger voltage to momentarily connect the output terminals of said filter to a utilization device at the termination of each discrete interval of time.

4. An active filter system according to claim 3 wherein said shunt impedance is a parallel resonant circuit tuned to the frequency of said alternating voltage.

5. An active filter system comprising a normally passive filter network having input and output terminals, means adapted to connect said input terminals to a source of voltage which is generated in successive discrete intervals of time with a random probability of the existence of voltage in any particular time interval, switching means actuated in synchronism with the start of each discrete time interval to momentarily shunt said output terminals and reduce the filter output voltage to zero, and sampling means synchronously actuated at the termination of each discrete time interval to momentarily connect the said output terminals to a utilization device.

6. An active filter system comprising a normally passive filter network having input and output terminals, means adapted to connect said input terminals to a source of voltage which is generated in equal successive discrete intervals of time with a random probability of the existence of voltage in any particular time interval, a source of trigger voltage synchronized with said discrete intervals of time, synchronous switching means responsive to said trigger voltage to momentarily shunt said output terminals at the start of each time interval, and a second switching means responsive to said trigger voltage to momentarily connect the said output terminals to a utilization device at the termination of each discrete time interval.

References Cited in the file of this patent UNITED STATES PATENTS 2,157,312 Wright May 9, 1939 2,258,877 Barber Oct. 14, 1941 2,293,135 Hallmark Aug. 18, 1942 2,416,308 Grieg Feb. 25, 1947 2,500,536 Goldberg Mar. 14, 1950 2,532,338 Schlesinger Dec. 5, 1950

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