US2106785A - Electric filter - Google Patents

Description

Feb. 1, 1938. H. w. AUGUSTADT 2,105,735

ELECTRIC FILTER Filed May 23, 1936 FIG.

INSERT/0N LOSS 0B N u Q Q Q l l 400 600 800 I000 FREQUENCY-CYCLES PER SECOND INVENTOR H. W AUGUSTADT ATTORNEY Patented Feb. 1, 1938 UNITED STATES ELECTRIC FILTER Herbert W. Augustadt, Valley Stream, N. Y., asslgnor to Bell Telephone Laboratories, Incorporated, New York, New York Application May 23,

7 Claims.

This invention relates to electrical filters and more particularly to filters for smoothing out fluctuations of a direct current.

An object of the invention is to reduce the cost and simplify the construction of current supply filters for amplifiers and other space discharge devices designed for energization from alternating current systems. Another object is to provide an enhanced degree of suppression of ripple currents in particular ranges of frequencies in the output of an alternating current rectifier. A further object is the provision of power supply circuits of aperiodic character which will be free from troublesome transients during operation.

An important application of the invention is found in current supply systems for vacuum tube apparatus, the energizing currents and voltages for which are obtained by rectification of alternating current from a power distribution network. The rectified current in such systems contains various harmonically related alternating components, the most important of which lie in the frequency range from 120 to 1000 cycles per second. To provide adequate suppression of the low frequency ripples with filters of the type heretofore commonly used, it has been necessary to employ choke coils of very large inductance and usually having windings of rather high resistance. Not only are such coils costly to manufacture and uneconomical of space and weight, but they also absorb a considerable portion of the rectified 'voltage and, with their associated condensers, tend to give rise to harmful voltage transients.

These difficulties are avoided in the filters of the invention by the use of a selective network comprising only resistance and capacity elements. The circuit configuration of the network is such as to permit the proportioning of the elements for the substantially complete suppression of currents of any selected frequency and, at the same time, to provide a large degree of attenuation over a relatively wide adjacent frequency range. By appropriate choice of the frequency atwhich complete suppression occurs, adequate attenuation of all of the noise producing ripples of a rectified current can be secured with a relatively small loss of the rectified voltage. If desired, the frequency of complete suppression may be made the same as that of the most troublesome ripple component, but experience has shown that this is not necessary although it is generally desirable to keep this frequency in the range from about 150 to 350 cycles per second when the power is obtained from a 60 cycle system.

N. Y., a corporation of 1936, Serial No. s1,4oo

The nature of the invention will be more fully understood from the following detailed description and from the accompanying drawing, of which:

Fig. 1 shows a filter network in accordance with Fig. 3 illustrates the performance of the filters 10 of the invention.

Referring to Fig. 1, the filter network comprises the circuits included between the terminals t1 t2 and is t4. It consists of two T-networks, one

comprising series resistances R1 and R1 and shunt capacity C1, and the other comprising series capacities C2 and C2 and shunt resistance R2. The two networks are connected to common input terminals t1 and t2 and to common output terminals t3 and t4 thereby providing a pair of parallel transmission paths. Network R1R1C1 is a filter of the low-pass type which transmits direct currents and low frequency alternating currents with relatively small loss and attenuates high frequency currents. filter which provides a high attenuation to low frequency currents. The two, in combination, provide a band elimination filter which may be proportioned to substantially suppress the transmission of alternating currents in any selected frequency range and to effect the complete suppression of a selected frequency in that range.

In the figure, the filter is shown connected in the power supply circuits of an audio-frequency amplifier, the plate current for which is obtained by the rectification of an alternating current.

The amplifier and the rectifier are of known types and will be described only briefly. The rectifier system comprises a full-wave rectifier I0 of the vacuum tube or gas discharge type coupled through supply transformer H to power input terminals l2 and I3. The transformer includes additional secondary windings for cathode heating purposes in accordance with common practice. push-pull amplification with resistance-capacity coupling and is provided with stabilizing feedback circuits in accordance with the principles described by H. S. Black in an article entitled Stabilized feed-back amplifiers, Bell System 50 Technical Journal, vol. XIII, No. 1, January 1934.

The first stage of the amplifier comprises two suppressor grid pentode tubes l4 and M, the signal input to which is supplied through transformer l5. Coupling to the second stage is ef- 5 Network C2C2R2 is a high-pass 25 The amplifier comprises two stages of ing feedback is transmitted from the plates of the second stage tubes through resistances 2i and 2i to cathode resistors 22 and 22' in the input circuits of the first stage, each side of the push-pull system being separately stabilized.

The energizing potentials for the anodes and screens of the vacuum tubes are taken from a potential divider comprising resistances 23 and 24 and condenser 25 connected across the output terminals of the supply filter. The anodes and screens of the second stage are supplied with the full rectified voltage through conductor 26 and the anodes of the first stage with a reduced potential through lead 21 and resistor 28. The screens of the first stage are connected-through lead 29 to the high voltage side of resistor 24. The steady bias of the grids of the first amplifier stage is furnished by the fall of potential in resistor 30 which carries the steady plate current of that stage. A filter'comprising capacities 3i and 32 and resistor 33 prevents plate current fiuctuation from affecting the steady grid bias. A similar arrangement is provided for polarizing the grids of the second stage tubes.

The amplifier described above is representative of a type having high gain and stability suitable for use in sound reproducing systems, and the like, where the highest degree of fidelity is required. In such systems, extraneous noises must be avoided, consequently, the requirements on the current supply filters are of the most severe character.

The filters of the invention operate to remove the noise producing fluctuations of the energizing current through the balancing of the output currents from the two parallel transmission paths provided by the low-pass and the high-pass component networks. At very low frequencies most of the current is transmitted through the lowpass network RiRi'Ci and such part as traverses the high-pass network CzCz'R: arrives at the output terminals in reversed phase. As the frequency increases, the output currents tend to become nearly equal in magnitude without substantial change in their relative phases, with the result that each substantially neutralizes the other in the output. By giving the elements suitable values, complete neutralization of the output currents may be secured at some selected frequency together with a high degree of suppression over a wide range including the selected frequency. Usually, the strongest component of the ripples in the rectified current is that corresponding to the second harmonic of the supply frequency, but it may frequently be the case that certain of the higher harmonic components are about equally effective in producing noise because of greater sensitivity of the car at these frequencies. I have found that the most effective noise reduction is achieved by selecting the frequency of complete suppression some where in the range from about 150 to 350 cycles per second.

The design relationships of the filter elements to effect suppression of a given frequency may be developed as follows:

It will be assumed that the network is of symmetrical configuration such that R1 and R1 are equal and also C: and C2. While other relationships may be given, these do not materially alter the properties of the filter and the symmetrical arrangement is one that will usually be preferred in practice. Under this condition, the symmetrical lattice equivalent to the network is readily obtained by standard methods. This is illustrated in Fig. 2 in which, for simplicity, only one of each of the equal pairs of branches is shown. The line branch impedances each consist of a parallel connection of resistance R1 and capacity C: and the lattice branches each comprise two parallel impedances, one consisting of resistance R1 in series with capacity /2C1 and the other consisting of resistance 2R: in series with capacity C2. The condition for complete suppression of the output current is that the admittance of the line branches of the equivalent lattice should be equal to the admittance of the lattice branches, in which case the circuit becomes a balanced bridge.

In terms of the element values, the foregoing condition is expressed by the equation:

imaginary parts. Upon simplification, these conditions are found to be:

(2) and where (0 denotes 21 times frequency.

If the frequency of complete suppression be assigned, there remain four variables, R1 C1 R2. and C: with only two equations relating to them. This leaves a considerable latitude in the design and permits variations to meet additional practical requirements. For example, since the series resistances determine the amount of the rectified voltage absorbed in the filter, their value may be chosen with respect to the direct current resistance of the load so as to limit the voltage loss to a predetermined value. In addition, one or the other of the capacities C1 and C: may be fixed arbitrarihr at some convenient value available in commercial condensers or the resistance R: may be assigned in accordance with a desired high frequency attenuation. In the latter connection it is to be observed that when the frequency is high enough to make the impedances of the several capacities negligibly small, the filter becomes equivalent to a simple resistance shunt of magnitude equal to the resistance of R1 R1 and R: all in parallel. With the series resistances fixed from other considerations, the high frequency attenuation may be controlled by adjusting the value of the shunt resistance R2.

In an amplifier system corresponding to that illustrated in Fig, 1, the output load on the filter had an impedance of approximately 12,000 ohms and the internal impedance of the rectifier was approximately 300 ohms. A filter suitable for use between these impedances has the following constants:

R1=250 ohms 01:2 microfarads R2=7.8 ohms 01:16 microfarads The insertion loss characteristic for this filter is shown by curve a of Fig. 3. The attenuation peak occurs at 100 cycles per second and the attenuation is well above 45 decibels over the frequency range from 100 to 1000 cycles per second. The total series resistance or 500 ohms is less than the direct current resistance of choke coils commonly used in rectifier filters. The sixteen microiarad capacity oi the condenser C: is well within the range of small sized electrolytic condensers. The loss characteristic of an alternative filter having an attenuation peak at 325 cycles per second is illustrated by curve b of Fig. 3. The constants for this filter were:

R1=100 ohms Ci=4 microfarads Ra=8 ohms Ca=12 microfarads As the frequency of complete suppression is increased, the value of the series resistances may be reduced with a consequent reduction of the voltage loss in the filter. Alternatively. the capacity of condensers C: and C1 may be diminished or the condenser capacity and resistance value may be reduced together. Some sacrifice of attenuation at the lower frequencies results, but this is not serious so long as the suppression frequency does not exceed about 350 cycles per second. It is generally desirable to have the shunt resistance Ra of the high-pass section quite low in order that the attenuation level may be kept high as the frequency increases. I have found values in the range from 5 to ohms to be suitable for this purpose. with values between these limits, the attenuation falls of! very slowly above the suppression range and may easily be maintained at a level of 40 decibels or greater throughout the whole audio-frequency range.

40 What is claimed is:

1. An electrical filter comprising a pair of symmetrical T-networks connected in separate paths between a pair of common input terminals and a pair of common output terminals, one of as said networks consisting 0! two series resistances and a shunt capacity, and the other of said networks consisting of two series capacities and a shunt resistance, said capacities and resistances being proportioned to provide maximum attenugo ation in the filter at a preassigned frequency.

2. An electrical wave filter comprising two T- networks connected in separate paths between a pair of common input terminals and a pair of common output terminals, one of said networks ll having series branches containing only substantially pure resistances and a shunt branch containing only capacitive impedance, and the other of said networks having series branches containing only capacitive impedance and a shunt branch consisting of a resistive impedance. said capacities and resistances being proportioned to provide maximum attenuation in the filter at a preassigned frequency.

3. In combination, a rectifier for alternating currents, a load impedance, and an electric filter network connected between the output terminals of said rectifier and the terminals of said impedance, said network comprising two parallel transmission paths, a low-pass wave filter in one of said paths, and a high-pass wave filter in the other of said paths, said low-pass and high-pass filters being proportioned relatively to each other whereby their output currents neutralize each other at a frequency in the range between the frequencies of the second and sixth harmonics of the alternating current supplied to said rectifier.

4. A system in accordance with claim 3 in which the said low-pass and high-pass filters contain resistance elements and reactance elements of only one kind.

5. In combination, an alternating current rectifier, a load, and an electric filter network connected between the output terminals 01' said rectifier and the terminals of said load, said network comprising two parallel transmission paths. a low-pass filter comprising only capacity and resistance elements connected in one of said paths, and a high-pass filter comprising only capacity and resistance elements connected in the other of said paths, said low-pass and high-pass filters being proportioned relatively to each other whereby their output currents neutralize each other at a frequency in the range between the second and sixth harmonics oi the alternating current supplied to said rectifier.

6. A combination, in accordance with claim 5, in which the said low-pass filter is a T-network of two series resistances and a shunt capacity and the said high-pass filter is a T-network of two series capacities and a shunt resistance.

7. In combination, a source of fluctuating direct current, a load, a low-pass filter comprising only capacity and resistance elements connected between said source and said load, a transmission path paralleling said filter, and a high pass filter comprising only capacity and resistance elements included in said transmission path, the capacities and resistances of said filters being proportioned to provide substantial suppression of current fluctuations o! a preassigned frequency.

T W. AUGUSTADT.

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