US2205075A - Variable width band-pass filter - Google Patents

Description

K. WILHELM 2,205,075

VARIABLE WIDTH BAND-PASS FILTER Filed July 10, 1937 INVENTOR KARL W/LHELM BY 7KZW ATTORNEY Patented June 18, 1940 UNITED STATES VARIABLE WIDTH BAND-PASS FILTER Karl Wilhelm, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H.,-Berlin, Germany, a

Germany corporation of Application July 10, 1937, Serial No. 152,926 In Germany July 10, 1936 3 Claims.

The present invention relates to band pass filters of adjustable band width.

The object of the present invention is to provide means for varying the band width ofthe filter without incidentally and undesirably varying the mid frequency of the transmitted band.

It is known in the art to vary the band width of a filter made up of coupled resonant circuits by varying the coupling between said circuits. It is further known to vary a coupling between two resonant circuits by varying an impedance in a link circuit utilized for coupling between two such circuits. This method is desirable in that there is no necessity for mounting one or more coils movable for the purpose of coupling variation, but this method as in the past suffered from the drawback that the tuning of the filter, or more precisely, the position of the mid frequency of the transmitted band is thereby altered. It is believed that the reason for this effect upon tuning is that the effective inductance in each of the resonant circuits coupled by a link circuit depends upon the impedance of the link circuit and hence in variation in the link circuit impedance effects the resonant frequency of the circuits coupled together'by the link. In accordance with the invention two pairs of tuned circuits are utilized, each pair being variably coupled by means of a variable link circuit, but the link circuits are so composed that the detuning effect of varying one link circuit is in the opposite direction from the detuning effect resulting from variation of the other link circuit. As a consequence the overall band-pass characteristic is merely spread out in both directions equally as the band width is increased. When the band width is considerably increased three maxima and two minima of transmission appear and in accordance with another object of the invention the transmission is made more uniform over a wide frequency range by employing still another filter arrangement having two maxima of transmission coinciding with the aforementioned minima so that signals traversing all three filter components successively will be substantially uniformly transmitted over a relatively high band.

In the accompanying drawing Fig. 1 illustrates the detuning effect in a single filter stage, Fig. 2 shows a two stage filter in which the two stages have oppositely directed detunling effects and in which the band width controls are interconnected for uni-control. Fig. 3 represents transmission characteristics of the composite filter of Fig. 2, and Fig. 4 represents the transmission characteristics of a well known type of coupled circuit filter adapted to be employed in connection with a filter of the type shown in Fig. 2 for the purpose of improving uniformity of signal transmission over a wide band.

Referring specifically to Fig. 2 the lefthand portion thereof shows a filter comprising a pair of resonant circuits coupled by mutual inductance M to an extent too small to produce bandpass characteristics and additionally coupled in the same sense by way of a link circuit including a variable series resistance B. When R is very large the transmission through the filter is sharply selective as shown in curve I of Fig. 1. The mid frequency of this curve is indicated by the letter a. When however R is very small the effective coupling between the resonant circuits is sufiiciently close to produce the familiar double humped response curve as shown in curve 4 of Fig. l. The reaction of the link circuit upon the associated resonant circuit is such as to reduce the effective inductance in each of the resonant circuits thereby causing the mid frequency of the transmitted band to increase from point a of Fig. 1 to point 0. This effect could be 'compensated by a variation in the electrical constants of the resonant circuits through a uni-control of such constants with the variation of R. In the preferred form of the invention however a second filter is utilized in cascade with the first filter, as shown in the right hand side of Fig. 2. In this second filter there is a small capacity coupling c between two resonant circuits and an auxiliary capacity coupling in the form of a link circuit'including a variable resistance W. The coupling 0 is a weak coupling adapted to produce a sharp selectivity characteristic similar to curve I of Fig. 1 when the resistance W is so large that the link coupling is ineffective. On the other hand when W is made very small so that the circuits are sufficiently tightly coupled to produce a double humped characteristic the effect of the link circuit upon the associated resonant circuits is to reduce their natural frequencies so that the transmission characteristic is the mirror image of curve 4 of Fig. 1 with respect to point (1.. Hence if resistance R and W are uni-controlled the overall transmission characteristic of Fig. 2 Varies from curve I of Fig. 3 to curve 4 of Fig. 3 as the resistances are simultaneously reduced from a large value to a very small value. It is seen that there is no shift of the mid frequency of the transmitted band in this case.

It will be observed that when a wide band is to be transmitted curve 4 of Fig. 3 departs from the ideal band pass characteristic in that there are two dips in transmission within the band. In order to further improve the transmission characteristic, in accordance with the invention, a third filter element of any suitable design having the transmission characteristic shown in Fig. 4 may be employed in cascade with the two elements shown in Fig. 2. The two transmission maxima of this third filter are chosen to coincide with the two transmission minima or dips of curve 4 of Fig. 3 so that the overall transmission characteristic of the entire system is rendered substantially uniform.

While I have indicated an arrangement for providing a variable band width with incidental detuning, it will be understood that any other type of filter having incidental detuning may be equally well employed. I therefore do not wish to be limited except in accordance with the following claims.

What I claim is:

1. A variable band Width filter comprising two portions each including pair of resonant coupled circuits, resistive means for varying the coupling between the pair of resonant circuits of one portion and incidentally shifting in one direction the mid frequency of the transmitted band of said filter portion, means for varying the coupling between the pair of resonant circuits of the second portion and incidentally shifting in the opposite direction the mid frequency of the transmitted band of said second filter portion, and means for simultaneously varying both of said means for varying said couplings in such a relation as to maintain constant the mid frequency of the transmitted band of said variable band width filter.

2. In combination, a variable band width filter comprising two portions, means for varying the coupling between elements of one portion and incidentally shifting in one direction the mid frequency of the transmitted band of said filter portions, means for varying the coupling between elements of the second portion and incidentally shifting in the opposite direction the mid frequency of the transmitted band of said second filter portion, means for simultaneously varying both of said varying coupling means in such a relation as to maintain constant the mid frequency of the transmitted band, and a second filter including two maxima of response, both of said filters being arranged for successive transfer of signals therethrough, the said maxima of response of Said second filter being arranged to coincide with the minima of response of said first named filter whereby to produce substantially uniform transmission of signals over the maximum band width of said first named filter.

3. In a variable width band-pass filter, a first pair of tuned circuits resonant to the same frequency, a link circuit including a resistance for coupling said tuned circuits, the variation of said resistance from high to low values resulting in respectively a highly selective response and a wide band response with incidental shift in one direction of the mid frequency of the transmitted frequency band, a second pair of tuned circuits resonant to the same frequency as the first mentioned circuits, means including a resistance for coupling said second pair of tuned circuits, the variation of said resistance from high to low values resulting in respectively a highly selective response and a wide band response with incidental shift of the mid frequency of the transmitted band in a direction opposite to that of the frequency shift of the first pair of tuned circuits, and means for simultaneously varying said resistances.

KARL WILHELM.

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