GB2210749A - Switching lowpass filter cut off frequencies to enable and disable selective passband operation - Google Patents

Abstract

A switchable passband enabling and disabling method and lowpass filter apparatus in which minimal number of network impedance components may be changed to enable a selected passband of fL to fH to be passed in one mode or position and to be disabled in another. In accomplishing this, the circuit impedance changes from a value fH/fL times the normal design filter characteristic impedance Ro to a circuit impedance value <IMAGE> with a cutoff frequency reduced to fL. <IMAGE>

Description

METHOD OF AND APPARATUS FOR SWITCHING LOWPASS FILTER CUT OFF FREQUENCIES TO ENABLE AND DISABLE SELECTIVE PASSBAND OPERATION The present invention relates to the switch-tuning of filter networks and the like as for such purposes as rendering television and other communications receiving systems enabled for reception or disabled to prevent reception of certain channels, being more particularly concerned with facile and economical switchable circuits of this character.

Earlier approaches to such and similar selectively switchable trap reception are represented, for example, by United States Letters Patent Nos. 3175033 and 4268860 of Isaac S.

Blonder of common assignee herewith. There are occasions, however, where lowpass filter networks are employed for selective passband operation and in which, if it is desired to change the cutoff frequency of the filter while maintaining effectively the same "frequency normalized" response shape, all the inductive and capacitive components or elements of the filter must be changed.

Such component changing may be effected by voltage-controlled varactors or the like to facilitate capacitance change or current-controlled inductors; or switching diodes as of the PIN type may be used.to switch in and out extra series or parallel reactances to change the resultant inductance and capacitance values, with choice of configuration depending upon.the sharpness of the cutoff desired, degree of attenuation in the stopband, and physical realization problems of size and stray inductance and capacitance, particularly at high frequencies. The requirement of changing substantially all the components, however, is onerous and costly.

Underlying the present invention, however, is the discovery of a remarkably useful compromise in which the filter cutoff frequency of a lowpass filter, as, for example, of the Cauer series-connected inductance, shunt-connected inductance, capacitance arm type, may be effected to a degree sufficient to provide, in practice, adequate passband enabling and disabling, without changing all of the network components; though, with a minor practical penalty of using a filter of a characteristic impedance R0 that is no longer equal to the actual operational circuit impedance Rt, and with the amount that the system return loss degrades dependent upon the ratio of the two cutoff frequencies and the design reflection coefficient of the filter.

Such has been found practically most useful and feasible.

An object of the present invention, accordingly, is to provide a novel method of and apparatus for switching lowpass filter cutoff frequencies, as for enabling and disabling selective passband operation, with only a few of the filter network components requiring effective value changes and with acceptable passband ripple and dependent upon the ratio of the two cutoff frequencies involved in the switched filter tuning and circuit impedance changes.

A further object is to provide a new and improved electronically controlled switchable cutoff frequency lowpass filter or the like of more general applicability as well.

Other and further objects will be explained hereafter and are more particularly delineated in the appended claims.

In summary, however, from one of its broader viewpoints, the invention embraces a method of switching the cut-off frequencies of a low pass filter, normally of characteristic impedance R0 to enable and disable a specific passband region, that comprises, predetermining the ratio of the lower frequency cutoff fL to the higher frequency cutoff fH of the filter; adjusting the filter elements to produce such lower and higher cutoff frequencies fL and H' respectively, to enable said passband with a circuit impedance Rt = R0 ( H ); and switching capacitance into the filter to tune the higher frequency cutoff to fL and Rt to RQ ( tH ) to disable operation in said passband. Preferred and best mode network details are later presented.

The invention will now be described with reference to the accompanying drawing, Fig. 1 of which is a circuit diagram of a lowpass filter circuit embodying the invention in preferred form; Fig. 4 is a fragmentary one filter section circuit of Fig. I showing the effects of parasitic inductance and stray capacitance; and Figs. 2 and 3 are graphs plotting filter response as a function of frequency for the passband enabling and disabling modes of the filter operation, respectively.

In Fig; 1, for illustrative purposes, the invention has been shown in the form of a Cauer (degree-9) low-pass filter comprising successive or serially connected series and parallel branches or arms, shown as successive series-connected inductors Ll, L3, L5, L7, L9, with shunt arms connected from the points of series connection of the series inductors to the ground side, and each comprising a series inductance-capacitor combination L2-C2, L4-C4, L6-C6 and L8-C8. The signal source is generally represented at S (such as the source of television signals) as applied with an internal impedance Rt, and with the filter being shown generically terminated in a load with its output circuit impedance Rt (such as a television receiver or the like).A much-used reference in the designing of this and other types of filters is "The Design of Filters Using the Catalogue of Normalized Low-Pass Filters" by R. Saal, 1963, Telefunken Gemblt, pages 364-369.

In accordance with the technique underlying the present invention, a preselected ratio of higher-to-lower cutoff frequencies f fH-fL is determined such that fH represents the normal upper lowpass filter cutoff for enabling the desired band of frequencies to be passed, Fig. 2. If the cutoff frequency becomes changed to f , Fig. 3, the circuit would block or disable the passing of the selected spectrum, fH-fL. Fig. 2 shows the response of the filter when set to pass all signals (say up to 450 Megahertz -- fH), and Fig. 3 shows the filter response with a cutoff at fL (say up to about 215-225 Megahertz) to include Channel 13 in the passband.Thus, the values of the above-named circuit components of the filter illustrated in the drawing are selected to tune the filter to pass up to an upper or higher frequency cutoff fH but with the circuit impedance presenting not the characteristic impedance Ro that would normally match the signal source and load impedances, but an impedance Rt = Ro(-fH) When it is desired to disable the reception of the selected band f fL, it has been found necessary appropriately to change only the values of the four shunt capacitances C2, C4, C6 and C8 (and not all the network components) by connecting, for instance, across the same, the respective supplemental capacitances C22, C44, C66 and C88, as by the simultaneous rendering conductive of respective PIN diodes D2, D4, D6 and D8 through the closing or switching of switch S1, andthe resulting corresponding removal of -2V back bias with application of a conduction current of about 1 ma in each diode for the circuit values shown. The supplemental capacitances (C22, etc.) are selected such that, when shunted across the respective capacitors (C > ; etc.) the resultant capacitance (C2, etc.) will be ( w ) times the value of the fL shunt arm capacitor (C2' = ( fH ) C2, etc.); in which event, the higher cutoff frequency of the filter becomes reduced to fL and the circuit characteristic impedance becomes Ro H ).These adjustments must take into account the significant effect at these high frequencies in practical circuit realization of parasitic inductance effects of the diodes, printed circuit board and of added capacitors such as C22, etc., shown schematically at L' in Fig. 4 for the first filter section, for example, and the stray capacitance C' of the printed circuit pad and diode, etc.

fL As an illustration, if ( f )=2 (such as, for example, a normal frequency fH of 450 MHZ and fL of 225 MHZ, Figs. 2 and 3, as for television or video-communication applications), with the supplemental capacitances switched across the shunt arm capacitors for a resultant shunt capacitance four times the value of the shunt are capacitor (C2, = C2 + C22 - 4C2, etc.), the selected passband fH - fL will be disabled as the filter assumes a lower cutoff f of 225 MHZ and a circuit impedance Rt = R0 #7 whereas with the bandpass enabling position with switch S1 open and diodes D2, etc. non-conducting or of high impedance value, the upper bandpass frequency was f = 450 MHZ and the circuit impedance Rt = R0 ff2.

If the filter had a ndrmal reflection coefficient of a theoretical zero value when run at its design characteristic impedance Rg, the return loss value resulting from using the approach of the invention, as for instance raising or lowering the cutoff frequency by 2, would degrade from infinity to a value of 20 log

or about 15.31 dB. In practice, the maximum reflection coefficient is specified in a design in order to obtain practical component values and improve the sharpness of cutoff. For instance, if a Cauer filter is designed with a reflection coefficient of K = 0.1 and maximum attenuation in the stop band of 60 dB, the worst pass band return loss for the 225 MHz cutoff setting becomes 8.0 dB, and for the 450 MHz cutoff, 7.7 dB.This compromise in operation, with its advantages of minimal filter component changes, has been found to be entirely acceptable in video passband selection systems producing, for the exemplary frequencies above-presented, satisfactory passband enabling and disabling operation.

A suitable switching diode is type BA284. The coils may be of simple air-wound helix form and the switched capacitors of conventional ceramic disc construction.

One computer-optimized implementation of this invention which includes compensation of the parasitic capacitance C' of the switched diodes and printed circuit board, as well as recognition of and compensation for the inductance L' of the printed circuit board, Fig. 4.

While the invention has been described with reference to the particular filter network configuration shown, it will be understood that the method underlying the invention may also be practiced with other well known filter configurations, such as a series inductance and just shunt capacitance filter with a Tschebaff or Butterworth type response, and that the impedance changes may be series rather than parallel-effected, and that other switching devices than the preferred PIN diodes may also be employed -- such modifications being considered to fall within the spirit and scope of the invention as defined in the appended claims.

Claims (9)

1. A method of switching the cut-off frequencies of a low pass filter normally of characteristic impedance R0 to enable and disable a specific passband, that comprises, predétermining the ratio of the lower frequency cutoff to the higher frequency cutoff fH of the filter; adjusting the filter elements to produce such lower and higher cutoff frequencies fL and H' respectively, to enable said passband with a circuit impedance Rt=Ro(H); and switching reactance into the filter to tune the L higher frequency cutoff to fL and Rt to Ro( fL ) to disable operation in said passband.

2. A method as claimed in claim 1 and in which the switched reactance is capacitance introduced at a few only of the filter elements.

3. A method as claimed in claim 1 and in which the normal reflection coefficient value of the filter when operated at its characteristic impedance R0 is designed at about 0.1.

4. A method as claimed in claim 3 and in which the ratio is about 2 and the degrading is of the order of 15dB.

5. A switchable cut-off frequency -low pass filter apparatus normally of characteristic impedance R0 having, in combination, a plurality of successively connected series-inductance shunt inductance-capacitance network arms normally tuned to provide a predetermined ratio of lower frequency cutoff fL to the higher frequency cutoff fob to enable a specific passband but with circuit impedance adjusted to Rt = Ro( ); a plurality of supplemental capacitance elements, one for each of the capacitors of the shunt network arms of the filter; and means for switching saidsupplemental capacitance elements across their corresponding shunt network arm capacitors, said supplemental capacitance elements being of value such that when so switched they tune the filter to a lower cutoff frequency of fL, and with a circuit impedance of R0 ( fL ) to disable said passband.

fH

6. A switchable lowpass filter apparatus as claimed in claim 5 and in which each of the supplemental capacitance elements when switched across its corresponding shunt fH network arm capacitor presents a capacitance value ( fH) times the value of said shunt network arm capacitor;

7.A switchable lowpass filter apparatus as claimed in claim 5 and in which the connection of each supplemental capacitance element across its corresponding shunt network arm capacitor is effected by switching diode means, with means provided for simultaneously rendering each diode means conductive to switch the filter to the bandpass disabling position, said last-named means being operable when desired to render said diode means non-conductive, effectively removing said supplemental capacitance elements from the filter circuit and enabling said bandpass operation.

8. A method of switching the cut-off frequencies of a low pass filter substantial]y as hereinbefore described with reference to and as illustrated in the accompanying drawings.

9. A switchable low pass filter substantially as herein before described with reference to and as illustrated in the accompanying drawings.