Parallel Pass And Filters Having Multiple Negative Feedback Paths

Abstract

An electrical frequency filter of the kind having several filter units covering adjacent passbands has negative feedback between adjacent units to tend to cancel unwanted frequencies.

Description

BACKGROUND OF THE INVENTION

In electrical frequency filters of the kind providing different outputs for the different passbands, it is difficult to provide filters which will block unwanted frequencies without unduly attenuating the wanted frequencies.

The main object of the invention is to provide improved filter apparatus for overcoming this difficulty.

SUMMARY OF THE INVENTION

According to the present invention electrical frequency filter apparatus comprises a plurality of band-pass filter units covering different frequency bands and adapted to be fed from a common input, each filter unit including a phase-inverting amplifier and connections whereby at least some of the filter units are fed with the phase-inverted outputs from other filter units so as to effect at least a partial cancellation of the other frequencies.

The invention is applicable, for example, to signal transmission systems such as speech frequency telegraph multiplex systems. It is also applicable to doppler radar for separating the frequency bands of the indicating signals. In such a case there may be a filter for each element to be detected, the apparatus being so designed that the elements of the periodic table produce different frequency signals.

The invention is especially applicable to filter units for audio and subaudio frequencies but is not limited to these frequencies and is applicable also to higher frequencies. Preferably the input of each filter unit is fed with the outputs of all the other filter units. However, for some applications this may not justify the cost for the purpose involved.

In other cases one or more unit filters on opposite sides of a passband may supply neutralizing signals.

BRIEF DESCRIPTION OF DRAWINGS

In order that the invention may be more clearly understood reference will now be made to the accompanying drawings, in which:

FIG. 1 shows in block form a three-filter unit arrangement,

FIG. 2 shows in greater detail an example of a single filter unit for use in the arrangement of FIG. 1,

FIG. 3 shows an alternative single-filter unit, and

FIG. 4 shows how a bank of units may be arranged to provide alternative characteristics.

FIG. 5 shows an arrangement having a high frequency filter unit and a low frequency filter unit.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 the common electrical input which extends over a wide frequency band is shown on the left; this is fed to three unity gain, phase-inverting amplifiers A1, A2 and A3, respectively, each of these amplifiers feeding an associated filter F1, F2 and F3, respectively. These filters cover adjacent frequency bands in the overall frequency band covered by the input signal.

As shown in the drawing each of the filters feeds a separate output; thus, filter F1 feeds output No. 1, filter F2 feeds output No. 2 and filter F3 feeds the output No. 3. In addition, however, filter F1 supplies a separate output x1 which is fed as a compensating input to the input terminals of the unity gain amplifiers A2 and A3 of the other two units. Similarly, the filter F2 supplies a compensating output x2 to amplifiers A1 and A3 and filter F3 supplies a compensating output x3 to amplifiers A1 and A2.

Preferably each summing amplifier filter combination produces a substantially complete phase reversal at the center frequency of the passband concerned but there will not normally be complete phase reversal at frequencies displaced from the center frequency of the passband. It follows, therefore, that if the input signal contains a frequency at the center frequency of the filter F2, then the compensating output x2 will be of equal amplitude and opposite phase to the input. As the output x2 is supplied to the inputs of the amplifiers A1 and A3, there will be zero transmission of this frequency through the first and third filter units.

A similar condition arises for signals at the center frequencies of the filters F1 and F3.

For frequencies near to but not quite at the center frequencies of the adjacent bands, the output from the appropriate filter will not cancel exactly with the signal frequency; however, there will be some reduction of the signal passing through the other amplifiers to which this frequency is fed. It follows, therefore, that the effect of the interconnection is to introduce zeros of transmission of the frequency characteristic of an individual filter at all center frequencies of the other filters and a general reduction of frequencies outside the pass band concerned. FIG. 2 shows a single-filter unit, assumed to be unit 1, forming a summing and inverting circuit in which the main input and the compensating inputs x1 and x2 from the other units are fed in through resistors R1 to a high gain inverting amplifier IA (having a gain of infinity or approaching infinity). A resistor R2 is connected across the amplifier IA, thus giving a summing and inverting amplifier. The output from IA is fed through a filter circuit formed by L, C and R3 to the output terminal.

FIG. 3 shows an alternative arrangement in which the filter circuit of FIG. 2 is replaced by an amplifier AMP having a fixed positive gain K and a network formed by C3, C4 and R4.

It will be appreciated that with such arrangements the filters may be made of a relatively simple construction and furthermore it is possible to predict their characteristics to a high degree of accuracy.

As a guide to the design of the filters, if the voltage transfer function of the nth unit filter is A.sub.n, then in a fully interconnected system, the modified transfer function f.sub.n may be shown to be ##SPC1##

A suitable design procedure is to find a selection of transfer functions A.sub.n such that f.sub.n has a (or approximates to) some desired characteristic within the effective passband of the unit. The resulting stopband behavior may then be computed and in a wide range of practical applications this is a useful improvement over what a single unit of similar complexity could achieve.

By using computerized design methods it is possible to obtain transfer functions of the unit filters such that they have a specified passband amplitude characteristic when they are interconnected.

By tabulating the resulting stopbands, the improvement over a comparable design of a filter with the same complexity may be seen.

In some arrangements additional units may be added at each end of the frequency band in order to improve the performance of the end units. The amplifiers may be transistor amplifiers.

In the arrangement above described there is an upper limit to the ratio of bandwidth to the spacing between the center frequencies of individual filters. This ratio limit may be increased by summing the output from adjacent filters in order to provide more useful characteristics.

FIG. 4 shows a 10-unit system in which pairs of adjacent outputs are summed to provide five outputs.

A particular application of the invention is to a system which includes a low-pass and high pass filter combination, as shown in FIG. 5. The output of the low-pass filter L.P.F. is fed through a mixer M2 to the input of the high pass filter H.P.F. is fed through a mixer M1 to the input of the low-pass filter L.P.F.

Claims

I claim:

1. Electrical frequency filter apparatus comprising:

a plurality of band-pass frequency filter units covering different frequency bands and adapted to be fed from a common input,

each filter unit including a phase-inverting amplifier and connections whereby at least some of the filter units are fed with the phase-inverted outputs from other filter units so as to effect at least a partial cancellation of the other frequencies.

2. Electrical frequency filter apparatus comprising:

a plurality of band-pass frequency filter units covering adjacent frequency bands in an overall frequency range and adapted to be fed from a common input but having separate outputs,

each filter unit including a substantially unity gain phase-inverting amplifier and

means whereby each filter unit is also fed with the phase-inverted outputs from at least one frequency filter unit covering an adjacent frequency band.

3. Electrical frequency filter apparatus comprising:

a plurality of band-pass frequency filter units covering adjacent frequency bands in an overall range and adapted to be fed from a common input but having separate outputs,

each filter unit including a substantially unity gain phase-inverting amplifier,

means whereby each filter unit is also fed with the phase-inverted outputs from each of the other unit filters to tend to cancel the other frequencies in the unit concerned.

4. Electrical frequency filter apparatus, as claimed in claim 3, comprising means for connecting the outputs of the unit filters in pairs to obtain required characteristics.

5. Electrical frequency filter apparatus, as claimed in claim 3, comprising additional frequency filter units covering frequency passbands outside each end of the overall frequency range, each unit including a unity gain phase-reversing amplifier and means connecting the output of each of said units to an input of the unit filter at the corresponding end of the overall frequency range.

6. Electrical frequency filter apparatus comprising:

a high-frequency band-pass filter unit including a substantially unity gain phase-inverting amplifier,

a low-frequency filter band-pass filter unit including a substantially unity gain phase-inverting amplifier,

said filter units having a common input but separate outputs,

means where the output from the high-frequency unit is also fed to an input of the low-frequency filter to tend to cancel high frequencies in the low-frequency unit output and

means whereby the output of the low-frequency unit is also fed to an input of the high-frequency unit to tend to cancel low frequencies in the high-frequency unit output.

7. In a signal transmission system electrical frequency filter apparatus comprising:

a plurality of band-pass frequency filter units covering different passbands in the frequency range of the signals

said filter units being fed from a common input but having separate outputs,

each filter unit including a substantially unity gain phase-inverting amplifier,

means whereby the output of each filter unit is also fed in phase opposition to an input of at least an adjacent passband unit to tend to cancel unwanted frequencies.

8. In a doppler radar system electrical frequency filter apparatus comprising:

a plurality of band-pass frequency filter units covering different passbands in the frequency range of the received frequency signals

said filter units being fed from a common input but having different outputs corresponding to different ranges,

said filter units each including a substantially unity gain phase-inverting amplifier,

means whereby the output of each filter unit is also fed in phase opposition to an input of at least an adjacent passband filter unit to tend to cancel unwanted frequencies in the output of the filter unit.

9. An analyzing apparatus in which the elements detected are distinguished by signals of different frequencies, frequency-analyzing apparatus comprising:

a plurality of band-pass frequency filter units covering different passbands in the overall frequency range of the signals,

said filter units being fed from a common input but having different outputs,

said filter units each including a substantially unity gain phase-inverting amplifier, and

means whereby the phase-inverted output of each filter unit is also fed to an output of at least an adjacent passband filter unit to tend to cancel unwanted frequencies in the output of the filter unit.