U.S. patent number 3,611,165 [Application Number 05/051,212] was granted by the patent office on 1971-10-05 for parallel pass and filters having multiple negative feedback paths.
This patent grant is currently assigned to National Research Development. Invention is credited to Michael Hills.
United States Patent |
3,611,165 |
Hills |
October 5, 1971 |
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.
Inventors: |
Hills; Michael (Colchester,
EN) |
Assignee: |
National Research Development
(London, EN)
|
Family
ID: |
21969986 |
Appl.
No.: |
05/051,212 |
Filed: |
June 30, 1970 |
Current U.S.
Class: |
327/557;
330/124R; 324/76.31; 330/109; 381/98 |
Current CPC
Class: |
H03H
11/34 (20130101) |
Current International
Class: |
H03H
11/34 (20060101); H03H 11/02 (20060101); H03b
001/04 () |
Field of
Search: |
;328/167,165,138,140
;307/229,233,295 ;329/142,140 ;330/21,31,107,109 ;324/77B,77E
;333/7R ;325/65,379,452,458,477,489 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Krawczewicz; Stanley T.
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.
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.
* * * * *