A notch filter for the selective attenuation of a narrow band of frequencies out of a larger band was developed. A helical resonator is connected to an input circuit and an output circuit through discrete and equal capacitors, and a resistor is connected between the input and the output circuits

Shelton, G. B.

English

NASA Technical Reports Server

United States Patent 1191 
Fletcher et al. 
[ I l l  4,007,434 
[45j Feb. 8, 1977 
1541 NOTCH FILTER 
[76] Inventors: 
[ 2 2 ]  Filed: 
James C. Fletcher, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of Glenn B. Shelton, 
Huntsville, Ala. 
Apr. 14, 1976 
[ 2 1 ] 
[ 5 2 ]  U.S. Cl. ............................... 333175; 333/70 R; 
[51 ]  Int. CL2 ..................... H03H 7/10; H03H 7/14; 
[58] 
Appl. No.: 676,957 
333176; 333/82 B 
H03H 13/00 
Field of Search .............. 333/70 R, 75, 74, 76, 
333/70 CR, 82 R ,  82 B; 331/107 R, 101, 138, 
140, 142; 330131, 56; 328f167 
(561 References Cited 
UNITED STATES PATENTS 
3,538,456 I 111970 Contus ............................. 333182 B 
Primary Examiner-Alfred E. Smith 
Assistant Examiner-Marvin Nussbaum 
Attorney, Agent, or Firm-L. D. Wofford, Jr.; George J. 
Porter; John R. Manning 
[571 ABSTRACT 
A notch filter for the selective attenuation of a narrow 
band of frequencies out of a larger band wherein a 
helical resonator is connected to an input circuit and an 
output circuit through discrete and equal capacitors, 
and a resistor is connected between the input and the 
output circuits. 
5 Claims, 3 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19770011363 2020-03-20T07:29:02+00:00Z
U.§. Patent Feb. 8, 1977 Sheet 1 of 2 4,007,4 34 
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U.S. Patent Feb. 8, 1977 Sheet 2 of 2 4,007,43 
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4.007,43 4 
NOTCH FILTER 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 
85-568 (72 Stat. 435; U.S.C. 2457). 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to frequency or band elimina- 
tion filters commonly called notch filters, and particu- 
larly to a filter of this class having improved rejection 
characteristics. 
2. General Description of the Prior Art 
The principal type of filter used as a trap or band 
elimination filter is a bridged-T filter consisting of two 
series capacitors bridged by a resistor at the top portion 
of the T configuration and an inductor connected be- 
tween the capacitors and a common terminal. A diffi- 
culty with this filter is that for some applications the 
slope of the rejection characteristic on each side of the 
design frequency is insufficient, that is, the cutoff is not 
sharp enough. While improved sharpness of cutoff can 
be achieved utilizing crystals, the problem with the 
crystal filters is that spurious responses are often passed 
which are sufficiently close to the notch frequency to 
be objectionable. 
Accordingly, it is an object of this invention to pro- 
vide a new and improved notch filter which approxi- 
mates the steepness of a crystal filter without the prob- 
lem of spurious responses. 
SUMMARY OF THE INVENTION 
In accordance with this invention, a notch fiter is 
constructed in the general configuration of a bridged-T 
filter but wherein the inductor of such a filter is re- 
placed by a resonating circuit consisting of a coil and 
an enclosing container comprising a helical resonator 
and capacitor connecting one end of the coil to the 
container, the other end of the coil being directly con- 
nected to the container, and a tap point on the coil 
connected to the capacitors of a bridged-T configured 
circuit wherein the capacitors are bridged by a resistor. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a combination pictorial-electrical schematic 
FIG. 2 is an electrical block diagram illustrative of a 
FIG. 3 is a plot of frequency response of the notch 
DETAILED DESCRIPTION OF THE DRAWINGS 
The notch filter shown in FIG. 1 will be described as 
configured for the elimination of a band of frequencies 
centered about a frequency of fo, in this case 24 MHz, 
and thus signal source 10 shall be assumed to contain a 
frequency or small band of frequencies in the vicinity 
of 24 MHz to be eliminated. A form of helical resona- 
tor is constructed of a coil 12 and cylinder or shell 14. 
Coil 12 would be held to within 0.45 and 0.6 of the 
diameter of cylinder 14, and in the instant case, coil 12 
was constructed of a dimensionally stable paper form 
one inch in diameter and 1.75 inch long. A spiral 
groove with 18 turns per inch is cut in the form to 
diagram of an embodiment of the invention. 
multi-stage notch filter circuit. 
filter depicted in FIG. 2. 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 
2 
receive AWG 22 magnet wire. The winding was started 
0.5 inch from bottom end 16 and continued for 27 
turns. Tap 18 is brought out four turns from bottom 
end 16 of coil 12. Cylinder 14 is made from 2-inch 
diameter copper tubing and is 2.5 inches long. Coil 12 
is electrically connected at top end 20 through trimmer 
capacitor 22 to cylinder 14, and the opposite end of 
coil 12 is connected directly to terminal 24 of cylinder 
14, which is a circuit ground or common point. These 
connections additionally function to provide physical 
support for coil 12. Coil tap 18 is connected at a se- 
lected impedance point spaced from end 16 of coil 12 
to match the input and output impedances of source 10 
and load 26 and connects through insulated feed- 
throughs 28 and 30 to a common terminal 32 of capaci- 
tors 34 and 34. Terminal 38 of capacitor 34 provides an 
input terminal between which terminal and circuit 
ground 24 input signals from source 10 are applied. 
The other terminal of capacitor 36, terminal 40, forms 
with circuit ground 24 and load 26 the output of the 
filter. 
Capacitors 34 and 36 are of equal values of five 
picafarads, and variable resistor 42 is connected across 
them, between terminals 38 and 40. This resistor has a 
maximum value of 500 ohms, substantially less than the 
reactance of either of the capacitors at the center or 
design frequency of filter fo. Coil 12, together with the 
distributed capacitance of coil 12 with respect to cylin- 
der 14 and lump capacitance (0.8 to 10.0 picafarads) 
provided by variable capacitor 22, provides at fo a net 
inductance with a Q of approximately 500. The equiva- 
lent resistance of this combination at resonance, fre- 
quency fo, is made equal to the equivalent resistance 
produced by resistor 42 and capacitors 34 and 36 by 
precise adjustment of the value of resistor 42. 
While only a single stage filter is shown, in practice it 
is often desirable to cascade a number of filter stages. 
In fact, the problem giving rise to the present invention 
was that of determining a notch filter having a mini- 
mum rejection of 60 db for a bandwidth of 100 KHz 
with 6 db reduction points having a bandwidth of less 
than 300 KHz. As stated above, 24 MHz was the cen- 
ter, or design frequency. FIG. 2 illustrates in block 
diagram form the filter circuit which was constructed, 
employing seven cascaded notch filters 21s illustrated in 
FIG. 1, each of filters F1-F7 being inter-stage coupled 
by one of amplifiers A1-A6. Initial alignment of the 
circuit is accomplished with the aid of a high frequency 
oscilloscope (connected to output) and sweep genera- 
tor (connected to input), the function being to adjust 
resistor 42 of each of the cascaded filter stages. All 
resistors 42 are initially set to a mid-range position, and 
all capacitors 22 are determined from the center fre- 
quency. Then each capacitor 22 of each stage is tuned, 
one at a time, to 24 MHz; and its associated resistor is 
adjusted to provide the lowest dip on an oscilloscope 
display. The final alignment additionally requires a 
frequency counter and a spectrum analyzer connected 
to the output. The center of the spectrum analyzer is 
set at 24 MHz and set to provide a display width of 120 
KNz, and attenuators of the analyzer are set so that 
gain is decreased by at least 60 db. The signal generator 
is set for 24.000 MHz; and with all of capacitors 22 
detuned, generator output is adjusted to set a circuit 
output for a convenient reference level on the spec- 
trum analyzer display. Capacitor 22 of filter stage F1 is 
then adjusted for a minimum display, and resistor 42 of 
filter F1 is readjusted for a minimum display. The dis- 
4,007,434 
3 4 
play should now be more than 60 db below the refer- 
ence level. Using this same technique, stage F2 is 
aligned to 24.018 MHz, stage F3 to 23.982 MHz, stage 
Fq to 24.035 MHz, stage F5 to 23.965 MHz, stage F6 to 
24.046 MHz, and stage F7 to 23.954 MHz. As shown in 5 
FIG. 3, the 60 db bandwidth should be at essentially 
100 KHz with a response ripple no higher than 60 db 
below the reference, and the 6 db bandwidth should be 
less than 300 KHz. Amplification by the inter-stage 
amplifiers AI-& is set to provide an overall 40 db gain 10 
with a low frequency cutoff at 2,500 Hertz and a higher 2. A notch filter comprising: 
frequency cutoff beyond 50 MHz. a metal cylinder; 
The notch filter of this invention provides a number a helical coil positioned within said cylinder about an 
of advantages. An extremely high selectivity is axis concentric with and spaced from said cylinder; 
achieved, bandwidth is controllable by cascading iden- 15 a first variable capacitor connecting one end of said 
tical stages and stagger tuning of them, the depth of a coil to an adjacent end of said cylinder, and the 
notch is readily adjusted, excellent isolation between opposite end of said coil being connected to an 
stages is possible, the problem of spurious responses is adjacent end of said cylinder; 
essentially eliminated, and component values are quite a resistor having a first, input, terminal and a second, 
practical. 20 output, terminal; 
a second capacitor connected between a tap on said 
coil spaced from said opposite end of said coil and is: said input terminal; 
a third capacitor equal in value to said second capaci- 
tor connected between said tap and said output 
terminal; and 
a signal source providing signal frequencies including 
a frequency fo connected between said input termi- 
nal and said cylinder and an output load connected 
between said output terminal and said cylinder and 
wherein the impedance of said coil between said 
tap and said opposite end, the impedance of said 
source, and the impedance of said load are all 
at frequency fo is equal to the equivalent resis- 
tance provided by the combination of said resis- 
tor and said second and third capacitors at fre- 
quency fo; and 
a plurality of amplification means, each for amplify- 
ing the output of a said notch filter, a said amplifi- 
cation means being interconnected between the 
said second terminal of said resistor of one of said 
notch filters and a said first terminal of a following 
said notch filter. 
Having thus disclosed my invention, what is claimed 
1. A filter comprising: 
a plurality of notch filters, each comprising: 
a metal cylinder including a common circuit termi- 25 
nal, 
a helical coil positioned within said cylinder about 
an axis concentric with and spaced from said 
cylinder, the diameter of said coil being 0.45 to 
0.6 times the diameter of said cylinder, and the 30 
ratio of the length of said coil to the diameter of 
said coil being greater than 1, 
a first variable capacitor connecting one end of 
said coil to one adjacent end of saidcylinder, and 
the opposite end of said coil being directly con- 
nected to an adjacent end of said cylinder, 
a resistor having a first terminal and a second ter- 
minal, 
a second capacitor connected between a tap point 
on said coil spaced from said opposite end of said 
coil and said first terminal, 
a third capacitor equal in value to said second 
capacitor being connected between said tap 
point and said second terminal of said resistor, 
each of said second and third capacitors having a 
reactance at a selected frequency, fo, substan- 
tially greater than the resistance of said resistor, 
and 
the reactance of the combination of said coil and 
said cylinder and said first capacitor is inductive, 
and the equivalent resistance of said combination 
equal at said fo frequency. 
35 3. A notch filter as set forth in claim 2 wherein the ratio of the diameter of said coil with respect to the 
diameter of said cylinder is between 0.45 and 0.6, and 
the ratio of the length of said coil to the diameter of 
said coil is greater than 1. 
4o 4. A notch filter as set forth in claim 3 wherein: said resistor is of a value substantially less than the 
reactance of either of said second and third capaci- 
tors at said frequency fo; and 
the reactance of the combination of said coil and said 
cylinder and said first capacitor is inductive and is 
equal to the reactance of either of said second and 
third capacitors at said frequency fo, and the resis- 
tance of said combination at fo is equal to the resis- 
tance of said resistor. 
5. A notch filter as set forth in claim 4 wherein said 
45 
50 resistor is a variable resistor. * * * * *  
55 
60 
65 


Notch filter

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