Notch Filter

1. Field of the Invention

This invention relates to frequency or band elimination filters commonly called notch filters, and particularly 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 between the capacitors and a common terminal. A difficulty 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 provide a new and improved notch filter which approximates the steepness of a crystal filter without the problem of spurious responses.

In accordance with this invention, a notch filter is constructed in the general configuration of a bridged-T filter but wherein the inductor of such a filter is replaced 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 connected 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.

FIG. 1 is a combination pictorial-electrical schematic diagram of an embodiment of the invention.

FIG. 2 is an electrical block diagram illustrative of a multi-stage notch filter circuit.

FIG. 3 is a plot of frequency response of the notch filter depicted in FIG. 2.

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 f.sub.0, 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 resonator 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 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 selected 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 capacitors 34 and 36. 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 f.sub.0. Coil 12, together with the distributed capacitance of coil 12 with respect to cylinder 14 and lump capacitance (0.8 to 10.0 picafarads) provided by variable capacitor 22, provides at f.sub.0 a net inductance with a Q of approximately 500. The equivalent resistance of this combination at resonance, frequency f.sub.0, 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 minimum 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 center, or design frequency. FIG. 2 illustrates in block diagram form the filter circuit which was constructed, employing seven cascaded notch filters as illustrated in FIG. 1, each of filters F.sub.1 -F.sub.7 being inter-stage coupled by one of amplifiers A.sub.1 -A.sub.6. Initial alignment of the circuit is accomplished with the aid of a high frequency oscilloscope (connected to output) and sweep generator (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 frequency. 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 KHz, 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 spectrum analyzer display. Capacitor 22 of filter stage F.sub.1 is then adjusted for a minimum display, and resistor 42 of filter F.sub.1 is readjusted for a minimum display. The display should now be more than 60 db below the reference level. Using this same technique, stage F.sub.2 is aligned to 24.018 MHz, stage F.sub.3 to 23.982 MHz, stage F.sub.4 to 24.035 MHz, stage F.sub.5 to 23.965 MHz, stage F.sub.6 to 24.046 MHz, and stage F.sub.7 to 23.954 MHz. As shown in 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 A.sub.1 -A.sub.6 is set to provide an overall 40 db gain with a low frequency cutoff at 2,500 Hertz and a higher frequency cutoff beyond 50 MHz.

The notch filter of this invention provides a number of advantages. An extremely high selectivity is achieved, bandwidth is controllable by cascading identical stages and stagger tuning of them, the depth of a notch is readily adjusted, excellent isolation between stages is possible, the problem of spurious responses is essentially eliminated, and component values are quite practical.