FR2656175A1 - Adjustable notch filter or rejector - Google Patents

Abstract

Rejector including two variable capacitors (CV1,CV2) and a choke. The choke is hollow in order to be able to perform the adjustment of CV1 with a screwdriver TV1 which does not disturb the operation of the rejector while it is inserted momentarily into the hollow of the choke.

Description

DESCRIPTION "ADJUSTABLE BAND CUTTER OR REJECTOR".

 The subject of the present invention is a bandwidth filter or rejector, adjustable to connect between a high frequency transmission line and ground, comprising a first variable capacitor connected in series with a choke and a second variable capacitor connected in parallel, each capacitor comprising two connection ends of which one, the adjustable end, is provided with an accessible element for its adjustment.

 Such a notch filter has many uses in the processing of high frequency signals, in particular for eliminating an undesirable signal of too high intensity as is often the case in the presence of a close and powerful FM transmitter where the name of "FM rejector" used when the cut band is very narrow.

 Such an adjustable filter is much more flexible to use than a simple LC series because it allows on the one hand to vary the rejection depth for a given frequency to be ejected, on the other hand to obtain a rejection depth constant over a wide range of tuning frequencies.

 To date, the practical realization of such a filter encounters difficulties at the time of adjustment because, in view of the electrical diagram, it is not possible that the adjustable end of each of the two variable capacitors is to ground . Therefore, the momentary presence of an adjustment screwdriver causes a momentary detuning of the circuit at least for one of the two variable capacitors.

 The object of the present invention is to remedy this drawback.

 For this purpose, a filter conforming to the preamble is particularly remarkable in that said self is hollow from end to end, and in that on the one hand said second variable capacitor is positioned so that its adjustable end is connected to massage it on the other hand said first variable capacitor is positioned so that its adjustable end is located on the side of the inductor and accessible, for its adjustment, through said hollow of the inductor.

 Thus, due to the fact that the HF currents only flow on the external surface of the inductor, the presence of the adjusting screwdriver in the hollow of the inductor does not cause any detuning of the circuit. The adjustment is then carried out without successive groping and it is easily automated, which significantly reduces the manufacturing costs.

 The present invention will be better understood with a nonlimiting example of embodiment illustrated by a drawing.

 Figure 1 shows the diagram of a band filter or nozzle.

 Figure 2 shows in section the embodiment, according to the invention, of such a filter.

 Figure 1 shows a line (L) in which high frequency signals (SHF) circulate. This line is connected to ground (M) via a rejector composed of a first variable capacitor (cl1) mounted in series with a choke (S) and a second variable capacitor (CV2) mounted in parallel.

 Such a rejector is very flexible to use since, by adjusting CV1 and CV2 adequately, it allows either, for a given frequency to be rejected, to vary the rejection depth, or to obtain a constant rejection depth over a wide range tuning frequency range.

In this diagram1 the variable capacitor CV2 having one end El to earth, the momentary presence of an adjustment screwdriver acting in El does not cause any discord of the circuit. It seems impossible that it is the same for the adjustment of the capacitor CV1 since, none of its ends
E2, E3 is not grounded. As a result, such rejectors are not found commercially.

 FIG. 2 represents a practical embodiment which overcomes the impossibility already mentioned.

 The elements are represented there with the same references as in FIG. 1. The choke (S) consists of a hollow element from end to end whose at least the external surface is conductive for conducting the HF currents. And the CV1 capacitor adjustment tip can be introduced opposite, or in the hollow of the inductor so that a TV1 screwdriver can now access CVI for adjustment without detuning the circuit. Of course, the mounting of CV2 still allows the adjustment, without problem, of CV2 with a TV2 screwdriver.

 Thus, thanks to the presence of a hollow inductor, it is possible to make a very flexible employment rejector.

 The realization of the hollow choke is preferably tubular cylindrical, or slightly conical to guide the TV1 screwdriver. The taper is also an advantage in the case where the inductor S and the mass M are integrally molded either in a conductive material or in a plastic material subsequently coated with a conductive layer on the surface.

In practice, the following nonlimiting values can be used
- Self. Diameter from 4 to 6 mm
Length from 20 to 50 mm
- CVI. from 0.5 to 5 pF.

 - CV2. from 0.5 to 10 pF.

 With such values, the adjustment range can be from 400 MHz to 2 GHz; the rejection depth is very variable depending on the impedance of the transmission line and the tangent of loss of variable capacities; with an impedance of 75 Q in UHF, the rejection depth can be between 10 and 25 dB.

Claims (1)

CLAIM 1. Notch filter, or rejector, adjustable to connect between a high frequency transmission line and ground1 comprising a first variable capacitor (CV1) connected in series with a choke and a second variable capacitor (CV2) connected in parallel, each capacitor comprising two connection ends, one of which, the adjustable end, is provided with an element accessible for its adjustment, characterized in that said self is hollow from end to end, and in that on the one hand said second variable capacitor is positioned so that its adjustable end is connected to ground, on the other hand the said first variable capacitor is positioned so that its adjustable end is located on the side of the choke and accessible, for its adjustment, through the said hollow of the self.