GB2259819A

GB2259819A - Trap circuit for superheterodyne receiver

Info

Publication number: GB2259819A
Application number: GB9219682A
Authority: GB
Inventors: Toru Ohashi
Original assignee: Pioneer Electronic Corp
Current assignee: Pioneer Corp
Priority date: 1991-09-20
Filing date: 1992-09-17
Publication date: 1993-03-24
Anticipated expiration: 2012-09-17
Also published as: DE4231159A1; JPH0583149A; GB2259819B; GB9219682D0

Abstract

A trap circuit 2A is connected to a resonance circuit 1 for absorbing undesirable frequencies. The trap circuit is formed by a series resonance circuit 6 comprising a coil L2 and a capacitor C2 connected in series. A capacitor C7 is connected to the coil in parallel. The coil L2 and the capacitor 07 are arranged to have a resonance frequency substantially equal to a resonance frequency of the resonance circuit 1. <IMAGE>

Description

1 i 1 Trap Circuit The present invention relates to a trap circuit,

provided for example in a receiver to reject undesirable frequencies.

Referring to Fig. 3 showing a part of a superheterodyne receiver, the circuit has a tuning' circuit for receiving transmitted waves and a converting circuit for converting the received frequency to an intermediate frequency. A resonance circuit 1 (tuning circuit) comprises a coil Ll coupled with an antenna coil (not shown) and a variable capacitor Cl. -The input signal is applied to a base of a transistor Tr of a mixer through a capacitor C3 which removes a direct current component therefrom. The base of the transistor Tr is further applied with a signal from a local oscillator, thereby generating an intermediate frequency signal. The intermediate frequency signal is applied to an intermediate frequency transformer 3 comprising a capacitor, primary coil and an opposing secondary coil. The primary coil is connected to a source B through a resistor.

The circuit further has a series trap circuit 2 disposed between the resonance circuit 1 and the base of the transistor Tr. The trap circuit 2 comprises a 2 coil L2 and a capacitor C2 connected to the co-L1 L2 in series to limit the current flowing in the circuit.

The trap circuit 2 has a resonance frequency FB which is expressed as, FB = 112wJ1,2C2 where L2 and C2 are the inductance and the capacitance of the coil L2 and the capacitor C2, respectively. Undesirable and disturbing signals at the frequency FB to be applied to the transistor Tr are absorbed by the trap circuit 2. Thus, the undesirable signals do not enter the mixer.

However, in the conventional circuit, the inductance and the capacitance of the trap circuit 2 cause the resonance frequency FA of the resonance circuit 1 to deflect, so that the correct waves cannot be received. As a result, an incorrect intermediate frequency signal is provided.

Referring to Fig. 2, there is shown a relationship between impedance and resonance frequency of the parallel resonance circuit 1. Assuming that the resonance circuit 1 has a resonance frequency fO, the impedance becomes maximum at the frequency FO as shown by a line A. If the trap circuit 2 has a capacitive reactance at the frequency, the trap circuit 2 acts on the resonance circuit 1 so as to deflect the characteristics as shown by a line B. As a result, the 1 3 resonance circuit 1 resonates at a lower frequency than the frequency FO. On the other hand, if the trap circuit has an inductive reactance, the resonance frequency is deflected as shown by a line C. Hence 5 transmitted waves cannot be accurately received.

An object of the present invention is to provide a trap circuit where the reactance thereof do not affect an adjacent resonance circuit.

According to the present invention, there is provided a trapping circuit having a resonance circuit, and a trap circuit comprising a series resonance circuit having a coil and a capacitor connected in series.

The trap circuit is further provided with a capacitor parallely connected to the coil to form a parallel resonance circuit which generates substantially the same frequency as the resonance frequency of the resonance circuit.

Undesirable or disturbing frequency signals included in the resonance frequency signal from the resonance circuit are tuned to the resonance frequency signal generated by the series resonance circuit of the trap circuit so as to be absorbed. In the parallel resonance circuit of the trap circuit, the impedance becomes maximum when the resonance occurs at the 4 frequency substantially the same as the resonance frequency of the resonance circuit. Thus the resonance circuit is not affected by the trap circuit.

The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.

Fig. 1 shows a circuit provided in a receiver having a trap circuit of the present invention; Fig. 2 is a graph showing a relationship between impedance and resonance frequencies in a parallel resonance circuit; and Pig. 3 shows a circuit provided in a receiver having a conventional trap circuit.

Referring to Fig. 1, a circuit to which the present invention is applied is similar to the conventional circuit shown in Fig. 3, and hence, the same references shown in Fig. 3 designate the same parts as the parts in Fig. 3.

A circuit to which the present invention is applied has the parallel resonance circuit 1 having the coil Ll and the variable capacitor Cl. The resonance circuit 1 is connected to the base of the transistor Tr through the capacitor C3. The signal received at the resonance circuit 1 is mixed with the signal from the J R Z1 oscillator thereby producing an intermediate frequency signal. The collector of the transistor Tr is connected to the intermediate frequency transformer 3 comprising the capacitor and the primary and secondary 5 coils.

A trap circuit 2A according to the present invention has a series resonance circuit 6 comprising the coil L2 and the capacitor C2, a parallel resonance circuit 5 comprising the coil L2 and a variable capacitor C7 parallely connected to the coil L2. The variable capacitor C7 is connected to the variable capacitor Cl so as to be operated together with the capacitor Cl.

The parallel resonance circuit 5 is provided to have a resonance frequency substantially equal to the resonance frequency FA of the resonance circuit 1.

In operation, the received signal, which is at the frequency FA, is applied to the base of the transistor Tr through the capacitor C3. Undesirable frequencies FB in the received signal is absorbed by the series resonance circuit 6 of the trap circuit 2A. On the other hand, the parallel resonance circuit 5 resonates at the frequency FA of the resonance circuit 1. The impedance of the resonance circuit 5 becomes maximum a the resonance frequency, so that the capacitive reactance and the inductive reactance of the trap 6 circuit 2A become small. Therefore, the trap circuit does not affect the frequency FA. As a result, the resonance circuit 1 receives a desired signal without interference. Hence an accurate intermediate frequency signal is obtained by the transistor Tr and outputted through the intermediate frequency transformer 3.

The resonance circuit 5 may be so modified that a stray capacitance is used in place of the capacitor C7.

From the foregoing it will be understood that the present invention provides a trap circuit which do not affect the'resonance frequency of an adjacent resonance circuit. Thus signals at an accurate resonance frequency can be obtained.

While the-presently preferred embodiments of the present invention has been shown and described, it is to be understood that these disclosures are for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.

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Claims

CLAIMS:

1. A trap circuit for connection to a resonant circuit to absorb energy of a predetermined frequency, said trap circuit comprising a series resonance circuit comprising a coil and a capacitor connected in series and for connection to the resonant circuit, and capacitor means connected to the coil in parallel, the coil and the capacitor means being arranged to have a resonant frequency substantially equal to a resonant frequency of the resonant circuit.

2. A trap circuit according to claim 1 for connection to a said resonant circuit including a first variable capacitor, wherein said capacitor means is a second variable capacitor adapted to be operated together with the first capacitor.

3. A trap circuit for connection to a resonant circuit, substantially as described herein with reference to Fig. 1 of the accompanying drawings.