JPS6038892B2 - Bandpass filter with trap - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a trap-equipped bandpass filter suitable for application to a high frequency interstage tuning circuit of a UHF television tuner.

The present invention proposes a bandpass filter with a trap that can change the center frequency of the pass frequency band and the trabe frequency all at once by varying the capacitance of one variable capacitance element with a simple configuration. It is.

An embodiment in which the present invention is applied to a high frequency interstage tuning circuit of a UHF television tuner will be described below with reference to FIG.

In FIG. 1, T is an input terminal and T2 is an output terminal.

VC is a variable capacitance diode as a variable capacitance element. Note that this variable capacitance element may be a normal variable capacitor. K
v is a control circuit that controls the capacitance of the variable capacitance diode VC. The input terminal Tvc of the positive DC voltage for control applied to the cathode of the variable capacitance diode VC is connected to the capacitor (
It is connected to the input side of a bottom pass reactor waver consisting of a 100 F) C4, a resistor (2200) R, and a capacitor (3 F), and the output side is an inductor (coil) for blocking AC signals. It is connected to the cathode of variable capacitance diode VC through L4. In the bandpass filter with a trap of the present invention, the input terminal T is grounded through the capacitor C, and the input terminal T is grounded through the capacitor C. is connected to one end of a series circuit of an inductor (coil) L and a variable capacitance element VC, and the other end of the series circuit is grounded through an inductor (coil) - and connected to an output terminal through an inductor (coil) L3,
A capacitor C2 is connected in parallel to a series circuit of an inductor L, a variable capacitance element VC, and an inductor.

Capacitor C. The capacities of C2 and C2 are 100 F and 0, respectively.
．． It is 5pF. Input terminal T, is connected to capacitor C through coupling capacitor Co. , C2 and the inductor. The midpoint of the connection between the inductors L and L4 is connected to the cathode of the variable capacitance diode VC. The midpoint of the connection of inductors L2 and L3 is followed by the anode of variable capacitance diode VC. A midpoint between the inductor L3 and the capacitor C2 is connected to the output terminal L through the capacitor C3. In this trapped bandpass filter, capacitor C. , inductor L, , an ordinary bandpass filter in which the center frequency of the pass frequency band is variable is constituted by a horizontal circuit consisting of a variable capacitance diode VC and an inductor, and an inductor L3 and a capacitor C2 are added thereto. Accordingly, a trap-equipped bandpass filter can be obtained in which the trap frequency is also varied in the same direction as the above frequency and at approximately the same frequency interval. TRP is this troubleshooting circuit, and is composed of a parallel resonant circuit consisting of inductors L, L3, variable capacitance diode VC, and capacitor C2. In this example, the trap frequency is higher than the center frequency of the pass band frequency. Of course, a lower frequency may be used. When this bandpass filter with a trap is applied to a high frequency interstage tuning circuit of a UHF television tuner, an image signal having a frequency equal to the sum of twice the intermediate frequency and the frequency of the UHF television signal is trapped.

In addition, impedance matching with the next stage circuit (transformer) is sought using the inductor L3 and capacitor C3.

The inductors L2 and L2 are used to reduce changes in output impedance due to changes in signal frequency.
The above-mentioned UHF television tuner consists of a high frequency input tuning circuit - a high frequency booster circuit - a high frequency inter-stage tuning circuit - a transformer.
The mixing circuit consists of a cascade of an intermediate frequency filter, an intermediate frequency multiplier circuit, and a local oscillator. Incidentally, instead of connecting the connecting ends of capacitors C4 and C5 in FIG. 1 to the ground, a variable capacitance diode VC is connected as shown in FIG.
The middle point of the connection between the resistor R and the capacitor C4 may be connected to the terminal Tvc through the resistor R2.

According to the present invention described above, there is provided a bandpass filter with a trap that can change the center frequency and trap frequency of the pass frequency band at once by varying the capacitance of one variable capacitance element with a simple configuration. can be obtained.

[Brief explanation of the drawing]

1 and 2 are circuit diagrams showing different embodiments of the present invention. Co, C, ~C5 are capacitors, L, ~L4 are inductors (coils), VC is a variable capacitance diode as a variable capacitance element, and TRP is a trap circuit. Figure 1 Figure 2

Claims (1)

[Claims] 1. The input terminal is grounded through the capacitor C_1, the input terminal is connected to one end of the series circuit of the inductor L_1 and the variable capacitance element VC, and the other end of the series circuit is grounded through the inductor L_2 and connected to the output terminal through the inductor L_3. A bandpass filter with a trap, characterized in that a capacitor C_2 is connected in parallel to the series circuit of the inductor L_1, the variable capacitance element VC, and the inductor L_3.