JPH0370408B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a variable bandwidth filter with a simple configuration that can vary the bandwidth without changing the gain at the tuning point.

[Conventional technology]

In a superheterodyne AM receiver, the IF
Increasing the width of the (intermediate frequency) band can improve sound quality, but on the other hand, it has the disadvantage of increasing noise in weak electric fields and noise due to adjacent interference. In order to improve this point, conventional methods have been used to change the IF band between wide and narrow using a switch, making the band narrower in weak electric fields, or to keep the IF bandwidth constant and use ATC (auto) in the audio band after IF detection.・There is a way to apply tone control).

[Problem that the invention seeks to solve]

However, since the method requires manual switching, it is inconvenient for use in a vehicle and also impedes safe driving. Moreover, since noise is generated when switching the IF band, a circuit to mute this is required, and there are many other difficulties. On the other hand, method (2) is automated, but since the IF band is kept wide, suppression by adjacent stations (AGC function) increases, and the ATC circuit is as variable as the CR primary filter, so it is not effective. There are few drawbacks. Furthermore, when playing AM stereo, circuits for two channels are required, making the configuration complex. The present invention attempts to improve these points.

[Means for solving problems]

The present invention is a variable bandwidth filter in which an electronic variable resistor whose resistance value can be varied by a control voltage is connected between an input signal voltage source and a parallel tuned circuit, and the output is taken out from the end of the tuned circuit. The electronic variable resistor is composed of a voltage-controlled variable gain differential amplifier, and the input signal is applied to the non-inverting input terminal, and a current corresponding to the output signal is fed back to the inverting input terminal. This is a characteristic feature.

[Effect]

FIG. 1 is a diagram of the principle of the present invention, where a is an ideal form in which the loss resistance R is ∞ in parallel tuned circuits L and C.
VR is an electronic variable resistor connected in series between the signal voltage source e _i and the tuning circuits L and C. The output e ₀ is taken out as a voltage from the end of the tuned circuit, and this filter has a bandpass characteristic in which the bandwidth changes while keeping the gain at the center frequency constant depending on the value of the variable resistor VR. This is because the variable resistor VR enters the tank circuits L and C instead of the loss resistor R, and its selectivity Q is varied. Furthermore, the gain is constant at the center frequency because there is no loss due to the variable resistor VR. Figure b shows a specific example of this filter, consisting of a variable gain differential amplifier A and its feedback conductance.
gm constitutes the electronic variable resistor VR shown in FIG. The control voltage Vc in this case can be changed automatically depending on the electric field strength, the strength of the interfering station, etc., or can be changed by a manual volume or switch.

Figure 1c shows a circuit that compensates for loss (R) when there is loss in the parallel tuned circuit (resistance R is finite).
gm ₁ (≒1/R) is connected in parallel to variable resistor VR. Both gm ₁ and gm ₂ are conductances that convert a voltage source into a current source, and gm ₂ is the conductance shown in b in the same figure.
Equivalent to gm. The loss resistance R determines the selectivity of the tuned circuit. In this case, the transfer function from e _i to e ₀ is e ₀ /ei=gm1+gm2・A/(1/R+gm2・A)+j(
ωC−1/ωL) ω=2π: Input frequency. Since ωC=1/ωL at the tuning point, the gain becomes G ₀ =gm1+gm2·A/1/R+gm2·A. Therefore, by selecting gm ₁ =1/R, the influence of the loss resistance R is removed and G ₀ =1, and the gain G ₀ of the filter becomes constant regardless of the gain A of the differential amplifier.

〔Example〕

FIG. 2 is a circuit diagram of an embodiment of the present invention embodying FIG. 1c. In the figure, C ₁ is the feedback coupling capacitor, C ₂ is the bypass capacitor,
Q ₁ and Q ₂ are transistors that constitute the differential amplifier for gm ₁ , R ₁ is the emitter side resistance, and Q ₃ and Q ₄ are
A transistor constitutes a differential amplifier for gm ₂ and A, R ₂ is a resistor on its emitter side, Q ₅ and Q ₆ are transistors that constitute a differential circuit that varies the gain A with a control voltage Vc, and V _B is a transistor of each part. Bias voltage.

In the above configuration, if 2・R ₁ =R is set and the shunt ratio of Q ₅ and Q ₆ is K (=0 to 1), the gain of differential amplifier A is A=R/2・R ₂・It becomes K. Therefore, the value at K=1 is selected to be, for example, about A=20. If this is done, the bandwidth will be widened when Vc> _VB , and the bandwidth will be narrowed when Vc< _VB .

〔Effect of the invention〕

As described above, the present invention has the advantage that a filter whose bandwidth can be continuously varied by voltage control while keeping the gain constant at the tuning point can be realized with a simple circuit configuration.

[Brief explanation of drawings]

FIG. 1 is a diagram explaining the principle of the present invention, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. In the figure, gm ₁ is the first conductance, gm ₂ is the second conductance, A is the differential amplifier, L and C are the parallel tuning circuits, R is the loss resistance thereof, and e _i is the input signal source.

Claims (1)

[Claims] 1. A variable bandwidth type in which an electronic variable resistor whose resistance value can be varied by a control voltage is connected between an input signal voltage source and a parallel tuned circuit, and the output is taken out from the end of the tuned circuit. The electronic variable resistor is a filter, and the electronic variable resistor is composed of a voltage-controlled variable gain differential amplifier, the input signal is applied to a non-inverting input terminal, and a current corresponding to the output signal is fed back to an inverting input terminal. A variable bandwidth filter characterized by: 2. The variable bandwidth filter according to claim 1, characterized in that a circuit for compensating the loss of the parallel tuning circuit is connected in parallel to the electronic variable resistor.