JPS62226709A - Band width variable filter - Google Patents

Abstract

PURPOSE:To form a filter whose band width is varied continuously by voltage control while the gain at a tuning point is kept constant with simple circuit constitution by connecting an electronic variable resistor whose resistance is varied by a control voltage in series between a voltage source of an input signal and a parallel tuning circuit and extracting an output from the tuning circuit end. CONSTITUTION:If the parallel tuning circuit has a loss, a circuit gm1 to compensate the loss R is connected in parallel with a variable resistor VR. Values gm1, gm2 are conductances to convert the voltage source into the current source. The loss resistance R decides the selectivity of the tuning circuit. For example, in selecting 2R1=R and the current division ratio K of transistors Q5, Q6 as K=0-1, then the gain of a differential amplifier A is expressed as A=(R/2R2)K. Thus, the A=20 at K=1 is selected. Thus, the band width at Vc>VB is widened and the band width is narrowed at Vc<VB.

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 sound quality can be improved by widening the IF (intermediate frequency) band, but on the other hand, it has the drawback of increasing noise in weak electric fields and noise due to adjacent interference. In order to improve this point, in the past, a switch was used to
One method is to switch the F band between wide and narrow, making it a narrow band in weak electric fields, and one is to keep the IF bandwidth constant and apply ATC (auto tone control) to the audio band after IF detection. There is.

[Problem that the invention seeks to solve]

However, method (2) requires manual switching of the switch, which is inconvenient for use in a vehicle, and also impedes safe driving. Furthermore, since noise is generated when switching the IF band, a circuit to mute the noise is required, and there are many other drawbacks. On the other hand, method ■ is automated, but 1
Since the F band is widened (therefore, suppression by adjacent stations (AGC function) increases, and the ATC circuit is as variable as the CR primary filter, it is less effective.

Furthermore, when reproducing AM stereo, circuits for two channels are required, resulting in a complex configuration.

The present invention seeks to improve these points.

[Means for solving problems]

The present invention is characterized in that an electronic variable resistor whose resistance value can be varied by a control voltage is connected in series between the input signal voltage source and the parallel tuned circuit, and the output is taken out from the tuned circuit product 1. It is something to do.

[Effect]

Figure 1 is a diagram of the principle of the present invention, (al is a parallel tuned circuit,
This is an ideal form in which the loss resistance R becomes beautiful at C. (2) R is an electronic variable resistor connected in series between the signal voltage source C3, the tuning circuit, and C. The output eo 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 circuit C instead of the loss resistor R, and its selectivity Q is varied. Further, 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 differential amplifier A with a variable gain (q) and its feedback conductance g [which constitutes the electronic variable resistor VR in Figure (a). The control voltage Vc 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 1 (C) shows the case where there is loss in the parallel tuned circuit (resistance R
is finite), a circuit gm + to compensate for this loss (R)
(#1/R) is connected in parallel to the variable resistor VR. Both grn+ and gm2 are conductances that convert a voltage source into a current source, and g m 2 corresponds to gm in fb) in the same figure. The loss resistance R determines the selectivity of the tuned circuit. 01 to e in this case.

The transfer function to is ω=2 π f f: input frequency. Since ωC=1/ω at the tuning point, the gain is as follows. Therefore, if gml-1/R is selected, the loss resistance R
is removed, G o = 1, and the gain Go of the filter becomes constant regardless of the gain A of the differential amplifier.

〔Example〕

Figure 2 is an embodiment circuit diagram of the present invention embodying Figure 1 (C1). In the figure, C1 is a feedback coupling capacitor, C2 is a bypass capacitor, and QIrO2 is a gm
1 transistor that constitutes the differential amplifier for t, R1 is its emitter side resistance, C3 and C4 are transistors that constitute the differential amplifier for , is the gain A at the control voltage Vc
1 to transistors constituting a differential circuit that varies the voltage, and vB is the bias voltage of each part.

In the above configuration, 2・R+=H, and C++
, Q, etc., the gain of the differential amplifier A is as follows. Therefore, the value at K=1 is selected to be, for example, about A=20. In this way, the bandwidth is widened when V c > V B and narrowed when V c < V B.

〔Effect of the invention〕

As described above, according to the present invention, there is an 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, gml is the first conductance, [In2 is the second conductance, A is the differential amplifier, and L. C is a parallel tuned circuit, R is its ti loss resistance el and is an input signal source. Applicant Fujitsu Ten Ltd. Representative Patent Attorney h Minoru Yanagi (cA) r! Actual property shape principle diagram (b) Buried μ mouth Y valley diagram c Wa p1 (C) - Gulgata principle mouth ■ Honshakuta day 8 J, Fls Rin 1 day moon diagram Figure 1

Claims (2)

[Claims] (1) An electronic variable resistor whose resistance value can be varied by a control voltage is connected in series between the input signal voltage source and the parallel tuned circuit,
A variable bandwidth filter characterized in that an output is taken out from the end of the tuned circuit. (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 an electronic variable resistor.