JPS6214530A - Intermediate frequency band pass filter of variable band width type - Google Patents

Abstract

PURPOSE:To vary the IF band width of an AM receiver in accordance with the electric field intensity and prevent the variation of level in the center frequency by setting the negative feedback rate from the output of a filter to the input-side resonance circuit to a prescribed value. CONSTITUTION:The gain and the selectivity of a series resonance circuit Q for an input voltage Vi are set to desired values, and the resonance output is supplied to a forward differential voltage variable amplifier A. Values of resistances R11-R13 are set to prescribed values in the amplifier A, and a control voltage Vc is varied to control the current flowed to the resistance R12, thereby varying the amplifier A in the range of 1-20. In a feedback circuit beta, constants of a voltage control amplifier A' are set to the same values as those of the amplifier A. The output of a subtractor N2 of an output V0 of the circuit is inverted by T20 and is converted to a current by T5' and is added to the output of T20 to obtain a negative feedback voltage, the inverse of V0'. The feedback rate is obtained in accordance with beta=V0'/V0=(A'-1)/A'.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an intermediate frequency band-pass filter for an AM receiver whose bandwidth can be automatically varied according to a received electric field.

[Conventional technology]

In a superhetero gain type AM receiver, sound quality can be expected to improve by widening the width of 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
How to switch the F band between wide and narrow, and make it narrow in weak electric fields, and ■ ATC (auto tone control) in the audio band after IF detection while keeping the IF bandwidth constant.
There is a way to apply it.

[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. Moreover, 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 I
Since the P band is kept wide, suppression by adjacent stations (AGC function) increases, and since the ATC circuit is as variable as the -order filter of CR, it has the disadvantage of being less effective.

Furthermore, when reproducing AM stereo, circuits for two channels are required, making the configuration complicated.

The present invention seeks to improve these points.

[Failure to solve the problem]

FIG. 1 is a diagram showing the principle of the present invention, in which (a) is a block diagram of the entire IF bandpass filter, (bl is a detailed block diagram of its feedback circuit β). In the figure, Q is a resonant circuit (gain of 1 at peak ), A is a voltage controlled amplifier, and β is a feedback circuit, and these symbols Q, A, and β are also used to represent selectivity, gain, and feedback ratio, respectively.The transfer function G of this filter is expressed as Therefore, the feedback rate β is as follows.Here, if Q=1.G=1, then N1 is a subtracter.

The feedback circuit shown in FIG. 6(b) is constructed using an inverting amplifier (-1) and a voltage control amplifier A', and the input/output ratio (β) is expressed by the following equation.

Therefore, by setting A'#A, equation (4) becomes equal to equation (3). N2 is a subtracter and P+ is an adder.

The gain of the amplifier A on the forward side can be varied by the control voltage Vc. This control voltage Vc is the antenna input level (electric field strength)
When Vc is large, A is large, and when Vc is small, A is small.

The bandwidth of the negative feedback extraction point Vo is wide when A is large and narrow when A is small, so the higher the antenna input level is, the wider the band becomes, and conversely, the lower the antenna input level is, the narrower the band becomes. Figure 2 shows this frequency characteristic diagram.
The case of 0 is shown. In both cases, the center frequency (fa) is 450KHz, and Q=98. β=1.

As is clear from the figure, the bandwidth can be changed by varying A. However, if β is constant, the peak level of the center frequency will also vary, which becomes a noise factor.

Therefore, the frequency characteristics shown in Fig. 3 are such that this β is also variable with Vc, (al is β = 0 when A = 1, β is set to β = 0.8 when A = 5, (C1 is changed to β = 0.95 when A = 20, and the peak level is the same in both cases. In this way, level fluctuations due to changes in bandwidth will not occur. Below, the implementation Explain an example.

〔Example〕

FIG. 4 is a circuit diagram showing an embodiment of the present invention, in which the resonant circuit Q
consists of a series resonant circuit (fo=450 KHz) consisting of transistors TI and T2, resistors RI to R3, a capacitor C and an inductance L. The gain Go of this circuit at the resonance point fo is set to . Further, the selectivity Q is set to a desired value by ωL Q=-. The output of the subtracter N1 is applied to the base of the transistor T1, and the bias voltage Va is applied to the base of the transistor T2.

The forward voltage variable amplifier A is a differential type consisting of transistors T3 to T5 and resistors R11 to RI3, and is unbalanced by the resistor R12. The resistor R13 is provided to prevent oscillation due to A<1. A bias voltage Va is applied to the base of the transistor T5. Further, a reference voltage Vref is applied to the base of the transistor T3, and a control voltage Vc is applied to the base of the transistor T4 paired therewith. The output Vo is taken out from the collector of this T4. In this example, the current flowing through the resistor RI2 is controlled by varying the control voltage Vc, and A is varied in the range of 1 to 20.

The voltage control amplifier A' of the feedback circuit β is a transistor T3.
'~T5' and resistors R1'~R13', and are set to the same constant as amplifier A. The output a of the subtractor N2 is -(V o -V o'), which is inverted by the inverting amplifier (transistor T20) and converted into a current by the voltage variable amplifier A' by the transistor T5'. Since this current flows through T2O-R11' and R12'-T4', it is added to the output of the inverting amplifier (corresponding to adder P1), and an output voltage -Vo' for negative feedback is obtained. Since this output Vo' is Va' = a (LA'), it is obtained by substituting a = - (Vo - Vo').

FIG. 5 is an explanatory diagram of an AM receiver using the present invention, (a
) is a block diagram. In the figure, 1 is an antenna;
2 is a radio frequency (RF) circuit and a frequency conversion circuit, 3 is a wideband IF tuning circuit, and 4 is a variable bandwidth IF according to the present invention.
A bandpass filter, 5 a detector for detecting its output, 6
7 is an AGC amplifier that receives the signal level Vs and controls the gains of the RF circuit 2 and tuning circuit 3, and 7 is a voltage conversion circuit that converts the AGC voltage VAGc into a control voltage Vc for the filter.

The control voltage Vc undergoes the transformation VS -VAGC-'Vc, but in the end, it changes as shown in the figure (
It changes like bl. As described above, when Vc is large, A is large, and when Vc is small, A is small. Therefore, the characteristics of the filter 4 are that a strong electric field has a wide band, and a weak electric field has a narrow band. Same figure [C]
shows this for the cases of A=1 and A=20.

When A=20, it expands to the bandwidth of tuning circuit 3, and A=
When it is 1, the bandwidth is narrowed to the narrowest bandwidth of the filter 4.

Figure (d) shows the continuous change in this bandwidth, with β as a parameter. β=1 corresponds to FIG. 2, and β=(A-1)/A corresponds to FIG. When β=1, as mentioned above, the disadvantageous force 3 that causes fluctuations in the peak level
match. Yo [ffl (e) 4 Yo:, Wow, 7 Io, 1. Also, set the selector switch 8 to the middle point M
If connected to N, the filter band can be automatically varied by the control voltage Vc, but if connected to N, the filter band will be fixed at a narrow band, W
If you connect to , you can get fixed broadband.

〔Effect of the invention〕

As described above, according to the present invention, the IF bandwidth of the AM receiver can be varied according to the electric field strength, and there is an advantage that no level fluctuation occurs at the center frequency.

[Brief explanation of drawings]

Fig. 1 is a principle block diagram of the present invention, Figs. 2 and 3 are frequency characteristic diagrams thereof, Fig. 4 is a circuit diagram showing an embodiment of the present invention, and Fig. 5 is an AM reception using the present invention. FIG. In the figure, Q is a resonant circuit, A is a voltage controlled amplifier, β is a feedback circuit, and A' is its voltage controlled amplifier. Applicant: Fujitsu Ten Ltd. Representative Patent Attorney Minoru Aoyagi (α) Filter C (b) Feedback circuit β Figure 1P, a block diagram of the principle of the present invention
lcoC1.

Claims (1)

[Claims] A resonant circuit having a resonant point at the center frequency of an intermediate frequency band, and a variable gain voltage controlled amplifier that amplifies the output thereof;
A feedback circuit that negatively feeds back the output of the amplifier to the input of the resonant circuit is provided, and the gain A of the amplifier is varied by a control voltage that changes according to the received electric field strength, and the feedback circuit also has the same type of voltage control. 1. A variable bandwidth intermediate frequency band-pass filter for an AM receiver, characterized in that an amplifier is provided and the feedback factor β is set to β=(A-1)/A.