JPH0320171B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an intermediate frequency bandpass filter for an AM receiver that can automatically vary the bandwidth depending on the received electric field.

[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. To improve this point, conventional methods have been used to switch the IF band between wide and narrow using a switch, making the band narrower in weak electric fields, or keeping the IF bandwidth constant and using 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]

Figure 1 is a diagram of the principle of the present invention, where a is a block diagram of the entire IF bandpass filter, and b is its feedback circuit β.
FIG. In the figure, Q is a resonant circuit (gain of 1 at peak), A is a voltage controlled amplifier,
β is a feedback circuit, and these symbols Q, A, and β are also used to represent selectivity, gain, and feedback rate, respectively. The transfer function G of this filter is expressed as G=V ₀ /V _i =QA/1+βQA (1), so the feedback rate β is β=QA−G/QAG (2). Here, if Q=1 and G=1, then β=A-1/A (3). N ₁ is a subtractor.

The feedback circuit shown in figure b is constructed using an inverting amplifier (-1) and an electrically controlled amplifier A', and the input/output ratio (β) is
is expressed by the following equation.

β=V ₀ ′/V ₀ =A′−1/A′ (4) Therefore, by setting A′≈A, equation (4) becomes equal to equation (3). N ₂ is a subtractor and P ₁ is an adder.

The gain of amplifier A on the forward direction side can be varied by control voltage Vc. This control voltage Vc changes depending on the antenna input level (field strength), and when Vc is large, A is large;
When Vc is small, A is small. The bandwidth of the negative feedback extraction point V ₀ is wide when A is large and narrow when A is small, so the higher the antenna input level, the higher the band, and conversely, the lower the antenna input level, the narrower the band. Figure 2 shows this frequency characteristic diagram, a
indicates the case where A=1, b indicates A=5, and c indicates A=10. In both cases, the center frequency (f ₀ ) is 450 KHz, Q=98, and 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 also changes, which becomes a noise factor. Therefore, the frequency characteristic shown in Figure 3 is that this β is also varied by Vc.
is β=0 when A=1, and b is β when A=5.
= 0.8, c is varied 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. Examples will be described below.

〔Example〕

FIG. 4 is a circuit diagram showing an embodiment of the present invention, in which a resonant circuit Q includes transistors T ₁ and T ₂ and resistors R ₁ to _{R 3} ,
In addition, it consists of a series resonant circuit (f ₀ =450KHz) consisting of a capacitor C and an inductance L. The gain G ₀ at the resonance point f ₀ of this circuit is set to G ₀ =R ₁ /R ₂ =1. Further, the selectivity Q is set to a desired value as Q=ωL/R ₂ . 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 transistor T ₃
It is a differential type consisting of ~ _T5 and resistors _R11 ~ _R13 , and resistor _R12
The balance has been lost. Resistance R ₁₃ is A<1
This is to prevent oscillation. At the base of the transistor T ₅ there is a bias voltage
Va is applied. 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 V ₀ is taken out from the collector of this T ₄ . In this example, R ₁₁ + R ₁₂ / R ₁₃ = 20 R ₁₁ / R ₁₃ = 1 is set, the control voltage Vc is varied to control the flow flowing through resistor R ₁₂ , and A is variable in the range of 1 to 20. do.

The voltage control amplifier A' of the feedback circuit β is composed of transistors T ₃ ' to T ₅ ' and resistors R ₁₁ ' to R ₁₃ ', and is set to the same constant as that of the amplifier A. Output a of subtractor _N2
is -(V ₀ -V ₀ '), the inverting amplifier (transistor T ₂₀ ) inverts this, and the variable voltage amplifier A' converts it into a current with the transistor T ₅ '. This current is
Since it flows through T ₂₀ −R ₁₁ ′−R ₁₂ ′−T ₄ ′, it is added to the output of the inverting amplifier (corresponding to adder P ₁ ), and an output voltage −V ₀ ′ for negative feedback is obtained. This output V ₀ ' is V ₀ '=a(1-A'), so by substituting a=-(V ₀ -V ₀ '), β=V ₀ '/V ₀ =A'-1/A ′ is obtained.

FIG. 5 is an explanatory diagram of an AM receiver to which the present invention is applied, and 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, 4 is a variable bandwidth IF bandpass filter according to the present invention, and 5 is a detector for detecting the output thereof. The detector, 6, is an AGC amplifier that receives the signal level Vs and controls the gain of the RF circuit 2 and the tuning circuit 3, and 7 is also a voltage conversion circuit that converts the AGC voltage V _AGC into a control voltage Vc for the filter.

The control voltage Vc undergoes a transformation of Vs→V _AGC →Vc, but ultimately changes as shown in b in the figure with respect to the antenna input level. And as mentioned above, A with large Vc
Since A is small when the electric field is large and Vc is small, the characteristic of the filter 4 is that the stronger the electric field, the wider the band, and the weaker the electric field, the narrower the band.
Figure c shows this for the cases of A=1 and A=20. When A=20, the bandwidth goes up to the tuning circuit 3, and when A=1, it goes down to the narrowest bandwidth of the filter 4. Figure d shows a continuous change in this bandwidth, with β as a parameter. β=1 corresponds to FIG. 2, β=(A−
1) /A corresponds to FIG. When β=1, there is a disadvantage that the peak level fluctuates as described above. Figure e shows this.

If the changeover switch 8 is connected to the midpoint M, the filter band can be automatically varied by the control voltage Vc, but if it is connected to N, the band can be fixed to a narrow band, and if it is connected to W, the band can be fixed to a wide band.

〔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 level fluctuation does not occur at the center frequency.

[Brief explanation of the drawing]

Fig. 1 is a principle block diagram of the present invention, Figs. 2 and 3 are its frequency characteristic diagrams, Fig. 4 is a circuit diagram showing an embodiment of the present invention, and Fig. 5 is an AM reception to which the present invention is applied. This is a description of the machine. In the figure, Q is a resonant circuit, A is a voltage controlled amplifier, and β
is the feedback circuit, and A' is its voltage controlled amplifier.

Claims (1)

[Claims] 1. A resonant circuit having a resonant point at the center frequency of an intermediate frequency band, a variable gain voltage control amplifier that amplifies the output thereof, and negative feedback of the output of the amplifier to the input of the resonant circuit. a feedback circuit, 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 includes an amplifier whose gain is varied by a control voltage that changes according to the received electric field strength. A variable bandwidth intermediate frequency bandpass filter for an AM receiver, characterized in that the feedback rate β is set to β=A-1/A, and the gain at the center frequency is always constant. .