JPH0320172B2 - - Google Patents

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 that automatically varies the bandwidth and amplification depending on the received electric field.

[Conventional technology]

In a superheterodyne AM receiver, the IF
Broadening 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 change the IF band to wide or 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. 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. moreover,
When playing AM stereo, circuits for two channels are required. In addition, these band variable circuits and IF AGC circuits need to be provided separately, so
It is inevitable that the overall configuration of the receiver will become complicated.

The present invention aims to provide AGC characteristics by automatically varying the bandwidth of an IF band pass filter according to the received electric field, and by simultaneously varying its amplification degree.

[Means for solving problems]

FIG. 1 is a block diagram of the principle of the present invention, in which a is a basic type and b is an improved type. Q is a resonant circuit with a peak gain of 1. A is a fixed gain amplifier. β is a feedback circuit, which is a voltage-controlled amplifier (or attenuator) whose feedback rate is controlled by the control voltage Vc.
Use. In the following, Q, A, and β are selectivity, gain,
It is also used as each symbol for the feedback rate. N is a subtractor.

The basic transfer function of a is V ₀ /V _i =QA/1+βQA (1), and the frequency characteristics set to Q=98 and A=10 are as shown in FIG. For a to c in the same figure, β is 0.1 to
It can be varied within a range of 1, and three representative points are shown. When β = 0.1, the bandwidth is the narrowest, but
Highest gain. On the other hand, when β=1, the bandwidth is the widest, but the gain is the lowest. Therefore, the control voltage Vc should have a characteristic that β approaches 1 when the received electric field is high, and conversely approaches 0.1 when the received electric field is low.

The increased variable range of β from 0.1 to 1 is limited in consideration of the general need to set the IF AGC amount to about 10 dB. If this is done, the amount of change in bandwidth and the amount of change in gain will have a fixed relationship, but the improved type (b) breaks this relationship and widens the variable range of bandwidth. In this circuit, a voltage controlled amplifier (or attenuator) β' having the same characteristics as the feedback circuit β is connected in series with the basic type fixed gain amplifier A of a. The transfer function of this filter is V ₀ /V _i =β′QA/1+ββ′QA (2).

The above equation shows that the gain of A can be increased by the amount of attenuation due to β', and the negative feedback is increased accordingly, and the variable range of the bandwidth is expanded accordingly. As an example, A
Fig. 3 shows the frequency characteristics when β≈β' is set to 100 and is varied in the range of 0.1 to 1. In the same figure, b is A.
= 100, and β = 1. Comparing this with Fig. 2c, it can be seen that the band is sufficiently widened. Figure 3a is the same as Figure 2a, with the narrowest bandwidth (maximum gain).

〔Example〕

FIG. 4 is a circuit diagram showing one embodiment of the present invention, which is an improved example of FIG. 1b. In the figure, a series circuit of transistors T ₁ and T ₂ and resistors R ₁ to R ₃ , as well as a capacitor C and an inductance L constitutes a resonant circuit Q. The ratio R ₁ /R ₂ of resistors R ₁ and R ₂ is set to 100.
The gain of the amplifier with A=100 is also used. Note that the resonance point f ₀ of the LC is 450 KHz, and Q=ωL/R. Feedback circuit β is a transistor
This is a differential amplifier consisting of T ₃ to _{T 5} and resistors R ₁₁ to R ₁₃ , and its gain changes depending on the control voltage Vc. Vref is a reference voltage, and Va and Vb are bias voltages. In this amplifier, in order to vary β between 1 and 0.1, R ₁₁ +R ₁₂ /R ₁₃ =1 and R ₁₁ /R ₁₃ =0.1 are set. The forward voltage controlled amplifier β′ has the same configuration, T ₃ ′ to T ₅ ′ are transistors, and R ₁₁ ′ to
R ₁₃ ′ is the resistance.

FIG. 5 is an explanatory diagram of an AM receiver to which the present invention is applied, in which 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 an explanatory diagram of an AM receiver to which the present invention is applied.
5 is an IF pass filter according to the present invention, 5 is a detector,
6 is the signal Vs obtained from it as the AGC voltage
An AGC amplifier converts the voltage V _AGC to negative feedback to the RF circuit 2, and 7 is a voltage conversion circuit that converts the voltage V _AGC into a control voltage Vc for varying the bandwidth. This control voltage Vc is converted as Vs→V _AGC →Vc, and is basically in a proportional relationship with the antenna input level (field strength) as shown in d. b is the variable band characteristic, and c is the variable gain. In both cases, the solid line shows the basic type using only β, and the broken line shows the improved version using β'.

〔Effect of the invention〕

As described above, according to the present invention, since the IF bandwidth of the AM receiver can be continuously varied according to the electric field strength, the S/N ratio in the case of a magnetic electric field can be improved, and the amplification degree can also be changed at the same time. Since it has an AGC function, it has the advantage of eliminating the need for an additional IF band AGC circuit.

[Brief explanation of drawings]

Fig. 1 is a block diagram of the principle of the present invention, Figs. 2 and 3 show its frequency characteristics, Fig. 4 is a circuit diagram showing an embodiment of the present invention, and Fig. 5 is an AM to which the present invention is applied. FIG. 2 is an explanatory diagram of a receiver. In the figure, Q is a resonant circuit, A is a fixed gain amplifier, and β
is a feedback circuit, and β' is a voltage variable amplifier (or attenuator) with the same characteristics.

Claims (1)

[Claims] 1. A resonant circuit having a resonant point at the center frequency of an intermediate frequency band, a fixed gain amplifier that amplifies the output of the resonant circuit, and a feedback circuit that negatively feeds back the output of the amplifier to the input of the resonant circuit. A variable bandwidth intermediate frequency pass filter that changes the pass band width for the intermediate frequency in proportion to the received electric field by changing the feedback rate of the feedback circuit according to the received electric field strength, wherein the resonance A variable bandwidth intermediate frequency pass filter for an AM receiver, characterized in that a variable gain amplifier whose gain changes in proportion to the received electric field is provided between the circuit and the input of the negative feedback circuit.