JPH0127299Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an AM receiver, and particularly to an AM receiver that is most suitable for automatic channel selection.

(Prior art) Generally, an AM receiver has a receiver like the one shown in Figure 1A.
An AGC (automatic gain control) circuit is installed to ensure that appropriate reproduction sound is always provided even when the antenna input fluctuates.

In receivers that do not have an AGC circuit, the signal will become saturated at the intermediate amplification stage.
AM components cannot be detected and normal playback is no longer possible.

For this reason, an AGC circuit is indispensable in conventional receivers, but this AGC circuit has caused a problem in that the distortion rate of the modulated signal, especially on the low frequency side, deteriorates. One possible way to improve this distortion factor is to make the time constant of the time constant setting section T of AGC circuit A sufficiently large relative to the lowest modulation frequency, but this method results in a delay in the response of the AGC circuit. In an automatic tuning type receiver such as a synthesizer tuner, the automatic stop function will be inhibited. The automatic stop function rectifies the signal output from the final stage of the IF amplification through the narrowband bandpass filter BPF ₁₂ , and makes the bandwidth approximately ±3KHz. It automatically locks to the station's carrier frequency.

Therefore, in conventional receivers, it is necessary to set a time constant that gives priority to the automatic stop function at the expense of distortion rate.

Another problem is that the AM detection output Dout outputted from the mixer M via the bandpass filter BPF ₁₁ , the amplifiers A ₁₁ , A ₁₂ , A ₁₃ and the detector D ₁₁ and the AM detection output Dout output from the detector D ₁₁ through the bandpass filter BPF ₁₂ ,
S meter output Sout output via detector D ₁₂
varies depending on the characteristics of AGC circuit A,
There were problems such as difficulty in maintaining product uniformity.

(Purpose of the invention) The present invention was devised in view of the above points, and its purpose is to improve the distortion rate and fully operate the frequency scan automatic stop function, and to easily realize uniform characteristics. Our goal is to provide a receiver that allows you to

(Structure of the invention) A receiver according to the invention includes an intermediate frequency amplification stage configured by cascading a plurality of amplitude-limiting amplifiers;
A signal synthesizing section for synthesizing amplitude components of signals taken out from each amplitude limiting amplifier via a diode, and an S meter time constant circuit to which the signal from the signal synthesizing section is supplied are provided, and the amplitude limiting amplifier The configuration is such that a sufficiently limited signal is extracted from the final stage of the circuit.

(Example) Examples of the receiver according to the present invention are shown in Figs.
This will be explained based on the diagram.

In the figure, reference numeral 1 denotes an intermediate frequency amplification stage, which is composed of a plurality of amplitude limiting amplifiers A ₁ to _{A 4} connected in series.

Reference numeral 2 denotes a signal synthesis unit for synthesizing the signals extracted from the respective amplitude limiting amplifiers A ₁ to _{A 4} , and in the illustrated example, it extracts the signals detected via the diodes D ₁ to _{D 4} . It's summery. In the embodiment, the latter stage of the signal combining section 2 and the earth GND
An S-meter detection section 3 consisting of a resistor R and a capacitor C connected in series is provided between the two and is capable of detecting signal strength. In addition, a bandpass filter is installed after the intermediate frequency amplification stage 1.
BPF ₂ and detector D are arranged in series,
The output from this is input to an automatic stop function (not shown) to automatically stop the frequency scan.

An example of the operation of the above configuration will be explained based on FIGS. 3 to 5.

The amplitude-limiting amplifiers _A1 to _A4 exhibit detection output characteristics as shown in FIG. 3, _P1 to _P4 , respectively, depending on the electric field strength of the input signal, and their combined output is indicated by a dotted line P. In FIG. 3, the horizontal axis shows the electric field strength, and the vertical axis shows the detected output, and the graph drawn by the dotted line P shows that a constant detected output is always obtained when the electric field strength exceeds a certain level. Further, from the S meter detection section 3, a signal is obtained in which only the DC component is separated from the detection output, and as shown in FIG. 4, the signals _Q1 to _Q4 from the respective amplitude limiting amplifiers _A1 to _A4 are Of these, only the DC components are combined, and the electric field strength (horizontal axis)
The S meter output (vertical axis) is a linear function relationship Q
becomes.

Therefore, the linearity of the S meter can be increased and accurate display can be performed. In addition, since this kind of linearity can be obtained without adjustment, it is possible to
There is no fear that AM detection output characteristics or S meter characteristics will differ depending on the product, and products can be made uniform. The detector D outputs a signal to stop the frequency scan, and as shown in Figure 5, this signal exhibits excellent limiting characteristics against changes in the input electric field, and also has a time constant. Therefore, the frequency scan can be stopped at an accurate tuning position. Moreover, since it does not use an AGC circuit unlike conventional ones, the sound quality in the low modulation frequency range is also good, and the tuning characteristics and sound quality can be improved.

Figure 6 shows an example of application to an AM-FM receiver.
It is designed to also operate the S-meter amplification stage and IF amplification stage in an FM receiver.

In terms of configuration, the input signal source of the intermediate frequency amplification stage 1 can be selectively switched between an FM system receiving section and an AM system receiving section using a switch SW. The FM system receiving section includes an FM antenna FA, an FM front end F, a bandpass filter BPF ₁ , and an FM detector provided after the intermediate frequency amplification stage 1.
The AM receiving section consists of an AM antenna ANT, mixer M, and a bandpass filter.
BPF ₁ is the main component. in this way,
By configuring the receiver as a two-band receiver, the main circuits can be used for the same purpose, resulting in a significant cost reduction.

(Effects of the invention) According to the receiver according to the invention, the gain can be accurately limited without using an AGC circuit, so distortion caused by the AGC circuit does not occur, and especially automatic If applied to a tuning type receiver,
It has excellent features such as being able to achieve both frequency locking characteristics and low-frequency distortion characteristics.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the main parts of a conventional receiver, Figs. 2 to 6 show embodiments of the receiver according to the present invention, Fig. 2 is a basic block diagram, and Figs. FIG. 5 is an input/output characteristic diagram, and FIG. 6 is a block diagram showing another embodiment. 1... Intermediate frequency amplification stage, _A1 to _A4 ... Amplitude limiting amplifier, 2... Signal synthesis section, 3... S meter detection section.

Claims (1)

[Scope of utility model registration request] an intermediate frequency amplification stage configured by cascading a plurality of amplitude-limiting amplifiers; a signal synthesizing section for synthesizing the amplitude components of signals taken out from the respective amplitude-limiting amplifiers via diodes; An AM receiver comprising an S meter time constant circuit to which a signal is supplied, and configured to extract a sufficiently limited signal from the final stage of the amplitude limiting amplifier.