JPS5824521Y2 - FM receiver IF circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to an IF circuit for an FM receiver, and more particularly to an IF circuit that has an improved portion for extracting a reception state indication signal for selectivity indication or multipath indication.

A part of a conventional IP circuit is shown in FIG.

The intermediate frequency signal (hereinafter referred to as "rIF signal") amplified by the transistor Tr1, to which the DC power supply voltage element B is applied to the collector via the resistor R1, is supplied to the filter 1 via the resistor R2, where the waveform is shaped. After receiving, resistance R
3 and is supplied to the base of the transistor Tr2.

The output of the transistor Tr2 is then detected by a discriminator and guided to a primary stage circuit, such as an FM demodulator.

On the other hand, the signal appearing on the collector side of the transistor Tri passes through the capacitor C1.

The resulting rectified output is rectified by a rectifier circuit 2 consisting of diodes DI and D2 and a capacitor C2.

It is used as an instruction signal such as selectivity instruction or multipath instruction.

However, the transistor Tri from which the instruction signal is taken out
Collector vA! l (point a) has a relatively small resistance R
Because it is connected to the filter 10 input terminal through 2Y, it is affected by the operation of filter 1.

Indicates the tendency of amplitude characteristics to change depending on frequency.

Therefore, a disadvantage arises that the levels of the IF signal and the instruction signal do not correspond to each other.

Although the influence of the filter 1 can be reduced by increasing the value of the resistor R2, this causes a decrease in the input signal level to the filter 1 and a deterioration of the transmission characteristics of the filter 1.

This invention aims to provide an IF circuit that is modified so that an instruction signal having a level that accurately corresponds to the IP signal level can be obtained without adversely affecting the operation of a waveform shaping filter.

FIG. 2 shows the configuration of an IF circuit according to an embodiment of this invention.

The circuit of FIG. 2 has the following features: a power supply voltage element B is connected to the input terminal of the filter 10 via a resistor R1'aj, and a resistor is connected between the point a of the collector rigidity of the transistor Tr1 and the input terminal of the filter 10. This circuit differs from the circuit shown in FIG. 1 in two points: capacitor C3 is connected in parallel with R2.

In FIG. 2, parts equivalent to those shown in FIG. 1 are designated by the same reference numerals, and their explanations will be omitted.

In the circuit shown in FIG. 2, the +B constant power supply is directly supplied to the input terminal of the filter 10 via the resistor R1.

The influence of the operation of filter 1 on the IF signal level at point a is significantly reduced, and this effect becomes more pronounced as the value of resistor R2 increases, but on the other hand, the selectivity of the two signals and the capture ratio deteriorate. I'll come.

However, the capacitor C3 connected in parallel with the resistor R2 is
It has a function of effectively preventing this characteristic deterioration.

In other words, capacitor C3 works to lower the impedance for the signal supplied to filter 1 from point a, so even if resistor R2 is increased to a certain extent, the two-signal selectivity or capture ratio will deteriorate. There isn't.

The two-signal selectivity characteristic of the circuit shown in FIG. 2 is shown in FIG. 3, and the capture ratio characteristic is shown in FIG. 4 with the symbol A.

The curve indicated by the symbol B is. The characteristics of a circuit without capacitor C3 are shown.

As described above, according to this invention, the two conflicting conditions of accurately maintaining the correspondence relationship between the instruction signal level and the IF signal level and not deteriorating the two-signal selectivity characteristic and capture ratio can be met. The effect on improving the reception condition of the FM tuner is extremely significant.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a part of a conventional IF circuit, Fig. 2 is a circuit diagram showing a part of an IF circuit according to an embodiment of this invention, and Fig. 3 is a circuit diagram showing a part of an IF circuit according to an embodiment of this invention. Figure 4 also captures the two-signal selectivity characteristics. 3 is a graph showing each ratio characteristic. 1... Filter, 2... Rectifier circuit.

Claims (1)

[Scope of utility model registration request] A resistor that supplies the IF signal appearing on the collector side of a transistor that amplifies the input signal to the input terminal of a waveform shaping filter, and a rectifier circuit that rectifies a part of the IF signal to generate an instruction signal. In the FM receiver, the input end of the filter is connected to a DC power source, and a capacitor is connected in parallel with the resistor.
F circuit.