JPS6233399Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a multipath detection circuit for FM broadcast receivers and the like.

Conventional multipath detection circuits use a filter to extract high-frequency noise outside the modulation frequency band, rectify it, and detect it. When using this method, the operation speed is slow because it always operates or rectifies when multipath occurs, and it is not possible to increase the operation speed beyond a certain level. Therefore, when applied to a receiver installed in a car, etc.,
There were drawbacks such as malfunction due to ignition noise.

A detection method has also been proposed in which an intermediate frequency signal is detected from a portion of the intermediate frequency stage where the limiter does not work sufficiently, and multipath is detected using the envelope component of the intermediate frequency signal.

In this method, as shown in FIG.
Comparator 2 integrates the level detection output passed through
A comparator 2 compares the average value and peak value of the level detection output, and a multipath detection signal is obtained from the output of the comparator 2.

Therefore, when multipath occurs as shown in FIG. 1b, the input applied to the inverting input terminal of comparator 2 is as shown by curve A in FIG. 1b, and comparator 2 integrates this input. The input applied to the non-inverting input terminal of is as shown by curve B in FIG. 1b. Therefore, the output signal of the comparator 2 is the pulse C in FIG. 1b.
The occurrence of multipath is detected as shown in FIG.

Further, the relationship between the electric field strength and input A-input B is as shown in FIG. 1c.

However, the above-mentioned method of detecting multipath using envelope components has the drawback that when the electric field becomes weak, the comparison voltage difference becomes small and malfunctions occur due to noise and the like.

The present invention was conceived in view of the above, and aims to provide a multipath detection circuit that eliminates the above drawbacks and detects multipath using the envelope component of an intermediate frequency signal. .

The present invention will be explained below with reference to examples.

FIG. 2 is a circuit diagram of an embodiment of the present invention.

Reference numeral 11 denotes a PNP type transistor forming an emitter follower, and a level detection output obtained by envelope detection detected from the intermediate frequency stage where the limiter is not working is applied to the base of the transistor 11 through a resistor 12. The collector of transistor 11 is grounded, and the emitter is level shift diode 1.
3, 14, 15 and a load resistor 16 to the power supply +B, and a voltage is applied to the emitter. Further, an integrating circuit consisting of a resistor 17 and a capacitor 18 is connected to the emitter of the transistor 11, and the output voltage of the integrating circuit is applied to the non-inverting input terminal of the comparator 2. The voltage at the common connection point of the resistor 16 and the diode 15 applies the output voltage of the voltage dividing circuit of the resistors 19 and 20, that is, the level adjustment circuit, to the inverting input terminal of the comparator 2.

In this embodiment configured as described above, the level detection output is amplified by the emitter follower, integrated by an integrating circuit consisting of a resistor 17 and a capacitor 18, and applied to the non-inverting input terminal of the comparator 2. Output is diode 13, 14
and 15, and this level-shifted voltage is divided by a level adjustment circuit made up of resistors 19 and 20 and applied to the inverting input terminal of comparator 2. The voltage a applied to the inverting input terminal of the comparator 2 and the voltage b applied to the non-inverting input terminal correspond to the input voltages A and B in FIG.
The pulse waveform output C shown in FIG. 1C is outputted from the output end of the comparator 2 to detect multipath.

In this case, assuming that the resistance value of the resistor 19 is 0Ω and the resistance value of the resistor 20 is ∞Ω, if the input electric field suddenly drops, the emitter of the transistor 11 will have a voltage approximately 0.6 to 0.7V higher than the base potential. . Further, the voltage at the connection point between the resistor 16 and the diode 15 is level-shifted by about 1.8 to 2.1V in the example shown in FIG. At this time, the input voltage b does not change rapidly due to the time constant of the integrating circuit including the resistor 17 and the capacitor 18. Therefore, when input voltage a temporarily falls below input voltage b, the output of comparator 2 becomes high level, and multipath detection is performed.

Here, when the peak value voltage is adjusted using the level adjustment circuit of resistors 19 and 20, the input voltage a becomes
Assuming that the resistance values of and 20 are R ₁₉ and R ₂₀ , R ₂₀ / (R ₁₉ +
_R20 ).

Therefore, as mentioned above, when R ₁₉ = 0Ω R ₂₀ = ∞Ω, the characteristics of the input voltage a and the level detection output voltage applied to the input terminal IN of the input voltage b are shown by the straight lines α and β in Figure 3. It will be as shown. Further, when the input voltage a is adjusted by the level adjustment circuit, the characteristics with respect to the level detection output voltage are as shown by the straight line γ shown by the broken line in FIG.

On the other hand, it is generally known that as the distance from the transmitting station increases, the ratio of reflected waves to direct waves decreases and more multipaths occur. Therefore, it has been confirmed through experiments that in weak electric fields it is better to reduce detection sensitivity and switch antennas to only use multipaths that generate large noise, and in strong electric fields it is better to increase detection sensitivity so that even the slightest noise is not overlooked. being admired.

Therefore, according to this embodiment, by adjusting the level using the level adjustment circuit, the difference between the input voltages a and b when the level detection output voltage is small is large, and the difference between the input voltages a and b when the level detection output voltage is large is increased.
The difference between the two is small, and the above characteristics can be achieved with a simple configuration.

Furthermore, the number of level shift diodes does not need to be limited to three. If the number is increased, the fluctuation of the input voltage difference (a-b) due to the level detection output voltage will become larger, and if the number is decreased, this fluctuation will become smaller.

As explained above, according to the present invention, automatic sensitivity adjustment can be performed using the multipath detection circuit, and the circuit configuration for this purpose is also simple.

[Brief explanation of the drawing]

Figures 1a, b, and c are circuit diagrams of conventional multipath detection circuits and explanatory diagrams of their functions, Figure 2 is a circuit diagram of an embodiment of the present invention, and Figure 3 is a circuit diagram of an embodiment of the present invention. It is an explanatory diagram of the action. 2... Comparators, 13, 14 and 15... Diodes for level shifting, 17 and 18... Resistors and capacitors forming an integrating circuit, 19 and 20... Resistors for level adjustment.

Claims (1)

[Scope of utility model registration request] an emitter follower into which is input a level detection output obtained by envelope-detecting an intermediate frequency signal taken out before the limiter of the FM receiver operates; an integrating circuit connected to the output end of the emitter follower;
A level shift circuit connected to the output terminal of the emitter follower, a level adjustment circuit that divides the output voltage of the emitter follower level-shifted by this level shift circuit, and an output voltage of the integration circuit and an output voltage of the level adjustment circuit are compared. A multipath detection circuit comprising a comparator that detects multipaths using the output of the comparator.