JPS636174B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention is widely used in FM receivers.
Regarding FM demodulation circuit.

A conventional FM demodulation circuit is configured as shown in FIG. 1, for example. In this Figure 1,
An FM-modulated intermediate frequency signal supplied to an input terminal 1 is demodulated by a frequency discriminator 3 (hereinafter referred to as a discriminator) after an amplitude modulation component is removed by a limiter 2 that limits the amplitude. The output of the discriminator 3 is applied to a low-pass filter 4 that passes only audio signal components and a high-pass filter 5 that extracts noise components.

The output of the high-pass filter 5 is appropriately amplified by an amplifier (not shown), then rectified by a rectifier 6, and converted into a DC voltage containing noise. By controlling the switching means 7 that opens and closes the output of the low-pass filter 4 depending on the presence or absence (or magnitude) of this DC voltage, it is possible to
This prevents noise output from being transmitted to the output terminal 8 when the CN ratio is poor.

Next, the operation of the FM demodulation circuit shown in FIG. 1 will be explained. Figure 2 shows frequency vs. discriminator 3.
As shown in FIG. 2a, when the CN ratio is sufficiently large, the output of the discriminator 3 is only the signal _S1 . This signal _S1 is taken out by a low-pass filter 4 (according to the attenuation characteristic as shown by the solid line _S2 in FIG. 2a).

On the other hand, the output of the high-pass filter 5, which has the attenuation characteristic indicated by the broken line _S3 in FIG. By closing the opening/closing means 7 under this condition, the output of the low-pass filter 4 is sent to the output terminal 8.

In addition, when the above-mentioned CN ratio is small, the output of the discriminator 3 generates so-called triangular noise in which energy is concentrated in a high frequency region in addition to the desired signal _S1 , as shown in Figure 2b. do. Therefore, a large noise energy N ₁ is obtained at the output end of the high-pass filter 5, and a DC voltage is obtained at the rectifying means 6. By opening the opening/closing means 7 under this condition, it is possible to prevent the output (audio signal) of the low-pass filter 4 containing a large amount of noise from appearing at the output terminal 8.

In a conventional FM demodulation circuit, there is a disturbance wave nearby, and the ratio of the power of the required wave to that power (hereinafter referred to as DU ratio) is 1 or more (in other words, the power of the disturbance wave is weaker, and the DU ratio is usually 5. ~6 dB), the opening/closing means 7 would open and the desired wave could not be received.

The reason for this is that the beat _B1 generated by the interference between the interfering wave and the desired wave falls into the passband of the high-pass filter 5, as shown in Figure 2c, and it appears as if the noise has increased. .

The present invention was made to eliminate the above-mentioned drawbacks of the conventional art.The present invention has been made in order to eliminate the above-mentioned drawbacks of the conventional art. By extracting it with a high-pass filter, even if there is a nearby interference wave, the opening/closing means will not open due to the beat, and the DU ratio, which is the theoretical limit of the FM method, will be 1 (i.e., odB) or more. An object of the present invention is to provide an FM demodulation circuit that can ensure reliable reception and improve interference characteristics.

Embodiments of the FM demodulation circuit of the present invention will be described below with reference to the drawings. FIG. 3 is a block diagram showing the configuration of one embodiment. In Fig. 3, when explaining the configuration, the same parts as in Fig. 1 are given the same reference numerals to avoid duplication.
I will mainly describe the parts that are different from Figure 1.

As is clear from comparing FIG. 3 with FIG. 1, in FIG. 3, the output of the high-pass filter 5 is limited by the limiter 9, and the output of the limiter 9 is Frequency discrimination is now possible. The noise component in the output of the discriminator 10 is filtered through the high-pass filter 1.
It is designed to be taken out at 1. The output of the high-pass filter 11 is configured to be rectified by the rectifying means 6. The other configurations are the same as in FIG. 1.

Next, the FM of this invention configured as described above
The operation of the demodulation circuit will be described separately with reference to FIGS. 4 and 5.

(A) When there is no interference and a sufficiently large CN ratio can be obtained.

Since there is no noise component that falls into the passband of the high-pass filter 5, the operation is similar to that of a conventional FM demodulation circuit.

(B) A sufficiently large CN ratio has been obtained, but there is interference from nearby sources.

In this case, in the conventional FM demodulation circuit, the squelch opens and reception is not possible.As mentioned above, the beat with the nearby interference waves falls into the passband of the high-pass filter 5, The situation is shown again in Figure 4a. 4a and 5a both show the relationship between the output levels of the frequency discriminator 3, and S ₁₁
is the signal obtained by discriminator 3, S ₁₂
is the attenuation characteristic of the low-pass filter 4, _S13 is the attenuation characteristic of the high-pass filter 5, _B11 is the beat signal, and _N11 is the noise at the output end of the high-pass filter 11.

4b and 5b both show the relationship between the frequency and the discriminator 10, _{S21 shows the signal obtained by the discriminator 10, and S23 shows} the attenuation characteristic of the high-pass filter 11. , and N ₂₂ represents the noise at the output end of the high-pass filter 11.

Now, as is clear, the beat signal B ₁₁ shown in FIG. 4a is different from noise, and when it is demodulated by the limiter 9 and the discriminator 10, the beat signal B 11 shown in FIG. 4b is obtained.
Although the modulated signal transferred to B ₁₁ appears, there is no noise energy that falls into the passband of the high-pass filter 11 (the attenuation characteristic is shown by the dashed line). Therefore, the opening/closing means 7 is activated by the beat signal _B11 .
Never open.

(C) When the CN ratio is small regardless of the presence or absence of nearby interference.

In this case, a signal _S11 from the discriminator 3 is obtained, as shown in FIG. 5a. this house,
Only the noise _N11 component passes through the high-pass filter 5, and this is passed through the discriminator 10.
As shown in FIG. 5b, the demodulated result is still only noise _N22 , and operates in the same way as a conventional FM demodulation circuit.

Regarding the added elements in this invention,
The following characteristics are required: On the input side of the limiter 9, it is necessary to set a threshold for noise. In other words, since the limiter 9 operates as a high gain amplifier for very small signals, if the output of the discriminator 3 contains even a small amount of noise,
The opening/closing means 7 opens. Therefore, care must be taken to block noise within a range where a good signal-to-noise ratio (hereinafter referred to as SN ratio) can be obtained.

Further, since the frequency band handled by the discriminator 10 is close to the audio band, the ratio band (ratio of the upper limit to the lower limit of the frequency handled) is very large. For this reason, the discriminator 10 is required to be, for example, a broadband type using a differential type. In addition, the cutoff frequency of high-pass filter 11 must be lower than that of high-pass filter 5.

As described above, according to the FM demodulation circuit of the present invention, the output of the first high-pass filter for extracting the noise component from the output of the first discriminator for FM demodulation is limited in amplitude by the limiter, and the output of the second high-pass filter is Perform frequency discrimination using the discriminator and repeat the second
The high-pass filter extracts the noise component, and the output is rectified to pass or block the audio signal component of the output of the first discriminator, so there is no interference signal in the vicinity of the desired frequency signal. Even if there is a DU ratio of 1 or more, it is possible to obtain an FM receiver that can reliably receive the desired frequency signal.

With this, it is possible to suppress as much as possible the interference that will increase with the increase in the number of automobile radios that are expected to develop in the future, and is expected to make a contribution in this field.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the configuration of a conventional FM demodulation circuit, and Figures 2a to 2c are respectively
FIG. 3 is a block diagram showing the configuration of an embodiment of the FM demodulation circuit of the present invention, and FIG. 5a and 5a are characteristic diagrams showing the relationship between the frequency and the output level of the discriminator in the previous stage, respectively, to explain the operation of the FM demodulation circuit in FIG. 3, and FIGS. 4b and 5b, respectively. FIG. 4 is a characteristic diagram showing the relationship between frequency and output level of a discriminator in the subsequent stage for explaining the operation of the FM demodulation circuit of FIG. 3; 1...Input terminal, 2,9...Limiter, 3,10...
Discriminator, 4...Low pass filter, 1
1... High-pass filter, 6... Rectifying means, 7... Opening/closing means, 8... Output terminal.

Claims (1)

[Claims] 1. A first high-pass filter that extracts the noise component from the output of the first discriminator that frequency-discriminates the FM intermediate frequency signal from which the amplitude modulation component has been removed, and a limiter that limits the amplitude of the output of this first high-pass filter. and a second discriminator that again discriminates the frequency of the output of this limiter.
a second high-pass filter for extracting a noise component from the output of the second discriminator; a means for rectifying the output of the second high-pass filter; and a means for rectifying the output of the second discriminator; Consists of an opening/closing means for passing or blocking the passage of the audio signal from the output of the discriminator.
FM demodulation circuit.