JPS6311812B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a side peak reduction circuit that effectively reduces side peak output generated in an FM receiver.

As shown in Fig. 1, a conventional FM receiver includes a high frequency amplification/frequency converter 1, an intermediate frequency amplifier 2,
The FM signal is reproduced as audio through an FM detector 3 and an audio amplifier 4.

_In such an FM receiver, _O
When the frequency is changed over a range wider than the filter bandwidth, centered around (10.7MHz), the output of the FM detector 3 has an S-shaped curve (hereinafter referred to as the S-curve) as shown in Figure 2 A. A detection output with the characteristics of appears. The above-mentioned S is applied to a circuit having such characteristics.
When a normal FM signal modulated in a frequency deviation range sufficiently narrower than the frequency band in which the curve appears, an audio output of the level shown in Figure 2 B is detected and generated according to the slope of the S curve. be done. To explain this in more detail with reference to Figure 2, in the frequency section e-f-g of the S-curve, the S-curve output decreases as the frequency increases, and in the sections d-e and gh near both ends of the frequency band, the S-curve output decreases. On the contrary, it rises. Moreover, as you can see from this S curve,
The slopes at both ends of the frequency band, d-e and gh, are steeper than the section e-f-g. The degree of this slope means a difference in detection sensitivity. Therefore, the center frequency of the input FM signal is _O
(10.7MHz), it can be seen that the audio output can be obtained at the level shown in Figure 2 (b). In other words, assuming that the modulation degree is constant,
When the center frequency of the FM signal is near _O , the detection output level is V _a , and near the center of d-e and g-h of the S curve, the detection output level is V _b .

The most ideal tuning point is when the center frequency of the input FM signal reaches _O , but in the tuning adjustment process to reach this tuning point, the frequency of the FM signal moves from lower or higher than _O to _O. You will have to move around. For example, from the lowest frequency of the FM signal
Considering the case where the frequency of the FM signal is adjusted toward _O , when the frequency of the FM signal moves within the range de of the S curve, the detection sensitivity is high because the slope of the S curve is steep, and the detection sensitivity is high, and the level of V _b is reached. The detection output appears. However, since the bandwidth of the sloped portion is narrower than the frequency deviation width of the FM signal, the linearity of detection is poor. Also, e,
As a result of the slope of the S curve becoming zero near point g,
When the center frequency of the FM signal is at point g, the lower frequency components of the FM signal are detected by the detection curve d-e, and the higher components are detected by the detection curve ef. At this time, as can be seen from the figure, since the direction of the slope of the S curve is opposite above and below point g, the phase of the detection output is confused, which tends to reach the ears as noise. Moreover, the output level in this range
V _b is 10 dB or more higher than the normal output level V _a , resulting in extremely unpleasant noise.

Furthermore, when an inexperienced user adjusts the tuning, he may mistakenly mistake the central portion of de or gh, which deviates from the predetermined tuning point f, as the tuning point. Hereinafter, the output in such a range of de and gh will be referred to as side peak output.

In conventional FM receivers, in order to eliminate noise generated during detuning, a signal level detector 5 detects the level of the signal passing through the intermediate frequency amplifier 2, and the DC voltage obtained here is used to detect the level of the signal passing through the intermediate frequency amplifier 2. A method is used in which the audio output is cut off by a switch circuit 6 (Japanese Patent Laid-Open No. 16312/1983).

However, in such conventional receivers, the reproduced sound is output only when a predetermined tuning state is achieved, and is completely cut off at other times, making it extremely difficult to perform tuning adjustments.

Therefore, the present invention solves such conventional problems, reduces noise such as side peaks generated when the tuning is detuned from a predetermined tuning state, and reduces the occurrence of unpleasant noise. ,
It is possible to make the tuning adjustment operation easier by making some noise etc. audible even in the detuned state.
The purpose is to provide an FM receiver.

In order to achieve such an object, in the present invention, the electronic volume is controlled using the output that detects the magnitude of the detection output of the FM detector and the output that detects the magnitude of the FM intermediate frequency signal. The feature is that the audio signal is reduced to such an extent that it does not become inaudible, and the side peak portion is reduced by an even larger amount of attenuation to the extent that it does not become inaudible.

An embodiment of the present invention will be described below with reference to FIGS. 3 and 4.

In FIG. 3, numerals 1 to 3 and 5 correspond to blocks with the same symbols in FIG. 1, respectively. 8 is a reference power supply that generates a reference voltage, 9-2
3, 25 to 42 are transistors, resistors, and capacitors, respectively. Transistors 11, 15-23
constitutes an electronic volume that controls the magnitude of the audio signal, and transistors 9, 10, 12, 32, 33
constitutes a comparator that compares the detection output of the FM detector 3 with the reference voltage, and the transistors 13, 14, 2
7, 28, 30, 31, 34, resistance 25, 41,
42 constitutes an electronic volume control circuit.

In Figure 4, A is the envelope of the output of the intermediate frequency amplifier, B is the output of the level detector 5, and C is the detection characteristic (S
curve), D is the collector output of transistor 12,
E and F indicate the voltages at the bases of the transistors 13, 14, and 34, and G indicates the output of the output terminal 24, respectively.

Next, the operation of this embodiment will be explained. The FM signal received by the antenna is converted into an FM intermediate frequency signal by a high frequency amplification/frequency converter 1 and an intermediate frequency amplifier 2, and is FM detected by an FM detector 3 with an S curve characteristic as shown in Figure 4 C. .

The output signal of the FM detector 3 is transmitted through a resistor 39 and a capacitor 4.
The audio signal is removed by a low-pass filter (LPF) consisting of 0, and the average voltage with an S curve characteristic as shown in FIG.
Applied to the base of 10. On the other hand, the audio signal is extracted by a high-pass filter composed of a capacitor 36 and a resistor 37, and the electronic volume transistor 11
supplied to the base of

Centered around the center frequency _O of intermediate frequency amplifier 2
When the frequency of the FM intermediate frequency signal is changed up or down, the S-curve output moves up or down with reference to the center voltage V _O at _O , as shown in Figure 4 (c), so it corresponds to the upper and lower limits of the tuning state in advance. The reference voltage V _O
If the FM detection output voltage moves beyond ±V _S , transistor 9 or 10 of the bidirectional switch of the comparator
One of the transistors becomes conductive and drives a differential amplifier consisting of transistors 32 and 33. As a result, a switching voltage V _A appears at the collector of the transistor 12 at a portion that is more than V _S from the center voltage V _O of the S curve, as shown in FIG. 4D.

On the other hand, the audio signal component of the output of the FM detector 3 is guided to a differential amplifier composed of transistors 13 to 23 via an electronic volume transistor 11, and after the magnitude of the audio signal is controlled, the output terminal It is output from 24.

This electronic volume is the base voltage of transistors 13, 14, and 34 as shown in FIG.
It is controlled by the difference between V _c , V _D , and V _E . That is, V _c ,
If V _E ≫ V _D , transistors 16 and 17 are turned on, and I _B flows from the power supply through transistors 16 and 17 to transistor 19, and I _A flows from the power supply through transistors 22 and 16 to transistor 20. Therefore, since I _B containing the audio signal does not flow to the transistor 22, the audio signal is not output from the collector of the transistor 23 to the output terminal 24. Next, when V _c , V _E ≪ V _D , transistor 1
5 and 18 are turned on, the current I _B containing the audio signal
flows from the power supply to transistor 19 via transistors 22 and 18, and an audio signal is output from transistor 23 to output terminal 24. Output terminal 24
By setting I _A and I _B equal, the DC voltage can be kept at a constant value regardless of the audio signal. Furthermore, if the difference between V _c , V _E and V _D is selected to an appropriate value, I _A and I _B flow through the transistors 22 and 23 at a rate corresponding to the difference between V _c , V _E and V _D , and the audio The magnitude of the signal is increased or decreased, acting as an electronic volume.

However, transistors 13, 14, 34
When V _c , V _D , and V _E are applied so that the differential amplifier consisting of
-OFF operation, and the audio signal is also turned ON-OFF and cut off. When the electronic volume is off, the audio output is attenuated by more than 40 dB and is blocked, so even if there is a signal in the frequency band around this, it cannot be heard until it is completely tuned, making it difficult to tune. Furthermore, even if the signal is a normal signal, nothing can be heard until the device is completely in tune, and when the device is in tune, a loud sound is suddenly generated, making it difficult to use.

In order to prevent such a situation, the present invention prevents the audio signal from being attenuated more than a certain value even in the detuned state, and reduces the side peak noise signal and the output audio signal in the detuned state to an audible level. That's what I do. Next, the circuit operation for this purpose will be explained.

First, in order to arbitrarily attenuate the side peaks of the FM-detected audio signal by the necessary amount without making them inaudible, use the operating level setting resistor 2.
The configuration is such that a constant current I _c is caused to flow through the transistor 28 through the output voltage of No. 5 through the emitter follower of the transistor 27 and further through the resistor 42 having a resistance value R _A . In this way, the base voltage V _D of the transistor 14 is determined from the emitter voltage V _F of the transistor 27 set by the operating level setting resistor 25.
A voltage lower by R _A ×I _c is supplied. On the other hand, the base voltage of transistor 13 is
Due to the switching operation of the differential amplifier 31, the voltage changes by ΔV with respect to the emitter voltage of the transistor 27. The voltage of ΔV is resistor 4
1 and the current flowing through it.

The base voltage of the transistor 31 in the circuit configured as described above is set to V _B , and the base voltage of the transistor 30 on the other side is set to the switch voltage V _A
When switching is performed, the audio output including side peaks as shown by b in FIG. However, in the side peak part Z, an audio signal of approximately 0 dB, which is comparable to the output at _O , remains, and the side peak cannot be sufficiently reduced.

Next, the circuit operation for reducing the remaining side peaks that could not be sufficiently reduced by the above circuit will be further explained.

In FIG. 3, the output envelope for the frequency shift near the final stage of the intermediate frequency amplifier 2 is the fourth
The output of the level detector 5, which detects the signal level changes at each stage of the intermediate frequency amplifier 2 and extracts the overall signal level change, is as shown in FIG. 4B. That is, since the signal level decreases in the portion where the side peak remains, this detection output is passed through the transistor 35 to the transistors 13 and 1 described above.
4, that is, the base of the transistor 34 inserted in parallel with the transistor 13, the signal level detection output becomes higher than the signal level output V _cA in FIG. From the vicinity where the intermediate frequency signal begins to decrease, the voltage at the base of the transistor 34 V _E
becomes high as shown in Fig. 4, and the transistor 34
Current will begin to flow. Accordingly, the collector voltage of transistor 34 decreases and the collector voltage of transistor 14 increases, so the currents of transistors 15 and 18 forming the electronic volume decrease and the currents of transistors 16 and 17 increase. As a result, the current in the transistor 23 is reduced by the current I _B carrying the audio signal, and is replaced by a direct current
The component of _IA will increase.

As a result, the audio output in the shaded area in Figure 4G is a
The remaining side peaks are further reduced by approximately 10 dB, and the remaining side peaks are sufficiently attenuated without becoming inaudible, as shown in c.

Furthermore, in the completely detuned section W, the signal level detection output becomes higher than V _cA as shown in Figure 4 B, so the above operation continues and the audio output is attenuated by about 10 dB, which is within the range of not becoming inaudible. Noise generated in timing is also reduced.

As explained above, according to the present invention, noise and side peaks at a distance of more than a predetermined distance from a predetermined tuning frequency are reduced, and the excellent effect is that the tuning operation is easier because the noise and side peaks are not heard at all but are a little audible. Obtained and high quality FM
Can provide receiver.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional side peak reduction circuit, Fig. 2 is a detection output characteristic diagram thereof, Fig. 3 is a block diagram of a side peak reduction circuit according to an embodiment of the present invention, and Fig. 4 is its operating waveform. It is a diagram. 3...FM detector, 5...signal level detection circuit, 9-23, 27-35...transistor.

Claims (1)

[Claims] 1. An FM detector that performs FM detection of the FM intermediate frequency signal, an electronic volume that controls and outputs the magnitude of the audio signal output from the FM detector, and a detection output with S curve characteristics obtained from the FM detector. A comparator that generates a comparison output at a portion whose magnitude is separated from the center voltage by a predetermined level or more when compared with a reference voltage, and a comparator that detects the magnitude of the FM intermediate frequency signal and generates a comparison output when the magnitude is less than a predetermined value. A signal level detection circuit that sometimes generates a detection output, a comparison output of the comparator, and a detection output of the signal level detection circuit, and when only one of the two outputs is generated, the electronic The magnitude of the audio signal output from the electronic volume is attenuated by a certain amount to the extent that it does not become inaudible, and when the above two outputs are both generated, the magnitude of the audio signal output from the electronic volume is reduced by the above fixed amount. and a circuit for controlling the electronic volume so that the electronic volume is attenuated only by a large amount of attenuation and to an extent that does not make the sound inaudible.