JPS58213534A - Radio receiver - Google Patents

Abstract

PURPOSE:To increase SN even if electric field strength is weak, by changing the band width of an intermediate frequency amplifier circuit in accordance with an input signal level. CONSTITUTION:The intermediate frequency 1F amplifier circuit 5 is provided with two filters of wide and narrow bands, e.g. 250KHz and 150KHz e.g. The output of the 1F amplifier circuit 5 is applied to a stereophonic demodulating circuit 7 and a level detecting circuit 12. When the output of the circuit 5 is reduced in a weak electric field, the stereophonic demodulating circuit 7 is controlled to deteriorate separation between stereophonic signals and turn to monophonic reception. When the output level of the circuit 5 is reduced to lower than a level, the circuit 12 is switched to the narrow band filter, and at the level, a titled receiver has been already switched to the monophonic reception. Since the narrow band filter is used in the weak electric field, the threshold level is reduced. Since the receiver has been already switched to the monophonic reception in the weak electric field, the generation of large distortion is suppressed.

Description

[Detailed Description of the Invention] The present invention relates to a receiver that receives FM broadcasting or television audio signals, etc., and its purpose is to receive broadcast waves in a state where the electric field strength is weak. Also, the signal-to-noise ratio (hereinafter referred to as S/N ratio) is large, and the aim is to obtain good reception.

First, the configuration of a conventional FM receiver is shown in FIG. 1 is an antenna, 2 is a high frequency amplification circuit, 3 is a frequency conversion circuit, 4 is a local oscillation circuit, 5 is an intermediate frequency amplification circuit, 6 is a detection circuit, 7 is a stereo demodulation circuit, 8 and 9 are low frequency amplification circuits The circuit, 10°11 is a speaker. The band of the intermediate frequency amplification circuit 5 is normally selected to be about 20 KHz. Now, the FM detection noise decreases when the input (received field strength) of the antenna 1 decreases, and suddenly increases when it becomes below a certain limit point. That is, the S/N ratio of the detection output deteriorates rapidly. This is generally called the S/N improvement limit phenomenon in FM. The S/N improvement limit point is the point where the level of the signal at the output of the intermediate frequency amplification circuit 5 becomes equal to the peak value of the noise.

In the case of white noise, the peak value of the noise is four times the effective value, and the peak value of the signal is five times the effective value, so if you compare the effective values, the signal level will be 9 dB above the noise level. is the improvement limit point (hereinafter referred to as the threshold level). That is, the signal level at which the signal-to-noise ratio (hereinafter referred to as C/N ratio) at the output of the intermediate frequency increase 1] circuit 6 is C/N-9 dB is the threshold level. If the noise figure of the receiver is NF, the impedance of the antenna is Ra, and the middle band of the intermediate frequency amplifier 111 circuit 6 is B, the threshold level Eth is E th −', 25a= . . . ...
(1) where: Bolzmann constant, T: absolute temperature, and Eth is the open output of the antenna.

For example T = 300 cK SRa = 60Ω, N
If F = 2, B = 260KH2, k = 1, 3
8X10-''3 joules/OK, so Eth #1.8/JV=5.2dBμV Figure 4 shows the relationship between antenna terminal voltage and S/N.

From the EthO point, the S/N rapidly deteriorates as shown by the solid line. That is, conventional receivers have high sensitivity, and the practical sensitivity is limited to about sdBμ■.

The present invention is intended to improve the conventional drawbacks as described in section 2. An embodiment of the present invention will be described below with reference to FIG. 2. In FIG. 2, blocks having the same functions as those of the conventional receiver shown in FIG. 1 are designated by the same numbers. Block A surrounded by a dotted line is the main part of the present invention, and this block A
I will mainly explain.

The intermediate frequency amplification circuit 6 has two filters, a wide band filter and a narrow band filter, for example, a filter with a middle band of B=250 kHz and a filter with a band 1t]B=150 kHz. And these two filters are circuit 12
Which one to use can be switched depending on the signal from. Normally, a broadband filter is used in strong electric fields. Therefore, in a strong electric field, the circuit becomes equivalent to that shown in FIG. Next, when the electric field becomes weak and the output voltage of the antenna 1 decreases, the output level of the intermediate frequency increase circuit 5 decreases. Circuit 12 is a level detection circuit.

Now, the output level from the intermediate frequency amplification circuit is applied to the stereo demodulation circuit 7 and the level detection circuit 12.

When the output level of the intermediate frequency amplification circuit 5 decreases, the stereo demodulation circuit 7 is controlled to worsen the separation between stereo signals and switch to monaural reception. On the other hand, the level detection circuit 12 generates a control signal to switch the filter of the intermediate frequency amplification circuit 6 from a wide band filter to a narrow band filter when the output level of the intermediate frequency amplification circuit 6 falls below a certain level. Switch. At the level at which this filter switching is performed, the signal has already been switched to monaural reception. If the electric field becomes weak like this, use a narrow band filter t7. ;)
Kth, the threshold level Eth is lowered according to equation (1).

If B = 150 KHz, then from equation (1), F'
th = 3 dBμV Therefore, the 21B thread 7 hold level is lower than the conventional one. The dotted line in FIG. 4 shows the S/N characteristics. Eth shows the threshold level when B=160KHz. Now, narrowing the band will increase distortion, but in the area where the band is narrowed, the reception has already switched to monaural reception.
The generation of large distortions is suppressed and does not pose any practical problems. As described above, according to the present invention, in areas with strong electric fields, you can enjoy music, etc. with the same level of sound quality as before, but in areas with weak electric fields, where conventional receivers cannot receive it, there is a slight decrease in sound quality such as distortion. You can receive and enjoy the broadcast. Note that the present invention is particularly useful for mobile receivers such as in-vehicle audio equipment.

FIG. 3 is a block circuit diagram showing another embodiment of the present invention. The difference from the embodiment shown in FIG. 2 is that the output of the detection circuit 6 is fed back to the local oscillation circuit 4. be. This is what is called negative feedback. Circuit 13 is a switch circuit.

When there is no control signal from the level detection circuit 12, the switch 13 is open, and when a control signal is generated, the switch 13 is open.
3 closes. When the switch 13 is closed, the detection circuit 6
The output of F is applied to the local oscillation circuit 4, and the local oscillation frequency is
M modulates. Then, the local oscillation frequency is FM-modulated in a direction that lowers the degree of modulation of the output signal of the frequency conversion circuit 3. Therefore, in a weak electric field, the switch 13 closes and the degree of modulation of the input signal to the intermediate frequency amplification circuit 5 decreases.

Therefore, it becomes possible to make the band of the intermediate frequency amplification circuit 5 even narrower than in the case of the embodiment shown in FIG.

Note that since the degree of modulation is reduced, the output of the detection circuit 6 is also reduced. Therefore, the control signal from the level detection circuit 12 increases the degree of increase in the low frequency amplification circuits 8 and 9 to prevent a difference in level between the strong electric field and the weak electric field.

In the embodiment, the output of the intermediate frequency amplification circuit 5 is used as the input to the level detection circuit 120, but the noise level of the output of the detection circuit 6 may be detected, for example.

As described above, the present invention can achieve a large signal-to-noise ratio even when the electric field strength of broadcast waves is weak with a simple configuration that changes the band of the intermediate frequency amplification circuit according to the input signal level. It is possible to perform a good reception, and the effect is a dog.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram showing a conventional example, and FIG. 2 is a block circuit diagram showing a conventional example. FIG. 3 is a block circuit diagram showing an embodiment of the present invention, and FIG. 4 is a diagram showing antenna terminal voltage and signal-to-noise ratio. 1...Antenna, 4...Local oscillation circuit, 5...Intermediate frequency amplification circuit, 6...Detection circuit, End...Stereo demodulation circuit , 12...
Level detection circuit, 13... Switch circuit. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 111''-8 Figure 3 Figure 4

Claims (1)

[Scope of Claims] (1) A radio receiver that receives a frequency-modulated signal, which includes an output circuit that outputs a signal corresponding to a change in the input signal level of the receiver, and an output circuit that outputs a signal corresponding to a change in the input signal level of the receiver. A switching means for switching under control, and a switching means configured to switch between the bands of the intermediate frequency amplification circuit, and when the input signal level of the receiver becomes small, the switching means switches under the control of the output circuit. Switch to and use the means,
2. The radio receiver according to claim 1, wherein the radio receiver is configured to switch to a signal by narrowing the band 1 of the intermediate frequency amplification circuit. (3) At the same time as narrowing the band of the intermediate frequency amplification circuit, the output of the detection circuit is used to control the oscillation frequency of the local excavation circuit, so that the frequency deviation of the output signal of the frequency conversion circuit is reduced. A radio receiver according to claim 1.