JPS628623A - Fm receiver - Google Patents

Abstract

PURPOSE:To reduce multi-path noises by detecting an amplitude modulation component due to multi-path disturbance included in a reception signal and increasing the feedback quantity of a FM feedback loop when the detection level is increased. CONSTITUTION:When a reception signal wave is subjected to multi-path disturbance, it is converted into an intermediate frequency by the 2nd mixer 11, amplified by the 2nd intermediate frequency amplifier 13 and the amplitude modulation component having multi-path distortion is extracted by an amplitude modulation component detector 14. After the amplitude modulation component is amplified by an amplifier 15, the component is converted into a signal having a DC level corresponding to the quantity of the multi-path disturbance by a rectifier circuit 16. A feedback quantity control means U2 comprising a gain control circuit 17 and a low frequency amplifier 9 is operated by the DC level signal and the FM negative feedback loop NF is controlled to the feedback quantity corresponding to the amplitude of the DC level signal.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an FM receiver equipped with an FM feedback loop,
In particular, the present invention relates to an FM receiver that can reduce multipath noise.

[Summary of the Invention] The present invention detects amplitude modulation components due to multipath interference contained in a received signal, and when the detection level becomes large, the feedback amount of the FM feedback loop is increased to reduce multipath noise. This is how it was done.

[Conventional technology J FM receivers, when the received input power is sufficiently large,
Compared to a receiver, the S/N ratio is good and the quality is high.

by the way. In recent years, the number of vehicles equipped with FM receivers has been increasing. When receiving FM broadcasts with a vehicle-mounted FM receiver, there is a relatively large amount of noise due to the generation of a weak electric field reception area as the vehicle travels. These noises include skip noise in a weak electric field, pulse noise, multipath noise caused by multipath distortion, and the like.

As a conventional means of removing such various noises, a pulse noise canceler circuit has been proposed for pulse noise, which effectively removes pulse noise and improves the quality of the received signal. . Furthermore, high blend circuits and high cut control circuits have been proposed for skip noise and multipath noise. These circuits detect the received field strength and when the field strength becomes weak, control the stereo left/right separation to gradually switch to monaural reception, or cut high frequency components to improve the S/N ratio. It was designed to achieve this.

On the other hand, in addition to the various noise removal methods mentioned above, FM
FM receivers using an FM negative feedback system have been known as means for improving the S/N ratio by improving the threshold of the receiver itself.

Here, the threshold refers to the received power that overcomes the internal noise of the FM receiver and is the limit for improving the S/N ratio, and when the level of the received signal falls below this power level, the S/N ratio deteriorates rapidly. In the FM negative feedback method, the audio signal output is fed back to the local oscillator, and the oscillation signal of the local oscillator is FM-modulated with the audio signal, so that the frequency of the intermediate frequency signal is equal to the frequency of the received signal and the FM-modulated local oscillation signal. Negative feedback is applied so that there is a difference with the frequency. With FM negative feedback, the frequency deviation of the intermediate frequency signal is compressed, and compared to FM receivers that do not use FM negative feedback, it is possible to achieve high sensitivity with an improved threshold for radio waves that are only a few times weaker. An FM receiver is obtained.

[Problem to be Solved by the Invention 1] Multipath noise is noise caused by multipath distortion caused by interference between direct waves of broadcast radio waves and reflected waves from mountains, buildings, etc. As a means for removing this multipath noise, there are a high blend circuit and a high cut control circuit as described above.

However, when an FM receiver is mounted on a vehicle and the directions of direct waves and reflected waves change frequently as the vehicle travels, multipath noise frequently occurs. If the high-cut circuit repeatedly operates and deactivates in response to such frequent occurrences, there is a problem in that the reproduced sound from the receiver becomes so-called fluffy, resulting in an unnatural sound quality.

This invention was made in view of these conventional problems, and aims to reduce multipath noise by utilizing the FM negative feedback loop, which is originally used to improve the threshold. It is said that

[Means for Solving the Problem 1] In order to achieve the above object, the FM receiver according to the present invention has an FM receiver equipped with an FM negative feedback loop. It has a multipath interference detection circuit that detects a modulation component, and a feedback ω control means that controls the feedback amount of the FM negative feedback loop according to the detection signal level of the multipath interference detection circuit.

1 Effect] The multipath detection circuit detects the amplitude modulation component due to multipath interference contained in the received input signal, and when the level of the detected signal increases by the feedback amount control means, the negative feedback loop is activated. ffi is controlled to be large.

As a result, the output of the audio signal no longer depends on the amplitude of the received signal wave, and the audio signal is no longer affected by multipath interference.

[Example 1 Embodiments of the present invention will be described below based on the drawings.

First, to explain the configuration, the symbol RX in the figure is the main circuit of the FM receiver, NF is the FM negative feedback loop, and the main circuit RX of the FM receiver includes the high frequency amplifier 1 to which the antenna ANT is connected, and the first mixer 2. , local oscillator 3, center frequency is 10.
7 HI3 (intermediate frequency) first bandpass filter 4, first intermediate frequency amplifier 5 including a limiter function, and detector 6, and the audio signal Eo is output from the detector 6.
is output.

The FM negative feedback loop NF receives the audio signal E.

is fed back to the local oscillator 3, and the oscillation signal of the local oscillator 3 is FM-modulated with the audio signal Eo.
A feedback line branched from the detector 6 is connected to the FM modulator 7 via a low-pass filter 8 that band-limits the audio signal @Eo. Reference numeral 9 denotes a low frequency amplifier constituting a feedback amount control means to be described later.

Further, in this invention, in order to remove multipath interference by utilizing the above-mentioned FM negative feedback loop NF,
A multipath interference detection circuit U1 as shown below and an FM negative feedback loop NF according to the detection signal level of this detection circuit U1.
A feedback amount control means U2 for controlling the amount of feedback is provided.

The multipath interference detection circuit U1 is a second mixer 11.10.7 MH that inputs the FM reception signal from the high frequency amplifier 1 and the oscillation signal from the local oscillator 3 and converts it into an intermediate frequency.
A second bandpass filter 12 having a center frequency of z, a second intermediate frequency amplifier 13, an amplitude modulation component detector 14 connected to the output terminal of the intermediate frequency amplifier 13 via a capacitor C, an amplifier 15, and a rectifier circuit 16. It consists of
An output terminal of the rectifier circuit 16 is connected to a gain control circuit 17 constituting the feedback amount control means U2.

The feedback amount control means U2 is composed of the gain control circuit 11 and the low frequency amplifier 9 described above.

18 is a gain adjustment circuit, and 19 is a preamplifier for audio signal Ea. This is provided to compensate for a decrease in the output of the signal Ea.

Next, the effect will be explained.

Now, for the antenna ANT, e-Elcospt <7)
It is assumed that a received signal FM-modulated with a signal wave is input, and the following received signal wave e1 is output from the high frequency amplifier 1.

el −Acos(ω1 j + Xl 5inpt+
φ1)...(1) A: Received signal wave amplitude ω1: Carrier angular frequency x 1 = modulation index (-ρ1E1/p) ρ1: Proportionality constant El: Signal wave amplitude p: Signal wave angular frequency φ1 -Phase On the other hand, if eo is the oscillation signal from the local oscillator 3 that is FM-modulated with the audio signal Eo fed back from the FM negative feedback loop NF, then e O-BeO2(ω2
t + ρ2: Proportionality constant From both equations (1) and (2), the intermediate frequency output ei of the first mixer 2
is eix e 1 11 eo-A dispute Be
O2(((t) 1- ω 2)t+(Xl-X2
)Sinl)j+ (φ1−φ2)) (3) From equation (3), the modulation index of the intermediate frequency signal ei is different from that of the received signal wave.

Assuming that the detection characteristic of the detector 6 is linear, the amplitude Eo of the detection output (audio signal) is obtained by differentiating the phase of equation (3) with respect to time, so E(1-(ZaIAB(XI X 2)P) =(Zat
A B [(1) 1E 1/D) - (kρ2 E o
/I)) ID -αa, AB (ρIE1- to ρ2EO) (4) α: proportionality constant al: detection efficiency From the above equation (4), the audio signal output of the FM receiver using the FM negative feedback method is , Eo=αa1AB, 01E1/ (1+(Z a I k A B p 2 ) −(5
).

The above equation (5) shows that if the negative feedback loop NF is not added, the output of the FM receiver will be (αaIABρ1E1). By adding a negative feedback loop NF, the gain of the entire receiving system is reduced by the negative feedback factor (1+
αa1k A Bρ2).

If the amount of feedback is set large so that αa, k A Bρ2) 1, the above formula (5) becomes Ea-ρ1 E1/ρ2...(6)
It can be expressed as. .

From the above equation (6), when the FM negative feedback loop N feedback amount is increased, the audio output signal Eo is determined by the amplitude changes of the received signal wave e1 and the local oscillation signal eo, and the detection efficiency a
It no longer depends on 1.

Therefore, in the present invention, the level of the magnitude of multipath interference is detected by the multipath interference detection circuit U1, and based on this detection signal, the FM negative feedback loop N taste feedback amount is transmitted through the feedback amount control means U2. , and when the degree of multipath interference is large, the amount of feedback is increased to prevent the effects of multipath interference from appearing on the audio output Eo of the FM receiver.

Describing the operation of the multipath interference detection circuit U1, when the received signal wave e1 is subjected to multipath interference, this
After being converted into an intermediate frequency by the mixer 11 and amplified by the second intermediate frequency amplifier 13, the amplitude modulation component subjected to multipath distortion is extracted via the capacitor C by the amplitude modulation component detector 14.

The amplitude modulation component is amplified by an amplifier 15 and then converted by a rectifier circuit 16 into a DC level signal corresponding to the magnitude of multipath interference. This DC level signal operates the feedback amount control means U2 consisting of the gain control circuit 17 and the low frequency amplifier 9, and the FM negative feedback loop N is controlled to the feedback amount corresponding to the magnitude of the DC level signal. As a result, when the degree of multipath interference becomes large, IIIj is performed so that its influence does not appear on the audio signal output.

As the FM negative feedback loop N feedback amount increases, the frequency deviation of the intermediate frequency signal output from the first mixer 2 becomes smaller, the modulation index becomes smaller, and the detected output level of the detector 6 increases. That is, the output level of the audio signal Eo decreases, but at this time, the amplifier 4111 adjustment circuit 18 and the preamplifier 19 operate to compensate for the decrease in the output of the audio signal Eo.

[Effects of the Invention] As detailed above, according to the present invention, there is provided a multipath interference detection circuit that detects an amplitude modulation component due to multipath interference, and an FMM feedback loop that detects an amplitude modulation component due to multipath interference, and an FMM feedback loop that detects an amplitude modulation component due to multipath interference. Since the present invention is equipped with a feedback amount control means for controlling the amount of feedback of is obtained. Furthermore, even when the FM receiver is installed in a vehicle, unlike an FM receiver equipped with a conventional high-cut control circuit as a means for removing multipath noise, the reproduced sound does not become bouncy and give an unnatural sound quality.

[Brief explanation of drawings]

The figure is a block diagram showing an embodiment of an FM receiver according to the present invention. RX: Main circuit of FM receiver, NF: FMM return loop, ul: Multipath interference detection circuit, LJ2: 91 far distance control means.

Claims (1)

[Claims] An FM receiver equipped with an FM negative feedback loop includes a multipath interference detection circuit that detects an amplitude modulation component due to multipath interference contained in a received signal, and a multipath interference detection circuit that detects an amplitude modulation component due to multipath interference contained in a received signal, and An FM receiver comprising: feedback amount control means for controlling the feedback amount of the negative feedback loop.