JPH08111648A - Fm receiver - Google Patents

Abstract

PURPOSE: To obtain an FM receiver which is proper in sensitivity suppression even when an interference wave exists in the vicinity of the frequency band of a desired wave. CONSTITUTION: The S-curve detector 12 of a band detection circuit 11 outputs the S-curve shaped DC signal corresponding to the difference of the frequencies of the output of an FM detector 10 and the IF signal of a mixer 4. When this DC signal is deviated from the set range corresponding to the frequency band of the desired wave or when an interference wave exists in the vicinity of the frequency band of the desired wave, an interference wave detection signal is outputted from either one of two level comparators 13 and 14 to a switch circuit 16 and the switch circuit 16 is controlled to be opened. As a result, even when the level meter output of a level detector 15 reply to the desired wave and the interference wave, the level member output is not inputted in an AGC circuit 20, the gain of an AGC amplifier 17 becomes a minimum value and AGC sensitivity (sensitivity suppression) becomes the lowest. Therefore, the desired wave can be excellently received.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved FM receiver.

[0002]

2. Description of the Related Art In recent years, an AGC (Automatic Gain Control) has been used by using an output (level meter output) according to the intensity of a desired wave.
l) FM receivers that control circuits and improve sensitivity suppression are becoming popular.

The structure of a conventional FM receiver having an AGC circuit will be described below with reference to FIG.

In FIG. 2, the antenna 1 is a tuning circuit 2
Is connected to the input of the RF amplifier 3 via. A dual gate MOSFET is generally used as a device forming the RF amplifier 3. The RF signal of the RF amplifier 3 is input to the mixer 4 via a selection circuit (not shown), and the mixer 4 mixes the local oscillation signal with the RF signal to generate an IF signal. The output of the mixer 4 is fed through the IFT 5 and the ceramic filter 6 to the pre-I
It is input to the F amplifier 7. The output of the pre-IF amplifier 7 is input to the amplitude limiter (hereinafter, referred to as limiter amplifier) 9 via the ceramic filter 8, and the output thereof is input to the FM detector 10. The FM detector 10 extracts an audio signal from the input IF signal.

The output of the mixer 4 is also input to the AGC amplifier 17 of the AGC circuit 20, and the AGC amplifier 1
The output of 7 is input to the level detector 18 in the AGC circuit 20. One of the outputs of the level detector 18 drives a pin diode (not shown) in the tuning circuit 2,
The other output is the dual gate MOSF of the RF amplifier 3.
It is input to the gate of ET (not shown).

Further, a level detector 15 is connected to the limiter amplifier 9. The level detector 15 is
A DC signal (hereinafter referred to as a level meter output) corresponding to the intensity of the signal input to the limiter amplifier 9 is output.
The level meter output is used for various controls of the FM receiver, and one of them is used for controlling the gain of amplification of the AGC amplifier 17.

In the FM receiver having the above structure, when the antenna 1 simultaneously receives the desired wave and the disturbing wave, the desired wave and the disturbing wave are leveled from the output of the mixer 4 via the AGC amplifier 17 of the AGC circuit 20. It is input to the detector 18. Of these amplified desired and interfering waves,
When one of the signal strengths exceeds a level preset by the level detector 18, the level detector 18 outputs two DC signals (hereinafter referred to as DC signals). One of the output DC signals drives the pin diode in the tuning circuit 2 to attenuate the signal input from the antenna 1, and the other DC signal lowers the gain of the RF amplifier 3 and increases the sensitivity. Suppress.

Here, if the received desired wave is significantly smaller than the interfering wave, the desired wave becomes unreceivable due to the sensitivity suppression. Therefore, the level detector 1 should improve this.
The gain of the AGC amplifier 17 is controlled by using the level meter output corresponding to the intensity of the desired wave from the signal 5. As shown in FIG. 3, when the desired wave is small, the gain of the AGC amplifier 17 is controlled by lowering the gain of the AGC amplifier 17 so that the signal level of the DC signal output from the level detector 18 (hereinafter referred to as AGC sensitivity 19) A when the desired wave is large
It is smaller than the GC sensitivity 20.

[0009]

However, in the above-mentioned conventional FM receiver, since the band characteristic of the level meter output is determined by the ceramic filters 6 and 8,
As shown in FIG. 4, the zone 2 of the ceramic filters 6 and 8
If the desired wave 21 and the disturbing wave 22 both exist in the area 3, the level meter output, which should originally respond only to the desired wave, also responds to the disturbing wave, and as a result, the desired wave is small. First, the AGC sensitivity is set to a large value, the sensitivity is suppressed excessively, and it becomes difficult to properly receive the desired wave.

The present invention has been made to solve the above problems, and its object is to improve the AGC sensitivity even when a desired wave and an interfering wave exist in the band of a ceramic filter. It is to prevent the sensitivity from being set unreasonably large so that the sensitivity can be properly suppressed and the desired wave can be received well.

[0011]

In order to achieve the above object, the present invention detects a case where an interfering wave exists in the band of a ceramic filter, and at the time of this detection, the AG wave is detected.
The C sensitivity is set small.

That is, the configuration of the FM receiving apparatus according to the first aspect of the invention is to amplify the signal received from the antenna by R
An F amplifier, a mixer that generates an IF signal from the output of the RF amplifier, an amplitude limiter that receives the IF signal from the mixer via a bandpass filter, and limits the amplitude of the received IF signal, and the amplitude limiter. In an FM receiver equipped with an FM detector for extracting an audio signal from an IF signal from the mixer, the output signal of the FM detector and the IF of the mixer
A band detection circuit that generates a DC signal according to the difference in frequency with the signal, and outputs a disturbing wave detection signal when the generated DC signal exceeds a setting range corresponding to the reception range of the desired wave, and the amplitude limiter. A level detector for outputting a DC signal corresponding to the strength of the IF signal inputted to the switch, a switch circuit for interrupting the passage of the DC signal of the level detector by an interference detection signal of the band detection circuit, and an IF of the mixer. It has an AGC amplifier for amplifying a signal, and a level detector for outputting a signal according to the output of the AGC amplifier when the output of the AGC amplifier is equal to or higher than a set level, and the gain of the RF amplifier by the output of the level detector. Controlling AG
The AGC amplifier of the AGC circuit receives the DC signal of the level detector via the switch circuit, and the amplification degree of the IF signal is changed by the received DC signal.

Further, in the invention described in claim 2, the band detection circuit of the FM receiver according to claim 1 is specified, and the band detection circuit comprises the output signal of the FM detector and the I signal of the mixer.
A level detector that generates a DC signal according to the frequency difference from the F signal, and a first level comparison that outputs an interfering wave detection signal when the DC signal generated by the level detector exceeds the upper limit value of the set range. And a second level comparator which outputs an interfering wave detection signal when the DC signal generated by the level detector exceeds the lower limit value of the set range.

[0014]

With the above construction, in the inventions according to claims 1 and 2, in the band detection circuit, the DC signal corresponding to the frequency difference between the output signal of the FM detector and the IF signal of the mixer has a setting range. If it does not exceed, that is, I after passing through the bandpass filter
When the F signal does not include the interfering wave and the level meter output from the level detector responds only to the desired wave, the interfering wave detection signal is not output, and thus the switch circuit is in the closed state. As a result, as usual, the level meter output from the level detector is input to the AGC circuit, and sensitivity suppression according to the level meter output works.

On the other hand, in the band detection circuit, the FM
When the DC signal corresponding to the frequency difference between the output signal of the detector and the IF signal of the mixer exceeds the setting range, that is, the IF signal after passing the bandpass filter contains an interfering wave, and the level from the level detector When the meter output responds to the desired wave and the interfering wave, the interfering wave detection signal is output from the band detection circuit, and the switch circuit is controlled to the open state. As a result, in the AGC circuit, in the same situation as when the minimum level meter output is received from the level detector, the AGC sensitivity is set to the minimum and the desired wave is satisfactorily received.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG.

As shown in FIG. 1, the antenna 1 is connected to an RF amplifier 3 via a tuning circuit 2. As the RF amplifier 3, a dual gate MOSFET is generally used. The output of the RF amplifier 3 is input to the mixer 4. The output of the mixer 4 is input to the pre-IF amplifier 7 via the IFT 5 and the ceramic filter 6, and the output of the pre-IF amplifier 7 is band-passed by the both filters 6 and 8 (band filter) via the ceramic filter 8. Is limited, and then input to a limiter amplifier (amplitude limiter) 9. The output of the limiter amplifier 9 is input to the FM detector 10, and the FM detector 10 outputs the input IF.
Extract audio signal from signal.

The output of the mixer 4 is also input to the AGC amplifier 17 of the AGC circuit 20, and the output of the AGC amplifier 17 is input to the level detector 18 of the AGC circuit 20. One of the outputs of the level detector 18 drives a pin diode in the tuning circuit 2, and the other output is connected to the gate of the dual gate MOSFET of the RF amplifier 3.

The limiter amplifier 9 includes a level detector 1
5 is connected. The level detector 15 outputs a DC signal (level meter output) corresponding to the intensity of the signal input to the limiter amplifier 9, and the level meter output is
It is input to the AGC amplifier 17 of the AGC circuit 20.
A switch circuit 16 is arranged in the path for inputting the level meter output of the level detector 15 to the AGC amplifier 17, which is a feature of the present invention. The switch circuit 16 is
Opening / closing control is performed by the band detection circuit 11 as another feature of the present invention.

The band detection circuit 11 is connected to the subsequent stage of the FM detector 10. This band detection circuit 11
Is a level detector 12 (hereinafter referred to as an S curve detector)
And two level comparators 13 and 14. Above S
The curve detector 12 inputs the output of the FM detector 10 and the IF signal of the mixer 4, and changes in an S curve shape with respect to the frequency as shown in FIG. 5 according to the frequency difference between the both input signals. The DC signal 25 is generated. The one level comparator 13 operates when the S-curve DC signal is equal to or higher than the upper limit value 26 of the setting range as shown in FIG. 5, and outputs the interference detection signal. The level comparator 14 operates when the S-curve DC signal becomes equal to or lower than the lower limit value 27 of the setting range as shown in FIG. 5, and outputs the interference detection signal. The set range defined by the upper limit value 26 and the lower limit value 27 is the frequency band of the desired wave and is set relatively narrower than the band of the two ceramic filters 6 and 8 (band filter). Is set.

Next, the operation of the FM receiving apparatus having the above configuration will be described.

The RF signal from the antenna 1 is fed to the tuning circuit 2
After passing through the mixer, it is amplified by the RF amplifier 3 and the mixer 4
Is input to and converted into an IF signal. The IF signal appearing at the output of the mixer 4 is input to the pre-IF amplifier 7 via the IFT 5 and the ceramic filter 6. Pre-IF
The IF signal amplified by the amplifier 7 is input to the limiter amplifier 9 via the ceramic filter 8, and its output is input to the FM detector 10. The FM detector extracts a voice signal from the IF signal.

The output (IF signal) of the mixer 4 is also input to the AGC amplifier 17 of the AGC circuit 20, amplified, and then input to the level detector 18. When the IF signal appearing at the input of the level detector 18 exceeds the preset level of the level detector 18, the level detector 18 outputs D
The C signal is output, and one of the DC signals drives the pin diode in the tuning circuit 2 to attenuate the signal input from the antenna 1, while the other DC signal reduces the gain of the RF amplifier 3. Thus, the strength of the RF signal input to the mixer 4 is suppressed.

The level meter output of the level detector 11 is input to the switch circuit 16. Band detection circuit 1
The S-curve detector 12 of No. 1 outputs an S-curve DC signal corresponding to the frequency difference between the output of the FM detector 10 and the IF signal of the mixer 4, and this DC signal has an upper limit value 26 and a lower limit value 27. If the signal is outside the setting range divided by, that is, if there is an interfering wave in the vicinity of the frequency band of the desired wave and the signal appearing at the output of the mixer 4 is determined to be the interfering wave, two An interfering wave detection signal is output to the switch circuit 16 from one of the level comparators 13 and 14,
The switch circuit 16 is controlled to be opened. As a result, even if the level meter output of the level detector 15 is large, the level meter output is not input to the AGC circuit 20, the gain of the AGC amplifier 17 becomes the minimum value, and the AGC sensitivity (sensitivity suppression) becomes the minimum. . Therefore, it is possible to favorably receive the desired wave even if the level meter output includes the interfering wave.

On the other hand, when the S-curve DC signal generated in the band detection circuit 11 does not exceed the set range and both level comparators 13 and 14 do not operate, that is, the level of the level detector 15 When the meter output does not include an interfering wave and responds only to the desired wave, the band detection circuit 11 does not output the interfering wave detection signal, and the switch circuit 16 is closed. Therefore, the level meter output of the level detector 15 is A
The gain of the AGC amplifier 17, which is input to the GC amplifier 17, is well controlled by the level meter output corresponding to only the desired wave.

Therefore, according to the FM receiver of the present embodiment, the frequency band regarded as the desired wave can be set narrower than the conventional one, so that the AGC circuit 20 is generated by the interfering wave adjacent to the desired wave within the band of the ceramic filters 6 and 8. The AGC sensitivity can be prevented from being set unreasonably low, and the sensitivity can be suppressed appropriately at all times.

[0027]

As described above, according to the FM receivers of the first and second aspects of the present invention, when the interfering wave exists near the frequency band of the desired wave in the subsequent stage of the FM detector. A band detection circuit for detecting is provided, and when the interference wave is present, the AGC sensitivity (sensitivity suppression) of the AGC circuit is set to the minimum, so that the sensitivity suppression can always be set appropriately.
You can listen to the desired wave satisfactorily.

[Brief description of drawings]

FIG. 1 is a block diagram showing an FM receiver according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a conventional FM receiver.

FIG. 3 is a diagram showing a change in AGC sensitivity according to a level meter output.

FIG. 4 is a diagram showing a band in which a desired wave is detected.

FIG. 5 is a diagram showing an operation of the band detection circuit according to the embodiment of the present invention.

[Explanation of symbols]

 1 Antenna 3 RF Amplifier 4 Mixer 6,8 Ceramic Filter 9 Limiter Amplifier (Band Limiter) 10 FM Detector 11 Band Detecting Circuit 12 S Curve Detector (Level Detector) 13,14 Level Comparator 15 Level Detector 16 Switch Circuit 17 AGC amplifier 18 Level detector 20 AGC circuit

Claims (2)

[Claims] 1. R for amplifying a signal received from an antenna
An F amplifier, a mixer that generates an IF signal from the output of the RF amplifier, an amplitude limiter that receives the IF signal from the mixer via a bandpass filter, and limits the amplitude of the received IF signal, and the amplitude limiter. In an FM receiver equipped with an FM detector for extracting an audio signal from an IF signal from the mixer, a DC signal is generated according to a frequency difference between the output signal of the FM detector and the IF signal of the mixer, and the generation is performed. A band detection circuit that outputs an interfering wave detection signal when the DC signal exceeds a setting range corresponding to the reception range of the desired wave, and a level that outputs a DC signal corresponding to the strength of the IF signal input to the amplitude limiter. A detector, a switch circuit for blocking passage of a DC signal of the level detector by an interference detection signal of the band detection circuit, and an AG for amplifying the IF signal of the mixer. When the outputs of the C amplifier and the AGC amplifier are above a set level, the above A
An AGC circuit that has a level detector that outputs a signal according to the output of the GC amplifier, and an AGC circuit that controls the gain of the RF amplifier by the output of the level detector. An FM receiving apparatus, characterized in that it receives a DC signal from a receiver via the switch circuit, and the amplification degree of the IF signal is changed by the received DC signal. 2. The band detection circuit sets a level detector for generating a DC signal according to a frequency difference between an output signal of the FM detector and an IF signal of the mixer, and a DC signal generated by the level detector. A first level comparator that outputs an interference wave detection signal when the upper limit value of the range is exceeded, and a second level comparator that outputs an interference wave detection signal when the DC signal generated by the level detector exceeds the lower limit value of the set range. The FM receiver according to claim 1, further comprising a level comparator.