JPH0969788A - Fm reception equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve S/N of a detection output without being affected by the intensity of a reception radio wave by changing the inclination of the S curve of the detection output in accordance with the intensity of the reception radio wave. SOLUTION: The reference voltage of a comparator 7 is pre-liminarily set so that the output of the comparator 7 goes to the row level at the time when the FM signal inputted to an up amplifier 1 is made weaker than a point (point A) where the noise level exceeds the signal level for strong input. When the level of the FM signal is made lower than the point A because of the weak reception radio wave, the output of the comparator 7 goes to the low level, a switching transistor TR Q1 is turned off. Then, the resistance value of the whole of an oscillator 3 for detection is made larger than that for turning-on of the TR Q1 though the oscillation frequency of this oscillator is not changed, and the voltage level of the oscillation output is raised in accordance with this increase. As the result, the inclination of the S curve showing the detection output characteristic is changed to narrow the width of the dynamic range of detection output.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM receiver, and more particularly to improvement of the FM detection part thereof.

[0002]

2. Description of the Related Art Generally, in an FM receiver, an FM detector such as a ratio detector or a quadrature detector is used.
A signal is demodulated into a signal having a voltage level according to its modulation degree, but the so-called S curve that determines the output characteristic of the FM detection has always been a fixed characteristic regardless of the strength of the received radio wave. It is set to obtain.

[0003]

If the FM-modulated received radio wave is strong and the carrier is large, the S / N ratio is improved due to the limiter effect, but if the radio wave is weak, the limiter effect is lost and the amplitude modulation is performed. At the same time, the amount of phase modulation increases,
Noise (white noise) increases rapidly.

As described above, if the S-curve remains fixed regardless of the strength of the received radio wave, noise is relatively increased when the radio wave is weak, and the dynamic range of the detection output is increased to the full range. It gets bigger.

Therefore, in the prior art, the S /
It was difficult to improve the N ratio. This becomes remarkable especially when the degree of modulation is low.

Therefore, an object of the present invention is to improve the S / N ratio of the detection output in the FM receiver without being affected by the strength of the received radio wave.

[0007]

In order to solve the above-mentioned problems, the present invention adopts the following structure in an FM receiving apparatus for receiving an FM-modulated signal.

That is, according to the present invention, a detecting means for detecting the intensity of the FM-modulated received radio wave and an S-curve changing means for changing the inclination of the S-curve for FM detection based on the detection output of this detecting means. The S / N ratio is improved according to the strength of the received radio wave.

According to this configuration, when the received radio wave is strong to some extent, the S curve of the detection output is normal,
When the input is weak, the slope of the S curve of the detection output is increased. Then, the dynamic range of the detection output becomes narrower, and the noise level saturates at a low level than at the time of strong input and does not increase further, so the S / N ratio of the detection output at weak input is improved. It

[0010]

Embodiment 1 FIG. 1 is a block diagram showing the configuration of an FM receiving apparatus according to Embodiment 1. As shown in FIG.

In the figure, 1 is an IF amplifier, 2 and 3 are phase comparators and detection oscillators constituting a quadrature detector, 4 is a low-pass filter, 5 is a level adjuster, 6 is a level detector, and 7 is a comparator. Is.

The detection oscillator 3 is an LC oscillator which outputs a signal for giving the reference phase of the phase comparator 2,
It consists of a coil L ₀ , capacitors C ₀ and C ₁ and a resistor R ₀ . A switching means 8 for switching the voltage level of the oscillation output is connected to the detection oscillator 3.

That is, this switching means 8 is, in this example,
A series circuit including a switching transistor Q ₁ and a resistor R ₁ is connected in parallel to the resistor R ₀ of the detection oscillator 3.

The switching means 8 corresponds to the S curve changing means in the claims, and the level detector 6 corresponds to the detecting means for detecting the strength of the received radio wave.

Now, when the FM-modulated received radio wave is received and taken out as a frequency-shifted FM signal with a center frequency of 10.7 MHz by a front end (not shown), this FM signal is amplified by the IF amplifier 1. After being
It is input to the phase comparator 2.

The phase comparator 2 compares the phase of the input FM signal with the oscillation output of the oscillator 3 and generates an output voltage according to the phase difference. Then, the low-pass filter 4 removes the high-frequency component from the output voltage, and outputs the detected voltage as a detection output.

On the other hand, the level detector 6 detects the level of the FM signal input to the IF amplifier 1, converts it into a voltage having a magnitude corresponding to the level, and outputs it.

FIG. 2 is a graph showing the input / output characteristics of the received signal. The horizontal axis represents the level of the FM signal obtained by reception, and the vertical axis represents the (signal + noise) level and noise level of the detection output. And the detection output voltage from the level detector 6 are shown.

Here, in FIG. 2, assuming that the position where the noise level is higher than the signal level at the time of strong input is point A, the FM signal input to the IF amplifier 1 becomes weaker, and point A is exceeded. The reference voltage of the comparator 7 is preset so that the output of the comparator 7 becomes a low level when the input voltage becomes weak (the output voltage of the level detector 6 becomes V _S or less). deep.

With this arrangement, when a strong received radio wave is obtained and the FM signal level is higher than point A,
The output of the comparator 7 is maintained at the high level, and the switching transistor Q ₁ forming the switching means 8 is on. Therefore, since a state where the resistance R ₁ is connected in parallel switching means 8 to the resistance R ₀ of the detection oscillator 3, the value of the overall combined resistance of the detecting oscillator 3 is reduced, correspondingly As a result, the voltage level of the oscillation output decreases.

FIG. 3 is a graph showing the S curve of the detection output of the FM receiver.

Here, the received radio wave is strong and the transistor Q
It is assumed that the characteristic of the detection output when ₁ is in the ON state and the voltage level of the oscillation output is small is the S curve indicated by the symbol a in FIG.

Now, when the level of the FM signal becomes lower than point A due to weak received radio waves, the output of the comparator 7 becomes low level and the switching transistor Q ₁ is turned off. Then, the oscillation frequency of the detection oscillator 3 does not change at all, but the overall resistance value of the detection oscillator 3 becomes larger than that when the transistor Q ₁ is on, and the voltage level of the oscillation output increases accordingly.

As a result, in FIG. 3, the detection output characteristic changes from the S curve indicated by the symbol a in the figure to the S curve indicated by the symbol b.

The S curve indicated by the reference sign b has a larger slope than the S curve indicated by the reference sign a, and therefore, the width of the dynamic range of the detection output is narrow. For this reason, even if there is noise with a large frequency deviation, it saturates at a time when it is at a lower level than at the time of strong input and does not increase further, so the S / N ratio of the detection output at weak input is improved.

However, the slope of the S curve of the detection output is the sign.
When it becomes large as shown by b, the level of the detection output also becomes large, and the problem that the audio signal level changes due to the switching of the S curve occurs.

Therefore, in this example, in order to make the detection output level the same before and after the switching of the S curve, the output of the comparator 7 becomes a low level and the S indicated by the symbol b in FIG.
When a curve is selected, the level adjuster 5 lowers the detection output level accordingly.

Embodiment 2 FIG. 4 is a block diagram showing the configuration of an FM receiving apparatus according to Embodiment 2, and the portions corresponding to those of Embodiment 1 shown in FIG. 1 are designated by the same reference numerals. 5 is a graph showing the input / output characteristics of the received signal, and FIG. 6 is a graph showing the S curve of the detection output of the FM receiver of FIG.

In the first embodiment described above, the S curve is switched between two stages by operating the comparator 7 with the output of the level detector 6 to control the ON / OFF of the switching transistor Q _1. In the example 2, the switches SW _{1 to} SW _{n are} switched according to the detection output of the level detector 6.
The on / off control is performed to switch the S curve in multiple stages.

That is, in the second embodiment, n resistors R _{1 to} R _n and switches SW _{1 to} SW _n are connected to the resistor R ₀ of the detection oscillator 3, respectively, while FIG. As described above, the magnitude of the detection output of the level detector 6 at each point indicated by reference signs A _{0 to} A _n when the FM signal level is changed is obtained in advance and written in the memory 11 such as the E ² PROM. deep.

At the time of receiving the FM radio wave, the output of the level detector 6 is sequentially taken into the control unit 10 via the A / D converter 9.

When the level of the received radio wave is sufficiently high,
When it is larger than the point A ₀ , the control unit 10 turns on all the _n switches SW _{1 to} SW _n .
Therefore, the combined resistance composed of n + 1 resistances R _{0 to} R _n has the minimum value, and the voltage level of the oscillation output of the detection oscillator 3 becomes the minimum. As a result, the detection output characteristic becomes the S curve indicated by the symbol a ₀ in FIG.

When the level of the received radio wave gradually weakens to reach the point A ₀ in FIG. 2, the control unit 10 turns off the n-th switch SW _n , so that the combined resistance of the detection oscillator 3 is increased. It gets a little bigger. Therefore, the voltage level of the oscillation output of the detection oscillator 3 is also increased to some extent, and as a result, the detection output characteristic becomes the S curve indicated by the symbol a ₁ in FIG.

Next, when the level of the received radio wave becomes weaker and reaches the point A ₂ in FIG. 2, the control unit 10 sets both the n-th and the n−1-th switches SW _n and SW _n-1. Since it is turned off, the combined resistance of the detection oscillator 3 is further increased. Therefore, the voltage level of the oscillation output of the detection oscillator 3 also increases to some extent, and as a result, the detection output characteristic becomes the S curve indicated by the symbol a ₂ in FIG.

Thereafter, as the level of the received radio wave decreases, the slope of the S curve increases, and when all the _n switches SW _{1 to} SW _n are turned off, the symbol a _{n in} FIG.
It becomes the S curve shown by.

As described above, if the S-curves can be switched in multiple stages, each S-curve can be selected according to the strength of the level of the received radio wave, so that the S / N ratio can be maintained while ensuring an appropriate dynamic range. This is advantageous because the ratio can be improved.

In each of the first and second embodiments described above, the S / N ratio is improved when the received radio wave is gradually weakened. However, by applying this, the S curve is improved even when a strong input is applied. By changing the inclination of, it can be used for volume adjustment and telephone call.

For example, in the configuration shown in FIG. 4, a volume control button 12 is provided, and the S curve can be selected from a _{0 to} a _n by operating the volume control button 12. Then, as the S curve approaches a ₀ → a _n , the detection output level increases, so the volume also increases, and conversely, as the S curve approaches a _n → a ₀ , the detection output level decreases and the volume increases. Also becomes smaller.

Further, in an analog cordless telephone, when a telephone call is made by a handset, data communication such as confirmation of an ID, designation of a call channel, confirmation, etc. is carried out in advance with a base unit. In order to surely exchange data at the time of receiving a data signal before a call, it is possible to change the slope of the S curve between when the data is received and when the call signal is received. That is, when the data signal is received, the slope of the S curve is made larger than that when the normal audio signal is received, and the detection output level is raised. Then, since the S / N ratio is improved, the probability of failure in data communication between the parent and child is reduced. In addition, when receiving a normal voice signal, if the slope of the S curve is made small and the dynamic range is made wide, a reliable voice call becomes possible.

[0040]

According to the present invention, the following effects can be obtained.

(1) Since the FM receiving apparatus of the present invention changes the slope of the S curve of the detection output according to the strength of the received radio wave, the S / N of the detection output during weak input is particularly high.
It is possible to improve the ratio.

As a result, it is possible to improve the difficulty of hearing the voice at the time of weak input, which cannot be achieved by the conventional FM receiver.

(2) In addition, S / N at the time of weak input of received radio waves
Not only the improvement, but also by changing the slope of the S curve even at the time of strong input, it can be used for level adjustment and volume switching, or the reliability of the connection at the time of receiving a voice signal and a data signal can be improved.

[Brief description of drawings]

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

FIG. 2 is a graph showing input / output characteristics of the receiving device.

FIG. 3 is a graph showing an S curve of a detection output of the receiving device.

FIG. 4 is a block diagram showing a configuration of an FM receiver according to a second embodiment of the present invention.

FIG. 5 is a graph showing input / output characteristics of the receiving device.

FIG. 6 is a graph showing an S curve of a detection output of the receiving device.

[Explanation of symbols]

 1 IF Amplifier 2 Phase Comparator 3 Detection Oscillator 4 Low Pass Filter 5 Level Adjuster 6 Level Detector (Detecting Means) 7 Comparator 8 Switching Means (S Curve Changing Means)

Claims (2)

[Claims] 1. An FM for receiving an FM-modulated received radio wave.
The receiving device includes a detecting means for detecting the strength of the received radio wave and an S curve changing means for changing the inclination of the S curve for FM detection based on the detection output of the detecting means. An FM receiver characterized in that the S / N ratio is improved accordingly. 2. The FM receiver according to claim 1, wherein the S-curve changing means switches the voltage level of an oscillation output of a detection oscillator for giving a reference phase of a phase comparator forming a quadrature detector. An FM receiver comprising a means.