JPS6239853B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Field of the Invention The present invention relates to improving the S/N of a wireless telephone receiver. In particular, the present invention relates to a demodulated output noise reduction circuit suitable for mobile radio communications.

(2) Description of Prior Art Conventionally, as methods for reducing noise in radio telephone receivers, there have been methods such as using a bandpass filter that passes only the voice band of the telephone, and using an amplitude limiter that limits excessive amplitude. be. These methods have little effect on impulsive noise such as fading or ignition noise.

In addition, in order to deal with noise caused by a decrease in the receiving input level due to fading, proposed methods include detecting the receiving input level and using its output to attenuate the demodulated output during low electric fields, or narrowing the bandwidth of the demodulated output. has been done. However, these methods alone cannot reduce ignition noise and the like.

That is, FIG. 1 is a signal waveform diagram, where a is the electric field level of the received signal, and b is the detection level of the received signal after passing through the AGC circuit. FIG. 1c shows the demodulated audio output when there is no noise, and FIG. 1d shows the demodulated audio output with noise superimposed. In FIG. 1d, [A] and [C] are noises caused by a decrease in the input electric field due to fading, and [B] is external noise when the electric field is high. FIG. 1e shows the waveform of the demodulated audio output d after passing through an amplitude limiting circuit that limits excessive amplitude.

FIG. 1f shows the waveform after passing through a circuit that detects the received output level and uses the output to attenuate the demodulated output when the electric field is low. FIG. 1g shows the waveform after passing through a circuit that narrows the bandwidth of the demodulated output when the electric field is low based on the detection output of the received input level.

As can be seen from these waveforms, the amplitude limiting circuit may give significant distortion to excessively demodulated audio output, while the amplitude limiting circuit does not give any distortion, but has the disadvantage that the noise suppression effect is reduced. Furthermore, if a circuit is used that detects the received input level and uses its output to attenuate the demodulated output or narrow the bandwidth when the electric field is low, there is no effect in removing external noise when the electric field is high.

(3) Purpose of the present invention The present invention is an improvement on the above, and provides a radio telephone receiver that can suppress impulsive noise such as ignition noise and at the same time reduce noise caused by fading. The purpose is to

(4) Main points of the present invention The present invention is characterized in that it includes a slope limiting circuit through which a demodulated signal passes, and that the limiting limit of this circuit is controlled in accordance with the received signal level.

(5) Explanation based on an embodiment FIG. 2 is a block diagram of a receiver according to an embodiment of the present invention. In the figure, 1 is a high frequency amplification section, a frequency conversion section, and an intermediate frequency amplification section, 2 is a demodulator,
3 is a slope limiter, 4 is a low frequency amplification section, and 5 is a received signal electric field level detection section.

The received signal amplified by the amplifier 1 is sent out by the demodulator 2 as a noise-superimposed audio signal. Furthermore, the received electric field level detected by the electric field level detector 5 controls the slope limit of the slope limiting circuit 3. FIG. 3 shows an example of the configuration of the inclination limiter 3. This is constructed by cascading a differentiator 6, an amplitude limiter 7, and an integrator 8. FIG. 3a shows a device in which the limiting level of the amplitude limiter 7 is made variable, and FIG. 3b shows a device in which the gains of the differentiator 6 and integrator 8 are made variable in conjunction with each other.

FIG. 4 shows an example of a slope limiting circuit suitable for constructing this using an integrated circuit. In Figure 4,
Transistor 1 connected to the collectors of transistors 14 and 15 forming a differential amplifier
6 and 17 form a current mirror circuit, the current i _c5 that can flow into the phase correction capacitor 19 is determined by the current value i _c3 [A] of the current source made up of the transistors 12 and 13, so that −i _c3 ≦i Restricted to _c5 ≦i _c3 . Therefore, capacitor C[F]1
The rate of change of the voltage V _b [V] at the output terminal b point of the inverting amplifier 18 to which the other end of the inverting amplifier 9 is connected is dV _b /dt [V/sec
] is restricted to _−ic3 /C≦dV _b /dt≦ _ic3 /C.

On the other hand, since the transistors 12 and 13 form a current mirror circuit, i _c3 =i _c2 . Furthermore, transistor 1 through which current i _c2 flows
1 and the transistor 10 also constitute a current mirror circuit, so the current i _c1 flowing through the transistor 10 determines the current i _c3 . That is, i _c1 = i _c2 = i _c3 . The current i _c1 flowing through the transistor 10 is the voltage V _c [V] applied to the control terminal c and the resistance R [Ω].
It is determined by the base-emitter voltage V _be [V]≈0.7 [V] of the transistor 10. That is, i _c1 =V _c V _be /R [A] Therefore, the rate of change of V _b dV _b /dt [V/sec] is −V _c −V _be /RC≦dV _b /dt≦V _c −
V _be /RC. In other words, the limit value can be controlled by the voltage V _c [V] applied to the control terminal c.

If such a slope limiting circuit 3 is used, even if the rate of change of the input signal exceeds a certain value, the rate of change of the output signal will not exceed a certain value, and even if a large amplitude impulse is input, a small triangular wave will be output. It will only be done.

In the phase modulation method currently widely used in radio telephones such as mobile communications, an instantaneous frequency deviation control circuit is used on the transmitting side in order to limit the instantaneous frequency deviation to a certain value or less. As a result, the rate of change, or slope, of the modulated signal is limited to a certain value or less, but almost no restrictions are placed on normal audio input signals. Experiments have shown that even if the amplitude slope is somewhat limited by a slope limiting circuit, the intelligibility hardly deteriorates.

Therefore, the slope of the demodulated output signal is limited to a certain value or less even on the receiving side of a wireless telephone communication system using the phase modulation method, so even if an appropriate slope limiter is added after the demodulator of the receiver, it will not work. No distortion is given to the demodulated output signal. Furthermore, even with other modulation methods, almost no distortion occurs if the signal is a normal audio demodulated signal.

When noise is mixed in the wireless line, the amplitude may change rapidly and the slope of the demodulated output signal may exceed a certain value. Therefore, the slope limiter 3 removes the superimposed noise and reduces the noise. . In addition, if the S/N of the demodulated signal deteriorates significantly due to a decrease in the received input electric field level, the noise can be reduced accordingly by controlling the slope limiting circuit to reduce the rate of change of the output signal. . In other words, the noise is reduced as shown in Fig. 1 h, both for external noise such as [B] shown in Fig. 1 d, and for noise caused by a decrease in the input electric field such as [A] and [C]. It is possible to obtain output audio with almost no distortion applied to the audio.

(6) Explanation of Effects As described above, the present invention is effective against impulsive noise such as ignition noise and noise due to fading at the same time.
A wireless telephone receiver with improved S/N can be provided.

[Brief explanation of the drawing]

FIG. 1 is an explanatory waveform diagram of a received signal. a is the input signal electric field level of the receiver subjected to fading, b
is the received electric field detection circuit output waveform, c is the demodulated audio output waveform when there is no noise, d is the demodulated audio output waveform with superimposed noise, and [A] and [C] are caused by a decrease in the received input electric field level due to fading. [B] is external noise during high electric field. e is a waveform passed through an amplitude limiting circuit, and the broken line indicates the limiting level. f is a waveform passed through a circuit that attenuates the demodulated output when the electric field is low by the output b of the received electric field detection circuit. Similarly, g is a waveform passed through a circuit that narrows the bandwidth of the demodulated output when the electric field is low. h is the waveform after passing through the slope limiting circuit according to the present invention. FIG. 2 is a configuration diagram of an embodiment of the present invention, and is an example of the configuration of a receiver in which a slope limiting circuit controlled by the output of an electric field detection power circuit is provided after the demodulator. FIG. 3 is a diagram showing an example of the configuration of the slope limiter 3. FIG. 4 is a block diagram of a slope limiting circuit suitable for implementation as an integrated circuit. 1... High frequency amplification section/frequency conversion section and intermediate frequency amplification section, 2... Demodulation circuit, 3... Slope limiting circuit, 4... Low frequency amplification section, 5... Received electric field detection section, 6... Differential circuit , 7...amplitude limiting circuit, 8...
...integrator circuit.

Claims (1)

[Claims] 1. a slope limiting circuit that is inserted into the path of the demodulated signal obtained by demodulating the received signal and blocks the passage of a signal whose rate of change is greater than a predetermined limit value; and a level detection circuit that detects the electric field level of the received signal; and means for automatically controlling the limiting limit of the slope limiting circuit in accordance with the electric field level of the received signal detected by the level detecting circuit.