GB2305034A

GB2305034A - Attenuation control with lower attenuation below a threshold using a Schmitt trigger

Info

Publication number: GB2305034A
Application number: GB9617691A
Authority: GB
Inventors: Hidetoshi Okamura
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1995-08-24
Filing date: 1996-08-23
Publication date: 1997-03-26
Anticipated expiration: 2016-08-23
Also published as: KR970013830A; JP2778546B2; GB9617691D0; GB2305034B; JPH0964774A

Abstract

A device for attenuating a received signal in accordance with the present invention has a field intensity detector (4) for detecting the field intensity of a received signal, and an attenuator (2) for attenuating the received signal. A controller (5) controls the amount of attenuation of the attenuator (2) in accordance with the detected field intensity. Specifically, the controller (5) feeds a zero voltage to the attenuator (2) when the detected field intensity is lower than a threshold inclusive, or feeds the detected field intensity directly to the attenuator (2) when it is higher than the threshold.

Description

APPARATUS FOR ATTENUATING A RECEIVED SIGNAL The present invention relates to apparatus for attenuating a received signal axi, more pwtloisrly, to a device for controlling the attenuation of a signal received by a radio communication apparatus.

It is a common practice with a radio communication apparatus to amplify a received signal with an amplifier before feeding it to a demodulator. The problem with this scheme is that when the received signal has a high signal level, i.e., intense field strength, saturation occurs dfle to the nonlinearity of a transistor constituting an amplifier, shifting the operation point of the transistor. As a result, the gain of the amplifier for a desired wave contained in the received signal decreases, deteriorating the sensitivity to the desired wave.

Usually, to insure the sensitivity to the desired wave, an attenuator for attenuating the received signal precedes the amplifier. The attenuator attenuates the received signal and thereby prevents the gain of the amplifier from decreasing.

A conventional radio communication apparatus including the above attenuator has an amplifier and a field strength detector in addition to the attenuator. The amount of attenuation to be executed by the attenuator is determined by the output voltage of the field strength detector. Usually, when the field strength of the received signal increases, the output voltage of the field strength detector increases and causes t h e amount of attenuation of the attenuator to increases. In practice, however, even when the field strength of the received signal is low, the output of the field strength detector is apt to exceed the actual field strength due to, e.g., thermal noise. As a result, despite that the field strength of the received signal is low, the attenuator attenuates the received signal by more than a necessary amount.

It is therefore an object of at least the preferred embodiments of the present invention to provide a device for controlling the attenuation of a received signal and capable of causing a received signal to be attenuated in an optimal amount.

In accordance with the present invention, apparatus for attenuating a received signal has a field intensity detector for detecting the field intensity of the received signal to thereby output a field intensity signal representative of the field intensity. An attenuator attenuates the received signal by an amount set by the field intensity signal. A controller controls the attenuator such that when the field intensity indicated b y the field intensity signal is lower than a preselected value, the attenuator attenuates the received signal by an amount smaller than the amount set by the field intensity signal.

Preferably, the controller has a Schmitt trigger circuit for outputting a signal of high level when the field intensity is higher than the preselected value, or outputting a signal of low level when the field intensity is lower than the preselected value. A switch turns on when the signal of high level is input or turns off when the signal of low level is input. A resistor is connected at one end to the switch and at the other end to ground. When the field intensity is lower than the preselected value, said field intensity signal is fed to the attenuator while, when the field intensity is higher than the preselected value, a preselected signal is fed to the attenuator.

In the - above construction, when the field strength indicated by the field strength detector is lower than the predetermined value, the controller causes a field strength signal indicative of zero field strength to be output. Therefore, when the field intensity of the received signal is low, the attenuation amount of the attenuator is reduced.This prevents the field strength of the received signal from being excessively reduced, thereby enhancing the minimum operation sensitivity to the received signal level, e.g., the field strength of the received sginal Preferred features of the present invention will now be described, purely by way of example only, with reference to the accompanying drawings, in which:: FIG. 1 is a block diagram schematically showing an attenuation control device for a radio communication apparatus and embodying the present invention; FIG. 2 is a circuit diagram showing a specific configuration of an attenuator included in the embodiment; FIG. 3 is a block diagram schematically showing a specific configuration of a controller also included in the embodiment; FIG. 4 is a graph representative of the input/output characteristic of a Schmitt trigger circuit included in the attenuator shown in FIG. 2; FIG. 5 is a graph representative of the input/output characteristic of the controller shown in FIG. 1; FIG. 6 shows a relation between the voltage input from an antenna included in the embodiment to the attenuator and the amount of attenuation;; FIG. 7 is a graph representative of the input/output characteristic of the Schmitt trigger circuit and representative of an alternative embodiment of the present invention; and FIG. 8 is a graph showing the input/output characteristic of the controller and associated with the alternative embodiment.

In the drawings, identical references denote identical structural elernents.

Referring to FIG. 1 of the drawings, an attenuation control device embodying the present invention is shown. As shown, a signal coming in through an antenna 1 is attenuated by a n attenuator 2 and then fed to an amplifier 3. The amount of attenuation to be executed by the attenuator 2 is determined in accordance with a voltage output from a field strength detector 4. The amplifier 3 amplifies the attenuated signal and feeds the amplified signal to the field strength detector 4 and a demodulator 6. The field strength detector 4 transforms the amplitude of the received signal to a DC voltage and delivers the DC voltage to a controller 5. A preselected threshold VTH is set in the controller 5.The controller 5 feeds zero voltage to the attenuator 2 when the output voltage of the field strength detector 4 is lower than the threshold VTH, or feeds the output voltage of the detector 4 directly to the attenuator 2 when it is higher than the threshold VTH.

FIG. 2 shows a specific configuration of the attenuator 2.

As shown, the attenuator 2 includes a diode 21 having an anode connected to a constant power source terminal, a diode 22 having an anode connected to the cathode of the diode 21, and a resistor 23 having one end connected to the cathode of the diode 22. A transistor 24 has a collector connected to the other end of the resistor 23, a base connected to one end of a resistor 25, and an emitter connected to ground. The other end of the resistor 25 is connected to the field strength detector 4.

In the attenuator 2 shown in FIG. 2, the impedance of the transistor 24 varies in proportion to the DC voltage output from the field strength detector 4. As a result, a current flowing through the diodes 21 and 22 varies and causes the impedances of the diodes 21 and 22 to vary. Consequently, the impedance of the attenuator 2 varies. Specifically, the impedance of the attenuator 2 increases with an increase in the field strength of the received signal or decreases with a decrease in the field strength. Therefore, when the field strength of the received signal or voltage. input to the amplifier 3 is high, the received signal is attenuated in a great amount by the attenuator 2.

A reference will be made to FIG. 3 for describing a specific configuration of the controller 5. As shown, the controller 5 includes a switch 51 connected to the attenuator 2 and field strength detector 4 in series. A Schmitt trigger circuit 52 selectively turns on or turns off a switch 51 in accordance with the DC voltage output from the field strength detector 4. A resistor 53 is connected at one end between the attenuator 2 and the switch 51 and at the other end to ground. As shown in FIG. 4, the output voltage of the Schmitt trigger circuit 52 remains in a low level (VL) when the voltage output from the field strength detector 4 is between zero and the threshold VTH or remains in a high level (VH) when it is higher than the threshold value VTH.

The operation of the controller 5 will be described with reference to FIG. 5 showing a relation between the input voltage and the output voltage of the controller 5. The field strength detected by the field strength detector 4 is applied to the switch 51 and Schmitt trigger circuit 52. The trigger circuit 52 outputs the voltage of low level VL when the input voltage is lower than the -threshold VTH inclusive or outputs the voltage of high level VH when it is higher than the threshold VTH, as stated with reference to FIG. 4. The output voltage VH of the trigger circuit 52 turns on the switch 51 while the output voltage VL of the trigger circuit 52. turns it off. Therefore, when the input voltage to the trigger circuit 52 is higher than the threshold VTH, the switch 51 is turned on to apply the input voltage directly to the attenuator 2.At this instant, the input voltage is equal to the input voltage to the controller 5, as shown in FIG. 5. When the input voltage to the trigger circuit 52 is lower than the threshold VTH inclusive, the switch 51 is turned off to connect the attenuator 2 to ground via the resistor 53. As a result, the voltage input to the attenuator 2 becomes zero, as shown in FIG.

5.

FIG. 6 shows a relation between the voltage input to the attenuator 2 via the antenna 1 and the amount in which it is attenuated by the attenuator 2. As shown, when the voltage input to the attenuator 2 is low, i.e., lower than a voltage D, the amount of attenuation is close to zero. When the input voltage exceeds the voltage D, the amount of attenuation sharply increases. The input voltage D is of a level immune to thermal noise.

An alternative embodiment of the present invention will be described with reference to FIGS. 7 and 8. FIG. 7 shows a relation between the voltages input to and output from the Schmitt trigger circuit 52. Likewise, FIG. 8 shows a relation between the voltages input to and 'output from the controller 5.

In this embodiment, the controller 5 uses a threshold VTHH when the voltage input thereto increases or uses a threshold VTHL when it decreases.

Assume that the voltage input to the controller 5 increases. Then, the output voltage of the controller 5 is zero when the input voltage is lower than the threshold VTHH or is equal to the input voltage when the input voltage is higher than the threshold VTHH, as shown in FIG. 8. On the other hand, assume that the voltage input to the controller 5 decreases.

Then, the output voltage of the controller 5 is equal to the input voltage when the input voltage is higher than the threshold VTHL or is zero when the input voltage is lower than the threshold VTHL.

As stated above, in the illustrative embodiment, voltages input to and output from the controller 5 have a hysteresis characteristic. This obviates an occurrence that when the voltage input to the controller 5 is around the threshold, the voltage output from the controller 5 sharply increases from zero to a voltage equal to the input voltage. Consequently, a loop circuit consisting of the field intensity detector 4 and controller 5 has a stable characteristic.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

The text of the abstract filed herewith is repeated here as part of the specification.

A device for attenuating a received signal in accordance with the present invention has a field intensity detector for detecting the field intensity of a received signal, and an attenuator for attenuating the received signal. A controller controls the amount of attenuation of the attenuator in accordance with the detected filed intensity.

Specifically, the controller feeds a zero voltage to the attenuator when the detected field intensity is lower than a threshold inclusive, or feeds the detected field intensity directly to the attenuator when it is higher than the threshold.

Claims

1. Apparatus for attenuating a received signal, comprising: a field intensity detector for detecting a field intensity of the received signal to thereby output a field intensity signal representative of the field intensity; an attenuator for attenuating the received signal by an amount set by said field intensity signal; and a controller for controlling said attenuator such that when the field intensity indicated by said field intensity signal is lower than a preselected value, said attenuator attenuates the received signal by an amount smaller than said amount set by said field intensity signal.

2. Apparatus as claimed in Claim 1, wherein said amount smaller than said amount set by said field intensity signal is substantially zero.

3. Apparatus as claimed in Claim 1 or 2, wherein said controller comprises: a Schmitt trigger circuit for outputting a signal of high level when the field intensity is higher than said preselected value, or outputting a signal of low level when the field intensity is lower than said preselected value; a switch turning on when said signal of high level is input or turning off when said signal of low level is input; and a resistor connected at one end to said switch and at the other end to ground; and wherein when the field intensity is lower than said preselected value, said first intensity signal is fed to said attenuator while, when the field intensity is higher than said preselected value, a preselected value is fed to said attenuator.

4. Apparatus as claimed in Claim 3, wherein said Schmitt trigger circuit has an input/output characteristic having a hysteresis.

5. Apparatus as claimed in any preceding claim, wherein said attenuator comprises: a first diode having a cathode connected to a power source; a second diode having a cathode connected to an anode of said first diode; a transistor having one end connected to an anode of said second diode; a transistor having a collector connected to the other end of said resistor, and an emitter connected to ground; and a second resistor having one end connected to a base of said transistor to thereby feed said field intensity signal to said base of said transistor.

6. Apparatus as claimed in any preceding claim, wherein said field intensity detector transforms an amplitude of the received signal to a DC voltage to thereby output said field intensity signal.

7. Apparatus for attenuating a received signal substantially as herein described with reference to and as shown in Figures 1 to 3 of the accompanying drawings.