GB2266422A

GB2266422A - Attenuators

Info

Publication number: GB2266422A
Application number: GB9307670A
Authority: GB
Inventors: Koji Kayama
Original assignee: Pioneer Electronic Corp
Current assignee: Pioneer Corp
Priority date: 1992-04-22
Filing date: 1993-04-14
Publication date: 1993-10-27
Anticipated expiration: 2013-04-14
Also published as: GB2266422B; DE4313211C2; GB9307670D0; JPH05300043A; DE4313211A1

Abstract

In an attenuator to be used at the front end of a radio receiver, the input signal is attenuated by a first control signal from 8, 9 applied to a first attenuating element e.g. a PIN diode D2. As the attenuation increases, the input impedance of the attenuator also rises causing various problems such as increase in intermodulation, mismatch with the following RF tuning circuit 2 and attenuation of input signal to other receivers sharing the same antenna 1. A second control voltage applied to a shunt attenuating element D1 has the effect of reducing input impedance which counteracts the increase caused by D2. Diodes D1, D2 may be replaced by other semiconductor devices or relays. The attenuator may form part of an automatic gain control arrangement. Control voltages may be applied directly from the switch 9 with the CPU not included. A modified attenuator may include two PIN diodes with respective bypass paths connected back to back in the signal path. <IMAGE>

Description

2266422 1 Attenuator for a Receiver The present invention relates to an

attenuator provided in a receiver to reduce the intensity of an RF signal.

The intensity of the RF signal transmitted to a radio receiver varies in accordance with electric field strength, which varies from time to time and place to place. In an intense electric field, the RF signal becomes excessively intense, thereby causing intermodulation and cross modulation. In order to prevent these modulations, an attenuator is provided in the receiver.

Referring to Fig. 5 showing a part of a superheterodyne receiver, the circuit has an attenuator 2a for attenuating a transmitted RF signal received by an antenna 1, a tuning circuit 3 for tuning in the RF signal and for impedance matching. The received signal is applied to a high-frequency amplifier 4 where the RF signal is amplified toa predetermined level and to a mixer 5 so as to be mixed with a locally generated signal from a local oscillator 6, thereby generating an intermediate frequency signal IF of 10.7 megahertz.

The attenuator 2a comprises a Pin diode D2 having a cathode connected to the antenna 1 through a 2 capacitor C2, and an anode connected to the turning circuit 3 through a capacitor Cl. The cathode of the PIN diode D2 is grounded through a resistor Rl. The anode is connected through a coil L1 to a control voltage generator 7a controlled by a signal from a CPU 8. The capacitors Cl and C2 are provided for checking the direct current, and the coil Ll is provided for preventing the RF signal from passing therethrough. The PIN diode D2 is adapted to decrease the operational resistance thereof with the increase of the applied forward current.

When the control voltage generator 7a is operated by the CPU 8 to decrease the control voltage of the PIN diode D2, the resistance thereof increases, thereby is increasing the amount of attenuation of the RF signal passing through. When the control voltage is increased, the amount of attenuation is decreased.

Fig. 6 shows another example of the attenuator. The attenuator 2b has the PIN diode D2, the anode of which is connected to a junction between the capacitors Cl and C2, and the catItode of which is grounded. That is, the PIN diode D2 is provided in parallel to the RF signal to form a shunt circuit. The control voltage is applied from the control voltage generator 7a through the coil Ll to control the resistance.thereof.

1 3 In operation, with the increase of the control voltage, the forward current is increased. The resistance of the PIN diode D2 is hence decreased, so that the attenuation of the RF signal is increased as a result. Conversely, the control voltage is decreased to reduce the amount of attenuation.

Fig. 7 shows the intermodulation characteristics in the attenuator 2a shown in Fig. 5 when the RF signal is attenuated by various quantities. in the graph, the abscissa shows difference (Af) between the frequencies of desire RF signal and the undesire disturbance signal which occurs in an execively intense electric field. The ordinate shows the allowable level of input RF signal not to cause predetermined level of disturbance with respect to the difference Af and various amounts of attenuation.

The graph shows that, in general, the allowable level increases as the difference Af between the RF signal and the disturbance wave increases. This is due to tuning characteristics of the tuning circuit 3. Hence in principle, thdcurve lines of allowable levels are shifted upward substantially.in parallel as the amount of attenuation increases.

However. when attenuating a large quantity, such as 20 decibels, the allowable level decreases when the difference exceeds 1 megahertz. In this case, the 4 tendency is significant particularly when the difference Af is positive, so that the allowable value becomes lower than those at the attenuations of 10 decibels and 15 decibels.

The cause of the phenomenon is described as follows. When the control voltage generated at the control voltage generator 7a is decreased in order to increase the amount of attenuation, the input impedance at the PIN diode D2 rises. The disturbance signal may be introduced because of the stray capacitance which exists between the anode and the cathode of the PIN diode D2 and the magnetical coupling in space. Moreover, the PIN diode D2 and the tuning circuit are mismatched in impedance, so that the reflexion of the RF signal causes the standing wave to be generated.

Hence the intermodulation is liable to increased. Thus, although the nominal maximum attenuation of the attenuator 2a is 20 decibels, the intermodulation can be restrained only up to about 10 decibels. 20 In the attenuator 2b of Fig. 6, since the resistance of the diodd D2 cannot be made zero. the RF signal can only be attenuated about 10 decibels. Furthermore, if the antenna 1 is commonly used with other receivers, mismatching may occur in the signal circuit. In addition, the RF signal to other receivers is undesirably attenuated.

An object of the present invention is to provide an attenuator where the amount of attenuation thereof can be increased without causing the intermodulation to 5 increase at any attenuating level.

In accordance with the present invention there is provided an-attenuator for attenuating a level of an input signal applied to a receiver with a predetermined amount of attenuation, comprising first control means for controlling the flow rate of the input signal in accordance with a first control signal, and second control means for reducing impedance of an input side of the first control means to a predetermined value in accordance with a second control signal when the flow rate of the input signal is reduced by the first control means.

The first control means includes at least one first PIN diode the resistance of which is controlled in accordance with the first control signal, and the second control means includes a second PIN diode the resistance of which iscontrolled in accordance with the second control signal.

In the present invention, when the RF signal is to be attenuated with a large quantity, the impedance of the input side of the attenuator is de.creased to prevent introduction of disturbance signals.

6 These and other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.

Fig. 1 is a block diagram of a radio receiver having an attenuator of the present invention; Fig. 2 is a block diagram showing a second embodiment of the attenuator of the present invention; Fig. 3 is a table showing the operations of the diodes in the attenuator of Fig. 2; Fig. 4 is a graph showing characteristics of the intermodulation of the receiver having the attenuator of the present invention; Figs.- 5 and 6 are block diagrams of radio receivers each having a conventional attenuator; and Fig. 7 is a graph showing characteristics of the intermodulation of the receiver of Fig. 5.

Referring to Fig. 1, the radio receiver in which the attenuator of the p'r'esent invention is employed is similar to the receiver shown in Fig. 5. The same references shown in Fig. 5 designate the same parts.

The RF signal from the antenna 1 is applied to the tuning circuit 3 through an attenuator. 2A and amplified at the high- frequency amplifier 4. The attenuated RF 7 signal and a signal from the local oscillator 6 are fed to the mixer 5 to generate an intermediate frequency signal IF of 10.7 megahertz.

The attenuator 2A of the present invention has the PIN diode D2, the anode of which is connected-to the tuning circuit 3 through the capacitor Cl. The cathode of the PIN diode D2 is connected to the antenna 1 through the capacitor C2, and to the ground through the resistor Rl.

The output of the capacitor C2 is grounded through a capacitor C4, a resistor R4 and a PIN diode D1 to form a shunt circuit. Namely, the anode of the diode D1 is connected to the resistor R4. The resistor R4 has a resistance of about 100 ohms, the value of which is small enough not to affect the operation of the antenna 1. The capacitor C4 is provided for checking the direct current.

The PIN diode D2 is linearly controlled dependent on a control voltage applied to the anode thereof from a control voltage generator 7B through the coil Ll. The control voltage generator 7B is, for example, a D/A converter operated in accordance with a signal from the CPU 8 which is connected to a selector switch 9 so as to apply an appropriate current to the PIN diode D2 in dependency on the desired attenuation.set by the select switch 9. When the sound reproduced by the radio 8 receiver includes noises which are caused by the intermodulation of the RP signal, a listner gradually turns a knob of the select switch 9 to select an appropriate amount of attenuation. - The PIN diode D1 is applied with a control voltage from a control voltage generator 7A through a coil L2.

The control voltage generator 7A comprises, for example, a transistor adapted to apply a high-level voltage or a low-level voltage to the PIN diode D1 in accordance with a signal from the CPU 8, thereby to cause the PIN diode D1 to be conductive (ON) or unconductive (OFF). When the diode D1 is conductive, the resistance thereof becomes small, so that the RP signal applied to the tuning circuit 3 is reduced.

In operation, when the select switch 9 is operated to attenuate the RP signal with a small quantity, the CPU applies a signal to the control voltage generator 7B to feed a high control voltage to the PIN diode D2. Since the resistance at the diode D2 is small, the flow rate of the RP signal passing through diode D2 becomes large.

When the select switch 9 is operated to increase the amount of attenuation, the control voltage fed to the diode D2 is decreased. The resistance of the PIN diode D2 is increased, thereby increasing the amount of attenuation. At the same time. the control voltage 9 generator 7A is operated to apply the high-level voltage to the annode of the PIN diode D1 in accordance with the output signal.of the CPU 8, thereby reducing the resistance of the diode D1. Hence the diode D1 becomes conductive. As a result, the impedance of the shunt circuit decreases to a value corresponding to the sum of the resistance of the resistor R4, that is 100 ohms, and the resistance of the diode D1. The sum of the resistance is set to a preferable value, so that short-circuiting is prevented. Thus, the introduction of disturbance signal is prevented. Furthermore, if the antenna 1 is commonly used with other receivers, the attenuation of the RF signals to the other receivers is prevented. Since the fluctuation of the impedance of the attenuator 2A as viewed from the antenna decreases, impedance mismatch is removed, thereby restraining the reflexion of the RF signal. Consequently, the intermodulation is restrained, so that the allowable value is improved as shown in the graph of Fig. 4.

When the selector'hwitch 9 is operated so as not to reduce RF signal, the control voltage generator 7A is operated to generate the low-level voltage, thereby rendering the PIN diode D1 unconductive. Thus, the sensitivity of the receiver is prevented from deteriorating.

Referring to Fig. 2, an attenuator 2B according to the second embodiment of the present invention has an additional PIN diode D3 provided between the diode D2 and the capacitor Cl. The anodes of the PIN diodes D2 and D3 are connected with each other. The cathode of the PIN diode D2 is connected to the control voltage generator 7B through a coil L3 and the cathode of the diode D3 is connected to a control voltage generator 7C through the coil Ll. The control voltage generators 7B and 7C are controlled by the CPU 8 to produce a high-level voltage or a low-level voltage in accordance with the operation of the select switch 9. Hence, contrary to the PIN diode D2 of the first embodiment, each of the PIN diodes D2 and D31s adapted to increase the resistance thereof when the high-level voltage is fed.

Parallelly provided to the PIN diodes D2 and D3 are capacitor CS, resistor R5, resistor R6 and capacitor C6, which are connected in series. The anodes of the diodes D2 and D3 are connected to a power supply +B through a resistor R7 and to the ground through the resistor Rl.

The PIN diode D1 is provided in the same manner as in the attenuator 2A of the first embodiment and applied with a high-level or a low-level control voltage from the control voltage generator 7A.

11 The operation of the present embodiment is described hereinafter with reference to the table of Fig. 3.

When the select switch 9 is turned off the CPU 8 operates the control voltage generator 7A to apply the low-level voltage to the PIN diode D1, thereby preventing the RF signal to pass through. The control voltage generators 7B and 7C apply the low-level voltages to the respective PIN diodes D2 and D3. Since the resistances of the diodes D2 and D3 are small, the diodes are conductive (ON) to allow the RF signal from the antenna 1 to pass through the diodes D2 and D3 without attenuating.

When the select switch 9 is operated to select the attenuation of 10 decibels, the control voltage generator 7B generates the high-level voltage and the control voltage generator 7C generates the low-level voltage. As a result, the resistance of the PIN diode D2 becomes large so as to be unconductive (OFF).

Accordingly, the RF signal passes through the capacitor C5. resistor R5 and thd PIN diode D3, bypassing the PIN diode D2. The RF signal is thus attenuated by the resistor R5.

In order to attenuate the RF signal with 15 decibels, the control voltage generator 7B and the control voltage generator 7C are controlled by the CPU 12 8 to generate the low-level voltage and the high-level voltage, respectively. Consequently, the resistance of the diode D2 becomes small and that of the diode D3 becomes large, thereby rendering the RF signal to pass through the PIN diode D2, resistor R6 and the capacitor C6, bypassing the PIN diode D3. Hence the RF signal is attenuated 15 decibels by the resistor R6. At the same time, the PIN diode D1 is applied with the highlevel voltage from the control voltage generator 7A. The RF signal is conducted to the ground thereby reducing the impedance of the shunt circuit.

When the attenuation of 20 decibels is selected, the control voltage generators 7B and 7C are operated to generate the high-level voltages. Hence the resistances of the PIN diodes D2 and D3 becomes large, rendering the diodes OFF. Consequently, the RF signal is transmitted through the capacitor C5, resistors R5 and R6 and the capacitor C6, bypassing the diodes D2 and D3, so as to be attenuated by the resistances of the resistors RS and R6. The resistance of the diode D1 is kept small to dedrease the input impedance.

Hence the attenuation of the RF signals is controlled to preferably restrain the intermodulation in the same manner as in the first embodiment as shown in Fig. 4.

The PIN diode D1 in the embodiments may be linearly controlled, in which case the resistor R4 may 13 be omitted. Each of the PIN diodes D2 and D3 in the second embodiment may be connected in reverse direction so that the anode is applied with the control voltage. The control voltage generators 7B and 7C are operated reversely to render the PIN diodes conductive-in the same manner.

Moreover, the PIN diodes used in the embodiments may be substituted with other semiconductor devices and relays having a good high-frequency response. The select switch 9 may directly control the control voltage generators, thereby omitting the CPU 8. The embodiments may be further modified to automatically attenuate the RP signals by providing an automatic gain control, or by detecting a multipath transmission. The present invention may be applied to receivers where the received frequencies differ from those of the radio receivers.

From the foregoing it will be understood that the present invention provides an attenuator for attenuating an RP signal applied to a receiver to restrain intermodulatidh. In the present invention, the input impedance of input side of the attenuator is decreased when the desired attenuation is large, so that intermodulation is prevented. When the antenna is -commonly used with other receivers, the attenuation of RP signals to the other receiver is prevented.

14 While the presently preferred embodiments of the present invention have been shown and described, it is to be understood that these disclosures are for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.

Claims

CLAIMS:

1. An attenuator for attenuating a level of an input signal applied to a receiver with a predetermined amount of attenuation, comprising:

means for generating a first control signal and a second control signal; first control means for controlling the flow rate of said input signal in accordance with the first control signal; and second control means for reducing impedance of an input side of the first control means to a predetermined value in accordance with the second control signal when said flow rate of the input signal is reduced by the first control means.

2. The attenuator according to claim 1 wherein the first control means includes at least one first PIN diode the resistance of which is controlled in accordance with the first control signal.

3. The attenuator according to claim 1 wherein the second control means includes a second PIN diode the resistance ot which is controlled in accordance with the second control signal.

4. The attenuator according to claim 2 wherein the second control means comprises a capacitor, a resistor and a PIN diode connected in.series between an input side of the first PIN diode and a ground.

16

5. The attenuator according to claim 2 further comprising a series of a capacitor and a resistor connected in parallel to the first PIN diode.

6. An attenuator substantially as described herein with reference to Figs. 1 to 4 of the accompanying drawings.