AUTOMATIC GAIN CONTROL CIRCUIT

Abstract: An automatic gain control circuit that can be suitably used for communication equipment such as a radio receiver and communication equipment provided with such an automatic gain control circuit. Signals received by an antenna (20) pass through a first band-pass filter (21), an h-f amplifier stage (22), a mixer (24), a second band-pass filter (23) and an i-f amplifier stage (26), and are detected by a detector stage (27). The detected signals art output from an output terminal 28. A gain control voltage V AGC derived at the detector stage (27) from the output of the i-f amplifier stage (26) is fed to the first and second band-pass filters (21, 23) to shift the center frequencies in the passband of these filters from the received frequency by an amount corresponding to the voltage V AGC . Therefore, the gains decrease relative to the received signals in these filters.

Description: This invention relates to automatic gain control circuits suitable for use in communication apparatuses such as wireless receivers, and more particularly to such an automatic gain control circuit which exhibits a good gain control characteristic with respect to an unduly large input signal. This invention also relates to a communication apparatus which comprises an automatic gain control circuit of the abovementioned type. Background Art A communication apparatus such as a wireless receiver is generally provided with an automatic gain control circuit so that a constant output level can be obtained even when the level of receiving signal changes. Particularly when the communication apparatus is a wireless transceiver mounted on a car which is used in such circumstances where the level of receiving signal significantly changes when passing through tunnels, moving from a plain up to a hill, passing through between buildings or approaching the transmitting radio station, the apparatus requires an automatic gain control circuit which can achieve a good gain control over a range of from a very small receiving signal level to an unduly large receiving signal level. Fig. 10 shows a structure of a principal portion of an example of wireless receivers which comprises an automatic gain control circuit of the conventional type. This receiver comprises a high-frequency amplifier stage 11 for amplifying a receiving signal inputted through an antenna 10, a mixer 13 for converting an output of this high-frequency amplifier stage into an intermediate-frequency signal in accordance with a local oscillation signal from a local oscillator 12, an intermediate-frequency amplifier stage 14 for amplifying the intermediate-frequency signal, and a detector stage 15 for demodulating an output of this intermediate-frequency amplifier stage. The detector stage 15 converts an output of the intermediate-frequency amplifier stage 14 into a DC signal to thereby produce a voltage V AGC for the automatic gain control and feeds it back to the respective amplifier stages 11 and 14. In each of the amplifier stages 11 and 14, an amount of bias to an amplifying element thereof is changed in accordance with the voltage V AGC to thereby control its gain. A curve V AGC in Fig. 11 shows one example of the variation of the automatic gain control voltage V AGC in terms of the variation of the receiving signal level in an automatic gain control circuit having the above structure. In the case where an automatic gain control circuit of the type whose gain is controlled by changing a bias to an amplifying element thereof is employed as described above, an amount of reduction of a gain in one amplifier stage is limited to about 30dB due to the influence of parasitic capacitance or the like in its amplifier elements, for example, in a wireless receiver. The reducing amount is not sufficient particularly to an unduly large receiving signal level in a communication apparatus of the aforesaid type which is to be mounted on a car. An automatic gain control circuit of this kind also has a deficiency that distortion in the output increases with the reduction of gain as indicated by a curve d in Fig. 11. Furthermore, an automatic gain control circuit of this type brings about an increase of power consumption in its amplifier stage, if the gain control circuit is of the forward AGC type in which the gain is decreased by changing the bias amount in the forward direction, and is therefore not suitable for use particularly in a portable-type communication apparatus. If the gain control circuit is of the reverse AGC type in which the gain is decreased by changing the bias amount in the reverse direction, a power supply of a different polarity is needed to obtain a sufficient reduction of gain, and this brings about an increase of the manufacturing costs. It is therefore an object of this invention to provide an automatic gain control circuit which has a sufficiently broad gain control range as compared to the conventional automatic gain control circuits and is yet possible to produce an output having less distortion with regard to an unduly large input signal. It is another object of the invention to provide an automatic gain control circuit which can control the gain without any substantial increase in power consumption. It is still another object of the invention to provide an automatic gain control circuit which does not need a plurality of power supplies of different polarities for giving biases to the amplifier stages. It is a further object of the invention to provide a communication apparatus which comprises an automatic gain control circuit of the above-described type. Disclosure of Invention For the purpose of achieving the above objects, an automatic gain control circuit for controlling a gain between an input terminal and an output terminal of a high frequency signal channel based on a signal level in the signal channel according to the invention comprises signal level detecting means for detecting a level of a signal at a point on the signal channel to produce a detection signal representative of the signal level; and is characterised by filter means which is provided between the input and output terminals of the signal channel and whose frequency band is shifted in the direction of higher frequencies or lower frequencies in accordance with the detection signal. In an automatic gain control circuit of the above construction, since the gain in the signal channel is controlled in accordance with the shift of the frequency band of the filter means, it is possible to perform a gain control within a broader range without a substantial increase of signal distortion as compared to an automatic gain control circuit of the conventional type in which the gain is controlled by changing the bias to an amplifier within the signal channel. In this case, said filter means may be constituted by a filter circuit which includes an induction element and a variable capacitance element, wherein a capacitance value of the variable capacitance element is changed based on the aforesaid detection signal. According to the invention, a communication apparatus comprising an automatic gain control circuit of the above construction is also provided. With this kind of communication apparatus, when it is constructed, for example, as a wireless transceiver to be mounted on a car, since a sufficient gain reduction can be performed even when an unduly large signal is received, it is possible to carry out a stable signal reception with less distortion within a wide range of receiving signal levels. In the case where an automatic gain control circuit according to the invention is applied to a communication apparatus which comprises a high-frequency amplifier stage for amplifying a received high-frequency signal, a mixer for converting an output of the high-frequency amplifier stage into an intermediate-frequency signal, an intermediate-frequency amplifier stage for amplifying an output of the mixer and a detector stage for demodulating an output of the intermediate-frequency amplifier stage and wherein the detector stage comprises detecting means for producing a detection signal indicative of an output signal level of the intermediate both input signals in such a manner that the center frequency of the frequency band is determined by the signal representative of the reception frequency and that a shift amount of the center frequency is determined by the detection signal. In the case where this communication apparatus is of the lower heterodyne system, the combining means may combine both input signals such that the center freqency is further shifted in the higher-frequency direction as the output signal level of the intermediate-frequency amplifier stage increases. On the other hand, if the communication apparatus is of the upper heterodyne system, the combining means may combine both input signals such that the center frequency is further shifted in the lower-frequency direction as the output signal level of the intermediate-frequency amplifier stage increases. With this construction, effects of the image-frequency waves can be avoided as in the former case, Brief Description of the Drawings Fig. 1 is a block diagram of a communication apparatus to which an automatic gain control circuit according to a first embodiment of the invention is applied; Fig. 2 is a graph showing shifts of the frequency band of the filter 21 or 23 in the communication apparatus shown in Fig. 1; Fig. 3 is circuit diagram of an example of the filter circuit used in the filter 21 or 23 in the communication apparatus shown in Fig. 1; Fig. 4 is a graph showing the variation of gain in that part of the communication apparatus of Fig. 1 which includes the filters 21 and 23 and the high-frequency amplifier stage 22; Fig. 5 is a block diagram of a communication apparatus to which an automatic gain control circuit according to a second embodiment of the invention is applied; frequency amplifier stage, filter means whose frequency band is shifted in accordance with the detection signal may be provided on at least one of the input and output sides of the high-frequency amplifier stage. In this case, if the filter means are constituted by a cascade connection of a plurality of filter circuits each comprising an induction element and a variable capacitance element, the gain control range can further be increased. If the above communication apparatus is of the lower heterodyne type wherein image-frequency waves are included in the lower frequency zone, the frequency band of the filter means may preferably be shifted in the higher-frequency direction with the increase of the output signal level of the intermediate-frequency amplifier stage. On the other hand, if the communication apparatus is of the upper heterodyne type wherein image-frequency waves are included in the higher frequency zone, the frequency band of the filter means may preferably be shifted in the lower-frequency direction with the increase of the output signal level of the intermediate-frequency amplifier stage. With this structure, when the level of the receiving signal increases, the frequency band of the filter means is shifted in the direction further away from the frequency zone in which the image-frequency waves exist, so that effects of the image-frequency waves can further be reduced. In the case where the center frequency of the frequency band of the filter means should follow the target reception frequency, combining means for combining the detection signal with a signal representative of the target reception frequency may be provided between the detecting means and the filter means so that the frequency band of the filter means is shifted in accordance with an output from the combining means. In this case, the combining means may combine Fig. 6 is a circuit diagram of an example of the amplifier 43 in the communication apparatus shown in Fig. 5; Fig. 7 is a circuit diagram of an example of the combining circuit 42 in the communication apparatus shown in Fig. 5; Fig. 8 is a circuit diagram of another example of the combining circuit 42 in the communication apparatus shown in Fig. 5; Fig. 9 is a graph showing voltages at various points in the communication apparatus of Fig. 5 and the distortion in the output of the apparatus; Fig. 10 is a block diagram of an example of wireless receivers in which one conventional automatic gain control circuit is provided; and Fig. 11 is a graph showing the characteristic of the conventional automatic gain control circuit shown in Fig. 10. Best Mode for Carrying out the Invention Fig. 1 is a block diagram showing the structure of a main portion of a wireless receiver to which an automatic gain control circuit according to a first embodiment of the invention is applied. In Fig. 1, a high-frequency signal received through an antenna 20 is supplied to a high-frequency amplifier stage 22 via a first band-pass filter 21. The received signal thus amplified by the high-frequency amplifier stage passes through a second band-pass filter 23 and is then supplied to a mixer 24. This mixer is supplied with a local oscillation signal from a local oscillator 25 of the known structure. The mixer 24 produces an intermediate-frequency signal from an output of the filter 23 in accordance with the local oscillation signal and supplies the produced signal to an intermediate-frequency amplifier stage (or an IF amplifier stage) 26. An output signal of the intermediate-frequency amplifier stage 26 is demodulated at a detector stage 27 and is thence supplied through a terminal 28 to an audio amplifier stage and the like in this receiver which are not shown. The center frequency of the frequency band of each of the filters 21 and 23 is brought into agreement with the reception frequency of this receiver by means of a known circuit which is not shown. In this case, the high-frequency amplifier stage 22 and the intermediate-frequency amplifier stage 26 are not necessarily provided with means for controlling gains thereof. The detector stage 27 converts the output of the intermediate-frequency amplifier stage 26 into a DC signal to produce a voltage V AGC for automatic gain control (an AGC voltage), which corresponds to the receiving signal level, and supplies the voltage V AGC , as a voltage for shifting frequency band, to both filters 21 and 23. As shown in Fig. 2, each of the filters 21 and 23 shifts a frequency band (solid line A) thereof in the direction of higher frequencies or of lower frequencies (broken line B or B′) by a frequency amount (delta f) corresponding to the above voltage V AGC . A frequency fo in the figure indicates the center frequency of the frequency band of each of the filters 21 and 23 at the time when the above voltage V AGC is zero, the center frequency fo corresponding to the target reception frequency. Fig. 3 shows one example of actual filter circuits used in the filters 21 and 23 in which a coil 30 is provided between the ground and a connection line between input and output terminals a and b of the relevant filter. In parallel with this coil, there is provided a series connection of a capacitor 31 and a variable capacitance diode 32. A junction point of the capacitor 31 and the variable capacitance diode 32 is supplied through a terminal c and a resistor 33 with the aforesaid voltage VAGc. With the thus constructed bandpass filter, a capacitance of the diode 32 varies in accordance with the voltage V AGC , whereby a resonance frequency determined by an inductance of the coil 30 and capacitance of the capacitor 31 and the diode 32 changes, that is to say, the frequency band is shifted. Each of the filters 21 and 23 may be constituted by a series connection of a plurality of filter circuits, such as that shown in Fig. 3, along the signal path. Thus, an overall gain of the portion comprising the first filter 21, the high-frequency amplifier stage 22 and the second filter 23 is reduced, as shown in Fig. 4, by an amount delta G in accordance with the frequency amount delta f shifted by the voltage V AGC . A gain indicated by a reference character C in Fig. 4 represents a value obtained by subtracting losses in the filters 21 and 23 from the gain of the amplifier stage 22. In the embodiment of Fig. 1, the overall gain can be reduced by an amount of 60 to 80 dB when the voltage V AGC is changed from 0 volt to 3 volts by suitably selecting the filter characteristic of the filter circuit in each of the band-pass filters 21 and 23 and the number of such filter-circuit stages in each filter. A wireless receiver to which a second embodiment of the automatic gain control circuit according to the invention will now be described with reference to Fig. 5, wherein elements corresponding to those in Fig. 1 are denoted by like reference numerals. The wireless receiver shown in Fig. 5 differs from that shown in Fig. 1 in the following respects. Shown at 40 is a terminal to which a voltage V f accurately brought into correspondence with the receiving frequency in this receiver by means of a phase-locked loop or the like is applied. The voltage V f is supplied to a local oscillator 41 of the voltage controlled type as a control voltage. The voltage V f is also supplied to one input terminal 42a of a combining circuit 42. On the other hand, the voltage V AGC supplied from the detector stage 27 is amplified by an amplifier 43 to a voltage V C which is then supplied to the other input terminal 42b of the combining circuit 42. The amplifier 43 comprises, as shown in Fig. 6, an operational amplifier 53 whose inverting input terminal is supplied through a resistor 50 with the voltage V AGC at an input terminal 43a and whose non-inverting input terminal is supplied with a divided voltage V ref of a supply voltage +V B by resistors 51 and 52. The amplifier 43 further comprises a pnp transistor 56 having an emitter fed wlth the supply voltage +V B through a Zener diode 54, a base driven by an output of the operational amplifier 53 through a resistor 55 and a collector connected to an output terminal 43b thereof. With the amplifier 43 of this construction, it is possible to convert the voltage V AGC into the voltage V C , which has a value and a variation range suitable for shifting the frequency band of each of the filters 21 and 23, by suitably selecting the voltage V ref , a Zener voltage of the Zener diode 54, a resistance value of a resistor 57 interposed between the inverting input terminal and the output terminal of the operational amplifier 53 and so on. The combining circuit 42 combines the voltage V f with the voltage V C in the following manner and supplies the combined voltage to each of the filters 21 and 23 as a band-shifting voltage. More specifically, in the case where this receiver is of the lower heterodyne system wherein the image-frequency waves exist in the lower frequency zone, the combining circuit 42 comprises, as shown in Fig. 7, a diode 70 for passing the voltage V f at the terminal 42a in the forward direction thereof to a terminal 42c and another diode 71 for passing the voltage V C at the terminal 42b in the forward direction thereof to the terminal 42c. With this combining circuit 42, the center frequency of the frequency band of the filter 21 (or 23) set by the voltage V f is shifted in the direction of higher frequencies whereby the gain is decreased. In this case, the frequency band is shifted in the direction away from the frequency zone in which the image-frequency waves exist, so that effects of the image-frequency waves can further be reduced. If a more accurate gain control is required, the combining circuit 42 may comprise an adder circuit which adds both inputs at the terminals 42a and 42b together and outputs the addition result to the terminal 42c. In the case where this receiver is of the upper heterodyne system wherein image-frequency waves exist in the higher frequency zone, the combining circuit may be constructed as shown in Fig. 8 at 42′. The combining circuit 42′ shown in Fig. 8 comprises a series connection of a diode 73 and a resistor 74 which are interposed between an input terminal 42a′, to which the voltage V f is supplied, and an output terminal 42c′. An npn transistor 75 is connected between the output terminal 42c′ and the ground, which transistor has a base to which the voltage V C at another input terminal 42b′ is supplied through a resistor 76. With this combining circuit 42′, the center frequency of the frequency band of the filter 21 (or 23) set by the voltage V f is shifted in the direction of lower frequencies whereby the gain is decreased. In this case, the frequency band is shifted in the direction away from the frequency zone in which the image-frequency waves exist, so that effects of the image-frequency waves can further be reduced. If a more accurate gain control is required, the combining circuit 42′ may comprise a subtracter circuit which subtracts the voltage V C at the terminal 42b′ from the voltage V f at the terminal 42a′ and outputs the subtraction result to the terminal 42c′. Fig. 9 shows a characteristic of the automatic gain control circuit in the receiver according to the embodiment of Fig. 5, wherein a curve V AGC indicates the variation of the voltage V AGC in terms of the variation of the receiving signal level, a curve V O indicating an output level of this receiver, a curve d₁ indicating a distortion in the output of this receiver, a curve d₂ indicating a distortion in a receiver provided with the conventional automatic gain control circuit for comparison. Industrial Applicability An automatic gain control circuit according to the present invention is suitable for use in a communication apparatus such as an AM and an FM wireless receiver, Also, a communication apparatus according to the present invention can advantageously be used particularly under such circumstances as for a wireless transceiver mounted on a car where the radio-wave field strength abruptly and significantly changes.

Claims: An automatic gain control circuit for controlling a gain between an input terminal and an output terminal of a high frequency signal channel in accordance with a signal level on the signal channel comprising: signal level detecting means for detecting a signal level at a point on the signal channel to produce a detection signal indicative of said signal level; characterised by filter means which are interposed between the input and output terminals of the signal channel and whose frequency band is shifted in the direction of higher or lower frequencies based on said detection signal. An automatic gain control circuit according to Claim 1, characterized in that said filter means comprise a filter circuit which includes an induction element and a variable capacitance element, wherein capacitance of said variable capacitance element is changed based on said detection signal. A communication apparatus which comprises an automatic gain control circuit according to Claim 1 or Claim 2, wherein a received signal is amplified in the signal channel. A communication apparatus according to Claim 3, characterized in that the signal channel comprises high-frequency amplifier stage for amplifying a received high-frequency signal, a mixer for converting an output of said high-frequency amplifier stage into an intermediate-frequency signal, an intermediate-frequency amplifier stage for amplifying an output of said mixer and detector means for demodulating an output of said intermediate-frequency amplifier stage, said detector means comprising detecting means so constructed as to produce said detection signal in accordance with an output signal level of said intermediate-frequency amplifier stage, said filter means being provided on at least one of input and output sides of said high-frequency amplifier stage. A communication apparatus according to Claim 4, characterized in that said filter means are constituted by a series connection of a plurality of the filter circuits. A communication apparatus according to Claim 4 or Claim 5, characterized in that if said apparatus is of the lower heterodyne system in which image-frequency waves are included in the lower frequency zone the frequency band of said filter means is further shifted in the direction of higher frequencies as the output signal level of said intermediate-frequency amplifier stage increases, and in that if said apparatus is of the upper heterodyne system in which image-frequency waves are included in the higher frequency zone the frequency band of said filter means is further shifted in the direction of lower frequencies as the output signal level of said intermediate-frequency amplifier stage increases. A communication apparatus accordlng to Claim 4 or Claim 5, characterized in that combining means are provided between said detecting means and said filter means for combining said detection signal with a signal indicating a reception frequency in said communication apparatus, wherein the frequency band of said filter means is shifted in accordance with an output of said combining means. A communication apparatus according to Claim 7, characterized in that said combining means combine said detection signal with said signal indicating the receptlon frequency such that the center frequency of said frequency band is determined by said signal indicating the reception frequency and such that a shift amount of said center frequency is determined by said detection signal. A communication apparatus according to Claim 8, characterized in that if said apparatus is of the lower heterodyne system in which image-frequency waves are included in the lower frequency zone said combining means combine said detection signal with said signal indicating the reception frequency in such a manner that said center frequency is further shifted in the direction of higher frequencies as the output signal level of said intermediate-frequency amplifier increases, and in that if said apparatus is of the upper heterodyne system in which image frequency waves are included in the higher frequency zone said combining means combine said detection signal with said signal indicating the reception frequency in such a manner that said center frequency is further shifted in the direction of lower frequencies as the output signal level of said intermediate-frequency amplifier increases.

Inventors: ENDO KATSUHIRO, SUGAWA KATSUSHI, ENDO, KATSUHIRO, SUGAWA, KATSUSHI
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: H03G3
IPC codes: H03G3