RECEIVER WITH FEED BACK CIRCUIT FOR THE CONTROL OF THE GAIN

Abstract: Receiver comprising analog signal processing means, intermodulation interference detection means and an automatic gain control circuit for controlling the gain of said analog signal processing means, in which an output of the analog signal processing means is coupled through frequency selective means for channel selection and an amplitude detector to a first terminal of the intermodulation interference detection means. A signal generator is used to generate a gain modulation signal being coupled to a gain control input of said analog signal processing means as well as to a second terminal of the intermodulation interference detection means. The intermodulation interference detection means comprise a correlator being included in a feed back loop of said automatic gain control circuit supplying a DC gain control signal to the gain control input of the analog signal processing means at a detection of differences in relative variations between the gain modulation signal on the one hand and the output signal of the amplitude detector on the other hand for reducing said differences by a DC gain setting adjustment of the analog signal processing means.

Description: The invention relates to a receiver comprising analog signal processing means, a feed back control circuit for controlling the gain of said analog signal processing means, comprising intermodulation interference detection means. Such a receiver is on itself known from US patent 4,654,884. The radio receiver disclosed therein, comprises an automatic gain control circuit in which intermodulation interference is detected by comparing the signal level within the bandwidth of a wanted radio channel with that of interfering radio-channels deviating from the wanted radio-channel by a frequency distance allowing intermodulation interferences to occur within the wanted radio-channel. An intermodulation interference indicative signal is generated when the signal level of wanted radiochannel becomes smaller then the signal level of one of the interfering channels. The intermodulation interference indicative signal is used to control the gain of the RF analog signal processing means. The automatic gain control obtained therewith requires accurate matching between the specific characteristic of the intermodulation interference indicative signal being generated and on the one hand and the deterioration of the useful signal caused by the initiating intermodulation interferences on the other hand. Due to element tolerance deviations a safety margin has to be taken into account in defining the specific characteristic of the intermodulation interference indicative signal to prevent either over- or underdosing the gain control, respectively resulting in loss of useful signal components or insufficient suppression of intermodulation interferences. This safety margin therewith limites the accuracy in gain control. Furthermore, to measure the signal level of an interfering channel, the receiver has to interrupt its correct tuning position, in which a wanted radio-signal is received, and to switch over its tuning to the interfering channel. To avoid the switching of the tuning and the reception of interfering signals during level detection from being heard, the receiver has to be muted. To come to a reliable level detection, the period during which the receiver has to stay tuned to said interfering channel, has to exceed a certain level detection time constant, therewith defining a minimum duration to the mute period. Furthermore, especially when the receiver is used in a fast changing environment, e.g. when being mounted on a moving object such as an automobile, the interference channel level check has to be carried out rather frequently, therewith putting a minimum repetition putting frequency to the mute actions. This all strongly deteriorates the overall receiver performance. Therefore, it is a first object of the present invention to provide a receiver with an automatic gain control circuit which allows to eliminate intermodulation interferences more accurately than the receiver disclosed in the above referenced US patent. A second object of the invention is to provide a receiver with an automatic gain control circuit without the necessity to interrupt the processing of the wanted signal and to switch tuning. A receiver of the above referred type according to the invention is therefore characterized in that an output of the analog signal processing means is coupled through frequency selective means for channel selection and an amplitude detector to a first terminal of the interfering signals detection means, a signal generator for generating a gain modulation signal being coupled to a gain control input of said analog signal processing means as well as to a second terminal of the interfering signals detection means, the intermodulation interference detection means comprising a correlator being included in a feed back control circuit supplying a DC gain control signal to the gain control input of the analog signal processing means at a detection of differences in relative variations between the gain control signal on the one hand and the output signal of the amplitude detector on the other hand for reducing said differences by a DC gain setting adjustment of the analog signal processing means. A variation of the gain of the analog signal processing means will cause the level of the useful signal to vary linearly with said gain variation, whereas intermodulation interference components and harmonic distortion products will vary to a second or higher order with said gain variation. According to the invention, this difference in gain variation is used to detect whether the output signal level of the analog signal processing means is predominated by the useful signal or by intermodulation and/or harmonic distortion interferences. The occurrence of a signal level at the output of the analog signal processing means varying disproportionally, i.e. to a second or higher order, with a gain variation of the analog signal processing means, is accurately detected in the correlator, and fed back as a gain control signal to the RF analog signal processing means. Due to this feedback, the accuracy in gain control is immune for element tolerances. Furthermore, the detection of intermodulation and/or harmonic distortion interferences is applied during correct tuning at a wanted radio-signal, which, in contrast to the above mentioned known method of intermodulation interference detection, does not require to change tuning and mute the receiver. In the following the description given with respect to the detection and elimination of intermodulation interference also applies to harmonic distortion interferences. A preferred embodiment of a receiver according to the invention is characterized in that gain modulation signal comprises a bandwidth limited bandwidth limited pseudo random carrier frequency. The use of such gain control signal prevents the gain variation from becoming noticeable in the useful signal. For the same purpose, the amplitude of the gain modulation signal is preferably chosen smaller than 2 dB. Another preferred embodiment of a receiver according to the invention, in which the effect of the gain variation on the signal output level of the analog signal processing means is prevented from becoming noticeable in the useful signal, is characterized in that the signal generator is coupled through inverting means to a modulation signal input terminal of an amplitude modulator following said analog signal processing means. With this measure, said gain variations can be completely eliminated. Preferably, a delay circuit is coupled between the signal generator and said modulation signal input terminal of the amplitude modulator to compensate for the signal delay occurring in the analog signal processing means. For a cost effective realisation the analog signal processing means preferably comprise an controllable attenuator preceding a broadband amplifying stage coupled to the gain control input of the analog signal processing means providing a signal attenuation varying with said gain control signal. In a further preferred embodiment the frequency selective means for channel selection comprise a mixing stage followed by an IF circuit for IF channel selection, an output of said IF circuit being coupled through said level detecting means to the first terminal of the correlator. Another preferred embodiment of a receiver according to the invention is characterized in that an output of the signal generator is coupled to a first input of signal combining means, an output of the correlator being coupled to a second input of the signal combining means and an output of said signal combining means being coupled through said gain control input of the analog signal processing means to said controllable attenuator. The above and other objects, features and advantages of the present invention will become more apparent from the following description referring to the accompanying drawings, in which a preferred embodiment of the present invention is shown by way of illustrative example and in which identical reference indications are used for corresponding circuitry. Figure 1a and 1b are diagrams illustrating the phenomenon of intermodulation interference. Figure 2 is a block diagram showing the concept of an AGC circuit according to the invention. Figure 3 is a block diagram of a receiver according to a preferred embodiment of the present invention. Figure 4a, 4b and 4c are timing charts illustrative of the various signals occurring in the receiver of Figure 3. Figures 1a and 1 b are diagrams illustrating the phenomenon of intermodulation interference, in which Figure 1a shows the differences in amplitude increase between a wanted first order signal s and unwanted second and third order intermodulation products d2 and d3, respectively, as a function of the fieldstrength of the received RF frequency band. These higher order intermodulation products are caused by intermodulation interference between all RF carrier signals within the received RF frequency due to non-linearities in the signal processing within (the amplifiers, mixer and demodulator) of a receiver. Where wanted RF signals vary proportionally with variations in the reception fieldstrength, said higher order intermodulation products vary non linearly (i.e. to their order of harmonic distortion) with variations in the reception fieldstrength. As indicated in Figure 1a, the RF input signal amplitude is stabilised at a value Vst at which the wanted first order signal component s dominates over the intermodulation products d2 and d3 and is sufficiently large to be reproduced without being noticeably distorted by intermodulation interferences. The occurrence of unwanted higher order intermodulation products within the received RF signal band is in practise unavoidable. Suppose RF carrier signals f1 and f2 simultaneously occur within the received RF frequency band as shown in Figure 1b. Then intermodulation products occurring nearest to those carrier frequencies f1 and f2, are a second order intermodulation product d2 occurring at f1±f2, and two third order intermodulation products d3 occurring at respectively 2f1±f2 and 2f2±f1. Because there are many RF carrier frequencies, these intermodulation products will be spread over the full RF input frequency band. Once occurring within a wanted signal channel, the intermodulation products cannot be filtered out or suppressed without losing useful signal components. The effect of such intermodulation products on the reproduced signal is reduced according to the invention by an automatic gain control setting the RF input signal amplitude to a value corresponding to the abovementioned value Vst. In the determination of this value Vst, the above difference in amplitude increase between the wanted RF signal and the unwanted higher order intermodulation products is used to distinguish between signals predominantly determined by wanted signals and signals predominantly determined by unwanted intermodulation products. Figure 2 shows the basic concept of an AGC circuit according to the invention applied to analog signal processing means ASP, which at least comprise a controllable broadband RF amplifier, but which may comprise various other analog receiver circuitry as will be explained in more detail with reference to Figure 3. The analog signal processing means ASP are gain controlled by means of a gain control signal combining various control signal components supplied to a gain control input of said analog signal processing means ASP. For the determination of a gain set point whereat intermodulation products occurring within the frequency range of a wanted signal are dominated by the wanted signal, i.e. at an RF input level value of Vst, the RF input signal of the analog signal processing means ASP is incrementally varied in amplitude with a gain modulation signal provided by a signal generator 10 through signal summing means 11 to a gain control input of said analog signal processing means ASP. The amplitude variations of the output signal of the analog signal processing means ASP level within the channel bandwidth of the wanted signal resulting therefrom is being measured in an amplitude detector 13 functioning as level detection means, and supplied to a first input of a correlator 12, which may comprise a signal multiplier. The signal generator 10 also supplies said gain modulation signal to a second input of the correlator 12. The correlator 12 provides an output signal comprising an unwanted DC signal component indicative for the differences in amplitude variation of the signal supplied to its first and second inputs and therewith for the overall level of unwanted intermodulation and harmonic distortion products occurring within the bandwidth of the wanted signal and an expected DC signal component resulting when no intermodulation interferences occur in the analog signal processing means ASP, i.e. when the amplitude variation of the signal supplied to the first and second inputs of the correlator 12 are mutually corresponding. In this, the correlator 12 functions as intermodulation interference detection means. The correlator 12 is coupled through a lowpass filter 15 to a differential stage 20, in which the expected DC signal component in the output signal of the correlator 12 is compensated by a preset DC voltage supplied by an adjustable preset DC voltage source 17, resulting in a DC output signal of the differential stage 20 corresponding to the above unwanted DC signal component indicative for the overall level of unwanted intermodulation and harmonic distortion products occurring within the bandwidth of the wanted signal. This DC output signal is supplied via an integrator 19 to the signal summing means 11, in which it is added to the gain modulation signal of the signal generator 10 into a combined gain control signal, which is supplied to the gain control input of said analog signal processing means ASP. A feedback loop is therewith obtained including the correlator 12, in which the differences in relative amplitude variations between the gain modulation signal on the one hand and the output signal of the amplitude detector 13 on the other hand are automatically reduced, causing the AGC to stabilise the gain of the analog signal processing means ASP at a gain set point where intermodulation and harmonic distortion products occurring within the frequency range of a wanted signal are dominated by the wanted signal, i.e. at an RF input level value of Vst. To make this automatic adjustment of the gain set point effective at various gain setting values within a certain control range, the preset DC voltage supplied by the adjustable preset DC voltage source 17 can be chosen to differ from the above expected DC signal component in the output signal of the correlator 12, resulting in an adjustable offset in the gain set point of the analog signal processing means ASP. Figure 3 shows a receiver according to a preferred embodiment of the present invention comprising analog signal processing means 1-8 substantially corresponding to the above analog signal processing means ASP and having subsequently connected to antenna means A, an RF input stage 1; controllable RF amplifier means 2, 3 comprising a controllable attenuator 2 and an RF amplifier 3; frequency selective means 4 comprising a mixer 5 coupled to an IF filter 7, the mixer 5 being supplied with a local oscillator signal from a tunable local oscillator 6 and an amplitude modulator 8 followed by IF and audio signal processing means 9. In these analog signal processing means 1-8 the RF frequency band comprising a wanted RF signal modulated on an RF carrier frequency is being supplied from the antenna means A to the RF input stage 1, comprising a passive, selective matching network, and broadband amplified in the controllable RF amplifier means 2, 3. An RF to IF conversion of the wanted RF signal is provided in the mixer 5 with a local oscillator signal deviating in frequency from the carrier frequency of the wanted RF signal over a frequency distance equal to the IF frequency. An IF channel selection of the so converted IF signal is provided in the IF filter 7. Further processing of the IF signal into an audiosignal suitable to be reproduced in audible form is being provided in the IF and audio signal processing means 9. Said processing of a wanted RF signal into an audio signal is on itself known and doesn't need further explanation for a proper understanding of the invention. The output of the controllable RF amplifier means 2, 3 is fed back through a level detector 20, the inverting input of said differential stage 18 and an integrator 19 to its gain control input terminal, therewith providing an automatic gain control (AGC) stabilising the output signal level of the controllable RF amplifier means 2, 3 to a certain set value. As already mentioned above, the RF to IF conversion of the so stabilised RF signal in the mixer 5 is followed by an IF channel selection of the wanted signal. The selected IF signal is applied through the amplitude modulator 8 to intermodulation interference detection means 11-14 comprising level detecting means 13, 14 having a level detector 13 followed by a bandpass filter 14, being coupled to a first terminal of a correlator 12. The intermodulation interference detection means 11-14 also comprise a signal generator 10 for generating a gain modulation signal being coupled to a gain control input of the controllable attenuator 2 of said controllable RF amplifier means 2, 3 as well as to a second input terminal of the correlator 12. The correlator 12 may comprise a signal multiplier supplying an output signal, which is indicative for the overall level of the intermodulation interference occurring in the circuitry 2-8 of the analog signal processing means 1-8 and which is coupled through an integrator 15 to a control input of the above mentioned adjustable DC voltage source 17. The so obtained DC gain control signal at the control input of the above mentioned adjustable DC voltage source 17 varies with the overall level of the intermodulation interference occurring in the circuitry 2-8 of the analog signal processing means 1-8. Said DC gain control signal and the gain modulation signal from the signal generator 10 are respectively coupled to first and second inputs of a signal multiplier 11, providing in its output signal a signal product comprising the sum or superposition of said DC gain control signal and said gain modulation signal. This sum signal is supplied to the noninverting input of the differential stage 18 and therewith injected in the abovementioned AGC loop of the controllable RF amplifier means 2, 3. This results in an automatic setting of the gain of the controllable RF amplifier means 2, 3 to a value at which the first order wanted signal at the output of the amplitude modulator 8 is substantially greater than the higher order intermodulation products occurring within the channel bandwidth of this wanted signal. To prevent IF amplitude variations caused by the gain modulation signal from passing through to the IF and audio signal processor 9 the amplitude modulator 8 is modulated with the inverse gain modulation signal of the signal generator 10. The signal inversion needed therefor is provided in the inverting stage 16 coupled between an output of the signal generator 10 and a modulator input of the amplitude modulator 8. A reduction of the effect of the gain modulation provided by the controllable attenuator 2 on the signal to be reproduced can alternatively be achieved by limiting the incremental gain modulation signal amplitude variations and/or to modulate the gain modulation signal with a bandwidth limited, bandwidth limited pseudo random frequency. The AGC according to the invention can be further improved e.g. by using delay circuits (not shown) to compensate for differences in signal transition time between the various signal paths, such as occurring in the signal path from the signal generator 10 passing through circuits 11, 18, 19, 2-8, 13 and 14 to the first input of the correlator 12 on the one hand and in the direct signal path from the signal generator 10 to the second input of this correlator 12. Furthermore, many changes and modifications may be made in the above embodiments without departing from the scope of the appended claims. For instance, the controllable attenuator 2 may well be replaced by a controllable amplifier, the RF to IF conversion may well be replaced by a direct conversion, etc.. Figure 4a illustrates an example of the IF signal supplied by the IF filter 7 of the frequency selective means 4 to the amplitude modulator 8. This IF signal varies incrementally in amplitude due to the gain modulation signal supplied to the gain control input of the analog signal processing means ASP, respectively 1-8. In this example, an increase in the level of higher order intermodulation and harmonic distortion products occurs after a point in time t0 reflected in a likewise increase of the incremental amplitude variations. Figure 4b illustrates the effect of the amplitude modulation of the signal of Figure 4a in the amplitude modulator 8 with the inverse gain modulation signal of the signal generator 10 provided by the inverting stage 16. The non-zero amplitude variations mark the time interval t1-t0, in which the AGC according to the invention is active. Figure 4c illustrates the variation of the gain control signal supplied to the gain control input of the analog signal processing means ASP in the receiver concept of Figure 2 and to the gain control input of the controllable attenuator 2 of the analog signal processing means 1-8 as a result of the variation in the level of higher order intermodulation products as shown in Figure 4a. The increase in the level of higher order intermodulation products causes the DC level of said gain control signal to reduce after a certain control time constant, i.e. after t1. As a result thereof the overall gain of the analog signal processing means ASP, respectively 1-8, is reduced causing the level of higher order intermodulation products to decrease during a time interval starting at t1 and closing at t2. After t2 the overall gain of the analog signal processing means ASP, respectively 1-8 is stabilised to a new gain set point, where the level of higher order intermodulation products d2, d3, etc. is decreased below the level of the wanted first order signal s.

Claims: Receiver comprising analog signal processing means (ASP), a feed back control circuit for controlling the gain of said analog signal processing means (ASP), comprising intermodulation interference detection means (11-14) characterized in that the analog signal processing means (ASP) comprise a controllable RF amplifier means (2, 3) and a frequency selective means (4) for channel selection, the intermodulation interference detection means (11-14) comprise an amplitude detector (13) and a correlator (12), an output of the controllable RF amplifier means (2, 3) being coupled through the frequency selective means (4) and the amplitude detector (13) to a first terminal of the correlator (12), a signal generator (10) for generating a gain modulation signal being coupled to a gain control input of said analog signal processing means (ASP) as well as to a second terminal of the correlator (12), the correlator (12) being included in said feed back control circuit, said feed back control circuit being adapted to detect differences in relative variations between the gain modulation signal on the one hand and the output signal of the amplitude detector (13) on the other hand and to supply a DC gain control signal to the gain control input of the analog signal processing means (ASP) for reducing said differences by a DC gain setting adjustment of the analog signal processing means (ASP). Receiver according to claim 1, characterized by a signal multiplier (11) included in said feed back control circuit having first and second terminals respectively coupled to outputs of the signal generator (10) and the correlator (12), an output of the signal multiplier (11) being coupled to said gain control input of said analog signal processing means (ASP). Receiver according to claim 2, characterized in that the output of said controllable RF amplifier means (2, 3) is coupled to an input of a level detecting means (20), an output of said level detecting means (20) is coupled to a first terminal of a signal combination circuit, the signal multiplier (11) being coupled to a second terminal of the signal combination circuit, the signal combination circuit supplying the difference between the output signals of the signal multiplier (11) and the level detecting means (20) to the gain control input of said analog signal processing means (ASP). Receiver according to one of claims 1 to 3, characterized by an amplitude modulator (8) following the frequency selective means (4), the signal generator (10) being coupled through an inverting means (16) to a modulator signal terminal of said amplitude modulator (8), an output of the amplitude modulator (8) being coupled through the amplitude detector (13) to said first terminal of the correlator (12). Receiver according to one of claims 1 to 4, characterized in that the analog signal processing means (ASP) comprise an controllable attenuator (2) preceding a broadband amplifying stage (3) coupled to the gain control input of the analog signal processing means (ASP) providing a signal attenuation varying with a gain control signal supplied to said gain control input. Receiver according to one of claims 1 to 5, characterized in that the frequency selective means (4) for channel selection comprise a mixing stage (5) followed by an IF circuit (7) for IF channel selection, an output of said IF circuit (7) being coupled through said amplitude detector (13) to the first terminal of the correlator (12).

Inventors: SESSINK FRANCISCUS JOSEPHUS AL, SESSINK, FRANCISCUS JOSEPHUS ALPHONSUS MARIA
Applicants: SIEMENS AG, SIEMENS AKTIENGESELLSCHAFT
Application year: 1999
Main classification: H04B1
IPC codes: H03G3, H04B1