AU2010244693A1 - System and method for protecting against interference generated by FM signals in receiving band III signals - Google Patents

Abstract

The invention relates to a system for protecting against the interference generated by signals (6) in receiving a listening signal (16) belonging to the band III frequency domain, comprising a band III tuner (14) for receiving at least one listening signal (16) transmitted in the band III frequency domain, an FM tuner (4) for scanning an FM frequency band in order to identify signals (6) in the FM frequency band, and a rejection module (20) for preventing an interval of frequencies from switching over to the band III tuner. The system comprises a processing module (8) for analysing the signals (6) according to the properties of the listening signal (16) so as to identify the reception of at least one interference signal (6) capable of interfering with the listening signal (16), the processing module (8) being suitable for controlling the rejection module (20) in order to prevent an interval of frequencies from switching over to the band III tuner (14) when the reception of at least one interference signal (6) is identified.

Description

System and method for protecting against interference generated by FM signals in receiving band III signals 5 The invention pertains to a protection system intended to limit the disturbances engendered by signals belonging to the so-called "FM" ("Frequency Modulated") frequency band on the reception of digital listening 10 signals belonging to band III. The present invention relates to the technical field of radiophony aboard motor vehicles allowing the reception of signals belonging to the FM band, that is to say 15 between 87.5 MHz and 108 MHz, as well as digital listening signals belonging to the so-called band III radiofrequency band, that is to say between 174 MHz and 230 MHz. 20 More particularly, the invention pertains to a protection system intended to limit the intrusion of strong signals belonging to the FM frequency region on listening signals belonging to the band III frequency region. 25 Having regard to the development of digital radiobroadcasting (DAB), more and more motor vehicles are being equipped with devices making it possible to receive band III digital listening signals. 30 Now, it has been noted that the devices for receiving band III listening signals exhibit inadequacies as regards the power of certain FM transmitters. Indeed, sufficiently powerful FM transmitters generate FM 35 signals that are liable to disturb the reception of listening signals belonging to the band III frequency region. More particularly, the reception of listening signals of frequency Fi belonging to this band III may -2 be disturbed by the emission of an FM signal of harmonic two, exhibiting a frequency F 2 substantially equal to F 1 /2. 5 For example, a tuner adapted for the reception of digital listening signals of frequency F 1 equal to 200 MHz is disturbed by the emission of an FM signal of frequency F 2 equal to 100 MHz. For most products existing on the market today, this disturbance is more 10 particularly felt when the power ratio of the FM signal emitted to the listening signal is greater than 60 dB. One solution for avoiding these phenomena consists in using a high-pass filter upstream of the band III tuner 15 so as to prevent the passage of the waves exhibiting a frequency belonging to the FM region. However, the use of a high-pass filter exhibits several drawbacks. First of all, inserting a high-pass filter such as this involves systematic insertion losses that are liable to 20 induce a significant loss of sensitivity as well as a loss of gain over the whole of the signal. Moreover, a high-pass filter acts on the whole of the spectrum of the signal received by the band III tuner 25 whereas only a part of this spectrum belonging to the FM band ought to be attenuated. Moreover, the production of a high-pass filter involves using a more significant number of components, thereby increasing the cost of manufacture of the protection system. 30 The present invention is aimed at eliminating the drawbacks of the prior art by proposing a protection system making it possible to limit the intrusion of FM signals on the reception of band III signals without 35 unduly limiting the sensitivity of the receiver. Furthermore, the invention is aimed at producing a protection system which is quite non-intrusive on the -3 reception circuit during its use and which disturbs only a modest part of the signal received by the band III tuner. 5 Moreover, the invention is also aimed at limiting the number of components used so as to reduce production costs. Accordingly, the present invention proposes to use the 10 FM tuner available in the car radio to identify possible signals belonging to an FM frequency band which are liable to disturb the reception of the band III listening signal. If appropriate, the protection system can then prevent the reception of these 15 disturbing signals by the band III tuner. In this respect, the invention relates to a system for protecting against the disturbances engendered by signals on the reception of a listening signal 20 belonging to the band III frequency region, comprising a band III tuner able to receive at least one listening signal emitted in the band III frequency region; an FM tuner able to scan an FM frequency band so as to identify signals present in the FM frequency band; a 25 rejection module able to prevent the passage of an interval of frequencies to the band III tuner. The system comprises a processing module able to analyze the signals as a function of characteristics of the listening signal so as to identify the reception of at 30 least one disturbing signal liable to disturb the listening signal, the processing module being able to control the rejection module so as to prevent the passage of an interval of frequencies to the band III tuner when the reception of at least one disturbing 35 signal is identified. Advantageously, the processing module comprises a first calculation unit able to determine the value of a -4 disturbing transmit frequency FEp according to the transmit frequency FE Of the listening signal through the following relation FEP = FE/ 2 , the processing module being able to identify the reception of at least one 5 disturbing signal when this disturbing signal exhibits a transmit frequency lying in a frequency band of 2 MHz around the disturbing transmit frequency FEP Preferably, the processing module comprises a second 10 calculation unit able to determine the value of a disturbing transmit power PEP according to the transmit power PE Of the listening signal through the following relation: PEP PE + 60 dB (60 dB being the system's initial rejection value), the processing module being 15 able to identify the reception of at least one disturbing signal when this disturbing signal furthermore exhibits a transmit power greater than or equal to the disturbing transmit power PEP 20 According to advantageous embodiments: - the rejection module is composed of a high-pass filter able to prevent the passage of the signals belonging to the FM frequency band. 25 - the rejection module is composed of an adjustable rejector filter able to prevent the passage of a signal belonging to a frequency band of 2 MHz around the disturbing transmit frequency FEP. - the FM tuner is able to scan the FM frequency band 30 periodically to identify signals, and preferably every fifteen seconds. The invention also relates to a method of protecting against the disturbances engendered by signals on the 35 reception of a listening signal belonging to the band III frequency region, consisting in receiving, by way of a band III tuner, at least one listening signal emitted in the band III frequency region; scanning an -5 FM frequency band so as to identify signals belonging to the FM frequency band. The method also comprises the following steps: analyzing the signals as a function of characteristics of the listening signal so as to 5 identify the reception of at least one disturbing signal liable to disturb the listening signal; and preventing the passage of an interval of frequencies to the band III tuner when the reception of at least one disturbing signal is identified. 10 According to another aspect, the invention relates to a motor vehicle radio comprising a protection system according to the abovementioned characteristics. 15 The use of a processing module able to analyze the characteristics of the signals received by the FM tuner makes it possible to define whether there exists a disturbing FM transmitter as a function of the frequency of the band III signal to be received. This 20 processing module also makes it possible to determine whether the power ratio of the signal of the FM band to the band III listening signal is high enough for the reception of this band III signal to actually be disturbed by the signal. It is thus possible to limit 25 the use of the rejection module to the periods during which a disturbing signal affects the reception of the band III listening signal. Systematic insertion losses are thus avoided. 30 The use of a rejection module composed of a high-pass circuit makes it possible to prevent passage of the whole set of signals in the FM frequency region. The use of a rejection module composed of a notch 35 filter makes it possible to prevent just the passage of the frequency band to be rejected and therefore to limit the effects of the rejection module on the band III listening signal.

-6 Regularly scanning the FM frequency region makes it possible to avoid the appearance of a disturbance induced by an FM transmitter that has entered the field 5 of reception of the car radio, especially while a vehicle is moving. The use of such a protection system in a motor vehicle car radio is particularly advantageous insofar as, on 10 account of the movement of the vehicle, disturbances related to the emission of strong signals are regularly encountered. Other characteristics and advantages of the invention 15 will emerge on the reading which follows of two detailed exemplary embodiments, with reference to the appended figures which represent respectively: - Figure 1, a schematic representation of a first 20 embodiment of the invention; and - Figure 2, a schematic representation of a second embodiment of the invention. Figure 1 represents a first embodiment of the 25 protection system according to the invention. According to this embodiment, the protection system comprises a first antenna 2 linked to an FM tuner 4 making it possible to receive FM signals 6 whose 30 transmit frequency belongs to the FM frequency region, that is to say from 87.5 MHz to 108 MHz. These FM signals 6 are more particularly characterized by their transmit frequency and their transmit power. 35 The FM tuner 4 is also linked to a processing module 8. Thus, the FM tuner 4 can transmit to the processing module 8 the whole set of information relating to the characteristics of the FM signals 6 received.

-7 The protection system also comprises a second antenna 12 linked to a band III tuner 14 capable of receiving a listening signal 16 in a frequency region going from 5 174 MHz to 230 MHz. This band III tuner 14 is also connected to the processing module 8. Thus, the band III tuner 14 communicates to the processing module 8 information relating to the frequency FE and to the power PE Of the listening signal 16 at an instant t. 10 It should be stressed that, according to this first embodiment, the second antenna 12 is linked to the band III tuner 14 by way of a normally non-deactivated high pass filter 20. 15 This high-pass filter 20 may be constructed with the aid of a passive component of inductor or capacitor type. The order of the filter and therefore the number of elements will depend on the level of desired 20 additional rejection. This high-pass filter 20 is turned on/off with the aid of switches, generally based on active elements (diode, transistors), the processing module 8 being in charge of the control of these switches. 25 According to this first embodiment, the FM tuner 4 regularly carries out a scan of the frequencies of the FM band so as to identify the reception, if any, of FM signals by the first antenna 2. The FM tuner 4 thus 30 acquires information relating to the characteristics, and more particularly to the transmit frequency and power, of the FM signals 6 received by the first antenna 2. This information is thereafter transmitted to the processing module 8 for analysis. 35 According to an exemplary usage the band III tuner 14 is tuned to a listening frequency FE equal to 197.648 MHz, for a power PE measured at -80 dBm. This -8 information relating to the characteristics of the listening signal 16 by the band III tuner is also transmitted to the processing module 8 for analysis. 5 Thus, the processing module 8 determines, as a function of the characteristics of the listening signal 16, the characteristics of an FM signal which would be liable to disturb the reception of this listening signal 16. In the exemplary usage hereinabove, the processing 10 module 8 selects a frequency band of 2 MHz around the disturbing transmit frequency FEP liable to disturb the listening signal 16, that is to say 98.242 MHz. Advantageously, the processing module 8 also determines a threshold power PEP above which an FM signal would 15 actually be disturbing. Preferably, this threshold power PEP is 60 dB greater than the power PE Of the listening signal 16. In the example hereinabove, this threshold power PEP would be -20dBm. 20 The processing module 8 thereafter analyses the data originating from the FM tuner 4 as a function of the data calculated on the basis of the characteristics of the listening signal 16. Each FM signal received by the FM tuner is thus analyzed so as to verify whether its 25 frequency and power characteristics define a disturbing FM signal 6 liable to disturb reception of the listening signal 16. If one of the FM signals 6 exhibits a transmit power 30 greater than the disturbing transmit frequency PEP (i.e. -20 dBm) and a transmit frequency situated in a frequency band of 2 MHz around the disturbing transmit frequency FEP (i.e. 99.324 MHz), then this FM signal 6 is considered to be a disturbing FM signal 6. The 35 processing module 8 then triggers the activation of the rejection module 20.

-9 If, conversely, none of the FM signals 6 received by the FM tuner 4 exhibits the abovementioned characteristics, then the processing module 8 does not trigger the activation of the rejection module 20. 5 There is then no attenuation on the FM band of the spectrum of the listening signal 16 and the sensitivity of the band III tuner 14 is maintained. According to this first embodiment, the rejection 10 module 20 used is a high-pass filter. The activation of this high-pass filter therefore makes it possible to prevent the passage of the whole of the FM band of the spectrum of the listening signal 16 to the band III tuner 14. 15 According to an exemplary usage, the band III tuner 14 is tuned to 197.648 MHz for a field measured at -60 dBm. So, if the FM tuner 4 detects a transmitter at 98.5 MHz of -10 dBm, the high-pass filter is activated 20 and adds -30 dB of rejection at the cost of an insertion loss of just a few dB. Figure 2 presents a second embodiment of a protection system according to the invention. 25 This second embodiment exhibits substantially the same architecture as the first embodiment. Indeed, the protection system also comprises a first 30 antenna 2 linked to an FM tuner 4 making it possible to receive FM signals 6 whose transmit frequency belongs to the FM region. The FM tuner 4 is linked to the processing module 8, thereby allowing it to receive information relating to the characteristics of the FM 35 signals 6 received by the FM tuner 4. Moreover, the protection system also comprises a second antenna 12 linked to a band III tuner 14 capable of - 10 receiving a listening signal 16 in the band III frequency region. This band III tuner 14 is also connected to the processing module 8. 5 However, according to this second embodiment, the rejection module 20 is composed of an adjustable rejector filter, also dubbed a notch filter, making it possible to prevent the passage of an adjustable frequency band. 10 Thus, when the processing module 8 detects a disturbing FM signal 6, a signal for activating the notch filter is emitted. This notch filter then makes it possible to prevent a frequency band of the spectrum of the 15 listening signal 16 from being transmitted to the band III tuner 14. Advantageously, the width of this frequency band attenuated by the notch filter exhibits a width of 2 20 MHz centered on the disturbing transmit frequency FEP calculated by the processing module 8. However, the characteristics of this frequency band attenuated by the notch filter are dependent on the characteristics of the FM signal 6. Thus, only a part of the spectrum 25 of the listening signal 16 is affected by the use of the rejection module 20. This matched (notch) filter may be made with the aid of a fixed inductor and a variable capacitor (varicap). 30 The processing module 8 is in charge of controlling the voltage required to match the filtering circuit to the frequency to be rejected or to disable the rejector. The summary table presented hereinbelow illustrates the 35 results obtained through the use of the two rejection modules 20 of the embodiments of Figures 1 and 2.

- 11 Rejection Rejection Rejection module width (dB) Band III Number of (MHz) insertion components loss (dB) 20 15 0.7 4 High-pass 20 20 0.7 6 filter 20 30 0.8 8 20 40 1 10 20 50 1.6 12 Notch 2 30 1.8 3 filter Table 1: Results obtained through the use of two types of rejection modules 5 It emerges from this table that the use of a notch filter is advantageous insofar as only a reduced part of the spectrum of the listening signal 16 is attenuated. Furthermore, the number of components used is reduced, thereby limiting the system production 10 costs.

Claims (8)

1. A system for protecting against the disturbances engendered by signals (6) on the reception of a 5 listening signal (16) belonging to the band III frequency region, comprising: - a band III tuner (14) able to receive at least one listening signal (16) emitted in the band III frequency region; 10 - an FM tuner (4) able to scan an FM frequency band so as to identify signals (6) present in the FM frequency band; - a rejection module (20) able to prevent the passage of an interval of frequencies to the band III tuner; 15 characterized in that it comprises a processing module (8) able to analyze the signals (6) as a function of characteristics of the listening signal (16) so as to identify the reception of at least one disturbing signal (6) liable to disturb the listening signal (16), 20 the processing module (8) being able to control the rejection module (20) so as to prevent the passage of an interval of frequencies to the band III tuner (14) when the reception of at least one disturbing signal (6) is identified. 25

2. The protection system as claimed in claim 1, in which the processing module (8) comprises a first calculation unit able to determine the value of a disturbing transmit frequency FEP according to the transmit frequency FE of the listening signal (16) 30 through the following relation: FEP = FE! 2 ; the processing module (8) being able to identify the reception of at least one disturbing signal (6) when this disturbing signal (6) exhibits a transmit 35 frequency lying in a frequency band of 2 MHz around the disturbing transmit frequency FEP

3. The protection system as claimed in claim 2, in which the processing module (8) comprises a second - 13 calculation unit able to determine the value of a disturbing transmit power PEP according to the transmit power PE of the listening signal (16) through the following relation: 5 PEP = PE + 60 dB; the processing module (8) being able to identify the reception of at least one disturbing signal (6) when this disturbing signal (6) furthermore exhibits a transmit power greater than or equal to the disturbing 10 transmit power PEP

4. The system as claimed in any one of claims 1 to 3, in which the rejection module (20) is composed of a high-pass filter able to prevent the passage of the FM signals belonging to the FM frequency band. 15

5. The system as claimed in any one of claims 1 to 3, in which the rejection module (20) is composed of an adjustable rejector filter able to prevent the passage of a signal belonging to a frequency band of 2 MHz around the disturbing transmit frequency FEP. 20

6. The system as claimed in any one of claims 1 to 5, in which the FM tuner is able to scan the FM frequency band periodically to identify signals (6), and preferably every fifteen seconds.

7. A method of protecting against the disturbances 25 engendered by signals (6) on the reception of a listening signal (16) belonging to the band III frequency region, consisting in: - receiving, by way of a band III tuner, at least one listening signal (16) emitted in the band III frequency 30 region; - scanning an FM frequency band so as to identify signals (6) belonging to the FM frequency band; characterized in that the method also comprises the following steps: 35 - analyzing the signals (6) as a function of characteristics of the listening signal (16) so as to identify the reception of at least one disturbing - 14 signal (6) liable to disturb the listening signal (16); and - preventing the passage of an interval of frequencies to the band III tuner (14) when the reception of at 5 least one disturbing signal (6) is identified.

8. A motor vehicle radio comprising a protection system as claimed in any one of claims 1 to 6.