Signal acquisition chain comprising a frequency-selective attenuator
The present invention relates to a signal acquisition chain comprising a frequency-selective attenuator. It applies notably to radioelectric reception chains for reducing the power of signals present in certain frequency bands.
Modern radioelectric receivers are designed to process frequency acquisition bands that are becoming ever wider with respect to the channelling of signals. One of the difficulties encountered in the production of reception chains is the acquisition of a weak signal in the presence of strong signals, while preserving information on the strong signals. With the emergence of new generations of digital receivers, this problem must notably be dealt with for the direct digitization of wide frequency bands in digital reception chains. Specifically, it is always possible to amplify weak signals, but this solution comes up against the limits of analogue-digital converters (ADC) which accept only a limited voltage at input. For example, a direct-digitization receiver must be able to digitize a band of frequencies lying between 1 and 30 MHz, each signal being channelled on a 3kHz channel, the whole in a spectral environment disturbed by strong signals. By way of example, the signals emitted on the radiobroadcasting or telebroadcasting bands (referred to as broadcast bands), are particularly powerful and can be considered to be jammer signals in relation to off-broadcast signals. In order to allow the ADC to fully utilize the range of voltages accepted at input when sampling all the signals of a wide band of frequencies, it is desirable to decrease the power of these strong signals.
Currently, it is known to use attenuators to decrease the power of signals. However, attenuators are frequency-invariant functions, this implying that all the signals of the spectrum undergo the attenuation. Now, generally, it is not appropriate to attenuate the weak signals for fear of no longer being able to process them subsequently. A combination of attenuators and of amplifiers would only culminate in an increase in the noise factor, and would therefore have no beneficial effect.
It is also known to eliminate the disturbing signals present on a given band of frequencies by using rejector filters of Notch type, which reject a frequency band while allowing through the remainder of the spectrum.
These filters are particularly bulky and expensive. Moreover, the use of this type of filter renders any processing of signals in the rejected band of frequencies impossible. Specifically, Notch filters exhibit a Standing Wave Ratio (SWR) of much greater than 1 in the rejected frequency band and > therefore introduce phase discontinuities which prevent certain types of processing from being performed, notably goniometric calculations. Additionally, this poor impedance matching over a portion of the frequency spectrum prevents the cascading of these filters, this turning out to be a hindrance when one wishes to eliminate several undesirable signals of different frequencies.
An aim of the invention is to attenuate the power of signals present
on certain portions of a frequency band without affecting the signals
occupying the other parts of the spectrum, and while allowing the processing
of the attenuated signals. For this purpose, the subject of the invention is a
signal acquisition chain, characterized in that it comprises at least one
frequency-selective attenuator of the power of an incoming signal, the
selective attenuator comprising at least one power attenuator module
provided at least with an input, with an output and with a third port, a pair of
rejection modules, one of the said modules being centred on one or more
frequencies F0, F1 and disposed between the input and the output of the
power attenuator module, and another of the rejection modules being centred
substantially on the same frequencies F0, F1 and placed between the third
port and an electrical ground, the rejection modules operating according to
two modes:
o when the frequency of the incoming signal is close to one of the rejection frequencies F0, F1, a first rejection module of the pair toggles to open circuit mode, a second rejection module of the pair toggles to short-circuit mode, o when the frequency of the incoming signal is far from the rejection frequencies F0, F1, the second rejection module of the pair toggles to open circuit mode, the first rejection module of the pair toggles to short-circuit mode.
According to an embodiment, the power attenuator module and the rejection modules of the selective attenuator are four-poles, one of the rejection modules of the pair being mounted in parallel with the power
attenuator module to form a third four-pole, the said third four-pole being mounted in series with the other rejection module of the pair.
According to an embodiment, the first rejection module of the selective attenuator comprises at least one parallel RLC circuit and the second rejection module of the selective attenuator comprises at least one series RLC circuit, the tuning frequencies of the RLC circuits being equal to the rejection frequencies.
According to another embodiment, the first rejection module of the selective attenuator comprises at least one series RLC circuit and the second rejection module of the selective attenuator comprises at least one parallel RLC circuit, the tuning frequencies of the RLC circuits being equal to the rejection frequencies.
According to an embodiment, at least one series RLC circuit and at least one parallel RLC circuit comprises a variable-capacitance diode to vary the tuning frequency F0.
According to an embodiment, the acquisition chain according to the invention is suitable for receiving and processing radioelectric signals in the HF band.
According to an embodiment, the signal acquisition chain according to the invention is reversible, the latter being suitable for receiving and emitting signals travelling through the frequency-selective attenuator.
A radiofrequency reception chain comprising a frequency-selective attenuator can, for example, be used to decrease the amplitude of signals emitted on already known broadcast bands.
Other characteristics will become apparent on reading the detailed description given by way of nonlimiting example which follows offered in relation to appended drawings which represent:
- Figure 1, an exemplary frequency-selective attenuator structure used in an acquisition chain according to the invention,
- Figure 2, a first embodiment of a frequency-selective attenuator used in an acquisition chain according to the invention,
- Figure 3, a power attenuation curve corresponding to the first embodiment,
- Figure 4, a second embodiment, making it possible to obtain a dual attenuation function to that of Figure 3,
- Figure 5, a power attenuation curve corresponding to the second embodiment,
- Figure 6, a generalization of the frequency-selective attenuator structure presented in Figure 1,
- Figure 7, a third embodiment of a frequency-selective attenuator used in an acquisition chain according to the invention comprising two attenuation zones,
- Figure 8, a power attenuation curve corresponding to the third embodiment,
- Figure 9, a generalization of the frequency-selective attenuator structure presented in Figure 4,
- Figure 10, a fourth embodiment of a frequency-selective attenuator used in an acquisition chain according to the invention comprising two non-attenuation zones,
- Figure 11, a power attenuation curve corresponding to the fourth embodiment,
- Figure 12, a schematic of a radiofrequency reception chain according to the invention comprising a frequency-selective attenuator.
Figure 1 presents an exemplary frequency-selective attenuator structure used in an acquisition chain according to the invention. The frequency-selective attenuator 100 is, in the example, a four-pole comprising three four-poles: two rejectors 104, 106 and a power attenuator 102. The first rejector 104 is mounted in parallel with the power attenuator 102, thus forming a four-pole, which is mounted in series with the second rejector 106.
To make the description specific, the frequency-selective attenuator 100 comprises two inputs 100a, 100b and two outputs 100c, 100d, the first rejector 104 comprises two inputs 104a, 104b and two outputs 104c and 104d, the second rejector 106 comprises two inputs 106a, 106b and two outputs 106c, 106d, and the power attenuator 102 comprises two inputs 102a, 102b and two outputs 102c, 102d. The first input 104a of the first rejector 104 is linked to the first input 102a of the power attenuator 102, these two inputs 104a, 102a being linked to the first input 100a of the frequency-selective attenuator 100. The first output 104c of the first rejector 104 is linked to the first output 102c of the power attenuator 102, these two outputs 104c, 102c being linked to the first output 100c of the frequency-
selective attenuator 100. The second input 104b of the first rejector 104 is linked to the second input 102b of the power attenuator 102 and to the first input 106a of the second rejector 106. The second output 104d of the first rejector 104 is linked to the second output 102d of the power attenuator 102 and to the first output 106c of the second rejector 106. The second input 106b of the second rejector 106 is linked to the second input 100b of the frequency-selective attenuator 100 and the second output 106d of the second rejector 106 is linked to the second output 100d of the frequency-selective attenuator 100.
The power attenuator 102 has notably the effect of decreasing the power of an incoming signal. This function is frequency-invariant, stated otherwise, the power attenuator 102 attenuates the power of an incoming signal in the same manner whatever the frequency of this signal. The first and the second rejector 104, 106 are designed to operate in concomitance and in a complementary manner. For example, when the first rejector 104 establishes a short-circuit between the first input and output 100a, 100c of the frequency-selective attenuator 100, the second rejector 106 isolates the said input and output from the second inputs and outputs 100b, 100d. The two rejectors 104, 106 are made so that the power transmitted by each of them evolves in a complementary manner as a function of the frequency of the input signal. For example, the two rejectors 104, 106 can be tuned to one and the same frequency F0, in such a way that a signal entering the frequency-selective attenuator 100, whose frequency is close to F0, is very strongly attenuated at output 100c, 100d and that an incoming signal whose frequency is far from F0 is transmitted almost without attenuation at output.
The selective attenuator 100 is made so that its input and output impedances are controlled. For example, for a radioelectric signal reception chain, an impedance value equal to 50 ohms is generally chosen. Generally, the attenuation level of the power attenuator 102 is chosen initially, then the values of the rejection frequencies of the two rejectors 104, 106 are chosen.
The installing of a selective attenuator 100 in a signal acquisition chain, for example a radioelectric signal acquisition chain, makes it possible to attenuate disturbing signals without degrading their informative content, so that the attenuated signals can still be analysed, unlike an equalizer circuit, which behaves as a level compensator.
Figure 2 presents a first embodiment of a frequency-selective attenuator used in an acquisition chain according to the invention. In the example, the first rejector 104 is a four-pole comprising a parallel RLC circuit. Thus, a circuit RPLPCP formed of a resistor Rp, an inductor Lp and a capacitor Cp mounted in parallel links the first input 104a with the first output 104c of the first rejector four-pole 104. The second input 104b is linked to the second output 104d by a wire.
According to the example of Figure 2, the first input 106a of the second rejector 106 is linked to its first output 106c by a wire, just as its second input 106b is linked by a wire to the second output 106d. A circuit RsLsCs, formed by a resistor Rs, an inductor Ls, and a capacitor Cs mounted in series, is placed between the first input and output 106a, 106c and the second input and output 106b, 106d. In the example, the second input and output 106b, 106d of the second rejector are grounded.
The power attenuator 102 presented in Figure 2 is a T-type attenuator, comprising two resistors R1, R2 placed in series between the first input 102a and the first output 102c. The power attenuator 102 also comprises a third resistor R3 linking the junction of the first two resistors R1, R2 with the second input and output 102b, 102d, the said input and output being linked together by a wire. According to another embodiment, the power attenuator 102 exhibits a n-type structure.
The RLC circuits present in the first and the second rejector 104, 106 are designed to have substantially the same tuning frequency F0. Additionally, the values of the inductors and capacitors of these circuits RPLPCP, RSL.SCS have an impact on the quality factor, that is to say the selectivity of the filter. On the one hand, the tuning frequency of the circuit RpLpCp of the first rejector 104 is dependent on the product LP.CP. On the other hand, the tuning frequency of the circuit RSLSCS of the second rejector 106 is dependent on the product LS.CS. Thus, the pairs of values (LP,CP) and (LS.CS) are notably determined as a function of the sought-after tuning frequency Fo and of the desired selectivity. In the example, the frequency Fo corresponds to the frequency of the disturbing signals to be attenuated and the selectivity corresponds to the frequency band to be covered around this frequency F0. The series resistor Rs is the loss resistor due notably to the resistive aspect of the series inductor Ls. The value of this series resistor Rs
depends on the value of the quality coefficient of the series inductor Ls. The inductor Ls is chosen so that the value Rs is quasi-zero. In an analogous manner, the value of the parallel resistor Rp depends on the value of the quality coefficient of the parallel inductor Lp, which is chosen so that the value Rp is much greater than the values of the resistors R1, R2, and R3 of the power attenuator 102. By way of example, for a frequency-selective attenuator 100 whose tuning frequency Fo is approximately equal to 7.5 MHz, the width of whose attenuated band is about 300 kHz, with an attenuation of 14 dB at F0, the following values may be used: R1=33 ohms, R2=33 ohms, R3=21 ohms, Rp=1000 ohms, Lp=33.77 nH, Cp=13.35 nF, Rs«0 ohm, Ls=33.17uH, Cs=13.59pF.
According to another embodiment, the rejectors can comprise several inductors and/or several capacitors so as to obtain a more significant rejection selectivity. In this case, the inductors belonging to one and the same rejector can be aligned during hardware installation so as to create a mutual inductance.
The capacitors Cs, Cp used in the RLC circuits can be fixed or variable. These capacitors are current or voltage tunable. For example, to vary the tuning frequency F0, a variable-capacitance diode can be used.
In the hyperfrequency domain, the RLC circuits can be replaced with other resonant circuits comprising, for example, ceramic bars.
When the frequency of the incoming signal is markedly less than the tuning frequency Fo, the first rejector 104 tends to short-circuit its first input 104a with its first output 104c, by virtue of the quasi-zero impedance of the inductor l_p at low frequencies. Concomitantly, the second rejector tends to open the circuit between its first input and output 106a, 106c and its second input and output 106b, 106d, the capacitor Cs blocking the flow of the electric current at low frequencies. Thus, when the frequency of the incoming signal is markedly less than F0, the frequency-selective attenuator 100 tends to behave as a simple transmission line, allowing the signal through without appreciably modifying it.
When the frequency of the incoming signal is markedly greater than Fo, the first rejector 104 tends to short-circuit its first input 104a with its first output 104c, by virtue of the quasi-zero impedance of the capacitor Cp at high frequencies. Concomitantly, the second rejector tends to open the circuit
between its first input and output 106a, 106c and its second input and output 106b, 106d, the inductor Ls blocking the flow of the electric current at high frequencies. Thus, when the frequency of the incoming signal is markedly greater than Fo, the frequency-selective attenuator 100 tends to behave as a simple transmission line, allowing the signal through without appreciably modifying it.
When the frequency of the incoming signal is close to the tuning frequency F0, the circuit RPLPCP of the first rejector 104 behaves substantially as an open circuit in relation to the power attenuator 102, since the parallel resistor Rp is of much greater resistance than those making up the attenuator 102. In parallel, at the level of the series circuft RSLSCS of the second rejector 106, a short-circuit is almost effected between the power attenuator 102 and the electrical ground, since the series resistor Rs is very weak. Thus when the frequency of the incoming signal is close to F0, the influence of the two rejector circuits 104, 106 is largely disabled and the frequency-selective attenuator 100 behaves substantially as a conventional attenuator.
Figure 3 illustrates, through a curve, the evolution of the direct transmission coefficient S21 of the frequency-selective attenuator according to the first embodiment presented in Figure 2 as a function of the frequency of the incoming signal. By virtue of the association of the two rejectors 104, 106 around the attenuator 102, only the signals whose frequency is close to the frequency F0 are attenuated, the remainder of the frequency spectrum not being altered. Furthermore, the SWR, not represented in the figure, remains almost equal to 1 whatever the frequency of the input signal and the phase of the signals is hardly modified. Additionally, the reflection coefficient S11 is close to the characteristic impedance of the four-pole 100.
Figure 4 presents a second embodiment of a frequency-selective attenuator used in an acquisition chain according to the invention. In this embodiment, the sought-after objective is to attenuate the signals of all frequencies, with the exception of those whose frequency is close to the tuning frequency F0. The sought-after function is therefore the dual function of the transmission function illustrated in Figure 3. In this second embodiment, the frequency-selective attenuator 100' comprises two rejectors 104', 106'. The first rejector 104' comprises a series circuit RSLSCS placed in oarallel with the power attenuator 102, while the second rejector 106'
comprises a parallel RPLPCP placed in series with the power attenuator 102. With respect to the first embodiment presented in Figure 2, the first rejector four-pole 104 and the second rejector four-pole 106 have each been replaced respectively with their dual four-pole 104' and 106'.
In a more detailed manner, the first rejector 104' comprises a series circuit RSLSCS placed between its first input 104a' and its first output 104c', while the second input and output 104b', 104d' are linked by a wire. The second rejector 106' comprises a parallel circuit RPLPCP placed between the first input and output 106a', 106c' and the second input and output 106b', 106d', the first input 106a' being linked by a wire to the first output 106c' and the second input 106b' also being linked by a wire to the second output 106d*.
When the frequency of the incoming signal is markedly less than the tuning frequency F0, the series circuit RSLSCS of the first rejector 104' behaves almost as an open circuit on account of the presence of the capacitor Cs. In parallel, the circuit RPLPCP of the second rejector 106' behaves almost as a short-circuit on account of the presence of the inductor Lp. Thus when the frequency of the incoming signal is markedly less than the tuning frequency F0, the influence of the two rejector circuits 104', 106' is largely disabled and the frequency-selective attenuator 100' behaves substantially as a conventional attenuator.
When the frequency of the incoming signal is markedly greater
than the tuning frequency F0, the series circuit RSLSCS of the first rejector 104'
behaves almost as an open circuit on account of the presence of the inductor
Ls. In parallel, the circuit RPLPCP of the second rejector 106' behaves almost
as a short-circuit on account of the presence of the capacitor Cp. Thus when
the frequency of the incoming signal is markedly greater than the tuning
frequency F0, the influence of the two rejector circuits 104', 1.06' is largely
disabled and the frequency-selective attenuator 100' behaves substantially
as a conventional attenuator. ;
When the frequency of the incoming signal is close to the tuning frequency F0, the first rejector circuit 104' tends to short-circuit its-first input 104a' with its first output 104c', since the resistor Rs of the series circuit RSLSCS is very weak. Concomitantly, the circuit RPLPCP of the second rejector 106' opposes the flow of the current with a significant resistance Rp. Thus,
when the frequency of the incoming signal is close to the tuning frequency F0, the frequency-selective attenuator 100' tends to behave as a simple transmission line, allowing the signal through without appreciably modifying it.
Figure 5 illustrates, through a curve, the evolution of the direct transmission coefficient S21 of the frequency-selective attenuator according to the second embodiment presented in Figure 4 as a function of the frequency of the incoming signal. With the exception of a frequency band around F0, the signals of all the frequencies are attenuated. Furthermore, just like in the first embodiment, the SWR remains, in the case of this embodiment, almost equal to 1 whatever the frequency of the input signal and the phase of the signals is hardly modified, including around the tuning frequency F0. Additionally, the reflection coefficient S11 is close to the characteristic impedance of the four-pole 100.
Figure 6 presents a generalization of the frequency-selective attenuator structure presented in Figure 1. To create several frequency attenuation zones, it is possible to associate several rejectors. For example, Figure 6 illustrates a frequency-selective attenuator comprising two frequency attenuation zones. Thus, the frequency-selective attenuator 600 comprises a first rejection module 611 comprising two rejector four-poles 604, 605 mounted in cascade, the said module being mounted in parallel with a power attenuator 102. A second rejection module 612 comprises two rejector four-poles 606, 607 mounted in parallel, this second module being mounted in series with the power attenuator 102.
More precisely, the first output1604c of the first rejector four-pole 604 is linked to the first input 605a of the second four-pole 605 and the second output 604d of the first rejector four-pole 604 is linked to the second input 605b of the second four-pole 605. The first rejection module 611 thus formed replaces the rejector four-pole 104 of Figure 1. According to the same principle, the second rejection module 612 replaces the second rejector four-pole 106 of the structure of Figure 1, the other connections being identical to those of the first embodiment.
This example is not limiting. For example, a larger number of rejectors can be associated in cascade and in parallel. The rejectors are associated in pairs, each pair corresponding to a power attenuation
frequency zone. To add such an attenuation zone to the spectrum, it is therefore necessary to add a new pair of rejectors, the first of which being mounted in cascade with the rejectors 604, 605 of the first rejection module 611, and the second being mounted in parallel with the rejectors 606, 607 of the second rejection module 612.
Figure 7 presents an embodiment corresponding to the
generalized structure of Figure 6. In this example, two frequency attenuation
zones are obtained, this is why the frequency-selective attenuator 600
comprises two rejector pairs. The rejector four-poles 604, 605 linked to the
first input and output 600a, 600c of the frequency-selective attenuator 600
are, in the example, the same as the first rejector four-pole 104 of Figure 2.
The rejector four-poles 606, 607 linked to the second input and output 600b,
600d of the frequency-selective attenuator 600 are, in the example, the same
as the second rejector four-pole 106 of Figure 2. This frequency-selective
attenuator 600 is distinguished therefore from the first embodiment presented
in Figure 2 by the addition of two RLC circuits. A parallel circuit Rp'Lp'Cp' is
associated in series with the circuit RPLPCP already present in the first rejector
104 and a series circuit Rs'Ls'Cs' is mounted in parallel with the circuit RSLSCS
already present in the second rejector 106. Each pair of circuits
RsLsCs/RpLpCp and Rs'Lg'Cs'/Rp'Lp'Cp' operates independently of one another,
the first pair RsLsCs/RpLpCp operating at one tuning frequency F0, and the
second pair Rs'Ls'Cs7Rp'Lp'Cp' operating at another tuning frequency Fi. On
the one hand, the cascading of the circuits RPLPCP, Rp'Lp'Cp' makes it
possible to perform a logical "or" on the function for opening the circuit
between the first input and output 102a, 102c of the power attenuator 102.
Moreover, placing the series circuits RSLSCS,; Rs'U'Cs' in parallel makes it
possible to perform a logical "or" on the function for short circuiting the
second input and output 102b, 102d of the power attenuator 102 with the
electrical ground. Thus, the frequency-selective attenuator 600 attenuates
only the signals whose frequencies are close to the frequencies F0 and F-i.
Figure 8 illustrates, through a curve, the evolution of the direct transmission coefficient S21 of the frequency-selective attenuator according to the third embodiment presented in Figure 7 as a function of the frequency of the incoming signal. With the exception of a frequency band around Fo and Fi, the signals of all the frequencies are attenuated.
In order to attenuate a larger number of frequency bands, it is also possible to associate several frequency-selective attenuators in series, each of the said attenuators being designed to attenuate one or more frequency bands.Figure 9 presents a frequency-selective attenuator structure making it possible to obtain a dual attenuation function to that presented in Figure 8. The sought-after objective is to attenuate the signals of the whole spectrum with the exception of two zones for which the signals are not attenuated. The frequency-selective attenuator 900 comprises a first rejection module 911 comprising two rejector four-poles 904, 905 mounted in parallel, the said module being mounted in parallel with a power attenuator 102. A second rejection module 912 comprises two rejector four-poles 906, 907 mounted in series, this second module being mounted in series with the power attenuator 102.
Figure 10 presents an embodiment corresponding to the structure
of Figure 9. The rejector four-poles 904, 905 linked to the first input and
output 900a, 900c of the frequency-selective attenuator 900 are, in the
example, the same as the first rejector four-pole 104' of Figure 4. The
rejector four-poles 906, 907 linked to the second input and output 900b, 900d
of the frequency-selective attenuator 900 are, in the example, the same as
the second rejector four-pole 106' of Figure 4. This frequency-selective
attenuator 900 is therefore distinguished from the first embodiment presented
in Figure 4 by the addition of two RLC circuits. A first series circuit RS'LS'CS' is
mounted in parallel with the circuit RSLSCS already present in the first rejector
104' and a second parallel circuit Rp'Lp'Cp' is associated in series with the
circuit RpLpCp already present in the secohd rejector 106'. Each pair of
circuits RsLsCs/RpLpCp and Rs'Ls'Cs'/Rp'Lp'Cp' operates independently of one
another, the first pair RsLsCs/RpLpCp corresponding to one tuning frequency
F0, and the second pair Rg'Ls'Cs'/Rp'Lp'Cp' corresponding to another tuning
frequency F1. On the one hand, placing the circuits RPLPCP, RP'LP'CP' in series
makes it possible to perform a logical "or" on the function for opening the
circuit between the second input and output 102b, 102d of the power
attenuator 102 and the electrical ground. On the other hand, placing the
series circuits RSLSCS, RS'LS'CS' in parallel makes it possible to perform a
logical "or" on the function for short circuiting the first input and output 102a,
102c of the power attenuator 102. Thus, the only signals not attenuated by the frequency-selective attenuator 900 are those whose frequencies are close to the frequencies F0 and F-i.
Figure 11 illustrates, through a curve, the evolution of the direct transmission coefficient S21 of the frequency-selective attenuator according to the fourth embodiment presented in Figure 10 as a function of the frequency of the incoming signal. With the exception of a frequency band around Fo and F^, the signals of all the frequencies are attenuated.
Figure 12 is a schematic of a radiofrequency reception chain according to the invention comprising a frequency-selective attenuator. The reception chain 1200 comprises a programmable attenuator 1204 receiving a signal arising from an antenna 1202. The attenuation level applied by the programmable attenuator 1204 is adjusted via a control signal 1205. The signal arising from the said attenuator 1204 is thereafter received by a first filter 1206 making it possible to preserve only a part of the frequency spectrum of the signal. At this juncture, the signal still comprises frequency components whose power is too high. To decrease the power of these components, the signal arising from the first filter 1206 is received by a frequency-selective attenuator 1208. The frequency-selective attenuator 1208 attenuates only the frequency components emitted with too large a power. An amplifier 1210 is placed at the output of the frequency-selective attenuator 1208, then the signal arising from this amplifier is received by a second programmable attenuator 1212 performing automatic gain control regulated by a control signal 1213. The output of the- programmable attenuator 1212 is linked to a second filter 1214, which is an anti-aliasing filter making it possible to comply with Shannon's condition before the sampling performed by an ADC 1216 placed at the output of this second filter 1214. The frequency-selective .attenuator 1208 can be deactivated in the reception chain by virtue of a gating module 1209 making it possible to transmit the signal directly from the first filter 1206 to the amplifier 1210. According to another embodiment, several selective attenuators 1208 can be placed in cascade. The frequency-selective attenuator 1208 is, in the example, placed among the first stages of the reception chain 1200 since it hardly affects the noise factor of this chain 1200.
According to another embodiment of the signal acquisition chain,

the latter is reversible. Stated otherwise, the selective attenuator 1208 can also be used alternatively in receive mode and in emit mode so as, for example, to attenuate any spurious spectral lines generated by the reversible acquisition chain when it is in emit mode. Specifically, a selective attenuator 1208 can operate in both directions of transmission receive/emit, its input impedance being equal to its output impedance.
The frequency-selective attenuator used in an acquisition chain according to the invention can be made with discrete components such as coils and capacitors, or with any other technology allowing the association of four-poles. It requires few components, thereby rendering it compact and inexpensive.
One of the significant advantages of the use of a frequency-selective attenuator in an acquisition chain, is that it makes it possible to obtain very good intermodulation performance.
Additionally, it is possible to associate several selective attenuators in series in an acquisition chain without appreciably degrading the noise factor of the said chain, the SWR remaining close to 1 for each attenuator.
Finally, the attenuator structure can be applied in acoustic signal acquisition chains.

CLAIMS
1. Signal acquisition chain, characterized in that it comprises at least one frequency-selective attenuator (1208) of the power of an incoming signal, the selective attenuator comprising at least one power attenuator module (102) provided at least with an input (102a), with an output (102c) and with a third port (102b, 102d), a pair of rejection modules, one of the said modules (611, 911) being centred on one or more frequencies F0, F1 and disposed between the input (102a) and the output (102c) of the power attenuator module, and another of the rejection modules (612, 912) being centred substantially on the same frequencies F0, F1 and placed between the third port (102b, 102d) and an electrical ground.
2. Signal acquisition chain according to Claim 1, characterized in that the
power attenuator module (102) and the rejection modules (611, 612, 911, 912) of the frequency-selective attenuator (1208) are four-poles, one of the rejection modules (611, 911) of the pair being mounted in parallel with the power attenuator module (102) to form a third four-pole, the said third four-pole being mounted in series with the other rejection module (612, 912) of the pair.
3. Signal acquisition chain according to one of the preceding claims, characterized in that the first rejection module (611, 911) of the selective attenuator comprises at least one parallel RLC circuit and the second rejection module (612, 912) of the selective attenuator comprises at least one series RLC circuit, the rejection frequencies being equal to the tuning frequencies of the RLC circuits.
4. Signal acquisition chain according to one of the preceding claims, characterized in that the first rejection module (611, 911) of the selective attenuator comprises at least one series RLC circuit and the second rejection module (612, 912) of the selective attenuator comprises at least one parallel RLC circuit, the rejection frequencies being equal to the tuning frequencies of the RLC circuits.
5. Signal acquisition chain according to one of Claims 3 and 4, characterized in that at least one series RLC circuit and at least one parallel RLC circuit comprises a variable-capacitance diode to vary the tuning frequency Fo.
6. Signal acquisition chain according to one of the preceding claims, characterized in that it is suitable for receiving and processing radioelectric signals in the HF band.
7. Reversible signal acquisition chain according to one of the preceding claims, characterized in that it is suitable for receiving and emitting signals travelling through the frequency-selective attenuator (1208).