Abstract:
The invention relates to a multichannel radio-frequency receiver for electromagnetic waves, having a radio-frequency analogue section, which has an input for an electrical signal from a reception device, and having a lower-frequency section, which is connected downstream of the radiofrequency analogue section and has a plurality of parallel channels ( 6   b,    6   c;    7   b,    7   c ) for in each case different signal levels and an evaluation circuit, in which, in the radiofrequency analogue section in order to split the signal in accordance with a predeterminable division ratio into signal elements which can be supplied to radio-frequency analogue channels ( 6   a,    7   a ), downstream from which the channels ( 6   b,    6   c;    7   b,    7   c ) of the lower-frequency section are respectively connected, and the channels ( 6   b,    6   c;    7   b,    7   c ) of the lower-frequency section each have an evaluation circuit for detection of the phase and amplitude of the respective signal element.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation in part of U.S. patent application Ser. No. 11/989,089 filed Jan. 18, 2008 and entitled “MULTI-CHANNEL RADIOFREQUENCY RECEIVER,” which is a U.S. National Stage entry of PCT/EP2006/006865 filed Jul. 13, 2006 and entitled “MULTI-CHANNEL RADIOFREQUENCY RECEIVER,” which claims priority to German Patent Application No. 10 2005 034 032.6 filed Jul. 21, 2005 and entitled “MULTI-CHANNEL RADIOFREQUENCY RECEIVER”, the complete disclosures of which are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a multichannel radio-frequency receiver. 
     In radio-frequency receivers for radar systems, communication appliances, test equipment etc., the dynamic range is limited by the electronic components that are used, and in the case of digital processing in particular the analogue/digital converters that are used. 
     In order to increase the dynamic range, it is known from GB 2 204 200 A for a received radio-frequency signal to be matched to the dynamic range of the components that are used by variable amplification after conversion to an intermediate frequency followed by conversion to a baseband frequency, by means of automatic gain control (AGC). However, the variable gain adversely affects the signal quality. In order to make it possible to react to rapid changes in the signal strength, the useful signal must also be delayed with respect to the actuating signal since, otherwise, the automatic gain control cannot carry out the control process before the useful signal arrives at an assembly which limits the dynamic range. This can be achieved only with a great amount of complexity. Finally, the gain setting of the AGC must be known accurately for calibration of measuring radars, in particular meteorological radars. 
     As an alternative to automatic gain control, EP 0 660 539 B1 proposes that a signal be split into three channels after the radio-frequency signal has been converted to an intermediate frequency. One channel has an amplifier and is supplied to one input of a multiplexer, a further channel is supplied without any change to a further input of the multiplexer, and the last channel has a detector for the signal strength and is connected to a control input of the multiplexer in order to pass on either the amplified channel or the unamplified channel to a common evaluation circuit for the signal, depending on the signal strength. However this requires an additional channel which is not used in the signal evaluation and, furthermore, reduces the signal strength on the other two channels. In addition, the multiplexer corrupts the signal that is passed on to the evaluation circuit, in particular during switching, so that the known receiver is not suitable for signals in which weak amplitudes frequently alternate with strong amplitudes. Finally, the components upstream of the signal divider must be designed for the entire dynamic range of the receiver, and are therefore subject to compromise. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention is therefore based on the object of providing a multichannel radio-frequency receiver that allows better-quality evaluation, with a simplified design. 
     This object is achieved on the basis of the features of Claim  1 . 
     This results in a multichannel radio-frequency receiver in which a signal divider for splitting a radio-frequency analogue electrical signal from a receiving device such as a radar antenna or a test equipment head into signal elements which can be supplied to radio-frequency analogue channels is actually provided in a radio-frequency analogue section downstream from each of which channels of a lower-frequency section of the radio-frequency receiver are in each case connected and each have an evaluation circuit for detection of the phase and amplitude of the respective signal element. 
     In the simplest case, all the sections which follow the signal divider are designed identically. This makes it possible to achieve a further cost reduction. 
     The splitting of the signal in the radio-frequency area itself between channels which are used exclusively for signal processing and evaluation allows optimum use of the available signal strength, as well as optimum design, without any compromises, of all the signal-processing, and in particular signal-evaluating components of the receiver, depending on the signal strengths to be evaluated in the respective channels. Noise, signal distortion and other signal corruption are therefore minimized. 
     It is possible to provide for the signal to be split into signal elements even before the first amplification process. This results in a further evaluation improvement. 
     It is possible to provide for a signal limiter to be connected downstream from the signal divider. This makes it possible to block or limit signals which are too strong for one channel or for a plurality of channels. The only signals which are preferably passed on for processing on a channel are those which do not overdrive the components in that channel. In addition to protecting the channels against overvoltage damage, the use of a signal limiter also makes it possible to detect signals on other channels during a blind time on one channel. The blind time is the time which a gas-discharge-based signal limiter requires in order to quench gas-discharge paths, and is normally longer than a transmission pulse from a radar apparatus. In radio-frequency receivers with an input signal limiter on the input side which blocks the entire radio-frequency receiver, it is therefore possible to avoid the occurrence of so-called blind spots or blind rings. 
     It is possible to provide for a plurality of signal dividers to be connected in series, in the form of a cascade, in order to scale the dynamic range virtually indefinitely. 
     It is possible to provide for a signal limiter to be connected downstream from the signal divider, and for a further signal divider to be connected downstream from the signal limiter. This makes it possible to use a single signal divider to protect a plurality of channels. 
     It is possible to provide for a frequency converter for converting the respective radio-frequency signal element to a signal element at an intermediate frequency to be provided in the lower-frequency section in each channel. The signal elements can be processed at the intermediate-frequency level using simple means and with high quality. 
     It is possible to provide for the evaluation circuits to be matched to the respective signal strength, with the matching being carried out in particular by the choice and design of the components used. The matching is then carried out by the division ratio of the signal or by the channels having permanently set different gains, or by both. 
     It is possible to provide for the evaluation circuits each to have an analogue/digital converter for digitizing the respective signal element. This allows independent digital further processing for each channel, in particular using a signal processor or computer. 
     It is possible to provide for the evaluation circuits each to have a demodulator. This allows independent processing for each channel. 
     The radio-frequency receiver may be designed for radio-frequency electromagnetic waves including the microwave range, or only the microwave range, and in particular for a radar device, for example a weather radar device. 
     The signal strength in this case optionally refers to the maximum amplitude or the maximum intensity of the signal. 
     Further refinements of the invention can be found in the following description and in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached figures. 
         FIG. 1  shows a block diagram of a radio-frequency receiver. 
         FIGS. 2 ,  3  and  4  illustrate signal elements at respectively different points in the radio-frequency receiver shown in  FIG. 1 . 
         FIG. 5  shows a block diagram of a radio-frequency receiver having signal dividers connected in series in the form of a cascade. 
         FIGS. 6 ,  7  and  8  illustrate signal elements at respectively different points in the radio-frequency receiver shown in  FIG. 5 . 
         FIG. 9  shows a block diagram of a further radio-frequency receiver. 
         FIG. 10  shows schematically a further embodiment of a receiver according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The radio-frequency receiver  1 , which is illustrated in a simplified form in  FIG. 1 , has a radio-frequency analogue section  2  with an input  3  for an analogue, radio-frequency, electrical signal of a receiving device  4 , for example in the form of a parabolic antenna for an electromagnetic radar beam at a wavelength in particular in the microwave range, having a signal divider  5  and having two channels  6   a,    7   a.  An analogue intermediate-frequency section  8  with channels  6   b,    7   b  follows the radio-frequency section  2 . Finally, a digital baseband-frequency section  9  with channels  6   c,    7   c  and outputs  10  for data signals which correspond to the analogue radio-frequency electrical signal, in particular in amplitude and phase, follows the analogue intermediate-frequency section  8 . 
     The expediently passive signal divider  5  is, for example, a directional coupler and in this case splits the signal received from the input  3  into two possibly in-phase signal elements with the same or a different signal element strength, which are each processed separately in the channels  6 ,  7 . If required, more than two signal elements and a corresponding number of channels are provided. 
     The dynamic range of the channels  6 ,  7  is in this case limited in particular by the dynamic range of the respective analogue/digital converters  11  and, to the extent described, the signal divider  5  is arranged upstream of low-noise amplifiers  12 , as well as by their dynamic range. In order to widen the dynamic range in comparison to single-channel evaluation, the channels  6 ,  7  are designed for respectively different signal strengths such that the dynamic range of the respective analogue/digital converter  11  and, if appropriate, low-noise amplifier  12  is used optimally. The channels  6 ,  7  can be designed for different signal strengths by suitable choice of the division ratio of the signal divider  5  and/or the gain of low-noise amplifiers  12 ,  13  provided in the channels. By way of example, the following text is based on the assumption that the aim is to process low signal strengths in the channel  6 , and high signal strengths in the channel  7 . 
     The division ratio, which can be predetermined by the configuration of the signal divider  5  and may additionally be variable, is, for example, 3:1, that is to say the strength of the signal element in the channel  6   a  is three times the strength of the signal element in the channel  7   a  and three quarters of the strength of the undivided signal, while the strength of the signal element in the channel  7   a  is one third of the strength of the signal element in the channel  6   a,  and one quarter of the strength of the undivided signal. The maximum signal element strength K max  and G max  of the signal elements in the channels  6   a  and  7   a,  respectively, for the maximum permissible signal strength S max  at the input  3  is illustrated in  FIG. 2 . Other division ratios, in particular 1:1, are likewise possible and may be combined with further gain ratios, with the gain ratios expediently being different if the division ratio is 1:1. 
     The low noise amplifiers (LNA)  12  which are in each case connected in the radio-frequency section downstream from the signal divider  5  in the channels  6   a,    7   a  have different gains in this case. The amplifier  12  in the channel  7   a  for high signal strengths has to provide less gain than the amplifier  12  in the channel  6   a.  The maximum signal strengths K′ max  and G′ max  of the amplified maximum signal elements that occur are illustrated in  FIG. 3 , with the magnitude ratios being distorted in comparison to  FIG. 2 , because the gain is expediently several orders of magnitude. Instead of or in addition to the amplifiers  12 , a low-noise amplifier can be provided upstream of the signal divider  5 . However, it is particularly advantageous to use identical gains in the channels, in order to allow the channels to be designed with the same circuitry at low cost, in which case the signal divider is then responsible for splitting the signal strengths expediently between the channels. 
     A mixer  14  is provided at each of the junctions between the channels  6   a,    7   a  of the radio-frequency section  2  and the downstream channels  6   b,    7   b  of the intermediate-frequency section  8 , and converts the radio-frequency signal elements to an intermediate frequency, using the frequency of an oscillator  15 . The signal elements which have been converted to the intermediate frequency are amplified by further low-noise amplifiers  13 , whose gain ratios may be different. The maximum signal strengths K″ max  and G″ max  of the signal elements which have been amplified by the amplifiers  13  are illustrated in  FIG. 4 , with the magnitude ratios being distorted in comparison to  FIG. 3 , because the gain is expediently several orders of magnitude. 
     At the junction between the channels  6   b,    7   b  of the intermediate-frequency section  8  and the downstream channels  6   c,    7   c  of the baseband-frequency section  9 , the analogue/digital converters  11 , which are clocked by an oscillator  16 , digitize the respective signal elements and pass digital signals to demodulators  17 , which are likewise connected to the oscillators  16 . The division ratio of the signal divider  5  and/or the gain of at least one amplifier  12 ,  13  are/is expediently designed such that the maximum signal element strength G″ max  in the channel  7  for strong signals makes optimum use of the dynamic range D of the analogue/digital converters  11 , while the maximum signal element strength K″ max  in the channel  6  for weak signals considerably exceeds the dynamic range D, see  FIG. 4 . The circuitry is expediently designed such that a signal element in the channel  7  for strong signals can be processed and in particular digitized completely in the channel  6  for weak signals with a signal element strength below a threshold value A, at which a predetermined resolution can still be achieved, see  FIG. 4 . 
     For this purpose, the amplifiers  12 ,  13  can provide linear amplification or, in particular, non-linear amplification, for example logarithmic amplification, such that the region above the threshold value A in the channel  7  and the region below the threshold value A in the channel  6  are amplified more strongly than the respective other region, in order to stretch the respective region of interest, for more accurate digitizing. 
     The demodulators  17  determined the amplitude and phase of the respective signal element. Known I/Q demodulators can be used for this purpose. The demodulators  17  are expediently implemented by a digital signal processor or a computer program in a computer connected downstream from the analogue/digital converters  11 . The computer may also be a microcontroller, an ASIC for example in the form of an FPGA or EPLD etc., or a digital signal processor or the like, in which case the software can be implemented as firmware. 
     The demodulators  17  are followed by a selection device  18  to whose input side the digital values for, for example, the amplitude and phase of the signal elements in the channels  6   c,    7   c  are supplied and which produces output signals at the outputs  10 . In the simplest case, the selection device  18  selects that signal element which makes best use of the dynamic range to be output at the outputs  10 , that is to say the signal element whose signal strength comes closest to the dynamic range D, without exceeding it. It is also possible to provide for the selection device  18  to identify those channels which are saturated. Furthermore, the phase differences and/or amplitude differences between the channels  6   c,    7   c  can be measured, in particular during a measurement and/or calibration time period, in order to correct the measured values on the basis of the differences during subsequent operation. The selection device  18  is expediently implemented together with the demodulators  17  as a computer program. 
     Instead of being followed by the intermediate-frequency section  5  as illustrated in  FIG. 1 , the radio-frequency section  2  can also be followed by some other lower-frequency section. In particular, the lower-frequency section may have a plurality of series-connected intermediate-frequency sections, no intermediate-frequency section but only one baseband-frequency section (zero-IF receiver, see  FIG. 9 ) or a section with analogue/digital converters to the intermediate frequency or baseband frequency. Furthermore, analogue I/Q demodulators can be provided instead of the analogue/digital converters, with the components in the channels being designed for their dynamic range. 
     In the radio-frequency receiver illustrated in  FIG. 5 , the signal divider  5  is followed by signal dividers  19  in the form of a cascade, in order to split the signal between four channels  6 ,  7 ,  20 ,  21  for graduated signal strengths. Channel  6  is intended for the lowest signal strength, while channel  21  is intended for the highest signal strength. The maximum signal element strengths G max , H max , I max , K max , corresponding to the respective channels  21 ,  20 ,  7 ,  6  after division, G′ max , H′ max , I′ max , K′ max  after amplification by the amplifiers  12  and G″ max , H″ max , I″ max , K″ max  after amplification by the amplifiers  13  are illustrated in  FIGS. 6 to 8 , which correspond to  FIGS. 2 to 4 . In this case, the division ratio of the signal dividers  19  is 1:1, so that the signal element strengths G max , H max , and I max , K max  are each the same. The components are designed such that a signal at the input  3 , which leads to an amplified signal element with a signal strength at the threshold value A, B or C as illustrated in  FIG. 8 , has a signal element strength in the channels  20 ,  7  or  6 , respectively, which is just below the dynamic range D of the respective analogue/digital converter  11 . 
     Furthermore, by way of example, a signal limiter  22  is provided for channels  6 ,  7  in  FIG. 5 . The signal limiter  22  prevents a signal with a strength which is above the greatest maximum permissible strength for the subsequent channels, that is to say in this case which is above the maximum permissible strength for the channel  7 , being passed through, while the signals with a strength which can be processed in channel  6  or channel  7  are passed through. Signal limiters can also be provided individually for individual channels, expediently in each case upstream of the first amplifier in the respective channel. This makes it possible to effectively prevent components being damaged as a result of overdriving or voltage spikes, since the only signals which are passed through are those which can be processed in the downstream channel or the downstream channels. For example, the signal limiter  22  passes on only signal elements with a strength which is within the dynamic range D of the channel  7  after amplification. While the signal limiter  22  is working, signal detection is still possible on the other channels, which are designed for higher signal strengths and have no signal limiters, or no signal limiters which limit a signal element at the same time. 
     The embodiment as illustrated in  FIG. 9  of a radio-frequency receiver  1  in the form of a so-called zero-IF receiver comprises a radio-frequency section  2  which is followed, directly via mixers  14 , by a lower-frequency section, for example a baseband-frequency section  9 , with A/D converters  11 . In this case, demodulation is carried out quasi-directly from the radio-frequency section  2 , without any intermediate-frequency section. Two mixers  14  are provided for this purpose in each channel, are fed with a phase shift of 90° from the oscillator  15  and carry out the demodulation process together with the A/D converters  11 , two of which are likewise provided for each channel, in order in this case to separately supply I/Q data to the inputs of the selection apparatus for each channel, as in the case of the demodulator  17  shown in  FIG. 1 . In an alternative embodiment more than two mixers and/or more than two A/D converters can be provided in each channel. 
       FIG. 10  shows a further embodiment of a receiver according to the invention. A radio-frequency analog signal from a receiving device  4  is input to a radio-frequency analog stage  2  of the receiver  1  at an input  3 . A signal divider  5  is provided to split the signal into two sub-signals that are passed to two channels  6   a,    7   a,  respectively. A low noise amplifier  12  is provided in the high frequency analog stage  2  of the receiver  1  to amplify the sub-signal in channel  6   a  before it is passed to a mixer  14  for down-conversion to lower frequencies. The sub-signal in channel  7   a  is passed to a second mixer  14  without amplification. The low noise amplifier  12  in channel  6   a  therefore causes the sub-signal in the downstream channels  6   b,    6   c  to have a higher signal strength than the sub-signal in the downstream channels  7   b,    7   c.  The signal divider  5  is preferably a symmetric signal divider, i.e. with a pre-established division ratio of 1:1. Alternatively the signal divider may be asymmetrical with a pre-established division ratio different from 1:1, preferably providing a higher signal strength to channel  6   a,  thereby further causing the sub-signals to have different signal strengths. By the different signal-processing, channel  6  is adapted to process input signals of low signal strength, while channel  7  is adapted for high input signal strengths. 
     Downstream from the mixers  14  in the lower frequency stages  8 ,  9  the receiver  1  corresponds to the embodiment illustrated in  FIG. 1 . The description pertaining to  FIG. 1  applies accordingly.