Radar receiver

Abstract

The disclosure relates to a radar receiver comprising an RF front end module having an N-path filter. Example embodiments include an FMCW radar receiver (300, 500) comprising: an antenna (301); an RF front end, RFFE, module (302) configured to amplify a radar signal from the antenna (101) and convert the radar signal to a baseband signal; a baseband processing module (303) configured to receive the baseband signal from the RFFE module (302) and digitise the baseband signal to generate an output digital baseband signal (308); and a local oscillator module (307) configured to provide a first local oscillator signal at a first frequency (fLO1) and a second local oscillator signal at a second frequency (fLO2), the second frequency (fLO2) being shifted from the first frequency (fLO1) by an offset frequency (Δffilter), wherein the RFFE module (302) comprises: an RFFE amplifier (304) arranged to receive the radar signal from the antenna (301); an N-path filter (305) arranged to receive an amplified radar signal from the amplifier (304) and the second local oscillator signal from the local oscillator module (307); and a downconverter (306) arranged to receive a filtered radar signal output from the N-path filter (305) and the first local oscillator signal and to provide the baseband signal to the baseband processing module (303).

Description

[0001] DESCRIPTION

[0002] RADAR RECEIVER

[0003] Field

[0004] The disclosure relates to a radar receiver comprising an RF front end module having an N-path filter.

[0005] Background

[0006] In FMCW (Frequency-Modulated Continuous-Wave) radar receivers, a challenge is to ensure linearity of the receiver is sufficient in the presence of strong jamming signals. In automobile radar systems, the radar receiver antenna is generally positioned behind the bumper of the vehicle, which results in a strong reflected signal. If the receiver is not sufficiently linear, this can result in intermodulation (IM) products being generated that degrade the signal to noise ratio (SNR) of the receiver and generate false targets. This is illustrated schematically in the example radar transceiver 100 shown in Figure 1 , in which the bumper 101 of a vehicle 102 provides a strong reflected signal 103 together with another reflected signal 104 from a target 105.

[0007] A transmitter 120 transmits a signal 121 via a transmitter balun matching circuit 122 and antenna 123, the transmitter 120 being provided with a transmitter signal by a local oscillator module 124. A controller 125 controls operation of the transceiver 100.

[0008] The receiver RF front end (RX-RFFE) 106 comprises an amplifier 107 that receives incoming signals 103, 104 via an antenna 108 and a balun matching circuit 109. The amplifier 107 provides an amplified radar signal at an RF frequency fRF and input power PIN to a downconverter 110. The amplifier 107 should add as little noise to the signals as possible. The downconverter 110 is also provided with a local oscillator signal having a local oscillator frequency fi_o. The resulting baseband signal at an intermediate frequency fiF is then filtered and amplified further by high pass filters 111 a, 111 b and variable gain amplifiers (VGAs) 112a, 112b in an analog baseband module 130 before being provided to an analog-to-digital converter (ADC) 113. The resulting digitised signal is then processed by a digital signal processor (DSP) 114 to determine targets from the incoming radar signals.

[0009] An example FMCW radar signal is transmitted as chirp signals with a constant output power and with an output frequency fi_o varying during each chirp within a band between 0.1 and 5GHz. A target reflected signal 104 may have its frequency ftarget shifted from 1 to 40 Mhz at the receiver input. The high-power reflection 103 from the bumper 101 may generate a frequency shift of around 20 kHz from fi_o.

[0010] Due to the strong reflected signal 103 from the vehicle bumper 101 , the linearity of the radar receiver 100 is limited, leading to a constrained RFFE gain and noise figure (NF). Techniques such as l / Q mixing could be used to reduce the receiver noise figure. The cascaded high pass filters 111 a, 111 b in the baseband analog module 114 can reduce the interfering signal and to some extent equalise the radar channels. Depending on the RFFE gain, however, these amplifiers 111 a, 111 b may face strong constraints on both noise performance and linearity.

[0011] Figures 2a-d illustrate signals at the points a-d indicated throughout the receiver 100 in Figure 1. Figure 2a illustrates example FMCW RF signals 201 , 202 received from a target 105 and bumper 101 respectively, having respective frequencies ftarget, fbump. and powers Ptarget, Pbump. After amplification by LNA 107, the RF signals become amplified RF signals 203, 204, with the LNA 107 introducing a noise floor 205, as shown in Figure 2b. After down conversion, the signals 203, 204 are converted to IF signals 206, 207 at frequencies IFtarget, IFbump and amplitudes Vtarget, Vbump. with an increased noise floor 208, as shown in Figure 2c. Intermodulation products 209a, 209b are also introduced at frequencies either side of the target IF frequency IFtarget. High pass filtering and amplification by the baseband analog module can reduce the relative magnitude of the IFbump signal 207 compared to the IFtarget signal 206 so that both the target signal and bumper signal are within an amplitude range of the ADC 113, resulting in the signals 210, 211 illustrated in Figure 2d. The intermodulation products, however, remain. The IF filtering and amplification steps do not remove the need for linearity and reduced noise on the RFFE. In deep CMOS technology, where flicker noise corner may be high in the MHz range, this noise / linearity trade-off is difficult to achieve, as gain is maintained low at the RFFE side so as not to limit the receiver linearity but the baseband module will tend to contribute to the receiver noise floor.

[0012] Summary

[0013] According to a first aspect there is provided an FMCW radar receiver comprising: an antenna; an RF front end (RFFE) module configured to amplify a radar signal from the antenna and convert the radar signal to a baseband signal; a baseband module configured to receive the baseband signal from the RFFE module and digitise the baseband signal to generate an output digital baseband signal; and a local oscillator module configured to provide a first local oscillator signal at a first frequency and a second local oscillator signal at a second frequency, the second frequency being shifted from the first frequency by an offset frequency, wherein the RFFE module comprises: an RFFE amplifier arranged to receive the radar signal from the antenna; an N-path filter arranged to receive an amplified radar signal from the amplifier and the second local oscillator signal from the local oscillator module; and a downconverter arranged to receive a filtered radar signal output from the N-path filter and the first local oscillator signal and to provide the baseband signal to the baseband module.

[0014] The baseband module may comprise a high-pass filter, a baseband amplifier and an analog-to-digital converter (ADC). The baseband amplifier may be a variable gain amplifier.

[0015] Where the RFFE amplifier is a first RFFE amplifier, the RFFE module may comprise a second RFFE amplifier between the N-path filter and the downconverter. The second RFFE amplifier may be a variable gain amplifier.

[0016] The local oscillator module may be configured to synchronously chirp the first frequency of the first local oscillator signal and the second frequency of the second local oscillator signal.

[0017] The N-path filter may comprise four filter paths.

[0018] The N-path filter may comprise a plurality of switches connected between an input and an output of the N-path filter, each of the plurality of switches being connected for being driven by a phase of the second local oscillator signal.

[0019] The N-path filter may comprise a plurality of adjustable capacitors for defining a bandwidth of the N-path filter.

[0020] According to a second aspect there is provided an FMCW radar transceiver comprising: a radar transmitter; and the FMCW radar receiver according to the first aspect, wherein the local oscillator module is configured to provide the first local oscillator signal to the radar transmitter for transmission of radar signals at the first frequency.

[0021] According to a third aspect there is provided a method of operating an FMCW radar receiver according to the first aspect, the method comprising: the local oscillator module providing the first local oscillator signal at the first frequency to the downconverter and the second local oscillator signal at the second frequency to the N-path filter; the RFFE module receiving the radar signal from the antenna; the RFFE amplifier amplifying the radar signal; the N-path filter filtering the amplified radar signal with a bandpass filter having a centre frequency defined by the second local oscillator frequency to provide a filtered radar signal; the downconverter mixing the first local oscillator signal with the filtered radar signal to provide a baseband signal to the baseband module.

[0022] The local oscillator module may synchronously chirp the first frequency and the second frequency.

[0023] The local oscillator module may provide the first local oscillator signal to a radar transmitter module for transmission of radar signals at the first frequency.

[0024] The N-path filter may comprise a plurality of adjustable capacitors for defining a bandwidth of the N-path filter.

[0025] The N-path filter may comprise a plurality of switches connected between an input and an output of the N-path filter, each of the plurality of switches being driven by a phase of the second local oscillator signal.

[0026] These and other aspects of the invention will be apparent from, and elucidated with reference to, the embodiments described hereinafter.

[0027] Brief description of Drawings

[0028] Embodiments will be described, by way of example only, with reference to the drawings, in which:

[0029] Figure 1 is a schematic diagram of an example automobile radar receiver;

[0030] Figures 2a-2d are schematic diagrams illustrating target and interfering received radar signals for the receiver of Figure 1 ;

[0031] Figure 3 is a schematic diagram of an example radar transceiver including an N-path filter in the receiver RF front end;

[0032] Figures 4a-4d are schematic diagrams illustrating target and interference received radar signals for the receiver of Figure 3;

[0033] Figure 5 is a schematic diagram of an alternative example radar transceiver incorporating a variable gain amplifier in the RF front end;

[0034] Figure 6 is a circuit diagram of an example N-path filter for the radar receiver of Figure 3 or Figure 5; Figure 7 is a simplified schematic diagram of an example radar receiver;

[0035] Figure 8 is a plot of RF signal power as a function of LNA gain for an example radar receiver RF front end;

[0036] Figure 9 is a plot of RF signal power as a function of LNA gain for an example radar receiver RF front end;

[0037] Figures 10 and 11 are plots of noise floor as a function of LNA gain;

[0038] Figure 12 is a circuit diagram of an example N-path filter; and

[0039] Figures 13a and 13b are plots of filter rejection as a function of RF frequency for an example radar transceiver implementation.

[0040] It should be noted that the Figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these Figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar feature in modified and different embodiments.

[0041] Detailed description of embodiments

[0042] Figure 3 illustrates an example radar transceiver 300 comprising a radar receiver 330 and a radar transmitter 320. The receiver 330 comprises an antenna 301 , an RF front end module (RX-RFFE) 302 and a baseband module 303. The RF front end module 302 is configured to amplify a radar signal from the antenna 301 and convert the radar signal to a baseband signal. The baseband processing module 303 is configured to receive the baseband signal from the RF front end module 302 and digitise the baseband signal to generate an output digital baseband signal 308.

[0043] The RF front end module 302 comprises an amplifier LNA 304 that receives the radar signal from the antenna 301 via a balun matching circuit 309. An N-path filter 305 is arranged to receive an amplified radar signal from the amplifier 304. A downconverter 306 is arranged to receive a filtered radar signal from the N- path filter 305 and provide the baseband signal to the baseband processing module 303. A local oscillator module 307 is configured to provide a first local oscillator signal at a first frequency fi_oi to the downconverter 306 and a second local oscillator signal at a second frequency fi.02 to the N-path filter 305. The second frequency fio2 is shifted from the first frequency fi_oi by a tuneable offset frequency Afniter. A bandwidth control signal BWBPF is provided to the N-path filter 305 by a controller 325 to enable the bandwidth of the N-path filter 305 to be adapted to a targeted IF band corresponding to a target distance range, enabling detection of targets within the targeted IF band and rejection of other signals such as the bumper reflection signal in an automotive radar system. The offset frequency Afmter being tuneable allows for selection of a range of targets.

[0044] Operation of the N-path filter 305 is synchronised with the downconverter 306 such that the centre frequency fi.02 of the N-path filter 305 varies along with the local oscillator frequency fioi during chirping.

[0045] Also illustrated in Figure 3 is a transmitter module 320, which also receives the first local oscillator signal fi_oi and transmits a radar signal via a transmitter balun 322 and transmission antenna 321 . The local oscillator module 307 provides the first local oscillator signal fi_oi synchronously to the transmitter and receiver modules 320, 300 and synchronously chirps the frequencies of the first and second local oscillator signals.

[0046] As with the transceiver 100 of Figure 1 , the transceiver 300 comprises a baseband analog module 303 that may include one or more high-pass filters 311a, 311 b and variable gain amplifiers 312a, 312b, which filter and amplify the baseband signal before providing the output baseband analog signal to an ADC 313, which outputs a digitized signal 308 to a DSP 314.

[0047] Figures 4a-d illustrate signals at the points a-d indicated throughout the receiver 330 in Figure 3, illustrating the target and interference signals at the RF front end, following filtering by the N-path filter 305 and after down conversion and further filtering. Figure 4a illustrates the incoming RF signals comprising target and bumper signals 401 , 402 at respective frequencies ftarget and fbump. and at respective powers Ptarget and Pbump. After being amplified by the LNA 304, the incoming radar signals are filtered by the N-path filter 305 having a bandwidth BWbpf defined by the controller 325 and a centre frequency defined by fi.02. The resulting filtered RF signal, illustrated in Figure 4b, has a reduced magnitude for the bumper signal 404 while the target signal 403 remains of similar magnitude.

[0048] Following down-conversion by the downconverter 306, the target and bumper signals 405, 406 are shifted to respective baseband or intermediate frequencies IFtarget, I Fbump and in this example to similar magnitudes, as illustrated in Figure 4c. Because the bumper signal has been reduced in magnitude by the N-path filter in the RFFE module, the intermodulation products 407a, 407b are significantly reduced, and in this example are reduced to below the noise floor 408 of the baseband signal. The target and bumper signals may then be further filtered and amplified to provide filtered and amplified target and bumper baseband signals 409, 410 that are within a range of the ADC 313, as illustrated in Figure 4d. The signals are then digitised by the ADC 313 and processed by the DSP 314 to extract target information.

[0049] The resulting baseband signals 409, 410 illustrated in Figure 4d may be similar to the resulting baseband signals 206, 207 illustrated in Figure 2 but by using the N-path filter in the RFFE 302 the linearity constraints on the baseband components 311 a, 311 b, 312a, 312b are relaxed. Reducing such constraints on the IF components and mixer can reduce costs, power and non-linearity effects of the receiver. A further advantage is that intermodulation products from the bumper reflection signal combined with real target reflections, which can result in unreal targets being identified, can be reduced.

[0050] Figure 5 illustrates an alternative example FMCW radar transceiver 500 that is similar to the radar receiver 300 of Figure 3 but with the addition of a second amplifier LNA2 501 between the N-path filter 305 and the mixer 306. Other components of the transceiver are as indicated above in relation to Figure 3.

[0051] The second amplifier 501 may be added to reduce mixer I baseband noise and to recover losses from the N-path filter 305. The second amplifier 501 may have a tuneable or variable gain to adapt to the degree of rejection of the bumper signal and the selected bandwidth of the N-path filter 305. Real mixing is implemented to reduce the overall receiver footprint while retaining a similar performance to IQ mixing, in particular relating to interference mitigation and image noise reduction.

[0052] Figure 6 is a schematic circuit diagram illustrating an example topology for a differential double switch 4-path bandpass filter that may be used for the radar receiver of Figures 3 or 5 described above. The input signal Vin is switched via a plurality of switches S1-S4 across switching resistors Rsw and a plurality of capacitors CBB, providing a differential filtered output signal vOut. Other N-path filter topologies and with different numbers of paths may alternatively be used.

[0053] Using simulations, comparisons were made between a receiver implementing an N-path filter and real mixing (i.e. according to the above described examples), a receiver implementing only real mixing, and a receiver implementing IQ mixing. Figure 7 illustrates schematically the various components of a simulated radar receiver 700. An input RF signal fRF is received by an aerial 701 and input to a first RFFE amplifier LNA1 704 before passing to an N-path filter 705. A second RFFE amplifier LNA2 710 receives the filtered output from the N-path filter 705 and outputs the filtered signal to a mixer 705, which outputs an IF signal to an IF amplifier 712. The IF amplifier 712 outputs an IF signal to be digitised.

[0054] The table below provides a summary of the figure of merit for the various components of the receiver 700 for out of band (OOB) and in band (IB) linearity.

[0055] Figure 8 is a plot of receiver out of band linearity, illustrating the RF ICP1 as a function of gain of the second RFFE amplifier LNA2 710 for a real mixer alone 801 and for an N-path filter in combination with a real mixer 802. RF ICP1 is a figure of merit corresponding to a 1 dB reduction in gain, typically based around a 3rd order interception point. Figure 9 is a plot of receiver in-band linearity, illustrating the RF ICP1 as a function of gain of the second RFFE amplifier LNA2 701 , again for a real mixer alone 901 and for an N-path filter in combination with a real mixer 902.

[0056] These results show that the out of band linearity can be improved by using N- path filtering. The bumper high power tone is filtered by the N-path filter, leading to improved OOB linearity. Extra gain in the second RFFE amplifier LNA2 is added with limited impact on the overall receiver RFFE linearity specification. The mixer deals only with in-band non-linearity but the specification is lower. Even if with the LNA2 extra gain, the specifications are fulfilled. Considering the LNA2 gain at 17 (Figure 8), this shows the remaining margin for the IF amplifier linearity is over 25 dB. The linearity constraint on the overall receiver RFFE is thereby reduced compared to a conventional downconverter.

[0057] Simulation results also show a trade-off between noise and linearity. Out of band linearity can be improved using N-path filtering, which provides flexibility to add extra gain using the second RFFE amplifier LNA2 710, which allows the baseband noise contribution to be reduced. The mixer 705 deals only with in- band non-linearity. High selectivity of N-path filtering improves the noise floor at low frequency offsets by reducing the contribution of 1 / f noise that is strongly present in advanced integrated technologies. Consequently, a different tradeoff in the receiver chain leads to higher RF gain in the receiver, reducing the noise constraint in the baseband.

[0058] The table below indicates example figures of merit for gain and noise floor for the various components of the receiver 700 illustrated in Figure 7. Figure 10 is a plot of noise floor as a function of LNA2 gain, illustrating the cumulative contributions made by package losses 1001 , LNA1 1002, filter and additional LNA2 noise 1003, additional noise up to the baseband output 1004, resulting in a total noise floor 1005. This illustrates that the LNA2 gain can be increased to minimise the noise impact of the mixer and analog chain.

[0059] Figure 11 is a further plot of noise floor as a function of LNA1 gain, illustrating simulation results from: i) a receiver with a real mixer 1101 ; ii) a receiver with an IQ mixer 1102; iii) a receiver with a real mixer and N-path filter, with a 0Hz filter offset 1103; and iv) a receiver with a real mixer and N-path filter with a 10MHz filter offset 1104. At higher LNA1 gains, the noise floor for the receiver with a real mixer and N-path filter with a 10 MHz offset is comparable to the receiver with the IQ mixer. Without the LO frequency shift for the N-path filter, the image band is not rejected and the noise floor remains higher at higher LNA1 gains. These results show that, as the receiver RFFE linearity constraints change, the LNA1 gain is not limited by the mixer ICP1 and can be increased but will be limited by the LNA1 linearity. The noise performance of the architecture will depend on the selected technology, but the overall architecture can lead to noise reduction.

[0060] Simulation results also show that the image band is rejected by the N-path filter, which leads to a good interference rejection similar to IQ down conversion but without the need to build a complete IQ downconverter. In example implementations, a 3dB SNR improvement may be obtained on average, up to 6dB when interference folding back is frequency coherent.

[0061] In an example implementation, simulation was performed on a receiver with an N-path filter having 4 paths, of the type illustrated in Figure 12. The first and second RFFE amplifiers LNA1 , LNA2 were designed to have a 7dB power gain. A passive mixer was used and loaded by a baseband TIA. The bandwidth of the N-path filter 1200 was selected through the capacitor CBB value. Switched capacitors may be used to allow for adjustable values. The filter LO inputs were in quadrature with the LO frequency, defining the central frequency of the bandpass N-path filter. Band selection of the filter 1200 is possible through changing the offset frequency and bandwidth. Figure 13a illustrates filter rejection as a function of RF frequency while varying the capacitor values, providing a Q of between 3000 and 38000 in the mmW band. Figure 13b illustrates filter rejection as a function of RF frequency with different offset frequencies, illustrating the ability of vary the centre frequency of the filter.

[0062] A radar receiver according to the present disclosure addresses various problems or disadvantages. Firstly, due to bumper proximity in a typical vehicle implementation, TX-to-RX leakage or a strong reflection, a higher power tone can constrain the linearity of the overall FMCW-radar RX chain, which leads to a difficult balance between linearity and noise performance. With the increased number of embedded radar modules in modem vehicles, and radar becoming popular in all car segments, modem FMCW radars will tend to suffer from mutual radar-to-radar interferences leading to a degradation of the signal integrity and target detection. Implementing an N-path filter in the RF Front-End can be used to effectively reduce or eliminate the bumper interference signal (up to 15dB rejection are shown in simulations), which allows the insertion of a second stage LNA in the RX chain, increasing RFFE gain and hiding mixer and baseband noise. The use of an N-path filter also opens the way to a different trade-off between noise & linearity, leading to less noise constraint on the baseband chain. The N-path filter implemented in the RF Front-End also allows for rejecting the image band, which leads to a radar-to-radar interference rejection similar to an IQ downconverter. A real receiver can then be used. With additional gain provided by a second RFFE amplifier and a different linearity trade-off in the RFFE, the noise figure can be improved compared to a conventional RX- RFFE. This benefit is even more visible when noise of BB circuits increases (which is mainly due to 1 / f noise in deeper CMOS). The use of an N-path filter directly in the RFFE also allows for a reduction of receiver overall form factor & power consumption, as a complex (IQ) down-conversion implementation can be avoided. The radar receiver may be used in next generation CMOS radar product receivers to achieve better compromise between linearity and noise (removal of bumper effect at mmw frequencies).

[0063] The radar receiver may also be suited for applications in future radar system which could run at higher frequency (e.g. 140GHz) and / or applications with higher bumper reflection constraints. The concept may also have additional benefits for future radar systems using deeper CMOS technologies (e.g. «28nm) as N-path filter performance improves naturally with process scaling, leading to lower losses and lower capacitances.

[0064] From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of radar systems, and which may be used instead of, or in addition to, features already described herein.

[0065] Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

[0066] Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. The applicant hereby gives notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

[0067] For the sake of completeness it is also stated that the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality, a single processor or other unit may fulfil the functions of several means recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

CLAIMS1 . An FMCW radar receiver (330) comprising: an antenna (301 ); an RF front end, RFFE, module (302) configured to amplify a radar signal from the antenna (301 ) and convert the radar signal to a baseband signal; a baseband module (303) configured to receive the baseband signal from the RFFE module (302) and digitise the baseband signal to generate an output digital baseband signal (308); and a local oscillator module (307) configured to provide a first local oscillator signal at a first frequency (fi_oi ) and a second local oscillator signal at a second frequency (fi.02), the second frequency (fi.02) being shifted from the first frequency (fi.01) by an offset frequency (Afniter), wherein the RFFE module (302) comprises: an RFFE amplifier (304) arranged to receive the radar signal from the antenna (301 ); an N-path filter (305) arranged to receive an amplified radar signal from the amplifier (304) and the second local oscillator signal from the local oscillator module (307); and a downconverter (306) arranged to receive a filtered radar signal output from the N-path filter (305) and the first local oscillator signal and to provide the baseband signal to the baseband module (303).

2. The FMCW radar receiver (330) of claim 1 , wherein the baseband module (303) comprises a high-pass filter (311 a, 311 b), a baseband amplifier (312a, 312b) and an analog-to-digital converter, ADC (313).

3. The FMCW radar receiver (330) of claim 2, wherein the baseband amplifier (312a, 312b) is a variable gain amplifier.

4. The FMCW radar receiver (330) of any preceding claim, wherein the RFFE amplifier (304) is a first RFFE amplifier, the RFFE module (302)comprising a second RFFE amplifier (501 ) between the N-path filter (305) and the downconverter (306).

5. The FMCW radar receiver (330) of claim 4, wherein the second RFFE amplifier (501 ) is a variable gain amplifier.

6. The FMCW radar receiver (330) of any preceding claim, wherein the local oscillator module (307) is configured to synchronously chirp the first frequency (fi_oi ) of the first local oscillator signal and the second frequency (fi.02) of the second local oscillator signal.

7. The FMCW radar receiver (330) of any preceding claim, wherein the N- path filter (305) comprises four filter paths.

8. The FMCW radar receiver (330) of any preceding claim, wherein the N- path filter (305) comprises a plurality of switches connected between an input and an output of the N-path filter, each of the plurality of switches being connected for being driven by a phase of the second local oscillator signal.

9. The FMCW radar receiver (330) of claim 8, wherein the N-path filter comprises a plurality of adjustable capacitors (CBB) for defining a bandwidth of the N-path filter.

10. An FMCW radar transceiver (300, 500) comprising: a radar transmitter (320); and the FMCW radar receiver (330) according to any preceding claim, wherein the local oscillator module (307) is configured to provide the first local oscillator signal to the radar transmitter (320) for transmission of radar signals at the first frequency (fi_oi ).

11. A method of operating an FMCW radar receiver (330) according to claim 1 , the method comprising:the local oscillator module (307) providing the first local oscillator signal at the first frequency (fi_oi) to the downconverter (306) and the second local oscillator signal at the second frequency (fi.02) to the N-path filter (305); the RFFE module (302) receiving the radar signal from the antenna (301 ); the RFFE amplifier (304) amplifying the radar signal; the N-path filter (305) filtering the amplified radar signal with a bandpass filter having a centre frequency defined by the second local oscillator frequency (fio2) to provide a filtered radar signal; the downconverter (306) mixing the first local oscillator signal with the filtered radar signal to provide a baseband signal to the baseband module (303).

12. The method of claim 11 , wherein the local oscillator module (307) synchronously chirps the first frequency (fi.01) and the second frequency (fi.02).

13. The method of claim 12, wherein the local oscillator module (307) provides the first local oscillator signal to a radar transmitter module (320) for transmission of radar signals at the first frequency (fi.01).

14. The method of any one of claims 11 to 13, wherein the N-path filter comprises a plurality of adjustable capacitors (CBB) for defining a bandwidth of the N-path filter.

15. The method of any one of claims 11 to 14, wherein the N-path filter (305) comprises a plurality of switches connected between an input and an output of the N-path filter, each of the plurality of switches being driven by a phase of the second local oscillator signal.

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