Chapter 7 Radar Receiver

Transcription

1 Chapter 7 Radar Receiver Receiver Types Superregenerative receiver - A single tube is used for the RF amplifier in RX and TX sources. - Advantages: Simplicity and low cost - Disadvantages: gain instability, poor selectivity, high receiver noise level Crystal Video Receiver - Advantages: Simplicity and low cost - Disadvantages: Poor sensitivity (No RF amplifier filter effect), Poor selectivity, poor pulse shape of video amplifier - 30 ~ 40dB loss than those achievable in Superheterodyne receivers. TRF Receiver - Add a RF amplifier prior to the detector in the Crystal Video Receiver - Improve sensitivity (reduce noise produced by the detector) and selectivity (RF amp. filtering), Reduce the video gain Superheterodyne Receiver 7 -

2 Superheterodyne Receiver The input at RF is down converted to an intermediate frequency (). Advantages: Excellent sensitivity, much lower conversion loss in detection. amplifier is more effective and stable than RF amplifier signal simplified filtering (narrow filter) improve selectivity LO OSC can be changed to track the TX frequency and filtering protect the sensitive mixer from saturation or burned not used due to unavailability of low-noise amplifiers and excellent of RF mixers Noncoherent: Low : lower cost High : Wideband Duplexer: switches the common antenna between TX and RX (TR switch). Input of RX to output of processor can vary from 00 to 200dB STC (sensitivity time control): gain as a function of time (range) AGC (automatic gain control): may 7-2 Performance Considerations Noise Characteristics Noise Figure Radar Receiver Noise Figure Dynamic Range - Bandwidth selection and Filtering Dynamic Range Noise BW Filter selection filtering Amp Radar receiver noise figure BW RF Amp 7-3

3 Considerations on Noise Usually the first characteristics specified for a radar receiver The understanding of the receiver noise as the ultimate limitation on radar range performance is important. The ability to detect received radar echoes is ultimately limited by thermal noise, even if receiver adds no additional noise The lowest-noise receiver may need great a sacrifice in system performance and cost It is seldom a dominant factor because the noise contribution has been reduced sufficiently. 7-4 Thermal Noise Characteristics The average thermal noise input to the receiver can be determined as N ktb, where k k w. T Noise Temperature of input impedance T o 290k : standard noise temp. B : Bandwidth We generally defined Noise Factor S F i N i , S S o G S i G i Noise Figure NF 0Log F(dB) S i N i T i Receiver or Device Gain Noise Noise Factor F Receiver or Device S o GFkT i B Output noise GFN i GFkT i B S i Gain S o Total equivalent input noise FN i Total equivalent input noise generated by receiver F N i Effective noise temp T eff F T i N i F N i F T i GFkT i B Equivalent input noise generated by the receiver 7-5

4 Noise of a Cascaded System System 2 System N S N G G S G 2 F G N F 2 G N F N S o System System 2 S i G G 2 N i F F 2 ktb ktb F ktb System G F 2 ktb System N G N Total output noise G G 2 G N F ktb F F N F ktb F 2 ktb G F 3 ktb G G 2 Total input noise F F ktb G 7-6 Cascaded Noise Figure T E F T System G System N G N Total input noise F F ktb G F F F T F F N + G + + G G 2 G N ktb T E F N T E2 Total output noise Whole system G T F T T E G G 2 G N F ktb F F 2 F N T ktb k T T E2 T G G G 2 G + + E N G + + EN G B k T + TE B G 2 G N The noise factor for a system consisting of The effective noise for a system consisting N cascaded network can be found to be of N cascaded network can be found to be If the first-stage network has adequate gain, he noise figure of the total network is primarily determined by the first stage. F T T E ktb Receiver Temp T EN T T E2 E G G G 2 G N F T T G T F T ktb 7-7

5 An Example (RF receiver) RF Amplifier Mixer Amplifier RF input A A A output G RF 30dB 000 F RF 2.3dB.698 G M 23dB 99.5 F M 7.5dB 5.62 G 30dB 000 F M.2dB.3 kt 0 B MHz 4dBm F F T F F N G G G 2 G + N dB T E F T T k k, if BW MHz N i equ F T ktb dBm N iequ G dBm 7-8 Sensitivity & Max. detection Distance) R max P t GA e S imin m 64.6km A 2 e G R max 4m 2 S o f 0G B 200MHz F 5dB RX S o 0dB min S o mindb S imin 66 G 30dB P t 80dBm 00kW 0 20dB S i N i 76dB for reliable detection S i N i S o S i F S o S i S imin N iequ F T ktb S o min FN i S o logB MHz + F TdB + S o mindb 4 + 0log dBm mw Bandwidth is too wide, such that the sensitivity is too high. Then, the maximum range is limited Maximize the receiver SNR Matched filter FN i B 7-9

6 Radar Receiver Noise Figure DSBNF(double sideband noise figure) radio astronomy noise figure radiometer noise figure Radiometer-type receiver: echo occupies upper and lower sidebands F DSB GkT o 2B f SSBNF(single sideband noise figure) Radar: echo occupies only noise at the signal frequency F SSB F GkT o B DSB 2 F DSBdB + 3dB NF (venders) +3dB for radar NF 7-0 Dynamic Range SFDR Dynamic Range The radar receiver is required to receives and detect signal levels near the receiver noise level, also be able to tolerate echo signals from large RCS target at close range Receiver Dynamic Range Nonlinearity Minimum signal level noise Maximum signal level no distortion to input power -db compression point SFDR (Spurious Free Dynamic Range) -db SFDR: 70~00dB Generally, is determined by mixer. Various stages following mixer do not saturate prior to mixer linearity from receiver noise level to a power of about -0db Without RF gain control, the useful dynamic range of a receiver is generally determined by mixer dynamic range. 7 -

7 Dynamic Range Improvement Various following the mixer ( amplifier, detector, video amplifier) do not saturate prior to the mixer in order to preserve dynamic range Power Control increasing dynamic range without degrading long-range detection performance AGC (Auto gain control): prior to the mixer will increase the effective dynamic range. - May fluctuate due to variations in the target cross section - Result in additional loss prior to the mixer. Noise level, sensitivity and DR. STC (Sensitivity Time control): may be better to vary gain as a function of time to provide more amplification for targets that are farther away far in a pulsed radar system - Receiver sensitivity is reduced in detecting small targets at close-in range, since gain is small at close-in range. - may be helpful in reducing the effects of closerange target without degrading the long-range detection performance Subsys Subsys 2 Dynamic Range D 3 D 2 D G R T G R 4 R DR is particularly important for processing multiple echoes. Large signals may cause saturation in the receiver distant echoes R 2 T 2 masking of more Provided the radar utilizes a pulse that is sufficiently short so as to discriminate at various ranges between R and R 2, Gain can be variable to keep received power P r Cons tan t. G Subsys 3 G 40LogR R 7-2 Bandwidth of signal The basic rule of the thumb for a pulse radar application is that receiver bandwidth B. B, noise.00ns pulse, B 0MHz Pulse has spectral characteristic H f, the matched filter should have spectral characteristic H f. selection Mixer/LO implementation: -, Mixer-and- LO induced noise. -, noise from amplifier, since frequency, noise. - 30M~4G are common in radar application Availability of processing components. - signal-processing components (Log s, pulse compression, surface acoustic wave devices SAW, limiters) are available at lower frequency (30~500M) Selection and Filtering The gain and filtering in the receiver usually distributed over stages of varying gains and losses. Gain must be distributed so that stages do not saturate prior to saturation at the RF converter (RF mixer) Narrowband filtering is most easily and RF Amp A 30dB Mixer Amp A 30dB 0dBm B Select components for their gain, BW and output power Transmitted waveform characteristics - At the higher millimeter wave frequency (40G), a higher is required. A minimum separation of LO and TX frequencies if 750M to 000M Hz is required. - Broadband systems also require higher frequency to minimize spurious response. 7-3

8 UP-Down Converter (Example) Three-stage up-down converter Flitter out Image, Local leakage, IM... f 2 f LO f 2 7. B400 f 2 f LO f f LO 70M VCO 8G f LO2 G f LO f 2 7. I/Q Demod f 2 f LO f 7-4 Receiver Components Mixer Single-ended, Balanced Amplifier RF Diode Limiter Accumulator Duplexer Oscillator Isolator Switch Phase shiftier Filter 7-5

9 Receiver Protection Duplexer A single antenna for both transmission and reception. responsible for protecting the receiver during transmission and for switching the antenna between TX and RX. Types Ant. duplexer TX High power radius employs power-sensitive gas discharge tubes to direct the TX or RX energy isolation - Transmit-receive (TR) - anti-transmit-receive (ATR) tubes Ferrite duplexer (circulator) RX - Do not employ gas tubes - use circulator, use ferrite materials - 25~ 30dB isolation 7-6 Receiver Protection Diode limiters Designed to perform the same function as the receiver Protector TR discussed above Reflect or absorb essentially all incident RF power above a certain level Ferrite or semiconductor tubes, Low insertion loss Pin Diode Switches - fast 5 to 25 ns switching time dB isolation - Insertion loss 2 ~ 4dB Open more reliable than TR duplexer TX Isolation switching time P o Ant. isolation 25~30dB Limiter Switch Insertion loss P i RX more isolation provided by switch 7-7

10 Receiver Protection Characteristics 7-8 Mixer At microwave frequency, mixer are usually obtained using point contact or schottky barrier diodes. in some applications, the mixer may be the first device in your receiver system NF (noise figure) db at 5G, 5 db at 95 G The nonlinear mixing process produces many sum and difference frequency of the signals, LO (local oscillation) and their harmonics. Mixing action generally described by F o : output Freq. F 2 : LO Freq. F : input Freq. I f v a 0 + a v + a 2 v a n v n Given that v t V RF sinw RF t+ V LO sinw LO t the primary mixing products, w LO w RF, come from the second-order term and proportional to in amplitude. a 2 7-9

11 Mixing v 2 t V RF sinw RF t+ V LO sinw LO t 2 V RF 2 sin 2 w RF t + V 2 LO sin 2 w LO t + 2V RF V LO sinw RF tsin w LO t V RF V LO cos w RF w LO t cosw RF + w LO t When the signal is mixed with the LO frequency of f RF f LO f andf RF + f LO results Similarly, when the image signal is mixed with the LO frequencyf RF 2f f LO f and f RF 2f + f LO may results But recall that because of the many other powers that are generated, many harmonics and intermodulation products (IMPs). e.g. cos 2 x 2 + cos2x, cos 3 x 4 cos 3x + 3cosx Non-linearity: Harmonics, Spurious RF 0dBm Image Mixer LO f RF 2f f B All harmonics and IMPs must be suppressed by filtering LO Signal f RF f 7-20 Mixer Dynamic Range In many applications dynamic range is limited by mixer dynamic range (DR) Three definitions for DR Low end: Thermal noise + NF mix, High end: saturated output ~Lo power conversion Loss Low end: Thermal noise + NF mix, High end: db compression point (input power at which conversion loss increases by db. Low end: Thermal noise + NF mix, High end: input power level at which two third-order IMP just equal mixer output noise level. conversion (Insertion) loss for (5 ~ 0 db) RF radiate RF/LO Isolation (2 ~ 23 db) LO Mixer LO/ Isolation (0 ~ 20 db) 5~3 dbm Harmonics down IMP down Low end Thermal noise DR DR2 DR3 SFDR 7-2 High end

12 Mixer Configurations Single-ended mixer The simplest form LO energy can be radiated by the receiver antenna (RF/LO isolation) All harmonics and IMPs will be suppressed by filtering, if required. Balanced mixer reduce spurious response, cancellation of DC components at the output, and convenient separation of LO and RF inputs. The even harmonics of one of the input signals are suppressed. Harmonics of the LO signal are suppressed Image-reject mixer Hybrid 90 o SSB mod. IMG. rej. mixer 90 o 0 o LO V RF cosw t + w LO t + 2 RF V LO cosw t + V IM cos w t + w LO t + 2 IM V LO cosw t Image Signal V RF sinw t + w LO t + V IM sin w t + w LO t + V LO f LO f + LO 2 RF V LO cosw t RF V LO cosw t IM V LO cosw t + 2 IM V LO cosw t 2 RF V LO sinw t RF V LO sinw t + 2 IM V LO sinw t V -- 2 IM V LO sinw t 0 cosw LO t 2 RF V LO sinw t + V -- 2 IM V LO sinw t 7-23

13 Detector Superheterodyne receiver At least two stages of down conversion in the detection process The first down conversion is accomplished by the first detector (Mixer) Information at consists of the phase, amplitude, and frequency of received echo signal. Second-stage detection Square Law detection: No LO signal, output voltage is proportional to input RF power (square of RF input voltage) - Tangential signal sensitivity (TSS): Synchronous detection: With an LO input, the detection process is linear - second LO (COHO) is at the same freq. as the. - I/Q provides amp. and phase information. Phase detection: signals are hard-limited (const. amp.), only phase information. increase RF input signal until minimum peak of video signal plus noise is the same as the maximum peak of the video noise Amplifier db compression point (Output) ~ 30dBm 3rd order 2-tone intercept point (Output) ~ 35 db db com. pt IMP3 Saturation Pt. NF ~ 7.5dB BW ~ 50 MHz Gain ~ 6 db P noise log (BW)+ NF -4 +0log(50) dbm db compression Dynamic Range P (input) DR db P (input) - P noise 4-(-84.5) 98dB Spurious Free Dynamic Range (SFDR) P dbm (input) SFDR 2/3(P - P noise ) 2/3 (9+84.5) 69 db Logarithmic amplifier/detector (log amp) Output video is proportional to the logarithm of the RF input. Extremely wide dynamic range (70 ~ 80 db) No AGC to achieve the wide dynamic range. P n -84.5dBm SFDR DR 98DB P 7-25

14 Fully Coherent Detection Coherent Radar Receiver Design RF transmitted signal the combination of a STALO (stable LO) and a COHO (coherent LO) oscillator The sum is formed in an up-converter and amplified using a pulsed RF amplifier. On receive, signal (60MHz ~ 4 GHz) is the same frequency as the COHO, signal is amplified, filtered..., and then downconverted to baseband Doppler by mixing with COHO. Orthogonal mixer I and Q signal components Signal-processing circuitry consist of MTI or pulse- Doppler filtering. Coherent on Receive maximum coherence or MTI improvement is not required. A noncherent transmitter can be employed 7-26 Coherent Receiver on Receive Tuned COHO receive phase tracks transmitter phase stability can be maintained in COHO during the time bet. transmission and reception, and repeatability of phase locking of COHO from pulse to pulse. Tuned COHO highly stable COHO to tune transmitter to match it in frequency stability can be realized. correct each target echo before applying the signal to moving target filter 7-27

15 Pulse Compression Allow a radar to use a long pulse to achieve high radiated energy and simultaneously to obtain range resolution Use freq. or phase modulation to wider the signal bandwidth Linear FM pulse compression A stable but noncoherent LO RF and processing circuitry must be broadband amplifier must have sufficient bandwidth and linear phase over the band Compressive filters used are surface acoustic wave (SAW) devices. analog device is used to obtained a compressed video output Frequency Stepped Coherent Receiver High-range resolution Wideband frequency stepped waveform processing the received echo using FFT Coherent or noncoherent detection Coherent processing can increase the receiver SNR STALO with a frequency synthesizer whose output frequency is selectable in N discrete steps of Total bandwidth f step size N f Wide bandwidth requirement for the receiver front end (circulator, protector, RF mixer effectively generates a wideband signal while maintaining a narrowband receiver. Range Resolution c 2B T S N impr 0logB T B B T N f c cm s B inverse of TX pulse wid (matched filter approx. ) Ex: 28 0MHz steps and a 00-ns pulse width 3 0 R 0 cm s res M.72cm S N impr 0log dB