A Dual-Band Through-the-Wall Imaging Radar Receiver Using a Reconfigurable High-Pass Filter

JOURNAL OF ELECTROMAGNETIC ENGINEERING AND SCIENCE, VOL. 16, NO. 3, 164~168, JUL. 2016 http://dx.doi.org/10.5515/jkiees.2016.16.3.164 ISSN 2234-8395 (Olie) ISSN 2234-8409 (Prit) A Dual-Bad Through-the-Wall Imagig Radar Receiver Usig a Recofigurable High-Pass Filter Duksoo Kim * Byugjoo Kim Sagwook Nam Abstract A dual-bad through-the-wall imagig radar receiver for a frequecy-modulated cotiuous-wave radar system was desiged ad fabricated. The operatig frequecy bads of the receiver are S-bad (2 4 GHz) ad X-bad (8 12 GHz). If the target is behid a wall, wall-reflected waves are rejected by a recofigurable Gm C high-pass filter. The filter is desiged usig a high-order admittace sythesis method, ad cosists of trascoductor circuits ad capacitors. The cutoff frequecy of the filter ca be tued by chagig the referece curret. The receiver system is fabricated o a prited circuit board usig commercial devices. Measuremets show 44.3 db gai ad 3.7 db oise figure for the S-bad iput, ad 58 db gai ad 3.02 db oise figure for the X-bad iput. The cutoff frequecy of the filter ca be tued from 0.7 MHz to 2.4 MHz. Key Words: Dual-Bad Receiver, FMCW Radar System, Gm C Filter, Recofigurable High-Pass Filter, Through-the-Wall Radar. I. INTRODUCTION Imagig radar uses radio waves to geerate a image of a distat target. Compared to covetioal optical imagig systems, radar-based imagig ca detect a target regardless of weather or time of day. Although the resolutio of the target image is lower tha with optical imagig, radio waves ca peetrate objects. Therefore, radar-based imagig is suited to security applicatios, because the system ca detect a target hidde behid a obstacle. This type of radar system ca be classified as through-the-wall radar [1 3]. Several through-the-wall imagig systems have bee researched; however, these are dedicated to a specific frequecy bad that determies the characteristics of the system. For example, the S-bad (2 4 GHz) is useful for detectig a target i a lossy medium, sice the S-bad sigal has good peetratio characteristics. However, it also has the drawback of limited image resolutio due to its relatively low frequecy. O the other had, the X-bad (8 12 GHz) ca achieve fier resolutio tha the S-bad, but the detectio rage of the target is restricted due to its high loss. Thus, a dual-bad imagig system is a attractive alterative, sice it ca flexibly select a suitable frequecy accordig to the situatio [4]. I this paper, a imagig radar receiver is desiged as part of a dual-bad through-the-wall imagig radar system. This receiver frot-ed ca operate i the S-bad or X-bad, ad has the characteristic of atteuatig wall-reflected waves whe the target is located behid a wall. To reject uwated reflected waves, a high-pass filter is desiged as a key part of the receiver. The filter has recofigurable frequecy respose by chagig the trascoductace of the circuit. I Sectio II, the desig of the overall imagig radar receiver system is des- Mauscript received November 9, 2015 ; Revised Jue 20, 2016 ; Accepted Jue 25, 2016. (ID No. 20151109-058J) School of Electrical Egieerig ad Computer Sciece, Istitute of New Media ad Commuicatios, Seoul Natioal Uiversity, Seoul, Korea. * Correspodig Author: Duksoo Kim (e-mail: dskim@ael.su.ac.kr) This is a Ope-Access article distributed uder the terms of the Creative Commos Attributio No-Commercial Licese (http://creativecommos.org/liceses/ by-c/3.0) which permits urestricted o-commercial use, distributio, ad reproductio i ay medium, provided the origial work is properly cited. c Copyright The Korea Istitute of Electromagetic Egieerig ad Sciece. All Rights Reserved. 164

KIM et al.: A DUAL-BAND THROUGH-THE-WALL IMAGING RADAR RECEIVER USING A RECONFIGURABLE HIGH-PASS FILTER cribed. Sectio III presets the implemetatio of the recofigurable high-pass filter. Sectio IV presets measuremets obtaied from the fabricated receiver system, ad Sectio V cocludes the paper. II. IMAGING RADAR RECEIVER DESIGN The proposed receiver system of a through-the-wall imagig radar is show i Fig. 1. The iput frequecy bads of the receiver are S-bad ad X-bad. The iput sigal travels from the atea, ad is amplified by a low-oise amplifier (LNA). The ateas are dedicated to oe frequecy bad, so the RF parts of the receiver are desiged for each bad separately. A switchig device is used to select the frequecy bad electrically. The amplified sigal is applied to a mixer ad directly dowcoverted to the basebad. The mixer has widebad characteristics to hadle the dual-bad iput, ad the receiver shares the circuit from the mixer stage to the basebad. The local oscillator (LO) sigal of the mixer is a frequecy-modulated cotiuous-wave (FMCW) sigal, which comes from the trasmitter chirp geerator. I a FMCW imagig radar system, the trasmitted wave is reflected from the target ad eters the receiver atea. The time-delayed received sigal ad the trasmitted sigal are multiplied i a mixer to produce a basebad beat frequecy, which is proportioal to the distace from the target [5]. Whe the target is located behid a wall-like obstacle, the wall-reflected wave appears at a lower frequecy tha the target-reflected wave. Therefore, the system requires a filter to detect the target o the other side of the wall [1, 6]. Moreover, if there is isufficiet isolatio betwee the trasmittig ad receivig ateas, the trasmitted sigal directly eters the receiver ad geerates a large basebad sigal. This directlycoupled sigal must be rejected, sice the large basebad sigal ca saturate the receiver circuits. To reject the direct couplig sigal ad the wall-reflected sigal, the filterig characteristics of the receiver become importat. I [1], a badpass filter is used to reject uwated sigals. However, sice the implemeted filter has a fixed frequecy respose, the system operatio should be adjusted to the filter specificatio. The receiver system i [1] employs heterodye architecture to adjust the itermediate frequecy (IF) to the ceter frequecy of the filter. The radar system i [6] uses a bak of badpass filters for rejectig the uwated sigal, ad a appropriate filter block is selected depedig o the distace to the wall ad the target. Rather tha adjustig the system operatio to the filter specificatio or usig a bak of filters, this paper proposes a receiver with recofigurable highpass filter to reject the uwated sigal. The proposed filterig topology employs a high-pass filter to reject the direct couplig sigal ad the wall-reflected sigal. The basebad sigal of the mixer output passes through the amplifier stage show i Fig. 2. The amplifier stage applies a secod-order high-pass filter to reject the direct couplig sigal. The distace betwee the trasmittig atea ad the receivig atea geerates a direct couplig sigal at a frequecy below 100 khz. Therefore, the cutoff frequecy is set to 100 khz, ad this basebad amplifier also amplifies those sigals higher tha the cutoff frequecy, with a gai of 15 db. Eve if the direct couplig sigal is rejected, the wall-reflected wave remais. Therefore, a additioal high-pass filter is ecessary to atteuate the wall-reflected wave. This filter should be desiged to be able to cotrol its cutoff frequecy, sice the distace betwee the radar system ad the wall ad target ca vary. This additioal high-pass filter, with its cofigurable cutoff frequecy, is located after the basebad amplifier stage. The cutoff frequecy ca be adjusted to atteuate the wall-reflected wave ad preserve the target-reflected wave. The output sigal of the filter is applied to the basebad buffer circuit i Fig. 1, ad the buffered sigal becomes the fial basebad voltage sigal. The resolutio of the dual-bad radar system is set to 75 cm for the S-bad ad 25 cm for the X-bad. To achieve this Fig. 1. Block diagram of imagig radar receiver system. Fig. 2. Block diagram of basebad amplifier stage. 165

JOURNAL OF ELECTROMAGNETIC ENGINEERING AND SCIENCE, VOL. 16, NO. 3, JUL. 2016 resolutio, a trasmittig FMCW sigal badwidth of 200 MHz at the S-bad ad 600 MHz at the X-bad are eeded, ad the receiver has to cover the required badwidth. The X- bad chirp sigal geerator ca be implemeted by a digital phase-locked loop (PLL) i [7], which has a chirp sigal badwidth of 600 MHz with ceter frequecy of 9.2 GHz. The S-bad chirp sigal ca be obtaied usig a divide-bythree circuit at the output of the X-bad chirp sigal geerator. The target rage is 3.5 m to 20 m from the radar system. The target detectio rage sets the cutoff frequecy of the highpass filter, which becomes 0.7 MHz to 2.4 MHz. Sice this radar system assumes that the distace from the system to the wall is kow, the high-pass filter ca be adjusted to appropriate cutoff frequecy. III. DESIGN OF RECONFIGURABLE HIGH-PASS FILTER To atteuate the wall-reflected wave, a recofigurable G m C high-pass filter [8, 9] is desiged. A filter of order N ca be sythesized usig N trascoductors ad N capacitors. A high-order admittace sythesis method for high-pass filter formatio is illustrated i Fig. 3. With iput voltage of V i ad iput curret of I i, the admittace Y i see from the iput becomes I G G G G G Yi() s G V sc s C C G G G. s C C C i m( 1) m( 2) m( 1) 2 i 1 m1 m2 1 2 3 Addig capacitor C 1 to the high-order admittace circuit forms a high-pass filter of order N. The iput voltage is divided ito C 1 ad Y i, such that the trasfer fuctio of the filter becomes Vout sc1 H() s Vi sc1 Yi () s G G 1 ( 1) G 2 s s s s C1 CC 1 G ( 2) G ( 1) G 3 Gm 1Gm2 G s. CC 1 1C CC 1 2 C (1) (2) Fig. 4. Block diagram of fourth-order high-pass filter. A fourth-order high-pass filter (Fig. 4) is desiged by Eq. (2). If the trascoductor circuits used i the filter have the same trascoductace value (G m ), the trasfer fuctio of the filter is expressed as 4 s H() s s c s c s c s c 4 3 2 41 42 43 44 where the ormalized coefficiets of the trasfer fuctio are 2 3 4 Gm Gm Gm Gm c41, c42, c43, c44. C1 CC 1 4 CCC 1 3 4 CCCC 1 2 3 4 (4) Eqs. (3) ad (4) show that the frequecy respose of the high-pass filter ca be scaled proportioally by chagig the G m value. The recofigurable high-pass filter is desiged to have Chebyshev frequecy respose, ad the coefficiets of the trasfer fuctio are give by c 41 = 0.2756, c 42 = 0.7426, c 43 = 1.4539, ad c 44 = 0.9528. The maximum cutoff frequecy of the filter is set to 2.4 MHz. The capacitor values to accomplish the maximum cutoff frequecy are C 1 = 270 pf, C 2 = 120 pf, C 3 = 39 pf, ad C 4 = 27 pf. The G m values of the trascoductors are cotrolled by a referece curret I ref. Fig. 5 shows the referece curret source ad a curret mirror circuit. The curret of the tuable referece curret source is duplicated by PMOS trasistors. To maitai idetical curret I ref, all trasistors have the same (3) Fig. 3. Block diagram of high-pass filer usig high-order admittace sythesis method. Fig. 5. Block diagram of referece curret source ad cotrol circuits for trascoductors. 166

KIM et al.: A DUAL-BAND THROUGH-THE-WALL IMAGING RADAR RECEIVER USING A RECONFIGURABLE HIGH-PASS FILTER width, but trasistor M 2 should have a fourfold chael width. The value of the resistor R 1 is four times larger tha the resistor R 2 to set the appropriate DC curret. IV. MEASUREMENT RESULTS A dual-bad imagig radar receiver was fabricated with recofigurable high-pass filter. The receiver was implemeted o a FR-4 prited circuit board (PCB) ad cosists of commercial devices. The size of the receiver is 13.7 cm 7.5 cm. Fig. 6 shows a photograph of the receiver. The VDDs for the commercial devices are 3.5 V ad 5 V. The LNAs of each bad are cascaded to have appropriate gai at the RF ad low oise figure. The X-bad LNA is selected to have higher gai tha the S-bad LNA, sice the X-bad sigal experieces higher loss. A sigle-pole doublethrow switch is used to select the RF iput frequecy path. The widebad mixer covers the S-bad ad X-bad sigals. The basebad amplifier stage i Fig. 2 is implemeted, ad commercial device OPA211 is used as the operatioal amplifier. I the recofigurable high-pass filter, LT1228 devices are used as the trascoductors. The referece curret source i Fig. 5 is realized by a digital-to-aalog coverter (DAC) with curret output. The trasistor M 2 is implemeted by a parallel coectio of four trasistors to have a fourfold chael width. Frequecy resposes measured from the receiver are show i Fig. 7. Sigle-toe RF ad LO sigals are applied to the receiver, ad the basebad sigal at 5 MHz is measured. The S-bad shows 44.3 db gai at 2.8 GHz. I the X-bad, gai reaches 58 db ad maitais a flat respose over a wide frequecy rage. I both frequecy bads, the desired badwidths of 200 MHz ad 600 MHz are achieved. Fig. 8 shows the coversio gai ad oise figure (NF) measuremets by sweepig the basebad frequecy. The mi- (a) (b) Fig. 7. (a) S-bad gai of the receiver. (b) X-bad gai of the receiver. (a) (b) Fig. 8. (a) S-bad coversio gai ad oise figure (NF) of the receiver. (b) X-bad coversio gai ad NF of the receiver. Fig. 9. Frequecy respose of the recofigurable high-pass filter. Fig. 6. Photograph of the imagig radar receiver. imum NFs of the receiver i each bad are 3.7 db ad 3.02 db, respectively. The frequecy respose of the recofigurable high-pass filter part is show i Fig. 9. The graph shows the chage of cutoff frequecy with the cotrol of the bias voltage. The cutoff frequecy of the filter ca be tued from 0.7 MHz to 2.4 MHz, with bias voltage of 0 V to 2 V. The locatio of the target ad the wall are assumed for two cases. The first case assumes the wall is located less tha 1.5 m away, ad the target rages from 7 m to 20 m. The secod case sets the wall s locatio at 1.5 m to 4 m, ad the target from 10 m to 20 m. The filter with bias voltage of 0.5 V correspods to the first case, ad the bias voltage of 1.5 V 167

JOURNAL OF ELECTROMAGNETIC ENGINEERING AND SCIENCE, VOL. 16, NO. 3, JUL. 2016 correspods to the secod case. I both cases, the atteuatio of the wall-reflected wave exceeds 20 db, compared to the target-reflected wave. V. CONCLUSION A dual-bad receiver of a through-the-wall FMCW imagig radar system was desiged, ad a recofigurable high-pass filter was implemeted as a key part of the receiver. The highpass filter stage is desiged usig a high-order admittace sythesis method, ad cosists of trascoductor circuits ad capacitors. A recofigurable high-pass filter-based receiver system was desiged, which ca operate at S-bad ad X-bad frequecies. The high-pass filter ca be tued to reject wallreflected waves at low frequecies, eablig effective target detectio. This work was supported by ICT R&D program of MSIP/IITP (No. B0717-16-0045, Cloud based SW platform developmet for RF desig ad EM aalysis). REFERENCES [1] G. L. Charvat, L.C. Kempel, E. J. Rothwell, C. M. Colema, ad E. L. Mokole, "A through-dielectric radar imagig system," IEEE Trasactios o Ateas ad Propagatio, vol. 58, o. 8, pp. 2594 2603, 2010. [2] P. H. Che, M. C. Shastry, C. P. Lai, ad R. M. Narayaa, "A portable real-time digital oise radar system for through-the-wall imagig," IEEE Trasactios o Geosciece ad Remote Sesig, vol. 50, o. 10, pp. 4123 4134, 2012. [3] J. Laviada, A. Arboleya, F. Lopez-Gayarre, ad F. Las- Heras, "Broadbad sythetic aperture scaig system for three-dimesioal through-the-wall ispectio," IEEE Geosciece ad Remote Sesig Letters, vol. 13, o. 1, pp. 97 101, 2016. [4] V. Jai, F. Tzeg, L. Zhou ad P. Heydari, "A sigle-chip dual-bad 22 29-GHz/77 81-GHz BiCMOS trasceiver for automotive radars," IEEE Joural of Solid-State Circuits, vol. 44, o. 12, pp. 3469 3485, 2009. [5] M.I. Skolik, Itroductio to Radar Systems, 3rd ed. New York: McGraw-Hill, 2001. [6] G. L. Charvat, Small ad Short-Rage Radar Systems. New York: CRC Press, 2014. [7] S. Ryu, H. Yeo, Y. Lee, S. So, ad J. Kim, "A 9.2 GHz digital phase-locked loop with peakig-free trasfer fuctio," IEEE Joural of Solid-State Circuits, vol. 49, o. 8, pp. 1773 1784, 2014. [8] M. Olsak, L. Matejicek, K. Vrba, ad Z. Smekal, "Realizatio of Nth-order electroically tuable highpass filter employig oly N OTAs," i Proceedigs of 10th Iteratioal Coferece o Telecommuicatios (ICT), Tahiti, 2003, pp. 671 676. [9] D. Kim, B. Kim, ad S. Nam, "A trascoductor ad tuable G m C high-pass filter liearizatio techique usig feed forward G m3 cacellig," IEEE Trasactios o Circuits ad Systems II, vol. 62, o. 11, pp.1058 1062, 2015. Duksoo Kim Byugjoo Kim received a B.S. degree i electrical egieerig from the Seoul Natioal Uiversity, Seoul, Korea, i 2011, ad is curretly workig toward a Ph.D. His mai research iterests are widebad RF receivers ad radar systems. received a B.S. degree i electrical egieerig from the Seoul Natioal Uiversity, Seoul, Korea, i 2009, ad is curretly workig toward a Ph.D. His mai research iterests are RF compoets ad radar systems. Sagwook Nam received a B.S. degree from Seoul Natioal Uiversity, Seoul, Korea, i 1981; a M.S. degree from the Korea Advaced Istitute of Sciece ad Techology (KAIST), Seoul, Korea, i 1983; ad a Ph.D. from The Uiversity of Texas at Austi, Austi, TX, USA, i 1989, all i electrical egieerig. From 1983 to 1986, he was a researcher with the Gold Star Cetral Research Laboratory, Seoul, Korea. Sice 1990, he has bee a Professor at the School of Electrical Egieerig ad Computer Sciece, Seoul Natioal Uiversity. His research iterests iclude aalysis/desig of electromagetic (EM) structures, ateas, ad microwave active/passive circuits. 168