Achieving First Pass Success in PCB-Based Filter And Matching Circuit Designs

Transcription

1 Achieving First Ps Success in PCB-Bed Filter And Matching Circuit Designs By Scott Muir, Larry Dunleavy, and Tom Weller Introduction Three fabrication Th paper provides a comparon three fabrication techniques used to techniques to generate generate a simple three element 250 MHz low ps filter (LPF) design meured and modeled to address in-band and out--band response through a low ps filter design. 5GHz. For each three scenarios, filter will be fabricated using values psive lumped elements obtained in ideal LPF design process; it expected that meured data will differ from ideal LPF design due to paritic effects present at high frequencies. Paritic elements are added incrementally to design in order to illustrate progression towards better meured to model agreement; final full paritic LPF model design will contain transmsion line (TL) elements and Modelithics CLR Library models for psive lumped elements. The purpose th work to provide insight into a good, better, best progression through fabrication techniques used to generate and meure a LPF. One objective th work to highlight how progressive modeling paritic effects present at high frequencies will provide better meured to model agreement. The best approach also utilizes on-board RF-probing and related thru-reflect-line calibration, where or two approaches utilize standard coaxial connectorized meurements using a commercial short-open-load-thru calibration along with subsequent connector modeling/de-embedding. The same techniques dcussed herein for a simple filter design apply equally for PCB-bed matching circuit designs and should be interest to anyone committed to improved simulationbed design success. Filter Design Approach The LPF shown in th document one three constant-k Butterworth 1 tee junction filter designs that belong to 5th laboratory signment Wireless Circuits & Systems Lab course fered at USF 2. A Butterworth filter a device designed to have flat a frequency response possible in ps band; for a LPF, ps band exts prior to cut-f frequency. The cut-f frequency a LPF frequency where signal power h been reduced by half ( frequency where signal experiences 3dB loss). Th 3dB loss seen in forward transmsion coefficient (S21) LPF S-Parameters. The S-Parameters LPF can be calculated using ABCD parameter equations derived for a general tee junction formed by three elements with given impedances. Also shown below a table lting capacitor and inductor values calculated to have LPFs with cut-f frequencies 100, 250, and 500 MHz; also 30 High Frequency Electronics Table 1 Calculated and vendor capacitor and inductor values used for three LPF designs.

2 Filter Design range interest to designer also may not be included in S2P data. The steps iteratively modeling paritic affects physical device will be demonstrated later in paper. Fabrication The first fabrication technique used a milling machine, with procedure performed by a University South Florida (USF) Center for Wireless and Microwave Information Systems (WAMI Center) teaching stant. Th let accurate approach to fabrication and labeled good approach. Th technique let accurate since it utilizes soldered slip-on coaxial connectors a means meurement and grounded with a wire soldered between TL and ground plane. Th rudimentary grounding not optimal and used in USF WAMI lab to reduce course costs. The substrate used in th fabrication 59 mil FR4 (Er = 4.3). The second fabrication technique w performed pressionalfigure 1 Fabricated 250 MHz LPFs. Good approach (top image), Better ly by an outside board manufacapproach (middle image), Best approach (bottom image); meurement turer with chemical etching well reference planes indicated by red arrows. plated through via holes; however soldered slip-on coaxial conshown in table are psive values selected from nectors are still used a means meurement. Th vendor daet. Note: psive lumped element valgrounding technique preferred method at high freues used in simulations and fabrication are values quencies since it provides reliable and repeatable fabriobtained from vendor daets for ideal design. cation well minimizing paritic inductance Once ideal design h been obtained, next step present between TL and ground plane. Th approach would be to begin accurately modeling paritic affects labeled better approach. The substrate used in physical device present at high frequencies in order th fabrication 60 mil Rogers 4003C (Er = 3.65). to better predict device behavior. These effects are The third fabrication technique uses same board caused by psive devices, pads each device and plated through via hole technology better mounted on, grounding and transmsion line elesemblies. Th technique labeled best due to ments and coaxial connectors (where applicable). use probe pads allowing ground-signal-ground RF wafer In some ces, a design engineer can obtain S2P data probes to be used a means meurement. The use for psive devices directly from vendor. While se probe tip structures provides yet anor degree reliable data files represent actual meured data for psive device being used in design, re a degree uncer- and repeatable meurements by removing uncertaintainty to how that data w obtained. The fixture used ties that can ext when using soldered slip-on coaxial conby vendor to obtain S2P data may be quite differ- nectors. Related information highlighting importance ent from fixture used by designer. Differences can meurement noninsertable devices can be found in ext in pad dimensions and substrate selection that product note Agilent can drtically affect device performance. The frequency 32 High Frequency Electronics

3 1 Num=1 L L1 R= C C1 C=27 pf L L2 R= 2 Num=2 Figure 2 Ideal LPF. Figure 3 Ideal good LPF containing TL and ground models. Good Approach: Meured vs Model Figures 2 through 4 show transition from ideal design to full paritic design. The dimensions TL elements were obtained from CAD design good LPF. Plots ideal LPF model performance can be seen in Figure 5; Figures 6 and 7 show plots meured data vs modeled performance good LPF. As paritic elements are added incrementally, meured to model agreement improves. However, even in final full paritic model, re are still dcrepancies between meured data and model performance. These dcrepancies can be attributed to less than ideal fabrication techniques used: in house milling on USF campus, use coaxial connectors a means meurement, and using a wire soldered between TL and ground planes. Better Approach: Meured vs Model Figure 8 shows full paritic design; ideal design can be seen in Figure 2. The dimensions TL elements were obtained from CAD design better LPF. Plots better LPF meured data vs modeled performance can be seen in Figure 9; compare th to ideal model performance in Figure 5. For better approach, re h been a drtic reduction in dcrepancies between full paritic LPF model performance and meured data when compared to good approach. Best Approach: Meured vs Model Figure 10 shows full paritic design; ideal design can be seen in Figure 2. The dimensions TL elements were obtained from CAD design best LPF. Plots best LPF meured data vs modeled performance can be seen in Figure 11; once again, compare th to ideal model performance seen in Figure 5. For best approach, agreement between full paritic LPF model performance and meured data h 34 High Frequency Electronics

4 1 2 Ref 2 Ref 1 S2P SNP20 File="Coax_Connector_Model_59mil_FR4.s2p" IND_CLC_0805_002_MDLXCLR1 CLC_0805CS_L6 Sim_mode=0 Tolerance=1.0 MTEE_ADS Tee4 W2=132 mil W3=132 mil IND_CLC_0805_002_MDLXCLR1 CLC_0805CS_L7 Sim_mode=0 Tolerance=1.0 TL37 L=611 mil S2P SNP19 File="Coax_Connector_Model_59mil_FR4.s2p" 5 Num=5 Connector model MTAPER Taper5 W2=20.0 mil L=72.0 mil TL34 L=611 mil Step29 Step30 CLR Library models TL35 L=197 mil Step34 W2=126 mil TL36 Step31 L=197 mil W2=126 mil Step32 CAP_ATC_800B_001_MDLXCLRATC1 ATC_800B_C2 C=27 pf Sim_mode=0 - Full Paritic Model Tolerance=1.0 Pad_Gap=1.6 mm Orient=0 - Horizontal Step33 MTAPER Taper6 W2=20.0 mil L=72.0 mil Connector model 6 Num=6 TL38 L=197 mil Step35 W1=200 mil W2=132 mil L L26 L= nh R= Figure 4 Full paritic good LPF containing TL and ground models well Modelithics CLR models for psive lumped elements (CLR models indicated by red arrows) and connector models (connector models indicated by green arrows). Figure 5 Ideal LPF model performance; S21 (blue line) and S11 (red line). Broad-band (left) and narrow-band (right). improved over better approach and much higher than good approach. Closing Remarks and Conclusions In th document, a comparon w performed three fabrication techniques used to generate a simple three element 250 MHz low ps filter (LPF) design. It w shown that using less than ideal calibration methods and fabrication techniques can lead to major dcrepancies between model performance and meured data. Th document also provided insight into how paritic effects will cause meured data to differ from ideal model performance, especially when viewed at high frequencies. To obtain improved meured to model agreement, proper modeling paritic effects in subsequent design iterations required. Once a full paritic model h been obtained, it possible to tune design in order to achieve ideal LPF performance in desired band. 35

5 Figure 6 Ideal good LPF with TL elements; meured data (symbols) vs model performance (solid line). S21 (blue) and S11 (red); broad-band (left) and narrow-band (right). Figure 7 Full paritic good LPF; meured data (symbols) vs model performance (solid line). S21 (blue) and S11 (red); broad-band (left) and narrow-band (right). Figure 8 Full paritic better LPF containing TL and ground models well Modelithics CLR models for psive lumped elements (CLR models indicated by red arrows) and connector models (connector models indicated by green arrows). 36 High Frequency Electronics

6 Figure 10 Full paritic best LPF containing TL and grounding effects well Modelithics CLR models for psive lumped elements (CLR models indicated by red arrows). Figure 11 Full paritic best LPF; meured data (symbols) vs model performance (solid line). S21 (blue) and S11 (red); broad-band (left) and narrow-band (right). About Authors Scott Muir w previously with Modelithics, Inc. He now with TriQuint Semiconductor in Apopka, Florida. Larry Dunleavy and Tom Weller are with Modelithics. Acknowledgements The milling good LPF w performed by Anand Kumar Santhanakrhna, a USF PhD student. The authors would like to thank Isabella Delgado and Alberto Rodriguez for ir helpful editing comments. The ADS and CAD files sociated with filters shown here are available upon request. References 1 David M. Pozar, Microwave Engineering, Second Edition, pp T. Weller and L. Dunleavy, Wireless and Microwave Education: From Circuits to Systems, Proceedings 1999 European Microwave Conference - Invited Paper, pp Keysight (formerly Agilent) Technologies, Meuring Noninsertable Devices, A new technique for meuring components using 8510C Network Analyzer Product Note. 38 High Frequency Electronics