Technieal Feature Comparing Microstrip and CPW Performance By buildingabetter electromagneti.c (EM) si,mulation madel, whl,ch includes the fficts of a PCB's m.etal surface roughness, microstrip and coplanar u-;aaeguide, ci,rcuits can be closely compared to find the best fit for different applications. f atchins a microware transmission- will be compared. Further analysis will be per- I tin" te?hnology to an application re- formed with the aid of electromagnetic (EM) l- quires carelul consideration of more models and EM simulation softwaie. The soft- than a few factors. Depending on the require- ware modeling will help validate the measured ments of an application, high-frequency circuit results and also show how effective software designers may be concerned with loss budgets, modeling can alleviate concer-rrs, when using propagation mode issues, radiation losses and new transmission-line approaches an&or cir- electromagnetic interference (EMI), and even cuit topologies. the printed-circuit-board (PCB) assembly lo- Microstrip and CPW formats are often se- gistics and the relative difficulty of adding com- lected over other high-frequency transmission- ponents to a PCB. Microstrip has been one of line options, such as stripline, due to their the most popular microwave transmission-line simolicitv.simplicity. StriolineStriplineStri can deliver excellent hish-high- formats for decades and is well characterized. [requency performance, with good noise im- Coplanar waveguide (CPW) transmission lines munity and isolation between adjacent circuit have also been used extensively in microwave traces. But it is also more dlfficult and expen- PCB applications, although they are not as sive to fabricate than microstrip or CPW Strip- well understood as microstrip lines. Typi- line is essentially a flat metal transmission line cally, conductor-backed coplanar waveguide between two ground planes, with the ground (CBCPW) circuits are often used in conjunc- planes separated by a dielectric substrate ma- tion with microstrip in microwave circuit terial. The width of the transmission line, the designs. A common approach is the use of thickness of the substrate, and the relative di- CBCPW in the circuit's signal launch area, electric constant of the substrate material de- transitioning to microstrip for the remainder of termine the characteristic impedance of the the circuit to enable simple component place- transmission line. Difficulties with stripline ment and PCB assembly. To help designers understand differences between microstrip and CPW transmission- ]ouN CooNnon Iine approaches, measurement data from dif- Rogers Corp., Chandler, AZ ferent test circuits fabricated with the same, BmeN Reurro well-known commercial substrate material Sonnet Software lnc., North Syracuse, NY MrcRowAVE JOURNAI I JULY 2012 k*hnFc*i Fsatur* hybrid transverse- circuit at the same frequencies. A magnetic modes are review of the practical tradeoffs of also possible with via placement for CBCPW circuits is microstrip, but these available in the literature.r Figure 2 modes are some- offers an overview of signal loss (S21) times the result of performance for microstrip, coplanar- undesired spurious launched microstrip, and CBCPW wave propagation. circuits fabricated on 3O-mil-thlck L nlg. I Cross-sectional aieu of a microstrip line (a) and. three- In general, CBCPW R04350BrM circuit-board material dimensional oieu; o.f a CBCPW line (b). circuits offer propa- from Rogers Co1p. gation behavior sim- GCPWG refers to a grounded co- include ground planes that must be ilartothatof microstripcircuits. planar waveguide and is actually the shorted together, requiring electrical For both microstrip and CBCPW same configuration as CBCPW. The via connections between the tr,vo met- circuits, spurious parasitic wave prop- top ground microstrip configuration al groundplanes andthe lackof direct agation can be a problem. As a gen- is essentially a coplanar-launched mi- access to the signal layer for compo- eral mle, the circuit geometry (that is crostrip circuit - a microstrip circuit nent mounting. Stripline's second its cross-sectional features) for either with a CBCPW configuration in the ground plane also results in narrower transmission-line approach should connector signal launch area. The transmissionline widths, for a given be less than 45' long at the highest cuwe-fit data for microstrip and co- substrate thickrress and characteristics operating frequency of interest- For planarJaunched microstrip are taken impedance, than for microstrip. microstrip, the circuit parameters of from the literature.l The traces reveal In contrast, microstrip and CPW concern include the thickness of the some interesting traits to consider for circuits feature an exposed signal substrate (that is the distance between the different transmission lines. For layer, greatly simplifying compo- the signal and ground planes) and the example. CBCPW tlpically suffers nent assembly on the PCB, Figure I width of the signal conductor (trans- higher loss than microstrip or copla- shows simple drawings of microstrip mission line width). For CBCPW, nar-launched microstrip. The GSG and CBCPW transmission lines. The attention must be paid to those two configuration of the CBCPW copla- microstrip circuit has a signal con- parameters, as well as to the distance nar layer exhlbits hlgher conduitor ductor on the top of the dielectric between the GSG spacing on the co- loss than microstrip-based circuits. substrate and a ground plane on the planar layer. SUll, the loss for CBCPW follows a bottom. In a CBCPW circuit, a copla- For proper grounding, CBCPW constant slope, while the loss curves nar layer with ground-signal-ground circuits employ vias to Connect the for microstrip and coplanar-launched (GSG) configuration replaces the sig- topJayer coplanar ground planes and microstrip undergo slope transitions nal layer of microstrip. The CBCPW the bottom-layer ground plane. The at approximately 27 and 30 GHz, circuit's top ground planes are tied to placement of these vias can be critical respectively. These loss transitions the bottom ground plane by means of for achieving the desired impedance are associated with radiation losses. vias. CBCPW is sometimes known as and loss characteristics, as well as for With proper spacing and via spacing, grounded coplanar waveguide. suppressing parasitic wave modes. CBCPW can be fabricated with mini- In terms of wave propagation, When grounding vias are effectively mal radiation loss. microstrip transmissionline circuits positioned in a CBCPW circuit, a In wideband applications, disper- generally operate in a quasi trans- much thicker dielectric substrate can sion can be important. Microstrip verse-electromagnetic (TEM) mode. be used at higher frequencies than transmission lines are dispersive by Hybrid transverse-electric (TE) and would be possible for a microstrip nature: the phase velocity for EM waves is different in the air above the signal conductor than through the dielectric material of the substrate. CBCPW circuits can achieve much o less &spersion when there is tight coupling at the GSG interfaces on -t.oo the coplanar layer, since more of the -2.Cb. E-field occurs in air to reduce the ef- a! -r.oo" fective inhomogeneity of wave travel ::4.OO through different media. ? *r.6O Using proper design techniques, .6,0O CBCPW circuits can achieve a much wider range of impedances mi- -ZOO than crostrip circuits. applica- .-8-00, In ad&tion, tions where crosstalk may be a con- cern, circuit performance can benefit ,r ,.'' r i , , . .r ,r , "' ., : ..'r.,1;', r.i;:tiilt:.:.ti=::.t' :::,ii-,ffF1l.:.ii!:li+:l#;:i+ii?,: from the coplanar ground plane sepa- rtg.flg. zZ Comparison.romparxson oJ resttest 2y2' CCPWG,2/2" anrl,2%" ration of CBCPW's neighboring signal ^L of ilatafor top grounil microstrip straight microstrip test boards conductors. Due to their significantly 76 MrcRowA\1E JOURNAL I JULY 2012 T**hnfcml ilem?*r* fective dielectric constant increases as There is also a real-life issue af- ED = 3.0 pm RMS roughness ED = 1.5 pm Rlttl5 the surface of the copper fecting most PCB circuits and espe- RT = 0.7 pm RMS increases, as indicated by copper sur- cially CBCPW, which can cause more RT = 0.5 pm RMS faces with higher root-mean-square variation 49' in circuit performance due (RMS) roughness values. to standard fabrication effects. This 5tU. z. !l* In addition to obserued dielectric is the conductor trapezoidal effect, 66 constant effects, surface rough- "edge uZ the or profile," where the PCB >o ness of a microstrip is known to impact conductors are ideally rectangular in buql insertion loss performance.3-7 The cross*sectional r a view but the actual r :..{O:r::rr5p gl lO 2O 3O.: i.6o. i topology of the circuit may be more circuits are trapezoidal in shape. This FnEauENCY icHt'i or less prone to such copper surface can cause the current densiW in the roughness effects, simply due to cur- coplanar GSG area to vary; 'an ldeai A mg. S Effecti,ae dielectric constant of a I rent and distribution within mil LCP lami,nate uith a 50 dl microstrip line E-field rectangular conductor structure will with different surface roughness. the circuit. For example, the copper have more current density up the surface roughness has less effect on a sidewalls of the adjacent conductors reduced radiation losses, dispersion tightly coupled CBCPW transmission in this region, whereas the trapezoi- and parasitic wave mode propaga- line than on a microstrip. In a CB- dal structure will have more current tion, CBCPW circuits are often used CPW circuit, the current and E-field density at the base (copper-substrate at much higher frequencies than mi- are tightly maintained within the GSG interface). When there is more cur- crostrip circuits. At millimeter-wave on the coplanar layer. For a microstrip rent density at the base due to the frequencies, for example, it is often circuit, the field and current move trapezoidal effect, the copper surface that a simple wire-bonded air bridge more toward the bottom of the metal, roughness will have more influence on will be used to connect the ground where the roughness lies.