RF Passive PCB Layout Designer

From a circuit schematic to PCB copper

Distributed passives such as Wilkinson dividers, branch-line, rat-race and edge-coupled couplers, stubs, tapers and coupled-line or stepped-impedance filters are defined by line impedances and electrical lengths. To build them you need physical widths and lengths on a given substrate. This tool converts the impedance of each section into a microstrip width with the Hammerstad-Jensen equations including copper thickness, takes the effective permittivity at your frequency from the Kirschning-Jansen dispersion model, and lays out the structure to scale with every dimension labelled. Coupled sections use the Hammerstad-Jensen even and odd mode model. Lumped-style elements are sized from published formulas: the modified Wheeler and current-sheet expressions for the square and circular spirals, partial inductances for the meander line, and the Alley formula for the interdigital capacitor.

How accurate is it?

The results are first-order starting values. Junction discontinuities, bend corners, pad parasitics, solder mask and etch tolerance are not modelled, and the layouts use ideal quarter-wave sections. For a production design, import the DXF into an electromagnetic simulator, tune the lengths for the centre frequency you want, and then lock the geometry. The element dimensions are most reliable on a substrate with a well-known permittivity, such as Rogers RO4003C or RO4350B; ordinary FR-4 has a permittivity that varies between batches and with frequency.

The 38 layouts you can generate

Pick any of the 38 RF passives below, enter frequency and substrate, and the designer returns a to-scale microstrip drawing with every length and width labelled. Click a name to open it in the tool.

Transmission lines

• Microstrip line: Width and length of a single microstrip line for a target impedance and electrical length, with loss and a cross-section view.
• Quarter-wave transformer: Matches two real impedances with a λ/4 section of impedance √(Zs·ZL). Shows all three line widths and the transformer length.
• Open or short-circuit stub: A shunt stub on a through line for a chosen impedance and electrical length. The open-end fringing field is subtracted from an open stub; a short stub ends in a via.
• Single-stub matching network: Shunt stub tuner: finds where to put the stub on the line and how long it must be to match a complex load to Z0, for an open or shorted stub.
• Mitred 90° bend: Optimally mitred right-angle corner (Douville and James) for a chosen line impedance, with the cut size dimensioned.
• Tapered line transformer: Smooth taper between two impedances: exponential, linear in impedance or linear in width, with the return loss at the design frequency.
• Multi-section quarter-wave transformer: Wideband match between Z0 and a load ZL with N cascaded λ/4 sections, binomial (maximally flat) or Chebyshev (equal ripple). Each section gets its own width and its own λg/4 length.
• Grounded coplanar waveguide (GCPW) with via fence: Centre strip with a coplanar ground strip on each side and a ground plane on the back. The gap is solved for the target impedance, and a via fence along each ground strip ties the top and bottom grounds together.
• Meandered delay line (phase trimmer): A line of a given electrical length folded into a serpentine to fit a small area, with ports aligned on one axis. The number of runs follows from the run length; the run length is then adjusted so that the folded path has exactly the target length.

Transitions and bias

• Radial (butterfly) stub: broadband RF short: A fan-shaped open stub that gives a much wider RF short than a straight λ/4 stub. The radius is solved from the radial-line (Bessel) model so that the stub looks like a short circuit at the design frequency.
• Bias tee: DC block, λ/4 choke and radial stub: RF line with a series DC-blocking capacitor and a DC feed through a high-impedance λ/4 choke that ends in a radial stub. The stub shorts the RF at the end of the choke, which then looks open at the through line.
• Substrate integrated waveguide (SIW) with microstrip tapers: Two rows of plated through-vias form a rectangular waveguide inside the board. The width follows from the cutoff frequency, and a tapered microstrip line launches the TE10 mode at each end.

Dividers and couplers

• Wilkinson power divider (equal split): Two λ/4 arms of impedance √2·Z0 joined by a 2·Z0 isolation resistor. The arms are drawn as arcs so the resistor can bridge the output lines.
• Branch-line coupler (3 dB, 90°): Square hybrid with two Z0/√2 series arms and two Z0 shunt arms, each λ/4. Equal power split with 90° between the outputs.
• Rat-race (ring) hybrid, 180°: Ring of circumference 1.5λ and impedance √2·Z0 with four ports. The sum port splits equally and in phase; the difference port splits equally and 180° apart.
• Edge-coupled coupler (λ/4): Quarter-wave parallel-line directional coupler. Even and odd mode impedances from the coupling, then the line width, gap and length on your substrate.
• T-junction divider with λ/4 transformers: Simple lossless T-junction power divider, equal or unequal split, with a quarter-wave transformer on each output. No isolation between the outputs.
• Gysel power divider (equal split): Equal-split divider whose isolation resistors go to ground, so they can be heat-sunk for high power. Two √2·Z0 input arms, then a Z0 loop of 3λ/4 joining the outputs, with a Z0 resistor to ground at each end.
• Lange coupler (4-finger, wire-bonded): Wide-band 3 dB quadrature coupler of four interdigitated fingers. A four-conductor capacitance model sets the finger width and gap for the coupling you choose; alternate fingers are tied with bond wires at both ends.
• Corporate power divider (binary-tree feed network): Equal-split 1 : N feed network built as a binary tree of T-junctions, for 2, 4, 8 or 16 outputs at a chosen output spacing. Each junction is matched either by a λ/4 transformer in the trunk or by 2·Z0 branches with λ/4 transformers; every output has the same path length, so all outputs are in phase.
• Wilkinson power divider (unequal split): Cohn unequal-split Wilkinson: two λ/4 arms of different impedance, an isolation resistor and a λ/4 output transformer on each side. P3 receives the larger share. Used to feed arrays with tapered amplitudes.
• Branch-line coupler (3 to 10 dB coupling): Square hybrid with the through and coupled powers set by the arm impedances. P3 is the coupled port; the 3 dB case reduces to the standard Z0/√2 and Z0 arms.

Filters

• Parallel-coupled-line band-pass filter: Staggered quarter-wave coupled-line band-pass filter. Chebyshev or Butterworth prototype, every coupled section synthesised on your substrate. The frequency above is the centre frequency.
• Stepped-impedance low-pass filter: Alternating high and low impedance sections from a Butterworth or Chebyshev prototype. The frequency above is the cutoff frequency.
• Hairpin band-pass filter (tapped): Compact band-pass filter of U-shaped half-wave resonators side by side, with tapped input and output lines. Gaps come from the coupling coefficients and the tap height from the external Q.
• Open-stub low-pass filter: Low-pass filter whose shunt capacitors are open-circuit stubs (one per side or split on both sides) and whose series inductors are short high-impedance lines. The frequency above is the cutoff frequency.
• Interdigital band-pass filter (tapped, via-grounded): Band-pass filter of λ/4 resonators side by side, each grounded by a via at alternate ends and open at the other. Gaps come from the coupling coefficients and the tap height from the external Q.
• Open-stub band-stop (notch) filter: Shunt open-circuit λ/4 stubs spaced a quarter wavelength apart on the through line. Each stub is a short circuit at the centre frequency, so the line is blocked there and passes at DC and at 2·f0.

Resonators

• Ring resonator (gap-coupled): Microstrip ring whose mean circumference is a whole number of guided wavelengths, coupled to the feed lines across small gaps. Used to measure εeff and loss, and as the basis of dual-mode ring filters.
• Square loop and open-loop resonator: Square microstrip resonator, either a closed loop one guided wavelength round (n = 1) or the open-loop type, a half-wave line folded into a square with a small gap. Coupled to feed lines across a gap at the sides.
• Half-wave line resonator (end or side coupled): Open-ended straight λ/2 line. Coupled across a gap at each end it forms a band-pass resonator; laid parallel to a through line it forms a band-stop notch. The open-end fringing field is subtracted from the length.
• Quarter-wave shorted resonator (gap-coupled): Line shorted to ground by a via at one end and open at the other, λ/4 long, coupled to a single feed across a gap at the open end. Half the length of a λ/2 resonator, and the first spurious response comes at three times the frequency.
• Stepped-impedance resonator (λ/2 type): Open-ended resonator with a wide, low-impedance centre section and narrow, high-impedance ends. For the same frequency it is shorter than a uniform half-wave line, and the first spurious response moves away from 2·f0.
• Dual-mode square patch resonator: Square patch resonator with two orthogonal, degenerate modes. Feeds at the middle of two adjacent sides excite one mode each; a cut corner couples them, so one patch acts as a second-order band-pass filter.

Lumped-style elements

• Square spiral inductor: Planar square spiral on the top layer with a via and underpass to bring the inner end out. Solve for inductance from geometry, or for the outer size from a target inductance.
• Interdigital capacitor: Series capacitor made of interleaved fingers with equal width and gap. Solve for the finger length from a target capacitance, or for the capacitance from the length.
• Meander-line inductor: Serpentine trace inductor. Inductance from the partial-inductance (Greenhouse) method. Solve for the inductance from the geometry, or for the run length from a target.
• Circular spiral inductor: Round (Archimedean) spiral with a centre via and bottom-layer underpass. Solve for the inductance from the geometry, or for the outer diameter from a target.

Frequently asked questions

What does the RF PCB layout designer calculate?

It converts the impedance and electrical length of each section of a distributed circuit into physical microstrip widths, lengths, gaps and spacings on your substrate, then draws the structure to scale with every dimension marked. You enter the centre frequency, the relative permittivity, loss tangent, dielectric height and copper thickness.

Which file formats can I export?

Every layout exports as an SVG drawing, a DXF file that opens in CAD and EM simulators, and Gerber copper layers for PCB fabrication. A mil display mode is available for US-style fabrication notes.

Can it design a corporate power divider with many outputs?

Yes. The corporate divider builds a binary feed tree for 2, 4, 8 or 16 outputs. You set the output port spacing and the trunk length, choose between a quarter-wave trunk transformer or 2·Z0 branches, and it reports the insertion loss, return loss and bandwidth.

How accurate are the dimensions?

They are first-order starting values from the Hammerstad-Jensen microstrip model with Kirschning-Jansen dispersion. Junction discontinuities and bends are not modelled, so tune the exported DXF in an electromagnetic simulator before fabrication.

Is it free?

Yes. It runs in your browser, needs no sign-up and nothing you type is uploaded.