RF View

RF tutorials

Practical RF guides on S-parameters, Smith charts, impedance matching and measurement, written from handset RF experience.

16 articles
  • Low-Pass to High-Pass Transformation: Swapping L and C2026-11-03The mapping ω → −ωc/ω turns a series-inductor element g into a series capacitor C' = 1/(Z0·ωc·g) and a shunt-capacitor element g into a shunt inductor L' = Z0/(ωc·g). For g = 1, 50 Ω and 1 GHz that gives 3.18 pF and 7.96 nH.
  • Unilateral Figure of Merit: When Can You Ignore S12?2026-11-03The true gain lies between 1/(1+U)² and 1/(1−U)² times the unilateral gain G_TU. For U = 0.1 that is −0.83 dB to +0.92 dB, and for U = 0.05 it is about −0.42 dB to +0.45 dB.
  • Hartley vs Weaver Image-Reject Receivers Compared2026-11-02Both receivers cancel the image by combining quadrature I and Q paths, but the rejection is always finite. For small errors IRR ≈ 4/(ε² + θ²), so a 1% gain error with a 0.01 rad phase error gives about 43 dB.
  • How Much Duplexer Isolation? TX Noise in the RX Band2026-11-02Required isolation equals the TX noise in the RX band minus the receiver noise floor, plus a margin. With -60 dBm of TX noise in 10 MHz, a -99 dBm floor and a 10 dB margin, the duplexer needs 49 dB of TX-to-RX isolation.
  • How to Pick Filter Order From a Rejection Requirement2026-11-02A Butterworth needs N ≥ log10(10^(LAs/10) − 1) / (2·log10(ωs/ωc)), so 40 dB at twice the cut-off needs N = 7. A Chebyshev with 0.5 dB ripple meets the same 40 dB at ratio 2 with N = 5.
  • Scaling a Low-Pass Prototype to 50 Ohm and Real Frequency2026-11-02Scale each prototype element with L = R0·g/ωc and C = g/(R0·ωc). At 1 GHz and 50 Ω, a g = 1, 2, 1 Butterworth becomes a 3.18 pF shunt C, a 15.9 nH series L and a 3.18 pF shunt C.
  • Turning a Low-Pass Prototype Into a Band-Pass Filter2026-11-02The band-pass mapping replaces each series inductor with a series LC and each shunt capacitor with a parallel LC, all resonant at ω0 = sqrt(ω1·ω2). For g = 1, 50 Ω, 1 GHz and Δ = 0.1, the series branch is 79.6 nH with 0.318 pF.
  • Reading a Front-End Module Data Sheet: Key Numbers First2026-11-02Read the band and test conditions first, then insertion loss, isolation and return loss. A 1 dB insertion loss means |S21| ≈ 0.891, so about 79% of the power gets through, and the number only counts under the stated conditions.
  • Test Cable Flexure and Phase Stability After Calibration2026-11-02A 1 cm change of electrical length in a cable with a 0.21 m wavelength shifts phase by about 17 degrees at 1 GHz, and the error grows in proportion to frequency. Keep the cable still after calibration and use phase to estimate how far it moved.
  • VNA IF Bandwidth and Averaging: Noise vs Sweep Time2026-11-02Each 10 dB of noise reduction costs about 10 times the sweep time, whether you cut the IF bandwidth by 10 or average 10 sweeps. Averaging cannot remove drift, systematic errors or crosstalk, so check those first.
  • SOLT Calibration Step by Step: Short, Open, Load and Thru2026-10-31A SOLT calibration measures an ideal short (reflection coefficient -1), an open (+1) and a load (0), then a thru between the ports. The result holds only for the frequency range, IF bandwidth, power level and cable setup used.
  • L-Network Matching: Low-Pass or High-Pass Version?2026-10-31Matching 50 ohm to 200 ohm needs Q = sqrt(3) = 1.732. The low-pass version uses 13.78 nH and 1.378 pF, the high-pass version 1.838 pF and 18.38 nH, and the choice depends on the DC path and the filtering you want.
  • Microstrip Width for 50 Ohm on a 1.6 mm FR-4 Board2026-10-31On FR-4 (er 4.4) a microstrip line reaches 50 ohm at a width-to-height ratio of about 1.9, which is about 3 mm on a 1.6 mm board. Wider strips lower the impedance and narrower strips raise it.
  • When to Put an Attenuator Pad in an RF Chain2026-10-31A fixed pad of A dB passes 10^(A/10) of the power, so a 10 dB pad passes 0.1 and turns the other 0.9 into heat. It can improve return loss by up to twice its attenuation, but it costs signal level and, in front of a receiver, noise figure.
  • How to Read One Line of an S2P File (MA, DB and RI Formats)2026-10-31A two-port S2P data line holds nine numbers: the frequency, then the pairs for S11, S21, S12 and S22 in that order. The option line tells you whether each pair is MA, DB or RI, and a magnitude of 0.99 is about -0.087 dB.
  • How to Open an S2P (Touchstone) File: 6 Ways and a Free Viewer2026-10-31An S2P file is a plain-text Touchstone file holding the two-port S-parameters of a device at many frequencies. You can plot it right here in your browser (the file never leaves your computer), read it in a text editor, or open it in RF View, Python, MATLAB or a spreadsheet.