RF circuits and components
RF circuits and components
This domain tests passive and active RF design: impedance matching, S-parameters, noise, compression, intermodulation, filters, and mixers. The exam favors cascade calculations and practical mismatch effects.
Core rules
- At a 50-ohm reference, S11 is input reflection and S21 is forward gain. Return loss=-20log|S11|; VSWR follows from |Gamma|.
- Cascaded noise figure: F=F1+(F2-1)/G1+(F3-1)/(G1G2)+...; use linear gain and noise factor, then NF=10logF.
- Thermal noise density is -174 dBm/Hz at 290 K. Sensitivity is -174+NF+10logB+required SNR.
- P1dB marks 1 dB gain compression; IP3 predicts third-order products. A 1 dB increase in input tone raises IM3 by 3 dB.
- An L-network matching Rhigh to Rlow has Q=sqrt(Rhigh/Rlow-1); a quarter-wave transformer uses Z0=sqrt(Zin ZL) for real impedances.
Worked example
An LNA has NF=1.5 dB and gain=15 dB; a following mixer has NF=8 dB and gain=10 dB. Convert: F1=1.413, G1=31.62, F2=6.31. F=1.413+(6.31-1)/31.62=1.581. NF=10log1.581=1.99 dB. The mixer adds only about 0.5 dB because the LNA gain suppresses later contributions. Reversing the order would give a much worse cascade.
Traps
Adding dB noise figures directly, forgetting to convert gain to linear in Friis, confusing voltage and power dB, and ignoring filter insertion loss ahead of the LNA. For IP3, track whether input or output intercept is quoted; output IP3 equals input IP3 plus gain.