Cascade Analysis
Understanding Cascade Analysis
Cascade analysis is the RF system engineer's primary tool for predicting system performance. By modeling each component and calculating the cascaded result, the engineer can optimize component selection and placement before committing to hardware.
Cascade Equations
G_total = G1 x G2 x G3 x ... (linear multiply)
G_total(dB) = G1(dB) + G2(dB) + ...
Cascaded NF (Friis):
NF_total = NF1 + (NF2-1)/G1 + (NF3-1)/(G1*G2)
Cascaded IP3:
1/OIP3_total = 1/OIP3_1 + 1/(OIP3_2/G1) + ...
Cascade Optimization
- Place lowest-NF, highest-gain stage first (LNA) to set system NF.
- Place highest-IP3 stage last (where signal levels are highest).
- Distribute gain to avoid compression at any stage.
- Use attenuators between stages to improve dynamic range (trade sensitivity for linearity).
Frequently Asked Questions
What is cascade analysis?
Cascade analysis calculates cumulative gain, NF, and IP3 through every stage of an RF chain. It uses the Friis equation for noise and IP3 cascade equation for linearity to predict system sensitivity and dynamic range.
Which stage dominates noise?
The first stage dominates if it has sufficient gain (15-20 dB). With 20 dB first-stage gain, the second stage NF contributes only 1% of its value to the system NF. This is why the LNA is the most important component for sensitivity.
Which stage dominates linearity?
The last stage (where signal levels are highest) typically dominates IP3. The last amplifier or mixer sees the highest signal power and generates the most intermodulation products. High IP3 at the output stage is critical.