- 2072 Asoj · 5 marks
Short note: LNA cavity device / insertion loss
Short answer
The insertion loss method designs filters from a specified power loss ratio PLR = 1/(1 − |Γ|²); choose Butterworth or Chebyshev response, find order N, read prototype g-values, then scale and transform and realise with high-Q cavity resonators. Low loss before an LNA is essential because filter insertion loss adds directly to the system noise figure.
Write a short note on LNA cavity device inserting loss method for filter designing.
The question is read as a short note on the insertion loss method of filter design, as used for the cavity and other filters around a low-noise amplifier (LNA) front end.
Insertion loss method: the filter is synthesised from a chosen power loss ratio:
PLR = 1/(1 − |Γ(ω)|²), IL = 10 log PLR (dB)
Butterworth: PLR = 1 + k²(ω/ωc)^(2N)
Chebyshev: PLR = 1 + k²·TN²(ω/ωc)
Steps:
- Specify cut-off/band, ripple, stopband rejection and R0.
- Choose the response and order N.
- Take low-pass prototype values gk from tables.
- Scale and transform (LP → BP for a receiver pre-select filter).
- Realise with transmission lines, coupled lines, or cavity resonators (high Q), then tune.
Why it matters for an LNA: a band-pass filter is placed before or after the LNA to reject out-of-band interferers and image signals. Filter loss before the LNA adds directly to the system noise figure (NF_total ≈ IL_filter + NF_LNA in dB), so a low-loss, high-Q cavity filter is preferred at the input. The insertion loss method lets the designer choose the lowest order that meets the rejection need, keeping passband loss small.
| Response | Benefit | Cost |
|---|---|---|
| Butterworth | Flat passband | Slower roll-off |
| Chebyshev | Sharp roll-off | Passband ripple |
✓ Answer checked by Claude Opus 5.5 (high effort)
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