Spot the mistake in this design: built exercise #4 (draft)¶
Drafted 8 Oct 2026 for issue #227. Nothing has been posted. A BUILT exercise under the "Built exercises" rule in marketing-block-1.md: made up on purpose, ONE planted mistake, never presented as an AI's answer. Same style as #3 (the owner's favourite): a system-level claim that a MagicON T/R budget disproves.
Post 1 (midweek): the puzzle¶
Spot the mistake in this design. 🔎
A made-up teaching example. A 9.4 GHz marine radar shares ONE antenna through a T/R switch. A limiter sits in front of the receiver's low-noise amplifier (LNA). Take every part figure as given. One sentence in the student's notes is wrong. Which one?
Answer on Friday.
Image (figures and notes):
| Item | Figure |
|---|---|
| Transmit leakage reaching the limiter during a pulse | 5.45 dBm |
| Limiter Qorvo TGL2209-SM | starts limiting at 20.90 dBm |
| LNA Qorvo QPA2609 | input 1 dB compression point −7 dBm |
Student's notes: "The limiter is rated far above the leakage, so it survives." (true) "And it clamps the leakage, so the LNA stays linear during each pulse." (planted mistake)
Post 2 (Friday): the answer¶
The second sentence is wrong. 5.45 dBm is far BELOW the 20.90 dBm where this limiter starts to clamp, so the limiter never acts: the leakage passes straight through to the LNA. The LNA compresses at −7 dBm, so it is driven 12.05 dB into compression on every pulse. It is not damaged, but it is deaf until it recovers.
A limiter protects the receiver from DAMAGE. Keeping it out of compression during transmit is a different job, called "blanking": a fast switch shuts the receiver off during each transmit pulse and turns it back on right after, so the leakage never reaches the LNA.
Lesson: check where a limiter starts cutting, not just where it breaks. A limiter can be completely safe and still do nothing, because it only cuts signals above its threshold (20.90 dBm here) and lets anything smaller through untouched.
Proof (MagicON result, for the answer image)¶
MagicON design_tr_module, run 2026-10-08 through the MagicON AI connector (live public server).
Inputs: frequencyGhz 9.4, targetOutputPowerDbm 40, transmitDutyCyclePercent 10, application
marine_radar, topology tr_switch; grounding caller_asserted (a made-up brief).
budgets.isolation: leakageDbm 5.45 (bindingPathdirect), headroomDb 34.55 against the limiter's 40 dBm CW rating, verdictsafe(the "survives" sentence is true).budgets.leakage: limiterOutputDbm 5.05, leakageTermpass_through, lnaInputP1dbDbm −7, lnaOverdriveDb 12.05, verdict lna_compressed. Disclosure: "Incident power of 5.45 dBm is below this limiter's clamping onset (20.90 dBm), so it is not limiting. The reported output is PASS-THROUGH, not the spike leakage rating of 20.50 dBm."- Warning, verbatim in part: "a limiter's clamping threshold protects the LNA from DAMAGE, and keeping the receiver out of compression during transmit is what blanking is for, not what a limiter does." Also: "During a transmit pulse the receiver is deaf, not damaged".
- Shared switch QPC2040 (isolation 36 dB at 10 GHz per its record), LNA QPA2609, limiter TGL2209-SM.
The planted change: none of the figures; only the second sentence of the notes is wrong.
Before posting¶
- The 5.45 dBm leakage assumes a 1.5:1 antenna (the tool's assumption, disclosed); it is the direct path that binds here, not the reflection.
- Not checked against datasheets: the part figures (the engine's). The post says "take every part figure as given".
- Images:
spot-the-mistake-built-04-puzzle.png,spot-the-mistake-built-04-answer.png, built bydocs/scripts/make-spot-the-mistake-built-04.py. - Check posted-log.md first; log both posts after posting.