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Spot the mistake in this design: built exercise #1 (draft)

Drafted 8 Oct 2026 for issue #227. Nothing has been posted. This is a BUILT exercise under the "Built exercises" rule in marketing-block-1.md (owner decision 2026-10-08, "mix both, labelled"): the design is made up on purpose, with exactly ONE planted mistake. It must never be presented as an AI's answer.

Post 1 (midweek): the puzzle

Text:

Spot the mistake in this design. 🔎

This is a made-up teaching example, not anyone's real design. A student needs 1 W (30 dBm) at 5.8 GHz for a drone video link, from a 12 V board supply, with 0 dBm from the radio. Below is the chain and the power plan. Take every part figure as given. One line is wrong. Which one, and why?

Answer on Friday.

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Image (chain and plan):

Stage Part Gain / loss Level after
Radio — — 0 dBm
Driver ADI HMC788A +14 dB 14 dBm
Pads PAT0816 + YAT-1A+ −11 dB 3 dBm
Power amplifier Qorvo QPA1019 +30 dB 33 dBm
Harmonic filter Knowles L090XE6S −1.5 dB 31.5 dBm
Isolator JQL JIM5700T5900VM2R −0.35 dB 31.15 dBm
Low-pass filter Mini-Circuits ALF-6000+ −0.7 dB 30.45 dBm
Substrate RO4003C-50ohm-SL −0.3 dB 30.15 dBm
Connector Amphenol 132203 −0.2 dB 29.95 dBm

Power plan: "The QPA1019 runs straight from the 12 V board supply. No other rails are needed." (planted mistake)

Post 2 (Friday): the answer

Text:

Spot the mistake in this design: the answer 🔎

The power plan is wrong. The QPA1019 is a GaN amplifier: its drain runs at 22 V, and its gate needs a NEGATIVE voltage that must be switched on BEFORE the drain. Qorvo's datasheet says so on page 1 and spells out the order on page 16: set the gate to −5 V first. Powering it "straight from 12 V" leaves out both the 22 V drain and the negative gate.

A working plan from a 12 V supply: a step-up converter for the 22 V drain, a negative supply for the gate, and a sequencer so the gate comes up first and goes down last.

Lesson: before trusting a power plan, read the bias section of every amplifier's datasheet.

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Image: the planted line next to the datasheet lines below.

Proof (for the answer image)

  • Datasheet: Qorvo QPA1019, "4.5 – 7.0 GHz 10 W GaN Power Amplifier", Data Sheet Rev. A, September 28, 2018, our copy data/datasheets/pa/1_6ghz/QPA1019.pdf, read 2026-10-08.
  • Page 1, Key Features: "Bias: VD = 22 V, IDQ = 290 mA, VG = −2.5 V typical".
  • Page 2, Absolute Maximum Ratings: "Gate Voltage Range (VG) −6 to 0 V".
  • Page 16, Bias-Up Procedure: "2. Set VG to −5.0 V" (before the drain is applied).
  • The planted change: the only change from the MagicON design is the power-plan sentence. The chain table is the engine's own power flow, row for row. (Until 2026-10-12 the substrate and connector rows were drawn as one "Board + connector −0.5 dB" row; MagicON check_grounding flagged that −0.5 dB as a figure no engine gave, so the two rows are now shown as the engine reports them.)
  • Images: spot-the-mistake-built-01-puzzle.png and spot-the-mistake-built-01-answer.png, built by docs/scripts/make-spot-the-mistake-built-01.py.

Source of the correct design (keep with the draft)

MagicON design_rf_chain, run 2026-10-08 through the MagicON AI connector (the live public server, not this repo's newest code). Inputs: 5.8 GHz, target 30 dBm, input 0 dBm, 12 V board supply, application drones_digital_video; grounding mode user_stated (the student question's own words). Results used above:

  • power_flow: External Input Power 0 → HMC788A +14 (14 dBm) → PAT0816 + YAT-1A+ −11 (3) → QPA1019 +30 (33) → L090XE6S −1.5 (31.5) → JIM5700T5900VM2R −0.35 (31.15) → ALF-6000+ −0.7 (30.45) → RO4003C-50ohm-SL −0.3 (30.15) → 132203 −0.2 (29.95). achievedOutputDbm 29.95.
  • bias_sequence: turn on Gate bias −5 V, then Drain 22 V; turn off Drain 22 V, then Gate bias −5 V.
  • Power parts in its BOM: TPS55289RYQR buck-boost regulator, ADP7142ARDZ-5.0 LDO, LTC3260EDE#PBF charge-pump regulator (negative rail), and a "GaN bias sequencer / supervisor (gate-before-drain ordering)" line with no part number chosen.
  • Warnings it gave: output aim clamped to the FCC 15.247 30 dBm conducted limit; 10.9 dB interstage attenuation computed vs 11.0 dB of pads; substrate loss is a 0.3 dB estimate (no datasheet loss).

Before posting

  • Not checked against datasheets: the other parts' figures in the table (they are the engine's). The puzzle is only about the power plan, so the post makes no claim about them, and since 2026-10-09 it says "take every part figure as given", as the other exercises do ("One line is wrong" invites a reader to challenge any row of the table).
  • âš  The catalog's QPA1019 P1dB was under review (issue #297: the store's Psat copied as P1dB; closed by 2026-10-12). The post does not use P1dB, so it is not affected.
  • Re-check 2026-10-12: MagicON design_rf_chain with the same inputs gives the same parts, the same power flow (29.95 dBm out) and the same bias_sequence (gate −5 V on, then drain 22 V). Answer text and puzzle table checked with MagicON check_grounding: grounded, 0 findings.
  • Image titles carry no number, because draft numbers do not match the posting order (built #2 went out first, 2026-10-12).
  • Ready-to-post folder (outside the repo): Documents/MagicON-posts/puzzle-built-01-drone-gate-supply/.
  • Check posted-log.md first; log both posts after posting.