Chapter 66: Magnetic Cores and Margins

17:30. Equipment storeroom. Iron door half-shut. Fluorescent tube flickering. Letting out a low-frequency hum. Lin Chen pushed the door open. Step length: thirty centimeters. Center of gravity shifted right. Left foot touched down. Pain shot up along the Achilles tendon. He paused. Adjusted his breathing. Did not frown.

Old Zhao sat behind a wooden table. Sleeve protectors on. Working an abacus. Hearing the footsteps, he looked up and handed over a form. Technical report advance request.

“I read the report,” Old Zhao said in a hoarse voice. “Filter network topology. Pi type. Parameter calculation. Logical closed loop. Advance: thirty. Sign here.”

Lin Chen stepped forward. Took the pen. Signed. His handwriting was neat. No connected strokes. Old Zhao stamped it, tore off the stub, and handed over three ten-yuan bills. Old notes. Curled edges. Faint smell of ink. Lin Chen took them and slipped them into the inner layer of his canvas bag, beside the ledger.

Balance on the books: forty-four yuan seven jiao. Gap: twenty-five yuan three jiao. Covered. Remaining: nineteen yuan four jiao. Emergency reserve.

“This weekend. A fellow townsman’s ride. Back to Qinghe,” Old Zhao added. “Medicine—I asked someone from the pharmaceutical company to hold two boxes of phenytoin sodium. They’re in the guardroom. Pick them up yourself.”

“Understood.” Lin Chen nodded, turned, and walked to the workbench.

The surface had been cleared. Anti-static mat laid down. Grid paper. Pencil. Straightedge. Vernier caliper. Soldering iron stand. Solder wire. Flux. He sat down, back straight, not leaning against the chair.

Seventy-two hours. Countdown. Start.

18:00. Design phase. Lin Chen drew the equivalent circuit. Input end: series inductor, shunt capacitor. Output end: shunt capacitor, grounded. Cutoff frequency: one hundred fifty megahertz. Stopband attenuation: forty decibels. Insertion loss: zero point five decibels.

He chose the magnetic core. Ferrite. Nickel-zinc material. Low high-frequency loss. Saturation flux density: zero point three tesla. He calculated the inductance: zero point five microhenries. Enameled wire diameter: zero point four millimeters. Number of turns: twelve.

He took the caliper and measured the toroid’s inner diameter, outer diameter, and height. Recorded them. He flipped through Design of High-Frequency Magnetic Components. Looked up formulas for distributed capacitance, interlayer winding capacitance, lead parasitic inductance. Worked through the calculation. Self-resonant frequency had to be above two hundred megahertz. Otherwise the stopband attenuation would roll off too early.

He crossed out the original parameters and rewrote them. Turns reduced to eight. Wire diameter increased to zero point five. Layers: single layer. Reduce interlayer capacitance. The shunt capacitor was changed to NP0 dielectric. Stable temperature coefficient. Capacitance would not drift at high frequency.

19:30. Materials prep. Lin Chen opened the parts cabinet. Took two toroids. One spool of enameled wire. Four NP0 capacitors. Two resistors. SMD. 0805 package. He took sandpaper and polished the surface of the toroids, removing the oxide layer until a gray-white base showed through. He took the enameled wire and wound it. Left hand holding the ring. Right hand guiding the wire. Tension even. No overlap. No crossing. Eight turns. Done.

He measured the inductance. LCR meter. Probes clipped to both ends. Reading: zero point four eight microhenries. Error: four percent. Within spec. He measured the Q value: one hundred twenty. At high frequency, loss was low. Usable.

20:15. Soldering. Lin Chen laid down the perfboard, copper side up. He inserted the components. Toroid in the center. Capacitors arranged symmetrically. Resistors for current limiting. He set the soldering iron temperature to three hundred twenty degrees. Fed solder. Pulled away. The solder joints were conical, bright, with no spikes, no cold joints.

He checked continuity. Multimeter. Buzzer sounded. Resistance: zero point two ohms. Within spec. He connected the signal generator and the spectrum analyzer. Swept from ten megahertz to two hundred megahertz.

The screen lit up. The curve was smooth. At one hundred fifty megahertz, attenuation: thirty-eight decibels. Short by two decibels. Not enough.

He stopped. Did not panic. Switched paths. Parasitic parameters. Actual values. Higher than estimated. Distributed capacitance coupling between the toroid and the perfboard. He removed the toroid and recalculated. Took insulating tape and raised the toroid two millimeters off the board surface to reduce coupling with the board. He soldered it again. Measured.

The curve updated. At one hundred fifty megahertz, attenuation: forty-one decibels. Passed. Insertion loss: zero point four two decibels. Passed.

He recorded the data. Paper smooth. Handwriting neat.

21:00. Shielding. Lin Chen took sheet aluminum. Cut it. Bent it into a box. Conductive foam lining inside. Ground terminal, M4. Soldered. He mounted the filter module inside and fixed it in place. Tightened the screws. No stripped threads. He connected the BNC jack. Output end. Ground clip hanging down.

He looked at his watch. 21:30. Sixty-nine hours to deadline. He took a mantou, broke it open, dipped it in the free soup, swallowed without chewing, replenishing carbohydrates. He drank water. Tap water. Cold. His stomach tightened. Tolerable.

22:00. Sleep. Lin Chen leaned over the desk, head on his arm, eyes closed. Breathing steady. In his mind, a timeline unfolded automatically. Tomorrow. Nine o’clock: retest. Twelve o’clock: optimize. Eighteen hundred: final test. Midnight: submit.

He slept four hours. Did not turn over. Did not dream.

02:00. Awake. Lin Chen opened his eyes. No transition. Sat up straight at once. Left foot. The pain had worsened. Blood had seeped through the edge of the gauze. He removed it. Changed the dressing. Iodophor. Cotton swab. Light wipe. The scab cracked. A faint sting. He taped on fresh gauze, fixed it in place, stood up. Step length: thirty centimeters. Did not step in the wet patches.

02:30. Retest. Lin Chen connected the equipment. Warm-up. Signal generator output. White noise. Amplitude: one volt. Spectrum analyzer sweeping. Curve smooth. No spikes. No burrs. He recorded the data. Filed it.

04:00. Optimization. Lin Chen noticed that at one hundred eighty megahertz, the attenuation flattened into a plateau instead of continuing downward. He checked the manual. At high frequencies, magnetic permeability dropped and loss rose. He switched toroids. Material changed to manganese-zinc. In the low-frequency band, permeability was high; in the high-frequency band, loss was high. Not a match. He switched back to nickel-zinc and adjusted the shunt capacitor, reducing its value by zero point five picofarads. Re-soldered. Measured.

The curve updated. At one hundred eighty megahertz, attenuation: forty-five decibels. The plateau disappeared. Roll-off steep. Passed.

He set down the pen and looked out the window. The sky was gray. Clouds low. Wind threading through the window crack with a low whistle.

06:00. Final test prep. Lin Chen tidied the workbench. Tools back in place. Grid paper stapled. Report printed. Signed. Stamped. He slung on the canvas bag and went out. Corridor empty. Terrazzo floor cold. He went downstairs. Step length unchanged.

08:00. Laboratory. Old Li was already waiting by the bench. Army coat still on. Chalk dust on the cuffs. Chen Hao stood to one side, load-matching network in hand. Black plastic box. Gold-plated connectors.

“Begin,” Old Li said. His voice was flat, without inflection.

Lin Chen stepped forward. Connected the equipment. Filter module. Input end to the signal generator. Output end to the spectrum analyzer. Ground clip to the signal source ground. Single-point grounding. He powered it on. Warm-up. The screen lit up.

Old Li adjusted the frequency. Ten megahertz. Fifty megahertz. One hundred megahertz. One hundred fifty megahertz. One hundred eighty megahertz. Two hundred megahertz. The curve was smooth. Attenuation met spec. Insertion loss stayed below zero point five decibels. Old Li wrote it down and closed the notebook.

“Passed.” Old Li raised his head. “Provincial competition. Eligibility retained. Monday next week. Training camp. Outline already issued. Miss once, and you’re disqualified.”

“Understood.” Lin Chen nodded, packed up the equipment, powered it down, and put everything back in place.

09:30. Corridor. A jumble of footsteps. Fellow townsman Old Zhou, wearing a jacket and carrying a woven bag, stood by the stairwell. Seeing Lin Chen, he waved.

“The car’s at the gate. I brought the medicine.” Old Zhou handed over a transparent plastic bag. Inside: two boxes of phenytoin sodium. White cartons. Batch number clear. Expiration date until next year.

Lin Chen took it. Light in weight, but the boundaries were clear. He handed over an envelope containing the remittance receipt and a letter, the handwriting neat, no connected strokes.

“Please take this to Dr. Zhao at the town clinic. The medicine—take it on schedule. Record the seizures. Time. Duration. Triggers. Keep paper and pen ready. Don’t save money on that.”

Old Zhou nodded and tucked it into his inner pocket. “Don’t worry. The ride’s steady. Won’t jolt. End of the month, when I come back, I’ll bring a reply.”

Lin Chen nodded, turned, and walked away. Step length: thirty centimeters. Did not step on the frost line.

10:00. Dorm room. Desk cleared. Lin Chen spread out paper and drew coordinate axes. Horizontal axis: time. Vertical axis: money. Marked the scale. Provincial competition. Training camp starts. Four hours every day. No part-time work. On the books, remaining balance: nineteen yuan four jiao. Emergency. Enough.

He stopped writing and looked out the window. The sky had cleared. Clouds dispersing. Sunlight slanted in, falling across the desk. Dust floated faintly.

He took the calendar and flipped to next month. Circled the date of the provincial competition in red. The pen tip paused for three seconds. He wrote: parameter—cutoff frequency, one hundred fifty megahertz; stopband attenuation, forty-one decibels; insertion loss, zero point four two decibels.

He closed the calendar, put it in the drawer, stood up, put on his shoes, tied the laces. Center of gravity shifted right. Step length: thirty centimeters. Did not step in the wet patches.

Ahead, the library lights were still on. The old librarian was waiting to take returns.

He pushed the door open and went in. Wind poured in. Warm.

He closed his eyes. In his head there were no curves, only magnetic cores, saturation, current, the tension in enameled wire, solder-joint temperature. Three lines crossing in the dark. Not colliding. Not tangling. Each moving forward on its own.

He opened his eyes. Step length: thirty centimeters. Did not step in the wet patches.

Forward.

14:00. Library. Third floor. Electronics engineering section. Old shelves. Faded spines. Lin Chen returned High-Frequency Circuit Design, Principles of Electromagnetic Compatibility, and Oscilloscope Probe Technical Manual. The old librarian checked them, registered them, stamped them, and handed back the library card, its edges worn smooth.

Lin Chen turned and walked toward the window. Sunlight struck the glass. The glare was sharp. He narrowed his eyes and looked at the sports field. Red track. White lines clear. A few students were running. Their steps were even. Their breathing regular.

He looked at his watch. The second hand ticked. Six days until training camp began. Twenty-one days until the provincial competition opened. How long until his left foot healed—unknown.

He took out his notebook, flipped to a blank page, and made a list.

1. Provincial competition outline received. System-level joint debugging. Anti-interference testing. Electromagnetic compatibility prescan.
2. Training camp: four hours every day. No part-time work. Time must be compressed. Sleep reduced to three hours.
3. Medicine costs already entrusted to a fellow townsman for delivery back. Risk of interruption in medication removed.
4. Remaining balance on the books: nineteen yuan four jiao. Emergency reserve. Enough to cover contingencies.

He checked the logic. Closed loop. Time windows overlapping. Execution had to be extreme. Physical limit near critical. Carbohydrates needed. Cafeteria mantou and free soup could sustain it.

He stopped writing and looked out the window. The sky was darkening. Clouds gathering. Wind rolled the dry grass at the edge of the field into little whorls.

He rose, slung on the canvas bag, and went out. Corridor empty. Terrazzo floor cold. He went downstairs. Step length unchanged. Did not step in the puddles.

16:00. School gate. Bulletin board. Crowd gathered. Lin Chen stepped forward and looked at the list for provincial competition eligibility review. White paper. Black print. Clear.

First line: Lin Chen. Intensive group. Practical operation. First place. Joint debugging passed. Additional test qualified.

He looked at it for three seconds. Did not smile. Did not sigh. Turned and left. The crowd parted. Footsteps mixed together. Wind lifted the edge of the notices with a faint rustle.

17:30. Dorm room. Desk cleared. Lin Chen spread out paper and drew new coordinate axes. Horizontal axis: time. Vertical axis: parameters. He marked the provincial competition schedule, training camp, joint debugging, prescan, finals. Drew connecting lines. Smooth. No jumps.

He took the ledger, turned to a blank page, and made a new list.

1. Provincial competition travel expenses reimbursed against receipts. No cash outflow.
2. Training camp stipend: fifty yuan per month. Issued at the end of the month.
3. Old instrument repairs can still be taken on through Old Zhao. Must avoid the training-camp time slot.
4. A reply from the fellow townsman is expected back at the end of the month, along with the medicine-cost record and seizure data.

He checked the logic. Closed loop. Time windows overlapping. Sleep had to be compressed. Physical limit near critical. Carbohydrates needed.

He stopped writing and looked out the window. Night was thick. No stars. Wind passed through the window crack with a low whistle.

He stood up, put on his shoes, tied the laces. Center of gravity shifted right. Step length: thirty centimeters. Did not step on the frost line.

Ahead, the laboratory lights were still on. Old Li was waiting with the outline.

He pushed the door open and went in. Wind poured in. Cold.

He closed his eyes. In his head there were no curves, only magnetic cores, saturation, current, the tension in enameled wire, solder-joint temperature. Three lines crossing in the dark. Not colliding. Not tangling. Each moving forward on its own.

He opened his eyes. Step length: thirty centimeters. Did not step on the frost line.

Forward.

19:00. Laboratory. Old Li handed over a document envelope. Provincial competition training outline. V2.0. System-level joint debugging. Anti-interference testing. Electromagnetic compatibility prescan. Miss once, and eligibility is canceled.

Lin Chen took it and read. Parameters explicit. Deadline clear. No room for ambiguity.

“Starts next Monday. Every day from four to eight in the afternoon. System-level joint debugging. Anti-interference testing. Electromagnetic compatibility prescan. Miss once, and you’re disqualified.” Old Li’s voice was flat, without inflection.

“Understood.” Lin Chen nodded and slipped the document envelope into the inner layer of his canvas bag, beside the ledger.

20:00. Corridor. Mixed footsteps. Chen Hao leaned against the window, load-matching network in hand. Black plastic box. Gold-plated connector. Seeing Lin Chen, he straightened up.

“I read the outline,” Chen Hao said. “System-level joint debugging. You handle the front end—filtering and shielding. I’ll handle the back end—matching and the power amplifier. Dual sign-off.”

Lin Chen took the report and read it. The division of labor was clear. Boundaries of responsibility clear. He signed. Handwriting neat. No connected strokes. Chen Hao signed too, his pen strokes crisp.

“I’ll front half the materials money,” Chen Hao said. “Travel for the provincial competition—my family can approve it. This little bit is nothing.”

“No need.” Lin Chen put away the report. “The accounts are balanced. Work-study will offset it. Settled at month’s end. Just do your part by the drawing. Don’t exceed tolerance.”

Chen Hao nodded and did not insist. The two looked at each other for three seconds. No extra words. Then turned and left in opposite directions.

21:00. Dorm room. Desk cleared. Lin Chen spread insulating paper and laid out the materials: sheet aluminum, copper mesh, BNC jack, capacitors, enameled wire, insulating sleeving. He took a steel ruler and a scribe and marked lines on the aluminum. Depth even. No drift. He took scissors and cut along the lines. Edges neat. He took a file and removed the burrs. Gentle movements. Did not cut his hand. Aluminum filings fell, fine and reflective.

He bent along the scored lines. Ninety degrees. Close fit. Flat and even. He drilled holes with a hand drill. Speed slow. No drifting. Hole positions lined up with the ground terminal. M4 threads. Smooth. He peeled the backing from the conductive foam, aligned it to the edge, and pressed it flat. No bubbles. He soldered the ground wire. Copper mesh overlapping. Soldering iron heating. Solder fed in. Pulled away. The joints were full. No cold joints. He tested continuity. Multimeter buzzer sounded. Resistance: zero point one ohm. Passed.

22:30. Probe assembly. BNC jack. Center pin soldered. Heat-shrink tubing over insulating sleeve. Heated and shrunk tight. He connected the enameled wire and wound the compensation coil. Twelve turns. Even spacing. He connected the trimmer capacitor in parallel. Fixed capacitor value matched the calculation. He installed it in the housing. Tightened the screws. No stripped threads.

He connected it to the oscilloscope and the signal generator. Ten megahertz square wave. Amplitude: one hundred millivolts. He looked at the screen. Rising edge slightly lifted. Overshoot: three percent. Within spec.

He recorded the data. Paper flat. Handwriting neat.

23:30. A letter was pushed through the crack under the door. Kraft envelope. Edges worn. Postmark: Qinghe County. Sender: Wang Guiying.

Lin Chen opened the letter. Thin paper. Crooked handwriting. Written in pencil.

“Chen. Medicine received. Xing didn’t convulse last night. Slept steady. Village doctor says take the medicine on time. Don’t stop. Study in peace. Home can hold out. Don’t worry.”

After reading it, he folded it and put it into his pocket. His fingers touched the edge of the ledger. Curled edge. Rough paper rubbing against the skin.

He sat up straight and looked out the window. Night thick. No stars. Wind passing through the window crack with a low whistle.

He took grid paper and drew new coordinate axes. Horizontal axis: time. Vertical axis: parameters. He marked points for probe calibration, the curve, shielding-box grounding, impedance, load matching. Drew connecting lines. Smooth. No jumps.

He stood up, put on his shoes, tied the laces. Center of gravity shifted right. Step length: thirty centimeters. Did not step on the frost line.

Ahead, the laboratory lights were still on. Old Li was waiting for the joint debugging.

He pushed the door open and went in. Wind poured in. Cold.

He closed his eyes. In his head there were no curves, only solder-joint temperature, time, molten solder, solidifying solder. Three lines crossing in the dark. Not colliding. Not tangling. Each moving forward on its own.

He opened his eyes. Step length: thirty centimeters. Did not step on the frost line.

Forward.