Chapter 65: Threshold and Reply

13:30. Third floor of the lab building. The window glass at the end of the corridor was filmed with dust. Water stains had dried, leaving irregular white marks. Lin Chen leaned against the wall, canvas bag slung across his body, an aluminum shielding box in his left hand, his right hand guarding the probe connector. His left foot hovered off the ground. Most of his weight pressed onto his right leg. The muscles in his calf twitched faintly from being tensed so long. He adjusted his breathing. A few seconds. One. Two. Three. The pain spread in a dull wave. Not sharp. Bearable. He checked his watch. The second hand ticked on. Thirty minutes remained before joint calibration.

He lowered his head and looked at the shielding box. The bent seams of the aluminum skin fit tightly. No burrs. The edge of the conductive foam was pressed flat, not lifting. The grounding terminal, M4 thread, had been coated with a thin layer of petroleum jelly against rust. The probe housing was wrapped in heat-shrink tubing. The seam was smooth. No exposed copper. He reached out and touched the BNC socket. The metal was cold. The center pin showed no oxidation. The insulator had no cracks. Tolerance: 0.1 millimeter. Within spec.

13:50. Footsteps at the end of the corridor. Old Li pushed the door open and came in. Chalk dust clung to the hem of his army coat. The cuffs were worn fuzzy at the edges. Chen Hao followed behind him, carrying the load-matching network in his arms, a black plastic box with gold-plated connectors. The two men nodded. No greetings. Lin Chen set the shielding box on the workbench. The metal base scraped against the antistatic mat with a dull friction sound. He removed the insulating paper, exposing the inner structure. The probe screwed into the BNC socket. Click. Locked tight. The grounding clip hung down. Wire diameter: 0.5 square millimeters. Insulation intact.

14:00. Old Li powered on the oscilloscope. Warm-up. The screen lit. Green spot. Scan line. Stable. He connected the signal source, outputting a ten-megahertz square wave with an amplitude of two hundred millivolts. The probe was connected. The waveform appeared. Steep rising edge. Stable top plateau. No ringing. Old Li adjusted the frequency. Twenty megahertz. Fifty megahertz. Eighty megahertz. The waveform amplitude attenuated, matching the frequency response curve of a ten-megaohm input impedance. He stopped at one hundred megahertz. The amplitude dropped to 0.707 times the original. Bandwidth met spec.

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

Lin Chen stepped forward and made a fine adjustment with a nonmetallic screwdriver. The shaft of the ceramic capacitor turned with even resistance. The leading corner of the square wave on the screen gradually flattened. Overshoot dropped from three percent to one point five percent. He stopped and recorded the data. The paper lay smooth. His handwriting was neat.

“Ground loop.” Old Li switched the probe’s grounding clip, attaching it first to the outside of the shielding box, then to signal-source ground, comparing the noise floor. With single-point grounding, peak-to-peak noise stayed below five millivolts. Multi-point grounding introduced mains-frequency spikes. Old Li nodded. “The shielding box workmanship passes. The probe compensation network is stable. Joint calibration passed.”

Chen Hao stepped forward and connected the load-matching network to the output end of the shielding box. Old Li switched to a high-frequency sine wave and swept from one hundred to one hundred fifty megahertz. Standing-wave ratio: 1.3. Reflection coefficient: within spec. No reflected signal. Energy transfer complete. Old Li wrote it down and closed his notebook.

“Provincial competition training starts next Monday. Every afternoon from four to eight. System-level joint calibration. Interference resistance testing. Electromagnetic compatibility prescan. Miss once, and you’re disqualified.” Old Li looked at the two of them. “Archive the drawings. Leave the hardware in the lab. Pick up the new outline tomorrow.”

“Understood,” Lin Chen and Chen Hao said in unison.

15:10. Joint calibration ended. The equipment was powered down. Lin Chen packed up his tools. The soldering iron cooled. The solder wire was put back in place. The graph paper was bound together. He swung his canvas bag onto his shoulder and went out. The corridor was empty. Wind poured in from the stairwell, carrying the dry chill of early winter. He went downstairs. Stride length: thirty centimeters. Avoiding puddles. Beneath his foot, gauze rubbed. The pain worsened. He paused, adjusted his center of gravity, and kept going.

At the school gate stood the post and telecommunications office. Green signboard. Paint peeling, exposing rusted sheet metal beneath. Lin Chen pushed the door open. The wind chime rang. Behind the counter, the clerk on duty wore sleeve covers and read a newspaper. A local radio station played the news through static. Lin Chen handed over the money order form. Amount: forty yuan. Message field: Medicine expenses. Urgent. He counted out the cash. Old paper bills. Flattened. Passed them over. The clerk stamped the form, tore off the receipt, and handed it back.

“The truck delivering medicine to the county got stuck in the mud last week. Delayed things,” the clerk said casually, eyes never leaving the newspaper. “The town clinic says sodium phenytoin is out of stock. You’ll have to wait until the end of the month for the municipal pharmaceutical company delivery. There is carbamazepine, though. But the dosage is hard to control. Big side effects.”

Lin Chen’s fingers paused slightly. The edge of the receipt paper nicked his fingertip. A faint sting. He nodded without answering, turned, and stepped outside. The wind swept the fallen leaves on the ground into spinning circles. He checked his watch. 15:40. Twenty minutes’ walk to the dormitory. Forty-eight hours until the training outline would be issued. Eight hours until he needed to change the dressing on his left foot.

16:10. Dorm room. The desktop cleared clean. Lin Chen spread out paper and drew coordinate axes. Horizontal axis: time. Vertical axis: funds. He marked the scale. Provincial competition training: four hours every day. No time for part-time work. Current cash on hand: fourteen yuan seven jiao. Money order sent: forty. Shortfall: twenty-five yuan three jiao. Risk of medicine running out: high.

He set down his pen and looked out the window. The sky was gray and dim. Clouds hung low. Wind rolled the dead grass beside the athletic field. He took out his account book, turned to a blank page, and made a list.

One. Library night shift. Possible, but must avoid training hours. Every Wednesday and Friday, 8 to 10 p.m. Income: eighteen yuan. Two. Old instrument repairs, at Old Zhao’s. Advance possible, but requires a technical report. Due Friday. Income: thirty yuan. Three. Provincial competition travel subsidy. Reimbursable only. No cash flow. Four. Ask a fellow townsman to bring medicine. Fare: five yuan. Deduct from remaining cash.

Total: forty-eight yuan. Covers the gap. Twenty-two yuan seven jiao left over for emergencies.

He checked it over. The logic closed. Time windows overlapped. Sleep would have to be compressed to four hours a day. Feasible. Physical capacity: near critical. He would need more carbohydrates. Cafeteria steamed buns and free soup could sustain him.

He took out letter paper and his fountain pen, unscrewed the cap. The ink was blue-black. He wrote the letter in neat characters, no cursive.

“Mother, I have sent forty yuan. Don’t panic about the clinic running out of medicine. For Xing’s episodes, record the time, duration, and triggers. Keep paper and pen ready. I will ask someone to bring sodium phenytoin from the county hospital. If they don’t have it, transfer to the municipal hospital. Keep the receipts. I’ll reimburse them. Don’t try to save money. Chen.”

He folded the letter, put it in an envelope, affixed the stamp, sealed it, and put it in the drawer. He would not mail it. He would wait until the weekend and ask a fellow townsman to carry it.

17:00. Footsteps in the corridor. Mixed, disorderly. The teacher on duty knocked and handed over a notice sheet. White paper. Black print. Provincial Competition Qualification Recheck — Additional Test Notice.

Lin Chen took it and read.

“To comply with new regional rules, an additional electromagnetic compatibility prescan will be required. Within seventy-two hours, submit a self-developed filter module prototype and test report. Those who fail to meet the standard will be replaced by alternates.”

He looked at his watch. The second hand ticked. Seventy-two hours until the deadline. Forty-eight hours until training began. Twenty-four hours until the dressing on his left foot had to be changed.

He slid the notice under the drawings, took out graph paper, and drew a new set of axes. Horizontal axis: frequency. Vertical axis: attenuation. He marked the scale. Then he wrote the parameters. Cutoff frequency: one hundred fifty megahertz. Stopband attenuation: greater than forty decibels. Insertion loss: less than zero point five decibels.

He stopped writing and looked out the window. Night had thickened. No stars. Wind slipped through the window cracks with a low whistling sound.

He stood up, put on his shoes, tied the laces. Center of gravity shifted right. Stride length: thirty centimeters. Avoid the frost line.

Ahead, the equipment storeroom light was still on. Old Zhao was waiting for the advance report.

He pushed the door open. Wind rushed in. Cold.

He closed his eyes. In his mind there were no curves, only inductance, turn count, core, saturation, current. Three lines crossing in the dark. Not colliding. Not tangling. Each moving forward on its own.

He opened his eyes. Stride length: thirty centimeters. Avoid the frost line.

Forward.