Chapter 78: Ripples and Reference Tables

The wind in the corridor died away. Lin Chen stepped back into the dorm room and locked the door behind him. The halo of the desk lamp pressed down over the graph paper. He crossed out "Find Old Zhao" and added two words beside it: functional assessment. Substitute certification. If the university clinic would not stamp it, the department could. Old Zhao had led three cohorts through the provincial competition; he knew exactly what the Academic Affairs Office cared about. Not whether there was a wound, but whether the hand was steady and the eye was precise.

At six the next morning, before dawn had fully broken, Lin Chen left twenty minutes early. Stride length: twenty-five centimeters. When his left foot touched the ground, a dull pain rose from the wound. He adjusted his center of gravity and shifted the weight onto the ball of his right foot. Around puddles. Across the athletic field. Dew soaked the cuffs of his pants and clung cold against his skin. Old Zhao's office was on the second floor of the lab building. The door was half open. Inside came the sound of papers turning. Lin Chen knocked. Three times. A pause. Then two more.

"Come in."

Old Zhao sat behind a desk piled with drawings, his glasses slipped halfway down his nose. Spread open on the desk was the manual for the new domestic-model spectrum analyzer. Beside it sat a cup of tea gone cold. Lin Chen handed over the graph paper and the clinic's preliminary record. "The doctor won't issue a no-injury certificate. But the wound is under control and won't affect fine operations. I need the department to issue a functional assessment."

Old Zhao took the papers. He didn't read the description of the wound. He flipped straight to the back and picked up a red pen. "The practical exam tests reading accuracy and attenuation settings, not a medical form." On the paper he wrote: After observation by the department competition group, this student's limb function shows no substantive impairment, and he is capable of independently completing practical RF measurement tasks. Signature: Competition Guidance Group, Department of Electronic Engineering. Then he stamped it with his personal seal. "Take this to Academic Affairs and exchange it for the provincial competition eligibility filing form. As for the clinic, I'll make a call. They'll just follow procedure and won't inspect the wound too closely."

Lin Chen took the paper. It was light in his hand. Heavy in what it meant. "Thank you, Professor Zhao."

"Don't thank me." Old Zhao removed his glasses and rubbed the space between his brows. "The equipment's been switched to a domestic unit. The manual's on the desk. Half-decibel steps. Your old compensation algorithm has to be recalculated. The thermal drift curve is different. The domestic machine has a higher noise floor. When you adjust attenuation manually, pointer jitter will interfere with the reading. And more troublesome than that is the power ripple. The old campus grid load is unstable. After two in the afternoon, the voltage drops by half a volt. Add ripple on top of thermal drift and your compensation values become useless. You get only three chances to test and adjust. One bad calculation and you're out."

Lin Chen nodded. He folded the paper and slipped it into the inner pocket closest to his body. Then he went back to the third floor of the library, to a seat by the window. Sunlight slanted in; dust floated in the beam. He spread open the manual for the domestic machine and turned to the page on electrical characteristics. Noise floor: -110 dBm. Frequency stability: ±0.01%. Manual attenuation step: 0.5 dB. Power ripple: ≤5 mV. He pulled out scratch paper and started writing formulas again.

His original compensation algorithm had been built around the HP8591E's linear thermal drift. The domestic machine's thermal drift was nonlinear. In the first ten minutes after a cold start, the noise floor decayed exponentially before leveling out. And with 0.5 dB steps, every attenuation adjustment amplified the reading error. He needed to fit both the thermal drift curve and the power ripple into the compensation formula at the same time. His pen moved quickly. Integrals. Derivatives. Boundary conditions substituted in. By the third pass, his left wrist had started to ache. The swelling pain from the wound in his left foot crawled up along his calf. He set down the pen, lifted his foot to let the blood flow back, took a deep breath, and continued. Fourth pass. The result diverged. He crossed it out and started again. Fifth pass. He introduced a piecewise function, splitting the post-cold-start timeline into three sections: 0-5 minutes, 5-12 minutes, and after 12 minutes. They corresponded to the high-ripple interference period, the thermal-drift-dominant period, and the stable period. This time the result converged. He arrived at a set of lookup values: at minute 3 after cold start, compensation +0.4 dB; at minute 7, compensation +0.25 dB; after minute 12, compensation returns to zero. If the voltage fluctuation exceeds the threshold, add an extra +0.1 dB.

He copied the lookup values onto the inside cover of his mistake notebook and boxed them in red. Muscle memory. Enter the room. Power on. Check the meter. Look up the value. Add it. Set it. Every step had a time point precise to the second. Nothing extra. The smell of old books and dust in the library settled heavy in his lungs. He closed his eyes and simulated the exam flow in his head. Pointer jitter. Voltage fluctuation. Fingertips making tiny adjustments on the knob. Breathing matched to movement. No emotion. Only steps.

At five in the evening, he stood in line at Academic Affairs with Old Zhao's assessment slip in hand. He submitted the form. The clerk checked the seal, stamped it, and the filing form was his. Procedure complete. No extra words. He returned to the university clinic. The doctor glanced at the filing form and signed the medical sheet. The loop was closed. He walked out of the clinic. The wind in the corridor was cool. Leaning against the wall, he rapidly reorganized the facts in his head. Forcing his way through would not work. Concealing it would not work. Changing the route would. But it cost time. He took out the graph paper and wrote: Filing complete. Medical clearance passed. Compensation algorithm reconstructed. Then he drew a horizontal line beneath it.

At eight that night, in the dorm under the yellow light of the desk lamp, Lin Chen sat at his desk and took off his shoes and socks. The yellowing at the edge of the wound had faded somewhat. There was less seepage. But the gauze was still damp. He cleaned it with an iodine swab, very slowly. The pain was real. He did not dwell on it. He focused only on the steps: disinfect, dry, cover, secure, put on shoes, tighten laces. Then he took out the graph paper and wrote: Date. Income: zero. Expenses: iodine, two yuan. Gauze, one yuan. Balance: one hundred eleven. Distance to eight hundred: six hundred eighty-nine. Days remaining: sixteen. The pen paused. He crossed out "old book market." The timeline was already full. There was no gap left in it.

He packed the filing form, the mistake notebook, the insulated box, spare gauze, iodine pads, and graph paper into the canvas bag one by one. The weight was distributed evenly. He adjusted the strap to the most comfortable position. Sitting on the edge of the bed, he closed his eyes and ran through tomorrow's sequence in his mind. Up at seven. Out at seven twenty. Reach the test site at seven fifty. Theory exam at eight. Practical in the afternoon. Equipment warm-up. Compensation lookup. Attenuation setting. Record the readings. He had rehearsed every step in his head seventeen times. His muscles already remembered. His breathing was already in sync.

He opened his eyes, walked to the window, and pulled the curtain aside. The night wind was cold. The streetlights in the distance stretched into a line, like signal cables in an exam room. He lowered his head and glanced at his left foot. Stride length: twenty-five centimeters. Don't step in standing water. The habit had already sunk into his bones. He went back to the desk and picked up the family letter—his mother's. He still had not opened it. The envelope flap was clean, without a crease. He took up the paper knife and slit it neatly along the edge. The paper inside was thin. The handwriting was tidy. Only three lines.

Chen'er. Everyone at home is fine. Xing'er drew a new picture. Come back when you can.

Tucked against the back of the letter was a crumpled remittance slip. Payee: Lin Chen. Amount: 50 yuan. Sender: Qingshi Village Clinic. Message: Subsidy. He stared at the slip. Fifty yuan. Distance to eight hundred: still six hundred thirty-nine short. Days remaining: sixteen. He tucked the remittance slip into the mistake notebook, sealed the letter again, and placed it in the innermost layer of the canvas bag. The zipper closed with a click.

He slung the bag over his shoulder and walked to the door, then stopped. The window at the end of the corridor stood open. Wind poured in. The air carried dust and diesel. He picked up the old radio from the desk, turned it on, and tuned the dial. Static, then a human voice. The provincial weather station forecast: cloudy tomorrow afternoon, thunderstorms by evening, brief heavy rainfall in some areas. Power-grid load warning. The old campus lines may have to operate under reduced voltage. He switched off the radio. Rain. Reduced voltage. The ripple would be worse. The tolerance of the 0.5 dB step would shrink even further. A twenty-five-centimeter stride would fail on flooded pavement. He would have to leave twenty minutes earlier and detour along higher ground. During the practical exam, if the voltage fluctuated, the compensation value would need to be manually raised by 0.15 dB. He went back to the desk and added one more line beneath the lookup values inside the front cover of the notebook: Rain. Reduced voltage. +0.15 dB. High-ground route. The pen came down. The handwriting was clear.

Outside, clouds covered the moonlight. The diesel smell in the wind faded. He capped the thermos and settled the canvas bag on his shoulder, the strap biting into his collarbone. He walked to the door, stopped, and looked back once at the mistake notebook and the insulated box on the desk. Then he opened the door and went out. The window at the end of the corridor was still open. Wind poured in. He took a deep breath. The air smelled of dust. He stepped forward. Stride length: twenty-five centimeters. Don't step in standing water. Tomorrow. See you in the exam room.