Chapter 67: Frequencies and Noise

16:00. The lab door opened. Cold air rushed at him, mixed with the smell of rosin and ozone. Lin Chen stepped over the threshold. Stride length, thirty centimeters. Center of gravity shifted right. When his left foot touched down, a dull pain came from the old wound under the gauze. He ignored it. Walked to Workbench Three.

The bench had already been cleared. Antistatic mat laid out. Oscilloscope. Spectrum analyzer. Signal generator. DC regulated power supply. Old Li stood in the middle. Military coat draped over the back of a chair. Sleeves rolled up. An old watch exposed. Scratches on the glass. Chen Hao was across from him, tuning the load network. The connectors on the black plastic box were gold-plated. The reflected light was cold and hard. The other students from the reinforcement group filed in one after another. Nine of them. Footsteps mixed together. Chairs pulled back. Metal legs scraped against the terrazzo floor. Harsh.

Old Li checked his watch. The second hand returned to zero. “Begin. System cascade tuning. Anti-interference testing. First item. Power-frequency suppression.” His voice was flat. No rise or fall. Lin Chen connected the power. The oscilloscope warmed up. The screen lit. The scan line stabilized. He connected the filter module. Input side. To the signal generator. Output side. To the spectrum analyzer. Ground clip. Single-point grounding. He adjusted the signal generator. Output a fifty-hertz sine wave. Amplitude one volt. Simulated mains interference.

The spectrum analyzer swept. The screen showed the fundamental. Fifty hertz. High amplitude. Harmonics. One hundred fifty hertz. Two hundred fifty hertz. Arrayed in order. Lin Chen recorded it. Data. The paper lay flat. He switched the filter module in. The curve changed. At fifty hertz. Attenuation, thirty-eight decibels. Up to standard. But at one hundred fifty hertz. A burr appeared. Peak rose sharply. Not up to standard.

Lin Chen stopped. Did not panic. Routing switch. Parasitic coupling. He checked the schematic. The filter module and the shielding box were too close together. Distributed capacitance. Coupling in the power-frequency harmonics. He took an insulating spacer. Thickness, two millimeters. Raised the module. Tested again. The curve updated. The burr disappeared. Attenuation, forty-two decibels. Up to standard. He recorded it. Filed it.

17:30. Old Li made his rounds. Stopped in front of Lin Chen’s bench. Looked at the screen. Looked at the record sheet. Nodded. “Second item. RF immunity. One hundred megahertz to two hundred megahertz. Sweep. Inject noise. Observe noise-floor lift.” Lin Chen switched the signal generator. Output broadband white noise. Amplitude, zero point five volts. The spectrum analyzer swept. The curve on the screen was smooth. Noise floor. Minus eighty decibels. Stable. He recorded it. Data. Handwriting neat.

18:15. The pain in his left foot worsened. Blood seeped through the edge of the gauze. He lowered his head. Looked. Did not treat it. Kept going. Right hand gripping the mouse. Left hand steadying the oscilloscope probe. Fingertips slightly cold. His stomach was empty. Carbohydrates exhausted. He picked up his water bottle. Took one sip. Tap water. Cold. His stomach contracted. Tolerable. He checked his watch. Until the end. One hour forty-five minutes. Sleep compressed to three hours. Physical strength at the edge. Must be maintained.

Over at Chen Hao’s station. Equipment alarm. VSWR over limit. He frowned. Took apart the load network. Retuned the matching capacitor. His movements were quick. But a little impatient. Lin Chen’s peripheral vision swept over him. Did not interfere. Focused on his own bench. Old Li walked past. Stopped. Looked at Chen Hao. “Matching point shifted. Recalculate the Smith chart. Don’t tune blind.” Chen Hao nodded. Pen tip scratching across paper. Rustling. Lin Chen pulled back his gaze. Continued recording.

19:00. Third item. Electromagnetic compatibility prescan. Old Li connected the radiating antenna. Set it outside the shielding box. Distance, thirty centimeters. The signal generator output pulse bursts. Simulated industrial-environment interference. The spectrum analyzer swept. The curve on the screen shook violently. Noise floor lifted to minus sixty-five decibels. Over the limit. Lin Chen stopped. Checked grounding. Single point. No loop. Checked shielding. Aluminum skin fitted tight. No gaps. Checked filtering. Parameters up to standard. The problem was not hardware. It was layout.

He stood. Stride length, thirty centimeters. Circled the workbench once. Looked at the cable routing. Power cable. Signal cable. Laid in parallel. Spacing insufficient. Cross coupling. Interference introduced. He took cable ties. Routed them again. Power line left. Signal line right. Crossed vertically. Pulled the spacing to five centimeters. Tested again. The curve updated. Noise floor dropped back. Minus seventy-eight decibels. Up to standard.

Lin Chen recorded it. Data. Paper. Flat. In his head. Automatic accounting. Time. Money. Stamina. Remaining balance on the books. Nineteen yuan four jiao. Tomorrow at the cafeteria. Steamed buns. Free soup. Cost. Zero point five yuan. Remaining, eighteen yuan nine. Medicine cost. Already entrusted. No new expense. Training stipend. Fifty yuan. Paid at month’s end. Enough to cover it. Logical loop closed. No holes.

He looked out the window. The sky was dark. Clouds hung low. Wind rolled dead leaves up against the glass. He closed his eyes. Three seconds. Breathing. Steady. In his mind. No curves. Only spacing. Coupling. Grounding. Impedance. Three lines. Crossing in the dark. Not colliding. Not tangling. Each of them. Moving forward.

19:45. Old Li clapped his hands. The whole room stopped. “Prescan over. Archive the data. Tomorrow. System cascade tuning. Full band. Under load. Test. Absence. Disqualification.” The students packed up one after another. Footsteps mixed. Chairs scraping. Lin Chen powered down. Put everything back. Tools. Graph paper. Bound the report. Signed. Slung his canvas bag over his shoulder. Went out. The corridor was empty. The terrazzo cold.

20:15. Dorm room. Desktop cleared. Lin Chen spread paper. Drew coordinate axes. Horizontal axis. Time. Vertical axis. Parameters. He marked. Full-band. Under-load. Test. Deadline. He took out his ledger. Turned to. A blank page. Listed. Inventory. One. Tomorrow. Under-load. Test. Requires. High-power. Load. Resistors. Two. Lab. Stock. Only. Two. Five-watt. Carbon-film. Units. Three. Need. Homemade. Water cooling. Heat dissipation. Or. Parallel connection. Derating. Four. Funds. Remaining. Eighteen yuan nine. To buy. Heat sink. Aluminum stock. Shortfall. Three yuan.

He stopped writing. Looked. Outside the window. Night. Thick. No stars. Wind. Through. The window crack. A low whine. He stood. Put on his shoes. Tied the laces. Center of gravity. Shifted right. Stride. Thirty centimeters. Did not step. On the frost line.

21:00. Equipment storeroom. Old Zhao. There. Waiting. Taking inventory. Lin Chen. Handed over. The requisition form. Five-watt. Carbon-film resistors. Two. Heat sink. Aluminum stock. One piece. Old Zhao. Looked at the form. Frowned. “Under-load. Testing. High power. Stock’s not enough. You’ll have to wait. For the provincial allocation. Next week.”

Lin Chen’s. Fingers. Paused slightly. The edge of the requisition form. Cut across. His fingertip. A slight pain. He. Nodded. Said nothing. Turned. Went out. Wind. Rolled up. Dead leaves. Spinning.

He. Checked his watch. 21:20. Until. Tomorrow’s. Test. Eighteen hours. Until. The dressing on his left foot. Needed changing. Twelve hours. Until. The sleep. Window. Six hours.

He. Returned. To the dorm. Desktop. Cleared. Spread out. Insulating paper. Set down. Materials. Old resistors. Five. Two-watt. Aluminum sheet. Scrap edges. Copper wire. He. Took. A steel ruler. A scriber. Drew lines. Depth. Even. He. Took. Scissors. Cut. Edges. Neat. He. Took. A file. Removed burrs. Movements. Light. Did not injure his hands. Aluminum filings. Fell. Fine. Reflecting light.

He. Calculated. Parallel. Resistance. Power. Distribution. Thermal resistance. Heat dissipation. Area. He. Worked through. Formulas. The paper. Flat. The handwriting. Neat.

He. Stopped writing. Looked. Outside the window. The clouds. Had broken. Moonlight. Slanted in. Fell across. The desktop. Dust. Floating slightly.

He. Took. The calendar. Turned to. Tomorrow. Circled it red. The pen tip. Paused. Three seconds. He. Wrote. Parameters. Under-load. Power. Ten watts. Temperature rise. Less than. Forty degrees. Resistance error. One percent.

He. Closed. The calendar. Put it. In the drawer. Stood up. Put on his shoes. Tied the laces. Center of gravity. Shifted right. Stride. Thirty centimeters. Did not step. In the water stains.

Ahead. The lab. Light. Still on. Old Li. Was waiting. For the schematics.

He. Pushed the door. Open. Went in. Wind. Poured through. Cold.

He. Closed his eyes. In his mind. No curves. Only. Thermal resistance. Parallel connection. Heat dissipation. Area. Three lines. In the dark. Crossing. Not colliding. Not tangling. Each of them. Moving forward.

He. Opened his eyes. Stride. Thirty centimeters. Did not step. In the water stains.

Forward.