Chapter 61: Copper Mesh and Ground Wire

00:20. Dormitory. Iron bunk bed. Lower bunk. Hard wooden board. Covered with old newspapers. Against damp. Lin Chen sat down. Canvas bag. On his knees. Zipper open. Metal sound. Cold. He took out graph paper. Drafting pencil. Eraser. Ruler. Compass. Arranged them. On the desk. Old. Paint flaking. Wood grain split. He pulled out a sheet. Smoothed it flat. Pinned it with the ruler. Drew axes. Horizontal axis. Frequency. Vertical axis. Attenuation. Marked scales. 50 Hz. 100 dB. 1 MHz. 60 dB. Power-frequency interference. Source. Mains coupling. RF interference. Source. Spatial radiation. He calculated. Low-pass filter. Cutoff frequency. 200 Hz. Order. Third. Chebyshev. Ripple. 0.5 dB. Formulae. L. C. Values. Computed. Draft pages. Three sheets. Numbers. Dense. No cross-outs.

The storeroom. Had no parts. Inventory list. Already checked. None. No ferrite rings. No shielding sheet metal. No filter capacitors. He stopped writing. Looked. Out the window. Night. Dense. No stars. Wind. Through the window cracks. A low howl. He. Did not hurry. Route. Switched. Backup plan. Activated.

00:45. He stood up. Put on clothes. Movements. Light. Did not touch. The bed frame. Cold water. On his face. Piercing. He wiped it. Dry. Changed the dressing. Iodophor. Cotton swab. Scab. Edges. Whitish. No seepage. Wrapped it. Tight. Put on shoes. Tied the laces. Canvas bag. Heavy. But. Steady. Out the door. Corridor. Empty. The sound-activated light. Broken. Footsteps. On terrazzo. Echoes. Short. Cold. Stride. Thirty centimeters. Did not step. On the frost line.

01:10. Old equipment storeroom. Side door. Iron lock. Rusted. Key. Turned. Stiff. Old Zhao. On duty. Cotton coat. Cuffs. Shiny with oil. Glasses. Filmed with white breath. “So late. What are you looking for?” Lin Chen. Handed over. The list. “Scrap parts. Picking. A few things.” Old Zhao. Looked. At the list. Frowned. “Ferrite rings. Cleared out. Long ago. Sheet metal. Only scraps left. Capacitors. Loose pieces. Dig through them yourself. Don’t. Mess up. The inventory.” Lin Chen. Nodded. Went in. Light. Dim yellow. Forty-watt. Incandescent bulb. Filament. Trembling faintly. In the air. Thick dust. Musty smell. Mixed with rosin. Old paper. Sourness.

He. Squatted down. Dug through. Wooden boxes. Loose parts. Jumbled together. Resistors. Capacitors. Inductors. Coils. He. Chose. Mica capacitors. 0.1 μF. Ten pieces. Rated. 250 V. He. Chose. Polyester capacitors. 1 μF. Five pieces. Rated. 400 V. He. Chose. Enameled wire. 0.3 mm. One spool. Insulation. Slightly cracked. He. Chose. A ferrite core. Broken. In two halves. He. Used. Sandpaper. Smoothed. The broken faces. Aligned them. Wrapped them tight. With tape. Fixed them in place. He. Found. Scrap sheet metal. Galvanized. Thin. Thickness. 0.5 mm. He. Measured. Dimensions. Length. Thirty centimeters. Width. Twenty centimeters. Two pieces.

02:30. Back to the dorm. Cleared the desktop. He. Spread. Insulating paper. Set down. The sheet metal. Marked lines. Ruler pressed down. Pencil. Cut deep grooves. He took. Scissors. Sheet metal. Hard. Difficult to cut. He. Added force. Steady. No slipping. The edge. Cut his hand. Blood. Seeped out. He. Ignored it. Kept cutting. Folded. The edges. Pliers. Bent them. Ninety degrees. Seams. Overlapped. Two centimeters. He. Measured. Dimensions. Length twenty. Width fifteen. Height ten. Centimeters. The rough shape. Of the shield box. Formed.

He. Wound. The inductor. Ferrite core. Closed together. Tape. Tight. Enameled wire. 0.3 mm. Wound densely. No overlap. No crossing. One turn. Two turns. Ten turns. Fifty turns. One hundred turns. Fingers. Numb with cold. Knuckles. Whitish. He. Breathed warm air on them. White mist. Scattered. Kept winding. The wire. Thin. Easy to snap. He. Controlled. The force. Even. Not tight. Not loose. Fingertips. Sensed. The tension. If it broke. All prior work. Gone.

At one hundred twenty turns. He stopped. Measured. Inductance meter. Reading. 12 mH. Too high. He. Unwound. Ten turns. Measured again. 10.8 mH. Acceptable. He. Connected. Capacitors. Mica. 0.1 μF. In parallel. Polyester. 1 μF. In series. Calculated. Resonant frequency. Cutoff point. Match. He. Soldered. Bent the leads. Fixed them. Soldering iron. Heated. Rosin. Smoked. Acrid. Fed solder. Pulled away. The joints. Full. No burrs. No cold solder.

04:10. Left foot. Stabbing pain. Worse. Gauze. Blood seeped through. Dried. Stuck fast. He stopped writing. Took off his shoe. Tore open. The sock. Scab. Split open. Deep crack. Edges. Whitish. No pus. He took. Iodophor. Cotton swab. Dabbed it on. Sharp pain. He bit his lip. Made no sound. Waited. Until dry. Replaced it. With fresh gauze. Wrapped it. Tight. Not too tight. Blood. Stopped. He put the shoe back on. Tied the laces. Tight. Shifted his weight. To the right. Continued. Assembly.

Filter network. Three stages. Cascaded. Input end. Connected to. BNC jack. Output end. Connected to. Test clips. He measured. Continuity. Multimeter. Beeper. Sounded. No short circuit. No open circuit. He measured. Impedance. Frequency counter. Swept the range. Curve. Steep. Ripple. 0.4 dB. Met spec.

05:20. Shield box. Assembly. Sheet-metal seams. Overlapped areas. He. Scraped off. The paint. Exposed. Bare metal. Copper foil tape. Covered it. Conductive adhesive. Pressed tight. No gaps. He. Opened holes. At the input and output ports. He. Left slits. Two millimeters wide. He took. Fine copper mesh. Dense. Cut strips. Embedded them. Into the gaps. Pressed them flat. Conductivity. Continuous. He measured. Shielding effectiveness. With a simple field-strength meter. Up close. Reading. Dropped. 40 dB. Baseline. Acceptable. Needed. Grounding. Optimization.

He took. Thick wire. 1.5 mm². One end. Soldered. To the inside. Of the shield box. One end. Connected. To an alligator clip. He marked. Ground point. Single-point grounding. To avoid loops. He drew. A wiring diagram. On paper. Parameters. Labeled clearly. Rolled the drawing. Secured it. Put it in. The inner layer. Of the canvas bag. The filter board. Packaged. Wrapped with insulating tape. Fixed in place. Interfaces. Tightened. Not loose.

06:10. Work finished. He turned off the light. Lay down. Wooden board. Hard. Cold. Air. Congealed. Breath. White mist. Dissipated. He closed his eyes. In his head. No curves. Only solder joints. Temperature. Time. Molten solder. Solidifying. Three lines. In the dark. Crossing. Without colliding. Without tangling. Each. Moving forward. He slept. Lightly. Easily woken. No dream. Only numbers. Jumping.

06:40. Awake. Alarm clock. Not yet rung. Body clock. Earlier. He opened his eyes. Ceiling. Water stains. Dried. He turned sideways. Left foot. Tested. Against the floor. Scab. Tightened. Stinging. He paused. Adjusted. His breathing. Shifted his center of gravity. Got up. Put on clothes. Movements. Light. Did not touch. The bed frame. Cold water. On his face. Piercing. He wiped it. Dry. Changed the dressing. Iodophor. Cotton swab. Scab. Edges. Whitish. No seepage. Wrapped it. Tight. Put on shoes. Tied the laces. Canvas bag. Heavy. But. Steady. Out the door.

06:55. Corridor. Empty. Sound-activated light. Broken. Footsteps. On terrazzo. Echoes. Short. Cold. Stride. Thirty centimeters. Did not step. On the frost line. Wind. Poured in. Through the window cracks. Low howl. He did not hunch. His shoulders. Breathing. Steady. White mist. Dissipated.

07:00. Laboratory. Iron door. Open. Heat. Not on. Room temperature. Below freezing. Group A. Reinforcement group. Already there. Nine people. Lin Chen. Sat. In the corner. Third bench. Set down the bag. Opened the case. Filter board. Shield box. Wiring diagram. Arranged them. On the desk. Neat. No clutter.

07:05. Old Li. Pushed open the door. Army coat. Collar turned up. Chalk dust. On his shoulders. He. Knocked. On the blackboard. Three times. The chalk. Snapped. White dust. Drifted down.

“Provincial mock contest. Begins. Interference sources. Already connected. Power-frequency. Fifty hertz. Injected into. The mains line. RF interference source. Antenna-coupled. Spatial radiation. Time limit. Six hours. Goal. Output signal-to-noise ratio. Greater than. Forty decibels. Waveform. No distortion. No self-oscillation. Instruments. Hook them up yourselves. Scheme. Adjust it yourselves. Record. The data. Out of tolerance. Eliminated. Begin.”

Stopwatch. Tick-tick. Metal casing. Reflecting light.

Lin Chen. Started working. Wiring. Banana plugs. Inserted. Tight. Not loose. Wire order. Red. Black. No crossing. Power. On. Indicator light. Lit. He connected. The interference source. Output terminal. To the filter network. Input end. Filter output end. To the system. Input end. Probe. Ground clip. Bit down hard. Metal. Cold. Shield box. Snapped on. Ports. Aligned. Gaps. Tight. He connected. The ground wire. Alligator clip. Clamped. Onto the lab’s grounding bar. Metal. Cold. Bite. Firm.

He adjusted. The signal generator. Frequency. 10.7 MHz. Amplitude. 10 mVpp. Waveform. Sine. Stable. He looked. At the oscilloscope screen. Green graticule. Lit. Waveform. Appeared. Sine wave. But. With spikes superimposed. 50 Hz. Power-frequency interference. Obvious. Large amplitude. He measured. Noise. Spectrum analyzer. Baseline. Raised. Spurious clutter. Dense. Reading. 18 dB. Not enough.

He stopped. Looked. At the wiring. Ground loop. He removed. The ground clip. Changed it. To the system chassis. Single-point ground. He measured again. Waveform. Spikes. Weaker. But. RF interference. Was coupling in. Through the shield box gaps. He checked. The seams. Copper mesh. Not pressed tight enough. He removed. The cover. Pressed the copper foil tape again. Reinforced it. Conductive adhesive. Spread evenly. Pressed flat. No gaps. He snapped. The cover back on. Ports. Aligned. Gaps. Tight.

He measured again. Waveform. Spikes. Gone. Sine wave. Smooth. No distortion. He measured. Signal-to-noise ratio. Reading. 42 dB. Up to standard.

Time. One hour twenty minutes. Left foot. Stabbing pain. Continuing. Gauze. Slightly wet. Sticky. He ignored it. Shifted weight. To the right. Breathing. Steady. Fingers. Numb with cold. Knuckles. Whitish. He. Breathed warm air on them. White mist. Scattered. Kept. Adjusting. Fine adjustment potentiometer. Knob. Dragging stiffly. He used force. Evenly. Did not strip it. Fingertips. Sensed. The change in resistance. Half a turn. Stop. Measured. Waveform. Stable. No self-oscillation.

Time. Two hours fifty minutes. Chen Hao. Turned in his work. Stood up. Left. Footsteps. Light. Confident. Door open. Door shut. Wind. Poured in. Cold.

Lin Chen. Did not look up. Kept going. Rechecked. Three sets. Of data. Recorded them. Cross-checked. Formulae. Verified the arithmetic. No errors. He checked. Wiring. No looseness. No short circuits. Probe. Ground clip. Bit tight. Metal. Cold. Shielding. Intact. Ground wire. Single point. No loop. Filter network. Temperature rise. Normal. No. Thermal drift.

Time. Four hours twenty minutes. He stopped writing. Powered down. Removed the wiring. Tidied. The desktop. No clutter. Wire ends. Straightened. Instruments. Returned to place. Knobs. Back to zero. Probe. Coiled up. Secured. Shield box. Removed. Filter board. Packaged. Put into. The inner layer. Of the canvas bag.

He stood up. Turned in his paper. Flat. Handed it over.

Old Li. Took it. Looked. Data. Three sets. Signal-to-noise ratio. 42 dB. 43 dB. 41.5 dB. Waveform. Stable. No distortion. He raised his head. Looked at Lin Chen. Dark circles. Lips. Chapped. Chilblains. Purpled. But his hands. Steady. Not shaking. Under his nails. Rosin. And copper rust.

“How did you handle. The ground wire?”

“Single point. To the chassis. Broke. The mains-ground. Loop. Avoided. 50 Hz coupling.”

“And the RF leakage. How did you suppress it?”

“Overlapped seams. Copper mesh. Embedded. Conductive adhesive. Pressed flat. No gaps. Shielding. Continuous.”

Old Li. Nodded. No praise. No criticism. “Sit down. Wait for the result.”

Laboratory. Quiet. Only. The faint sound. Of cooling fans. From the instruments. In the air. The smell of rosin. Growing thicker.

Six hours. Reached. Old Li. Collected the papers. Checked them. Scored them. Chalk. On the blackboard. Wrote. The rankings. Handwriting. Heavy. Thick.

Reinforcement group. Nine people. Lin Chen. First. Chen Hao. Second. Third through ninth. In order.

Old Li. Turned around. “Mock test. Passed. Tomorrow. You enter. The practical assembly assessment. System on bench. Running under load. Eight hours. Stability. And thermal drift. Assessed. Can’t do it. Eliminated.”

He paused. His gaze. Swept over. Lin Chen. “Lin Chen. Stay behind. There’s work.”

The students. Got up. Packed up. Left. Footsteps. Mixed. Door open. Door shut. Wind. Poured in. Cold.

In the laboratory. Only two remained. Old Li. Walked to. Lin Chen’s bench. Set down. An old oscilloscope. Casing. Deep scratches. Knobs. Loose. Model. Domestic make. Produced in 1990. Scan circuit. Aged. Trigger. Unstable. Waveform. Shaking.

“Old machine. Eliminated by the provincial team,” Old Li said. “If you can repair it. Repair it. Fix one. Fifty yuan. Cash. Paid on the spot. Can’t fix it. Work for nothing. Bring your own tools. Parts. From the storeroom. Three-hour limit. Start.”

Lin Chen. Looked. At the machine. Reached out. Touched. The casing. Cold. But. Slight vibration. Powered test. He did not hesitate. Opened the bag. Took out tools. Screwdriver. Multimeter. Soldering iron. Rosin. Solder. Insulating tape.

He. Opened the machine. Screws. Rusted. Hard to turn. He added force. Steady. No slipping. The casing. Opened. Dust. Thick. Circuit board. Yellowed. Solder joints. Oxidized. Blackened. He measured. Power supply output. Fifteen volts. Normal. Scan circuit. Aging. He checked. Time-base capacitor. Leakage. He removed it. Replaced it. Bent the leads. Fixed them. Soldering iron. Heated. Fed solder. Pulled away. Joints. Full. No burrs.

Trigger. Unstable. He adjusted. The trigger-level potentiometer. Knob. Stiff. He used force. Evenly. Did not strip it. Screen. Lit. Graticule. Stable. No flicker. He adjusted. The scan-speed switch. Poor contact. He disassembled it. Cleaned. The contacts. Alcohol. Cotton swab. Wiped them. Dry. Reassembled. Contact. Good.

Time. Two hours forty minutes. He assembled. The casing. Closed it. Tightened. The screws. Powered on. Tested. Signal. Input. Waveform. Stable. No shaking. Trigger. Accurate.

He stood up. Turned in the machine.

Old Li. Looked. At the screen. Nodded. “Fine. Fifty. Cash.” He took out. The money. Bills. Old. But flat. Handed them over.

Lin Chen. Took it. Slipped it into. The inner pocket. His fingers. Touched. The edge. Of the account book. Curled. Paper. Rough. Rubbing. Against the skin.

Old Li. “Tomorrow. Seven o’clock. Gather in the lab. Practical assembly assessment. Under load. Eight hours. System temperature rise. Will affect. Parameter drift. You need. Margin. If you can’t do it. Eliminated.”

“Understood.”

Lin Chen. Slung. The canvas bag. On his back. Went out the door. Corridor. Empty. Footsteps. On terrazzo. Echoes. Short. Cold.

He touched. His pocket. Fifty yuan. Plus. The previous. One hundred twenty-one yuan fifty. Total. One hundred seventy-one yuan fifty.

He felt. Neither joy. Nor sorrow. In his mind. Only equations. Routes. Countermeasures. Time windows. Physical limit. Three lines. In the dark. Crossing. Without colliding. Without tangling. Each. Moving forward.

He walked. Stride. Thirty centimeters. Did not step. On the frost line.

Ahead. Bulletin board. Posted. New notice. White paper. Black print. Outline for the practical assembly assessment. Issued. System under load. Running for eight hours. Must record. Thermal drift curves. And parameter attenuation. Deadline. Tomorrow. Fourteen hundred. Submit.

He stopped. Looked. Thermal drift curve. Parameter attenuation. Eight hours. He memorized. The parameter requirements. The route.

He turned. Went back. To the dormitory. Wind. Through the corridor. Low howl.

He closed his eyes. In his head. No curves. Only solder joints. Temperature. Time. Molten solder. Solidifying. Three lines. In the dark. Crossing. Without colliding. Without tangling. Each. Moving forward.

He opened his eyes. Stride. Thirty centimeters. Did not step. On the frost line.

Forward.