Chapter 92: Temperature Drift and Margin

The motion-sensor light in the stairwell was broken. Lin Chen braced himself against the wall, shifting his weight down one step at a time. Every time his left foot touched down, the flesh beneath the dressing felt like it was being scraped by a dull knife. He refused to dwell on the pain, focusing instead on the transfer of his center of gravity. Right foot bears the weight, left foot taps lightly, then switch. Breathing synchronized with his pace: one inhale for two steps, one exhale for two steps. Stepping out of the examination building, the wind hit him, carrying the provincial capital’s characteristic smell of coal smoke. He hunched his shoulders and zipped his old jacket all the way to the top. The coins in his pocket pressed hard against his thigh—seven yuan and three jiao. Hard, unyielding, not a single penny missing.

The guesthouse’s iron gate groaned on its hinges. At the end of the third-floor corridor, he slid the key into the lock and turned it twice. Pushing the door open, a wave of mildew mixed with cheap disinfectant washed over him. He closed the door behind him with his back and dropped his backpack onto the bed frame. He sat down and began to take off his shoes. The movements were deliberate and slow. The sock on his left foot had already fused with the gauze. He snipped the edges, poured saline over it, and used a cotton swab to peel it away bit by bit. The wound bed was red, with slight serous exudate at the margins, but no pus. He reapplied the ointment, re-bandaged it, and applied pressure. When he stood up, his vision blacked out for a second. He gripped the edge of the desk, waiting for his sight to clear. He poured a cup of hot water, fished out the last half of a compressed biscuit from his bag, broke it into pieces, and soaked it in the water. He chewed. He swallowed. A faint warmth settled in his stomach.

He opened his ledger. Seven yuan and three jiao. The practical exam was over. Return train ticket: three yuan and five jiao. Tomorrow’s breakfast: one yuan. Emergency reserve: two yuan and eight jiao. The accounts were clear, leaving zero margin. He closed the ledger and took out his error notebook. The pages were already curled at the edges. He needed to review. Not to comfort himself, but to quantify the error.

The pen tip touched the paper. He wrote out the practical steps from memory. Question One: Baseline calibration. Knob damping peak at 0.5 dB. Stepwise approximation. Error: 0.47 dB. Question Two: Attenuation linearity. The 30 dB range was critical; restored after retightening the connector. Question Three: Unknown signal capture. 14.23 MHz. Sine wave. Unmodulated. Data complete. Logical loop closed. He stopped writing. His gaze fell on a line of small text at the edge of his scratch paper: Exam room temperature. 8:00 AM. The corridor was cold. The equipment had just been powered on. The air blowing from the cooling vents was warm. He suddenly sat up straight. The internal crystal oscillator of the HP8591E was highly temperature-sensitive. A cold start meant the chassis temperature needed time to climb from 15°C to its operating range. The fifteen-minute warm-up merely powered the circuits. But the frequency stability of the oscillator was dictated by the ambient temperature gradient. The exam room window hadn’t been fully closed. A north wind was drafting in. The equipment’s left side was right in the path of the draft. When he manually compensated, he had only accounted for damping and scale backlash. He had missed the temperature drift.

He pulled out his calculator. The clicks of the keys were sharp in the quiet room. Crystal oscillator temperature drift coefficient, standard value ±0.5 ppm/°C. Exam room temperature differential: approximately 4°C. Frequency offset… He ran the conversion quickly. For a 14.23 MHz signal, under the influence of temperature drift, the actual reading could shift by 0.028 MHz. Converted to a dB error and superimposed on his manual calibration, the total error might approach 0.49 dB. The 0.5 dB deduction threshold. His fingers froze. The numbers on the calculator screen sat motionless. It wasn’t an equipment fault. It was pointer jitter. At the time, he’d assumed it was poor contact and had pressed the probe down harder. In reality, it was baseline noise fluctuation caused by temperature drift. He had missed an environmental variable.

He felt no frustration. He didn’t slam the desk. He simply set the calculator down, picked up his pen, and on a fresh page of the error notebook, wrote: Temperature drift compensation formula. Ambient temperature T. Initial equipment temperature T0. Time t. Heat dissipation coefficient k. Derivation. He wrote slowly. His handwriting was neat. Reality offered no “what ifs.” The exam paper was already handed in. The point deduction was a fait accompli. All he could do was patch the loophole. For next time. Or, if there was a next time.

Noise echoed from the corridor. Candidates from other schools were returning. Their voices were loud. Comparing answers. Complaining about the aging equipment. Some were crying. Some were laughing. Lin Chen shut the window tightly, sealing out the noise. He lay down. The springs of the iron bed frame gave a faint groan. His left foot still throbbed. He adjusted his posture, propping his foot up. He closed his eyes. There were no scores in his mind. Only formulas. Variables. Coefficients. That was how reality worked. Miscalculate one step, and the cost was already incurred. You could only accept it. Then keep moving forward.

4:00 PM. A broadcast crackled from downstairs. A notice from the Provincial Competition Committee. Comprehensive scores for theory and practical exams would be announced tomorrow at 9:00 AM. Those who passed would be added to the provincial team training roster. Those who did not could arrange their return travel today. Additional note: Due to batch discrepancies in the cold-start warm-up procedures for certain exam equipment, the committee would arrange an on-site re-evaluation tomorrow afternoon for candidates whose total scores fell within two points above or below the cutoff line. The re-evaluation would cover only Question Three, the signal capture frequency band. Relevant candidates were advised to monitor the bulletin.

Lin Chen opened his eyes. Sat up. Put on his shoes. His movements were slow but steady. He walked to the window and cracked it open a sliver. Cold air rushed in. He watched the bustling crowd below. Some were dragging luggage away. Others stood frozen in front of the bulletin board. He touched the coins in his pocket. Seven yuan and three jiao. He had to decide. Stay until tomorrow to see the results, or buy a ticket back today. Leaving meant forfeiting the chance for re-evaluation. Staying meant paying for another night’s lodging, which would drain his accounts to exactly zero.

He turned around. Took his notebook from his bag. Flipped to the last page. Wrote: Stay. Reason: Temperature drift error sits at the tolerance margin. Re-evaluation only tests Question Three. Requires on-site confirmation of scoring criteria and equipment status. If I pass, room and board for training are fully covered. If I fail, the loss is controllable. He closed the notebook. Walked to the sink. Splashed cold water on his face. Droplets traced down his chin. He dried off. Sat back down on the edge of the bed. Waited. The night was long. Tomorrow at 9:00 AM. The needle would point to a definitive number. He had to be ready to face it. Regardless of the outcome. The road was still beneath his feet.