Chapter 92: Temperature Drift and Margin

The motion-sensor light in the stairwell was broken. Lin Chen braced himself against the wall and edged his way down one step at a time. Each time his left foot touched the ground, the flesh beneath the dressing felt as if it were being scraped by a dull blade. He did not think about the pain. He focused only on shifting his center of gravity. Right foot bearing the weight, left foot touching down lightly, then switch. His breathing matched his cadence: inhale for two steps, exhale for two steps. When he walked out of the examination building, the wind rushed at him carrying the provincial capital’s familiar smell of coal smoke. He hunched his neck and pulled the zipper of his old jacket all the way up. The coins in his pocket pressed against his thigh: seven yuan and three jiao, hard and solid, not a single fen missing.

The iron gate of the guesthouse creaked. At the room at the end of the third-floor corridor, he slid the key into the lock and turned it twice. When he pushed the door open, a wave of mildew mixed with cheap disinfectant rushed at him. He closed the door behind him, tossed his backpack onto the bed board, sat down, and took off his shoes. He moved very slowly. The sock on his left foot had already stuck to the gauze. He cut open the edges, poured saline over it, and peeled it away bit by bit with a cotton swab. The wound surface was red, with a slight seep of tissue fluid at the edges, but no suppuration. He applied the medicine again, rebandaged it, and added pressure. When he stood up, his vision went black for an instant. He caught himself on the edge of the table and waited for his sight to return. He poured a cup of hot water and fished the last half of a compressed biscuit out of his bag. He broke it into pieces and soaked it in the water. Chewed. Swallowed. A little warmth returned to his stomach.

He opened his ledger. Seven yuan three jiao. The practical exam was over. Return fare: three yuan five jiao. Tomorrow’s breakfast: one yuan. Two yuan eight jiao held in reserve for emergencies. The accounts were clear. There was no margin. He closed the ledger and took out his notebook of corrected mistakes. The pages were already curling at the edges. He needed to review. Not to comfort himself, but to confirm the error.

The tip of his pen touched the paper. He wrote out the practical procedure from memory. First question: baseline calibration. Knob damping peak at 0.5 dB. Incremental approximation. Error: 0.47 dB. Second question: attenuation linearity. The 30 dB range was critical; after retightening the connector, it returned to normal. Third question: unknown-signal capture. 14.23 MHz. Sine wave. No modulation. Data complete. Logic closed. He stopped writing. His eyes fell on a small line at the edge of the scratch paper: exam-room temperature. Eight in the morning. The corridor had been cold. The equipment had only just been powered on. The air blowing from the cooling vent had been warm. He suddenly sat upright. The internal crystal oscillator of the HP8591E was temperature-sensitive. On a cold start, the temperature inside the chassis needed time to rise from fifteen degrees to its operating temperature. Fifteen minutes of preheating only meant the circuitry had power. The oscillator’s frequency stability was still affected by the ambient temperature gradient. The exam-room window had not been shut tightly. North wind had been pouring in. The left side of the machine had been near the draft. When he made his manual compensation, he had accounted only for damping and scale backlash. He had missed temperature drift.

He pulled out his calculator. The click of the buttons was sharp in the quiet room. Standard crystal-oscillator temperature-drift coefficient: plus or minus 0.5 ppm per degree Celsius. Temperature difference in the exam room: about four degrees. Frequency offset... He converted it quickly. For a 14.23 MHz signal, under the influence of temperature drift, the actual reading might be offset by 0.028 MHz. Converted into dB error and added to the manual calibration, the total error might approach 0.49 dB. The 0.5 dB deduction line. His fingers stopped. The numbers on the calculator screen stayed motionless. It had not been equipment failure. It had been pointer jitter. At the time, he had thought it was poor contact and pressed down on the probe again. In truth, it had been fluctuation in the baseline noise caused by temperature drift. He had missed an environmental variable.

He did not grow upset. He did not slap the table. He only set down the calculator, picked up his pen, and wrote on a new page of the notebook: temperature-drift compensation formula. Ambient temperature T. Initial equipment temperature T0. Time t. Heat-dissipation coefficient k. Derivation. He wrote very slowly. His handwriting was neat. Reality had no ifs. The exam paper had already been handed in. The deduction was an established fact. All he could do was patch the hole. Next time. Or, if there was a next time.

Noise came from the corridor. Candidates from other schools had returned. Their voices were loud. Comparing answers. Complaining about how old the equipment was. Some were crying. Some were laughing. Lin Chen shut the window tight, cutting off the noise. He lay down. The springs of the iron bed gave a faint groan. His left foot was still swollen and aching. He shifted his posture and propped the foot up higher. He closed his eyes. There were no scores in his head. Only formulas. Variables. Coefficients. That was how reality worked. If you miscalculated one step, the cost had already occurred. All you could do was accept it. Then keep walking forward.

At four in the afternoon, a broadcast came from downstairs. A notice from the provincial polytechnic admissions office. The combined results for the specialized theory and laboratory operation exams would be announced at nine o’clock tomorrow morning. Those who reached the interview cutoff would enter the final interview list. Those who did not could arrange their return today. In addition: because certain batches of equipment in some exam rooms had differed in cold-start preheating, the admissions office would arrange an on-site review tomorrow afternoon for candidates whose total scores fell within two points above or below the interview line. The review would concern only the signal-capture band in Question Three. Relevant candidates were asked to watch for the announcement.

Lin Chen opened his eyes. Sat up. Put on his shoes. His movements were slow, but steady. He walked to the window and pulled it open a crack. Cold air rushed in. He watched the bustling crowd below. Some were dragging luggage away. Some stood staring blankly at the notice board. He touched the coins in his pocket. Seven yuan three jiao. He needed to decide. Stay until tomorrow to see the result, or buy a ticket and go back today. To go back would mean giving up the chance for review. To stay would mean paying for another night’s lodging and reducing his accounts to absolute zero.

He turned around, took a notebook from his bag, flipped to the last page, and wrote: Stay. Reason: temperature-drift error lies at the edge of tolerance. The review covers only Question Three. Need to confirm the scoring criteria and equipment status on site. If I pass, I will still qualify for the final interview; if not, the loss is controllable. He closed the notebook, went to the sink, and washed his face with cold water. Droplets ran down from his chin. He dried himself off, sat back down by the bed, and waited. The night was still long. Tomorrow at nine. The pointer would settle on a definite number. He had to be ready to face it. Whatever the result. The road was still under his feet.