I started with a ship full of max-level veterans, and I'm just a complete noob.

The same autumn rain fell with a somber tone on the bluestone slabs of the front street post office, but on the windowpanes of the No. 1 workshop research room, it conveyed a sense of anxiety.

As the chalk drew its first arc on the blackboard, a dull thud was heard as Fang Xudong slammed his teacup onto the table.

Jiang Ming stood with his back to them in the middle of the blackboard, holding the chalk box in his left hand and wielding the chalk in his right. He first drew a square, marking each of the four vertices with a small circle. Then, starting from the top left corner, he drew a diode symbol on each of the four sides in a clockwise direction, with the arrows alternating in direction, forming a circular bridge connecting the beginning and the end.

"These are four Yuanlei No. 2 germanium diodes." Jiang Ming wrote the three letters "DBM" in the center of the bridge. "This is the core structure of a dual-balanced mixer."

Fang Xudong had already walked to the front of the blackboard, his gaze sweeping back and forth along the connection direction of the four diodes twice, his fingers unconsciously rubbing against the seam of his pants.

"RF enters from here." Jiang Ming drew an arrow on the left port, labeled RF. "Local oscillator enters from above." The upper port is labeled LO. "IF is taken from the right." The right port is labeled IF.

Wu Hanzhang stood near the door, his cotton-padded coat still damp with the dampness of the rain.

He kept his hands in his pockets, remained silent, and stared intently at the four alternating arrows on the blackboard.

Jiang Ming put down the chalk box, replaced it with a short piece of chalk, and drew a diagram of the direction of the current next to the bridge.

"Assuming the frequency sweep interference signal enters from the antenna end, sweeping between 5.2 and 5.4 MHz, with an amplitude much larger than the useful signal." He drew a large-amplitude waveform on the RF port. "After this interference signal enters the bridge circuit, it will pass through the two upper diodes and the two lower diodes simultaneously."

The chalk drew an electric current arrow on each of the four tubes, all pointing in the same direction.

"Due to the geometric symmetry of the bridge circuit, the interference current has the same amplitude and phase on the two paths of the intermediate frequency port." Jiang Ming drew two arrows in opposite directions on the IF port. "However, the windings of the intermediate frequency port are differentially connected, and the in-phase signals cancel each other out here."

Two opposing arrows combine to form zero, and Jiang Ming drew a large equal sign and the number zero next to it.

Fang Xudong's lips moved slightly, but he did not interrupt.

"Any useful signals?" Wu Hanzhang took two steps closer from the doorway and asked.

"The useful signal has a low frequency and small amplitude. When it passes through the four diodes, it is modulated by the action of the local oscillator switch." Jiang Ming drew a square wave at the local oscillator port. "The local oscillator is equivalent to a periodic switch, which flips the conduction direction of the diodes in the bridge circuit once every half cycle."

He quickly drew two diagrams: one showing the equivalent circuit during the positive half-cycle of the local oscillator, and the other showing the equivalent circuit during the negative half-cycle.

"The useful signal travels through the two upper transistors during the positive half-cycle and through the two lower transistors during the negative half-cycle. When it reaches the intermediate frequency port, the phases are exactly opposite. The differential windings add them together instead of canceling them out."

An amplified sine wave appeared next to the IF port.

"So the result is: the interference is canceled out, the useful signal is preserved and down-converted to the intermediate frequency." Jiang Ming wrote a line on the right side of the blackboard: common-mode rejection ratio greater than or equal to 20dB.

Fang Xudong finally reached out and picked up another piece of chalk, drawing a question mark next to the bridge and road map.

"What about the matching degree of the diodes? If the characteristics of the four tubes are not consistent, the cancellation will not be perfect."

"Of the tubes Lao Sun made yesterday, the forward and reverse characteristic deviations of the fifteenth to nineteenth tubes are within three percent." Jiang Ming turned to face Fang Xudong. "I picked the four closest ones, with a forward resistance difference of no more than five ohms, a reverse resistance difference of no more than eight hundred ohms, and a junction capacitance difference of no more than 0.05 picofarads."

Fang Xudong erased the question mark.

How is a balun transformer made?

"Double-wire parallel winding, high-permeability ferrite magnetic ring, twelve turns of winding, impedance transformation ratio of 1:1, bandwidth covering the mid-frequency band from four MHz to ten MHz." Jiang Ming had already drawn a schematic diagram of the balun's winding method below the blackboard, and the cross-section of the magnetic ring was marked with two colored lines to indicate the direction of the two wires.

Wu Hanzhang walked to the far right of the blackboard and pointed to the line "Common mode suppression greater than or equal to 20dB".

What does 20 decibels mean?

"The receiver at the front line overloaded under 1 kilowatt of interference and entered the nonlinear region, generating third-order intermodulation. The intermodulation products were only a dozen decibels lower than the useful signal, so the direction finding completely failed." Jiang Ming put down the chalk and wiped the white dust off his hands with his sleeve. "After adding a dual-balanced bridge, the interference signal was suppressed by more than 20 decibels before mixing. The front end was no longer overloaded, the intermodulation products disappeared, and the signal-to-noise ratio of the useful signal was at least restored to a level where the direction could be determined."

Fang Xudong twirled the chalk in his hand twice, put it back in the chalk tray, and took a step back to look at the entire blackboard.

From the bridge topology in the upper left corner, to the current direction analysis in the middle, to the common-mode rejection conclusion on the right, and then to the balun winding and intermediate frequency filter network at the bottom.

He stared at it for a long time, then let out a muffled syllable, as if he wanted to say something but swallowed it back.

Wu Hanzhang did not give him time to think.

"This diagram needs to be compiled into a document by this afternoon. It's classified as Top Secret and numbered 502." He turned to Jiang Ming. "Can you do it?"

"Tonight."

Wu Hanzhang nodded, turned and walked towards the door, but stopped and looked back after taking two steps.

"Where are Old Sun's four matching tubes now?"

"Locked in my drawer."

"Take it out, we'll start soldering this afternoon."

After the door closed, the research room was quiet for a few seconds.

Fang Xudong stood in front of the blackboard without moving, his gaze fixed on the word "DBM" in the middle of the bridge.

The autumn wind outside the window blew a few locust leaves from the yard down, which stuck to the glass window and were washed away by water droplets. The light and shadow flickered on the blackboard.

Fang Xudong finally turned around and looked at Jiang Ming's back as he packed up the chalk box.

Without saying a word, he simply turned to a new page in his notebook and wrote a line on it: "Dual balanced mixer, four-tube bridge cancellation, active preamplifier."

Then he closed the notebook, walked to the soldering station, and began cleaning the surface.

At 2 p.m., Jiang Ming took the four paired Yuanlei No. 2 tubes out of the cotton box and used tweezers to check the lead wire solder joints one by one to confirm that there were no poor solder joints or cracks.

The ferrite magnetic ring was found among the spare parts brought by the physics team. It has a diameter of eight millimeters, a magnetic permeability of three thousand, and a smooth, crack-free surface.

The double-wound wire used is enameled wire with a diameter of 0.2 millimeters. Old Sun helped by using a hand-held winding frame to tightly wind it around, round after round. After twelve rounds, the two wire ends were led out and coated with shellac to fix them.

The soldering lasted from 3 p.m. to 7 p.m. Xiao Zhao handed over solder wire and rosin, while Da Liu was responsible for cutting the shielding copper foil and bending the grounding terminal.

The first dual-balanced mixer was assembled at 8 o'clock. Fang Xudong connected the test instrument, set the local oscillator power to 10 milliwatts, and connected the RF end to a 5.25 MHz signal simulated by a signal generator.

The level meter pointer at the intermediate frequency output jumped up and stopped steadily at the expected position.

"Isolation level?" Jiang Ming asked.

Fang Xudong measured a set of data between the radio frequency end and the local oscillator end and recorded it: "Twenty-three decibels".

The second one was completed at 9:30, with an isolation level of 21 decibels.

Jiang Ming put the two mixers into wooden boxes lined with shock-absorbing cotton, closed the lids tightly, and sealed the outside with tape.

He wrote the numbers DBM-01 and DBM-02 on the box lids with a pencil, then placed the two wooden boxes side by side on the table, withdrew his hands from the lids, and his fingers paused on the edge of the table.

It was completely dark outside the window, and the streetlights in the yard cast a hazy, murky glow in the rain and mist.

Wu Hanzhang pushed open the door, took the wooden box, signed the handover form, and left quickly.

Jiang Ming sat at his worktable, with a sheet of graph paper spread out in front of him and a pencil lying motionless on it.

Fang Xudong was packing up the instruments on the other side, hanging the oscilloscope probe back on the rack in a loop, when he suddenly spoke.

"I just remembered someone while I was measuring the isolation level."

Jiang Ming looked up at him.

"Never mind." Fang Xudong covered the instrument with the cover and turned off the lamp. "We'll talk about it tomorrow."