Oscilloscopes and test instruments are the bench instruments that engineers use to see and measure electrical signals while chips, boards and phones are designed and made. The best known is the oscilloscope, which most engineering students meet in a college lab. It draws a graph of a voltage changing over time: voltage up the screen, and time across it.
A modern oscilloscope does not draw the signal directly. A probe touches a point on the circuit. The oscilloscope measures the voltage there billions of times a second, stores the readings, and draws them as a line. It can stop on a single event, such as the moment a voltage falls below a level that the engineer sets. This setting is called a trigger. Two numbers describe an oscilloscope. Its bandwidth is the fastest signal it can measure correctly. Its sample rate is how many readings it takes each second.
Other instruments measure other things. A spectrum analyser shows how strong a signal is at each radio frequency. Engineers use it to test a phone's RF front end, the chips between its antenna and its modem. A signal generator produces exact test signals. A power analyser measures how much current a device draws. Instruments that check fibre and network equipment have their own page, network test equipment.
Say a phone assembly plant near Chennai finds that some new phones restart by themselves when the camera opens. A hardware engineer takes one of the phones' main boards to the lab. She connects an oscilloscope probe to the 0.8-volt supply that feeds the phone's main processor. She sets the trigger to stop the display when the voltage falls below 0.75 volts. Then she opens the camera. The display stops on the moment: the supply falls to 0.68 volts for 3 microseconds, just as the camera module draws a burst of current. A microsecond is a millionth of a second. That dip is enough to make the processor restart. The cause is a capacitor on the board, a part that stores a little charge to cover such bursts. It has a lower value than the design specifies. The plant replaces that batch of capacitors, and the restarts stop.
Engineers also use these instruments on analog and mixed-signal ICs, the chips that turn real-world signals into numbers. At an OSAT (chip assembly and test) plant, engineers use them to set up and check the automated test machines. Most instruments can be controlled from a computer. So engineers write scripts that run hundreds of measurements and save the results. Labs connect their instruments through lab instrument integration, and analyse the readings in a numerical computing environment such as MATLAB.
The leading instrument makers have engineering centres in Delhi, Bengaluru and Kolkata, which write much of the software that runs inside the instruments.