What instrument is employed to measure frequencies already present in a circuit?

The appropriate instrument for measuring frequencies that are already present in a circuit is a frequency counter. This device is specifically designed to measure the frequency of an input signal and display it numerically.

Frequency counters operate by counting the number of cycles of a waveform within a specified time period, providing an accurate measurement of the frequency in hertz (Hz). Their ability to analyze signal frequency makes them ideal for applications where precise frequency measurement is necessary, such as in communications, RF design, and circuit troubleshooting.

While oscilloscopes can also display frequency information by allowing the user to visualize the waveform and calculate the frequency from the time base settings, they do not directly provide a numerical frequency reading as a frequency counter does. Signal generators generate signals at specific frequencies rather than measure existing ones, and multimeters typically focus on measuring voltage, current, and resistance but do not have the capability to measure frequency accurately.