Which device is used to measure frequencies that already exist?

The frequency counter is the device specifically designed to measure frequencies that already exist in a signal. It counts the number of cycles of a signal over a specific period of time and displays this as a frequency in hertz (Hz). This makes it particularly useful for accurately measuring the frequency of oscillating signals, which is key in various applications, including troubleshooting and maintaining electronic systems.

An oscilloscope, while it can display waveform shapes and measure time characteristics such as period and frequency indirectly, is not primarily designed for direct frequency measurement. A signal generator, on the other hand, is used to create signals of specific frequencies and does not measure existing frequencies. Similarly, a logic analyzer is geared towards analyzing digital signals and their timing but does not directly measure frequencies in the same manner as a frequency counter. Therefore, the frequency counter is the most appropriate choice for the question asked.