Frequency (f) is the number of repeating events per unit time.
So if something is vibrating at 20 times per second, then it’s said to have a frequency of 20 Hertz.
If something is vibrating at 200 times every 5 seconds, then it’s said to have a frequency of 40 Hertz.
From the examples above, you can see Frequency is measured in Hertz (Hz). And Hertz is named after the German physicist Heinrich Hertz.
One Hertz is defined as one cycle per second of a repeating event. In the old days, frequency was often called cps (cycles per second). You may see old, vintage electronics with marking in cps or kc (kilocycles). But starting in 1960, the cps unit was replaced officially by the Hertz.
Photo below of an RF Signal Generator, you can clearly see the frequencies marked in KILOCYCLES
and MEGACYCLES.
Now, the time duration of one (1) complete cycle is called it’s PERIOD, and is the reciprocal (or inverse) of frequency.
Mathematically, it’s stated as:
f = 1 / T
In the discussion below, we’ll be using a sinusoidal waveform (or sine wave for short).
In the example above, a total of (5) oscilloscope horizontal divisions is occupied by a single complete cycle of the waveform. And the oscilloscope time base is set to 2us per division. So one complete cycle of the waveform (or it’s PERIOD) takes up 5 div x 2us/div = 10us.
To get the frequency, we compute the reciprocal. The frequency of the above waveform therefore is:
f = 1 / 10us
f = 100 kHz
So… picture a bunch of sine waves, and if you cram 100,000 of these sine waves to occur in a single second, then what you have is a 100Khz sinewave. And if you dissect and determine how long a single sine wave cycle happens, you’ll see it happens every 10-microsecond.
Now the 100kHz example above is beyond human hearing. Human hearing range is much lower than that, approximately from 20Hz to 20kHz… give or take, depending on the listener’s age. Thus, the human ear can “hear” frequencies (another way of saying it is our eardrums can detect vibrations) whose time period is anywhere from 50 millisecond (ms) to 50 microsecond (us). Amazing if you really think about it.
