# Difference between Hertz and Duty Cycle

I'm super new to electronics, but I wanted to clarify something just in case I was wrong:

Duty Cycle is the measurement of what the percentage is High and Low from when a signal starts from low and goes high, stays high until it goes back low and ends right before the signal is about to go high again.

Hertz is the number of duty cycles that occur every second, so if there was only one duty cycle in one second, it would only have 1 hertz, if there were 50 duty cycles in one second, there would be 50 hertz?

• Look at the units. Duty cycle is expressed in %; whereas frequency is expressed in Hz or cycles per second. – jippie Feb 16 '15 at 22:57
• You kind of answered yourself in your question (in that your second paragraph makes no sense). If duty cycle is the high to low ratio of each pulse, what exactly is "50 duty cycles" supposed to mean? What you should say is 50 cycles at a % duty cycle. # of cycles is not the same thing as the duty cycle. – I. Wolfe Feb 16 '15 at 22:58
• For my opinion - and to be exact - the unit "Hertz (Hz)" is reserved for sinusoidal waveforms only. For all other waveforms (square, triangel,...) we should use the unit "repetition rate". – LvW Feb 17 '15 at 8:45

See the image:

Frequency = $\dfrac{1}{T_{cycle}}$ and Hertz is the unit of frequency. It gives the number of cycles in one second.

Duty cycle = $\dfrac{T_{high}}{T_{cycle}}$. It gives fraction of one cycle for which the signal is high.

Yes Hertz is cycles per second but refers more broadly to say sine waves, square waves, events, etc. Duty cycle is only loosely related in this context.

So 50 cycles per second of a square wave is 50Hz regardless of whether it has a 50/50 duty cycle or a 10/90, or whatever it is.

• 50 cycles per second* – I. Wolfe Feb 16 '15 at 22:51
• @I.Wolfe Good point, edited accordingly. – Some Hardware Guy Feb 16 '15 at 23:21

Hehehe, duty.

But in all seriousness, Hz is a measure of cycles per second. For example, driving a PWM servo motor, the standard period is 20 ms I believe, which is 50 Hz. Inside of that, the duty cycle is a measure of active period (could be active high or low) as a percent of the total period time.

The servo control comes from duty cycle. If there is 2 ms of asserted signal, and 18 ms of deasserted signal, you now have a 50 Hz signal, with a 10% duty cycle. Conversely, if you have 18 ms of asserted signal, followed by 2 ms of deasserted signal within the single 20 ms period; you still have a 50 Hz signal, but now you have a 90% duty cycle.