Square wave form factor

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negrocachuzo

Joined Oct 19, 2009
3
Hi everyone. I understand that the form factor of a square wave is 1, which means that it's RMS value equals it's average value. However if the square wave alternates between positive and negative values, say +5V and -5V with 50% duty cycle, wouldn't it's average value be 0 ? But it's RMS value would be 5V, right ? Then how would it's form factor be 1 ?


Thank you very much in advance :)
 

t_n_k

Joined Mar 6, 2009
5,448
Hi everyone. I understand that the form factor of a square wave is 1, which means that it's RMS value equals it's average value. However if the square wave alternates between positive and negative values, say +5V and -5V with 50% duty cycle, wouldn't it's average value be 0 ? But it's RMS value would be 5V, right ? Then how would it's form factor be 1 ?


Thank you very much in advance :)
I agree this can be confusing - the way I work out form factor for a simple symmetrical AC waveform with effective zero average value is to evaluate the form factor over a half cycle (or half period).

Alternatively evaluate the form factor for the entire period but "full wave rectify" the waveform to find the "average". This is what happens inside a simple averaging rectifier meter on AC range where the form factor is assumed to correspond to that of a sinusoid and this is applied to scale the indication.
Have you checked AAC site book on this?
See http://www.allaboutcircuits.com/vol_2/chpt_1/3.html
 

Ratch

Joined Mar 20, 2007
1,068
negrocachuzo,

Hi everyone. I understand that the form factor of a square wave is 1, which means that it's RMS value equals it's average value. However if the square wave alternates between positive and negative values, say +5V and -5V with 50% duty cycle, wouldn't it's average value be 0 ? But it's RMS value would be 5V, right ? Then how would it's form factor be 1 ?
You could ask the same question of a sine wave. By strict definition of the average, the sine wave average is 0. The resolution is that the definition of average has been modified for electronics by selecting the absolute values of each point on the wave. That takes care of excursions below zero.

Ratch
 
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