For instance, for a 50% duty cycle, what's the advantage/disadvantage of using 01010101 rather than 00001111?

  • \$\begingroup\$ The two are the same, just with a slower pulse rate. So, you could use a high pulse rate and do 00001111, or pulse at 1/4 the rate and do 01010101. \$\endgroup\$
    – JRE
    Oct 21, 2015 at 16:56

2 Answers 2


As in so many engineering questions, the answer is "it depends".

Suppose you are just generating a voltage by filtering the PWM. In that case, the cost of switching is very low and you will get less ripple in the output by using your first example.

However, in the case of PWM to a massive load, perhaps using relatively slow-switching devices such as IGBTs, the cost of each switch transition is high and you may wish to minimize switch transitions- perhaps even to the point of using "magic sinewaves".

  • \$\begingroup\$ Your link points back to this answer. Did you intend to make it a reference to Don Lancaster's site? \$\endgroup\$
    – Dave Tweed
    Oct 21, 2015 at 16:54
  • \$\begingroup\$ @DaveTweed Rather to the less relentlessly self-promotional article in EETimes- thanks for pointing out the bad link. \$\endgroup\$ Oct 21, 2015 at 18:03

In many cases, a device which is being controlled by a PWM signal will effectively lengthen or shorten pulses by some unpredictable amount. If one has the ability to turn the output on or off once per microsecond and a device may arbitrarily lengthen or shorten each pulse by up to 100ns, then a sequence of PWM output patterns which successively has 1, 2, 3, or 4 separate "1" intervals per 8us interval, and then 3, 2, or 1 separate "0" intervals per 8us interval, would yield average output values (in eights) of

1 +/- 0.1 = 0.9 to 1.1
2 +/- 0.2 = 1.8 to 2.2
3 +/- 0.3 = 2.7 to 3.3
4 +/- 0.4 = 3.6 to 4.4
5 +/- 0.3 = 4.7 to 5.3
6 +/- 0.2 = 5.8 to 6.2
7 +/- 0.1 = 6.9 to 7.1

There's a rather nasty bit of uncertainty in the middle, and if the device tends to ride the high or low side of the range the outputs will be decidedly non-linear. By contrast, if one only has one rising edge and one falling edge per 8us interval the output values would be:

1 +/- 0.1 = 0.9 to 1.1
2 +/- 0.1 = 1.9 to 2.1
3 +/- 0.1 = 2.9 to 3.1
4 +/- 0.1 = 3.9 to 4.1
5 +/- 0.1 = 4.9 to 5.1
6 +/- 0.1 = 5.9 to 6.1
7 +/- 0.1 = 6.9 to 7.1

Much more predictable, and much less likely to introduce non-linearity.

Reducing switching will thus minimize some kinds of non-linearity and unpredictability in the output response, but will also increase the amount of low-frequency content in the output, which may be harder to filter than high-frequency content would be.

A hybrid approach which may be useful if one e.g. wanted to output about 225 distinct PWM values using a 1us sampling clock but one wanted to minimize the amount of frequency content below 50Khz would be to have every 256us interval contain 16 groups of consecutive 1's, which could vary in length by 1us. The lowest output level (code 16) would have sixteen 1us pulses every 256us; the next higher value would have fifteen 1us pulses and one 2us pulse. The next would have seven 1us pulses, then a 2us pulse, then seven 1us pulses, then a 2us pulse. The next would have five 1us, one 2us, four 1us, one 2us, four 1us, one 2us. So long as one only uses code values within the middle 14/16 of the range, this approach would yield consistent linear behaviors while minimizing the amount of lower-frequency content. I don't know why I never see hardware to implement this, but it's certainly workable in software.

  • \$\begingroup\$ You have seen it, but perhaps you didn't recognize it for what it is. It's called a "delta-sigma modulator". \$\endgroup\$
    – Dave Tweed
    Oct 21, 2015 at 17:04
  • \$\begingroup\$ @DaveTweed: I thought delta-sigma modulators were normally used in cases where frequency-dependent non-linearity in the output transforms is well-characterized (and often deliberately introduced). What I had in mind would basically be the ability to cascade two 8-bit PWM generators such that one of them would control whether the other would have its output indicate whether the count was less than the programmed value, or indicate whether the count was less than or equal to the programmed value. \$\endgroup\$
    – supercat
    Oct 21, 2015 at 18:09
  • \$\begingroup\$ Yes. In both cases, you're doing what's called "noise shaping". It's just that the delta-sigma modulator does implicitly what you're trying to do explicitly. That's why everyone does it that way. \$\endgroup\$
    – Dave Tweed
    Oct 21, 2015 at 19:34

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