[time-nuts] The home time-lab

David davidwhess at gmail.com
Mon Jul 25 15:32:42 EDT 2016

The reason they call it a modified sine wave is that it is a square
wave with the same peak and RMS voltages as a sine wave.  Since the
RMS value of a (bipolar) square wave is equal to its peak value, it
has to include parts at zero or a lower voltage.  Some inverters use
additional voltage steps to more closely approximate a sine wave but I
think they are less common now that true sine outputs have become more

I do not know why Don Lancaster's Magic Sinewave idea is not used more
widely.  Does it have patent issues?  Maybe it is not economical
compared to a true sine output.

In the past I have used the predicable sine average responding and RMS
values of a square wave to calibrate its peak-to-peak value for use as
a calibration source.  This is useful for calibrating analog
oscilloscopes if you have an uncalibrated source but a good sine
average responding or RMS (or both for a sanity check) AC voltmeter.

On Mon, 25 Jul 2016 10:29:02 -0700, you wrote:

>Hi Chris:
>The APC RS1500 uses what they call modified sine wave, but I call modified square wave, i.e. it's a square wave with a 
>couple of parts that are at zero volts.
>Don Lancaster promoted "Magic Sinewaves" where a pulse modulated waveform drives an H-bridge.  The leading and trailing 
>edges are determined using the idea of FFT so that all the harmonics up to some number (typically 9 to thirty something) 
>are zero.  There were also 3-phase versions.  But he no longer sells any hardware.
>PS I'm looking for a source of 3-phase 400 Hz 115 VAC to power a North Finding Gryo.

More information about the time-nuts mailing list