I have a so-called "modified sine wave" inverter that is connected to 200+
amp hours of deep discharge battery located in about the center of the
house in the basement. SNIPPED

meanwhile is there a filter I can
apply to the output of the inverter to round off the corners of the
modified sine wave (which is really nothing but a stairstep square wave)
and eliminate the worst of the harmonic noise?
-----

It is generally quite easy to construct a filter to eliminate the square wave
component[s] in an approximated sine wave.

First, determine how many steps are used, what is the frequency of the square
wave, in one cycle to make the 'sine' wave?

Second, a three stage pi-filter is designed to severely reduce the conducted
interference.

The first stage is series resonant at the fundamental square wave frequency and
is across the hot line to return [not ground].

The second stage is parallel resonant in the both paths after the series filter.

The third stage is identical to the first stage, but located at the output of
the parallel resonant filter.
--L--
---------------| |---------- [Hot]
L --C-- L
| |
C C
| --L-- |
--------------| |----------- [Return]
--C--
NOTE: The Return must be an isolated wire just like the Hot line. The return
line MAY be connected to inverter chassis by a 'zero current' carrying
connection [approximately 50 Kohms] ONLY at one point.

It will take a little effort to locate suitable cores for the inductive
component. The capacitors can be electrolytic up to several 100 KHz. The key is
to establish RESONANCE at the fundamental component of the square wave.

----

An alternative may be to replace the square wave generator with a simple op-amp
and power stage configured as a WEIN Oscillator. A Wein oscillator has
negligible harmonic component. One op-amp and a couple of transistors should do
the trick.

/s/ DD, W1MCE


Rick (W-A-one-R-K-T) wrote:

About a 30-foot length of #14 wire goes to an
outlet box in the shack, down on one end of the house, and provides 110VAC
(of a sort) when the power is off and I don't feel like hauling out the
generator. :-)

Trouble is, whenever I turn on the inverter it spews harmonics all up and
down the HF band and makes it difficult to impossible to hear weak signals.

I realize that a pure sine wave inverter would mostly alleviate this but
that's not gonna happen any time soon...

Thanks...