Tom Bruhns wrote:
"Clifton T. Sharp Jr." wrote in message ...
Fred McKenzie wrote:
Full-wave unfiltered rectification, followed by bandpass filter?

Clifto-

Doesn't full wave produce a symmetrical waveform that minimizes even harmonics?

It's not exactly symmetrical. Rectifying a sine wave produces what looks
like a sine wave with pointy lower peaks, at twice the input frequency.
A little decent filtering at 2F, and the pointy lower peaks go away.

Well, actually the whole output wave is at 2* the input, which is I
believe what the OP wanted: a freq doubler. Note that rectifying a
sinewave is the same as multiplying by a square wave of the same
frequency which is +1 when the sine is positive and -1 when it's
negative. You can use trig identities and the fact that the square
wave is its fundamental and all odd harmonics to convince yourself
that the "ideal" full wave rectifier puts out DC, 2*fin, 4*fin, 6*fin,
... -- just the even harmonics and no odds. It's only because the
rectification is imperfect that the fundamental or odd harmonics get
through. The pointy lower peaks must represent higher order even
harmonics of the input frequency.

http://www.rfcafe.com/references/electrical/periodic_series.htm has
a nice exposition of Fourier series for various waveforms, including
half- and full-wave rectified sine waves. The Rubber Bible math table
book used to have the waveforms and Fourier series for them, too.

I first got interested in them when I was about 11, in 1957. Somewhat
later I took the math class where we derived them. It was sort of
interesting to see my childhood friends constructed on a blackboard.

--
Mike Andrews

Tired old sysadmin since 1964