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February 23rd 05, 11:35 PM

On 22 Feb 2005 15:42:00 -0800, wrote:

Paul, I don't want to sound critical here, but this newsgroup is
about "homebrew." In that light, I'd suggest a simple quad
op-amp circuited as two phase-shift oscillators (pick the AF
desired) out of two of the op-amps in the quad package, use
the third as a resistive summing mixer.

Did I miss something or are you suggesting using a Wien bridge circuit
as an audio oscillator ? How should the amplitude be stabilised in
order to get nearly a sine wave output ?

I suggested an R-C Phase Shift oscillator. With a resistor
feedback to set the gain of the op-amp to just overcome the
loss in the R-C network (3 caps, same value; 3 resistors,
same value) feeding back to itself. It isn't much used but it
has appeared in nearly all textbooks I've got. With stable
supply voltages, the sinewave output will be fairly pure and
reasonably constant in amplitude.

While I guess that quite a few radio amateurs with professional
background in electronics are following this newsgroup, do not forget
that most young radio amateurs (with limited test equipments) are much
more used to playing with computer programming and computer hardware
than working with circuit design.

Well that may be, but consider that a personal computer
CANNOT build all of the simple sources (and attenuator)
necessary for a two-tone IMD test of a transmitter.

Don't get me wrong on computers. I think they are one
of the miracles of technology of the last millennium. I've
written my own circuit analysis program(s) and wrote a
filter component synthesis (and response analysis)
program on my PC and have used them extensively. They
are excellent in SIMULATING a given circuit (with the
proper modeling). Computers cannot build a circuit and
sometimes need extra, out-board circuits to function in
a particular application (a 40 db attenuator for example).

A single DVM and a hot soldering iron are basic tools
for any hobby electronics workshop, but they have
limitations on what they can do. If I wanted a "first
look" on the purity of a sinewave, I'd check it with an
oscilloscope first. A DVM would read the average (or
whatever) amplitude, depending on its AC limitations,
but that's about the limit. A simple oscilloscope
doesn't cost the same as a couple of mortgage
payments but it WILL show the purity of a generated
sinewave, something necessary to prove that an IMD
two-tone test source IS good enough for test purposes.
There are limitations on what a particular hobby
workshop can do; sometimes what one wants to do
conflicts with what must be there to accomplish
something. That alone will determine what "the
future" planning must be.

Both the traditional approach as well as some more day to day approach
is required, in order to make amateur radio a viable hobby even in the
future.

I don't know what the "traditional approach" is in
regards to a hobby activity (homebrewing IS a
hobby activity). I've been a hobbyist in various
electronics since 1947, a computer programming
hobbyist since 1976. Doing some things requires
more workshop equipment than a DVM and a hot
soldering iron. Doing some things requires self-
education into the whichness of the what in
circuitry in order to accomplish something. That's
just the way things are in a technologically
oriented hobby...like amateur radio activity
beyond just using radio equipment to talk to
people far away.

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