In article ,
Gary Schafer wrote:

Yes I know it is a standard IMD test. :) That is why I asked what
they were doing with two tones without a spectrum analyzer.

You left out the last part of my post.
"To do much else with it you also need a spectrum analyzer."

Here's my guess: it's a standard, which would be available in almost
any professional radio test lab.

Hence, it may simply be a convenient signal to use when performing
ALC and audio-pathway calibrations.

As is sometimes said, "The best thing about a standard is, sometimes,
that it's a standard. If it happens to be a _good_ standard, so much
the better!"

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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.

wrote:



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.

Might not be able to 'build' a tone generator but can certainly generate
a pair of tones...adjustable to boot.

-Bill

I can get down to 0.01% THD with 4th order active filtering of a square
wave -- as measured on an HP339 THD analyzer. For the fellow who had asked
the original question I suggested the switched cap filter -- but there is
always going to be some noise clocking through. Using the interactive
filter software on Analog Devices or Texas Instruments website is pretty
easy, but necessitates more parts.

Sorry not to have included "odd harmonics" but I guess you know what I
meant.


"Tim Wescott" wrote in message
...
Michael Black wrote:

"John Walton" ) writes:

A Square Wave is all the harmonics of the fundamental, so a 1.9kHz
square
wave will have all the harmonics which will complicate the
measurement --

If you take a square wave and filter it you can get a relatively low
distortion fundamental -- it's much easier than you might think -- use
an
NE556 dual timer to generate 1.9kHz and 1.3kHz square waves, combine
with
10k resistors into an opamp buffer, filter with an MF-10 (now upgraded
to
LMF100) switched capacitor low pass filter -- voila.


And why not start with a sinewave oscillator in the first place? That's
the way it used to be done, two transistors each making up phase shift
oscillators.

A square wave oscillator followed by a filter is, in some ways, easier
to implement than a phase-shift oscillator -- particularly if you want
well controlled frequency and amplitude without having to use
sophisticated AGC circuits.

A sqarewave won't complicate measurements, it will downright give
different results. The whole point of two-tone testing is that the
first tone causes a "carrier" out of the SSB transmitter (the single
tone translates to an RF frequency), and the second tone adds
modulation.
If the tones aren't pure sinewave, the output of the transmitter will
be radically different.

That's probably why the guy doesn't advocate using it: he's recommending
a square wave that's heavily low-pass filtered to get rid of the
harmonics. If done right this will result in a clean stable sine wave
at the desired frequency.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com