# Above 108MHz with FM radio (or other)?

Discussion in 'Electronic Basics' started by Michael J., Dec 10, 2004.

3. ### Bob MyersGuest

A slight nit to pick here, John; there's "mixing" in the sense
that you use it here, and then there's modulation based on
multiplication, which may be something else entirely. A
"mixer", meaning the specific sort of circuit that this term is
generally applied to, does preserve the original frequencies.
However, a "balanced mixer" does not, and the difference is
that the latter actually DOES perform an operation that is
solely a multiplication. In mathematical terms:

sin(A) * sin(B) = 1/2[cos(A-B) - cos(A+B)]

which clearly doesn't have the original frequencies in the output.
So pure multiplication results in DSBSC, which is then often
converted to SSB by filtering out one of the sidebands.
To get the original carrier back, a constant term has to be introduced,
i.e.,

sin(A) * [K + sin(B)]

Here, the K term corresponds to the DC offset which exists
when , for instance, the modulating signal (here, sin(B)) is applied
to the carrier (sin (A)) via a modulation transformer (classically,
in the "plate" circuit of a class C output stage), as is the case very
commonly in commercial AM installations. The result is
conventional "AM", with a "full" carrier.

Just to clarify that for those who may be trying to follow the
terminology, since this IS sci.electronics.basics after all. I
know YOU know this stuff already...

Bob M.

4. ### Steve EvansGuest

whkatever.. i'm so happy for you.. it still doesn't alther the fact
that the majoritiy of his remarks I found on Google were simply
disparaging one-liners often made without the slightest provokation.

5. ### Steve EvansGuest

sorry, roger, but I think Bob Myers is the only person here to have
got it absolutely right.

6. ### Roger JohanssonGuest

Even a double balanced mixer cannot completely remove the two

In this case there is no need for that level of detail. The OP just
wanted a simple way to tune in the air band on a common FM radio.

solution there would have been a need to talk about double balanced
mixers.

7. ### Jonathan KirwanGuest

It's the "disparaging" and "without the slightest provocation" parts that I'd
probably not see eye to eye with you, about. When I first wrote my question
(and you can check google) on these phone indicators that I wanted to operate
without a battery, his reply wasn't exactly sugar and honey -- but it was
exactly the kind of information I didn't have and needed to hear about:
He made the job really hard for me to properly achieve. But then, learning to
get the job done well and right, even for a hobbyist, isn't a bad thing. One
grows from it and learns better why the standards *are* standards, too. In the
end, I developed a circuit that presents 20M continuous (it doesn't present
different momentary loads) to the phone line and still provides me with an LED
indicator. Don's challenge to me was from someone who knew the material and I
worked to meet it.

receiving broadcasts on bands where they aren't supposed to (for experimental
purposes when you are licensed it is one thing, but for general tapping in it is
another), when it looks even something like that's what the question is about.
He pushes those considering such ideas to find another avenue, just as I'm
certain he was pressing me to not improperly hook up equipment to my phone lines
that was ignorant of the intended use and might otherwise interfere with it.

I can respect that.

In any case, he's helped at least one person I know of. Me.

Jon

8. ### Steve EvansGuest

thats fair enouugh, Johathan. You gotta speak as you find.